diff --git a/src/libcore/ops.rs b/src/libcore/ops.rs
index 399aec9afd440..21af342b1bf78 100644
--- a/src/libcore/ops.rs
+++ b/src/libcore/ops.rs
@@ -1145,3 +1145,52 @@ pub trait FnOnce<Args> {
     /// This is called when the call operator is used.
     extern "rust-call" fn call_once(self, args: Args) -> Self::Output;
 }
+
+#[cfg(not(stage0))]
+mod impls {
+    use marker::Sized;
+    use super::{Fn, FnMut, FnOnce};
+
+    impl<'a,A,F:?Sized> Fn<A> for &'a F
+        where F : Fn<A>
+    {
+        extern "rust-call" fn call(&self, args: A) -> F::Output {
+            (**self).call(args)
+        }
+    }
+
+    impl<'a,A,F:?Sized> FnMut<A> for &'a F
+        where F : Fn<A>
+    {
+        extern "rust-call" fn call_mut(&mut self, args: A) -> F::Output {
+            (**self).call(args)
+        }
+    }
+
+    impl<'a,A,F:?Sized> FnOnce<A> for &'a F
+        where F : Fn<A>
+    {
+        type Output = F::Output;
+
+        extern "rust-call" fn call_once(self, args: A) -> F::Output {
+            (*self).call(args)
+        }
+    }
+
+    impl<'a,A,F:?Sized> FnMut<A> for &'a mut F
+        where F : FnMut<A>
+    {
+        extern "rust-call" fn call_mut(&mut self, args: A) -> F::Output {
+            (*self).call_mut(args)
+        }
+    }
+
+    impl<'a,A,F:?Sized> FnOnce<A> for &'a mut F
+        where F : FnMut<A>
+    {
+        type Output = F::Output;
+        extern "rust-call" fn call_once(mut self, args: A) -> F::Output {
+            (*self).call_mut(args)
+        }
+    }
+}
diff --git a/src/librustc/lib.rs b/src/librustc/lib.rs
index b3ad96c4b5f16..a4bb17bc35476 100644
--- a/src/librustc/lib.rs
+++ b/src/librustc/lib.rs
@@ -120,6 +120,8 @@ pub mod middle {
     pub mod traits;
     pub mod ty;
     pub mod ty_fold;
+    pub mod ty_match;
+    pub mod ty_relate;
     pub mod ty_walk;
     pub mod weak_lang_items;
 }
diff --git a/src/librustc/middle/infer/bivariate.rs b/src/librustc/middle/infer/bivariate.rs
index 17b0d788590c4..940dc75271c22 100644
--- a/src/librustc/middle/infer/bivariate.rs
+++ b/src/librustc/middle/infer/bivariate.rs
@@ -25,66 +25,54 @@
 //! In particular, it might be enough to say (A,B) are bivariant for
 //! all (A,B).
 
-use middle::ty::BuiltinBounds;
+use super::combine::{self, CombineFields};
+use super::type_variable::{BiTo};
+
 use middle::ty::{self, Ty};
 use middle::ty::TyVar;
-use middle::infer::combine::*;
-use middle::infer::cres;
-use middle::infer::type_variable::BiTo;
-use util::ppaux::Repr;
+use middle::ty_relate::{Relate, RelateResult, TypeRelation};
+use util::ppaux::{Repr};
 
-pub struct Bivariate<'f, 'tcx: 'f> {
-    fields: CombineFields<'f, 'tcx>
+pub struct Bivariate<'a, 'tcx: 'a> {
+    fields: CombineFields<'a, 'tcx>
 }
 
-#[allow(non_snake_case)]
-pub fn Bivariate<'f, 'tcx>(cf: CombineFields<'f, 'tcx>) -> Bivariate<'f, 'tcx> {
-    Bivariate { fields: cf }
+impl<'a, 'tcx> Bivariate<'a, 'tcx> {
+    pub fn new(fields: CombineFields<'a, 'tcx>) -> Bivariate<'a, 'tcx> {
+        Bivariate { fields: fields }
+    }
 }
 
-impl<'f, 'tcx> Combine<'tcx> for Bivariate<'f, 'tcx> {
-    fn tag(&self) -> String { "Bivariate".to_string() }
-    fn fields<'a>(&'a self) -> &'a CombineFields<'a, 'tcx> { &self.fields }
+impl<'a, 'tcx> TypeRelation<'a, 'tcx> for Bivariate<'a, 'tcx> {
+    fn tag(&self) -> &'static str { "Bivariate" }
 
-    fn tys_with_variance(&self, v: ty::Variance, a: Ty<'tcx>, b: Ty<'tcx>)
-                         -> cres<'tcx, Ty<'tcx>>
-    {
-        match v {
-            ty::Invariant => self.equate().tys(a, b),
-            ty::Covariant => self.tys(a, b),
-            ty::Contravariant => self.tys(a, b),
-            ty::Bivariant => self.tys(a, b),
-        }
-    }
+    fn tcx(&self) -> &'a ty::ctxt<'tcx> { self.fields.tcx() }
 
-    fn regions_with_variance(&self, v: ty::Variance, a: ty::Region, b: ty::Region)
-                             -> cres<'tcx, ty::Region>
-    {
-        match v {
-            ty::Invariant => self.equate().regions(a, b),
-            ty::Covariant => self.regions(a, b),
-            ty::Contravariant => self.regions(a, b),
-            ty::Bivariant => self.regions(a, b),
-        }
-    }
+    fn a_is_expected(&self) -> bool { self.fields.a_is_expected }
 
-    fn regions(&self, a: ty::Region, _: ty::Region) -> cres<'tcx, ty::Region> {
-        Ok(a)
-    }
-
-    fn builtin_bounds(&self,
-                      a: BuiltinBounds,
-                      b: BuiltinBounds)
-                      -> cres<'tcx, BuiltinBounds>
+    fn relate_with_variance<T:Relate<'a,'tcx>>(&mut self,
+                                               variance: ty::Variance,
+                                               a: &T,
+                                               b: &T)
+                                               -> RelateResult<'tcx, T>
     {
-        if a != b {
-            Err(ty::terr_builtin_bounds(expected_found(self, a, b)))
-        } else {
-            Ok(a)
+        match variance {
+            // If we have Foo<A> and Foo is invariant w/r/t A,
+            // and we want to assert that
+            //
+            //     Foo<A> <: Foo<B> ||
+            //     Foo<B> <: Foo<A>
+            //
+            // then still A must equal B.
+            ty::Invariant => self.relate(a, b),
+
+            ty::Covariant => self.relate(a, b),
+            ty::Bivariant => self.relate(a, b),
+            ty::Contravariant => self.relate(a, b),
         }
     }
 
-    fn tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> cres<'tcx, Ty<'tcx>> {
+    fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
         debug!("{}.tys({}, {})", self.tag(),
                a.repr(self.fields.infcx.tcx), b.repr(self.fields.infcx.tcx));
         if a == b { return Ok(a); }
@@ -109,17 +97,22 @@ impl<'f, 'tcx> Combine<'tcx> for Bivariate<'f, 'tcx> {
             }
 
             _ => {
-                super_tys(self, a, b)
+                combine::super_combine_tys(self.fields.infcx, self, a, b)
             }
         }
     }
 
-    fn binders<T>(&self, a: &ty::Binder<T>, b: &ty::Binder<T>) -> cres<'tcx, ty::Binder<T>>
-        where T : Combineable<'tcx>
+    fn regions(&mut self, a: ty::Region, _: ty::Region) -> RelateResult<'tcx, ty::Region> {
+        Ok(a)
+    }
+
+    fn binders<T>(&mut self, a: &ty::Binder<T>, b: &ty::Binder<T>)
+                  -> RelateResult<'tcx, ty::Binder<T>>
+        where T: Relate<'a,'tcx>
     {
         let a1 = ty::erase_late_bound_regions(self.tcx(), a);
         let b1 = ty::erase_late_bound_regions(self.tcx(), b);
-        let c = try!(Combineable::combine(self, &a1, &b1));
+        let c = try!(self.relate(&a1, &b1));
         Ok(ty::Binder(c))
     }
 }
diff --git a/src/librustc/middle/infer/combine.rs b/src/librustc/middle/infer/combine.rs
index 9aa17b2b1d9fe..86f12b669b35e 100644
--- a/src/librustc/middle/infer/combine.rs
+++ b/src/librustc/middle/infer/combine.rs
@@ -37,395 +37,21 @@ use super::equate::Equate;
 use super::glb::Glb;
 use super::lub::Lub;
 use super::sub::Sub;
-use super::unify::InferCtxtMethodsForSimplyUnifiableTypes;
-use super::{InferCtxt, cres};
+use super::{InferCtxt};
 use super::{MiscVariable, TypeTrace};
 use super::type_variable::{RelationDir, BiTo, EqTo, SubtypeOf, SupertypeOf};
 
-use middle::subst;
-use middle::subst::{ErasedRegions, NonerasedRegions, Substs};
-use middle::ty::{FloatVar, FnSig, IntVar, TyVar};
+use middle::ty::{TyVar};
 use middle::ty::{IntType, UintType};
-use middle::ty::BuiltinBounds;
 use middle::ty::{self, Ty};
 use middle::ty_fold;
 use middle::ty_fold::{TypeFolder, TypeFoldable};
+use middle::ty_relate::{self, Relate, RelateResult, TypeRelation};
 use util::ppaux::Repr;
 
-use std::rc::Rc;
-use syntax::ast::Unsafety;
 use syntax::ast;
-use syntax::abi;
 use syntax::codemap::Span;
 
-pub trait Combine<'tcx> : Sized {
-    fn tcx<'a>(&'a self) -> &'a ty::ctxt<'tcx> { self.infcx().tcx }
-    fn tag(&self) -> String;
-
-    fn fields<'a>(&'a self) -> &'a CombineFields<'a, 'tcx>;
-
-    fn infcx<'a>(&'a self) -> &'a InferCtxt<'a, 'tcx> { self.fields().infcx }
-    fn a_is_expected(&self) -> bool { self.fields().a_is_expected }
-    fn trace(&self) -> TypeTrace<'tcx> { self.fields().trace.clone() }
-    fn equate<'a>(&'a self) -> Equate<'a, 'tcx> { self.fields().equate() }
-    fn bivariate<'a>(&'a self) -> Bivariate<'a, 'tcx> { self.fields().bivariate() }
-
-    fn sub<'a>(&'a self) -> Sub<'a, 'tcx> { self.fields().sub() }
-    fn lub<'a>(&'a self) -> Lub<'a, 'tcx> { Lub(self.fields().clone()) }
-    fn glb<'a>(&'a self) -> Glb<'a, 'tcx> { Glb(self.fields().clone()) }
-
-    fn mts(&self, a: &ty::mt<'tcx>, b: &ty::mt<'tcx>) -> cres<'tcx, ty::mt<'tcx>> {
-        debug!("{}.mts({}, {})",
-               self.tag(),
-               a.repr(self.tcx()),
-               b.repr(self.tcx()));
-
-        if a.mutbl != b.mutbl {
-            Err(ty::terr_mutability)
-        } else {
-            let mutbl = a.mutbl;
-            let variance = match mutbl {
-                ast::MutImmutable => ty::Covariant,
-                ast::MutMutable => ty::Invariant,
-            };
-            let ty = try!(self.tys_with_variance(variance, a.ty, b.ty));
-            Ok(ty::mt {ty: ty, mutbl: mutbl})
-        }
-    }
-
-    fn tys_with_variance(&self, variance: ty::Variance, a: Ty<'tcx>, b: Ty<'tcx>)
-                         -> cres<'tcx, Ty<'tcx>>;
-
-    fn tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> cres<'tcx, Ty<'tcx>>;
-
-    fn regions_with_variance(&self, variance: ty::Variance, a: ty::Region, b: ty::Region)
-                             -> cres<'tcx, ty::Region>;
-
-    fn regions(&self, a: ty::Region, b: ty::Region) -> cres<'tcx, ty::Region>;
-
-    fn substs(&self,
-              item_def_id: ast::DefId,
-              a_subst: &subst::Substs<'tcx>,
-              b_subst: &subst::Substs<'tcx>)
-              -> cres<'tcx, subst::Substs<'tcx>>
-    {
-        debug!("substs: item_def_id={} a_subst={} b_subst={}",
-               item_def_id.repr(self.infcx().tcx),
-               a_subst.repr(self.infcx().tcx),
-               b_subst.repr(self.infcx().tcx));
-
-        let variances = if self.infcx().tcx.variance_computed.get() {
-            Some(ty::item_variances(self.infcx().tcx, item_def_id))
-        } else {
-            None
-        };
-        self.substs_variances(variances.as_ref().map(|v| &**v), a_subst, b_subst)
-    }
-
-    fn substs_variances(&self,
-                        variances: Option<&ty::ItemVariances>,
-                        a_subst: &subst::Substs<'tcx>,
-                        b_subst: &subst::Substs<'tcx>)
-                        -> cres<'tcx, subst::Substs<'tcx>>
-    {
-        let mut substs = subst::Substs::empty();
-
-        for &space in &subst::ParamSpace::all() {
-            let a_tps = a_subst.types.get_slice(space);
-            let b_tps = b_subst.types.get_slice(space);
-            let t_variances = variances.map(|v| v.types.get_slice(space));
-            let tps = try!(relate_type_params(self, t_variances, a_tps, b_tps));
-            substs.types.replace(space, tps);
-        }
-
-        match (&a_subst.regions, &b_subst.regions) {
-            (&ErasedRegions, _) | (_, &ErasedRegions) => {
-                substs.regions = ErasedRegions;
-            }
-
-            (&NonerasedRegions(ref a), &NonerasedRegions(ref b)) => {
-                for &space in &subst::ParamSpace::all() {
-                    let a_regions = a.get_slice(space);
-                    let b_regions = b.get_slice(space);
-                    let r_variances = variances.map(|v| v.regions.get_slice(space));
-                    let regions = try!(relate_region_params(self,
-                                                            r_variances,
-                                                            a_regions,
-                                                            b_regions));
-                    substs.mut_regions().replace(space, regions);
-                }
-            }
-        }
-
-        return Ok(substs);
-
-        fn relate_type_params<'tcx, C: Combine<'tcx>>(this: &C,
-                                                      variances: Option<&[ty::Variance]>,
-                                                      a_tys: &[Ty<'tcx>],
-                                                      b_tys: &[Ty<'tcx>])
-                                                      -> cres<'tcx, Vec<Ty<'tcx>>>
-        {
-            if a_tys.len() != b_tys.len() {
-                return Err(ty::terr_ty_param_size(expected_found(this,
-                                                                 a_tys.len(),
-                                                                 b_tys.len())));
-            }
-
-            (0.. a_tys.len()).map(|i| {
-                let a_ty = a_tys[i];
-                let b_ty = b_tys[i];
-                let v = variances.map_or(ty::Invariant, |v| v[i]);
-                this.tys_with_variance(v, a_ty, b_ty)
-            }).collect()
-        }
-
-        fn relate_region_params<'tcx, C: Combine<'tcx>>(this: &C,
-                                                        variances: Option<&[ty::Variance]>,
-                                                        a_rs: &[ty::Region],
-                                                        b_rs: &[ty::Region])
-                                                        -> cres<'tcx, Vec<ty::Region>>
-        {
-            let tcx = this.infcx().tcx;
-            let num_region_params = a_rs.len();
-
-            debug!("relate_region_params(\
-                   a_rs={}, \
-                   b_rs={},
-                   variances={})",
-                   a_rs.repr(tcx),
-                   b_rs.repr(tcx),
-                   variances.repr(tcx));
-
-            assert_eq!(num_region_params,
-                       variances.map_or(num_region_params,
-                                        |v| v.len()));
-
-            assert_eq!(num_region_params, b_rs.len());
-
-            (0..a_rs.len()).map(|i| {
-                let a_r = a_rs[i];
-                let b_r = b_rs[i];
-                let variance = variances.map_or(ty::Invariant, |v| v[i]);
-                this.regions_with_variance(variance, a_r, b_r)
-            }).collect()
-        }
-    }
-
-    fn bare_fn_tys(&self, a: &ty::BareFnTy<'tcx>,
-                   b: &ty::BareFnTy<'tcx>) -> cres<'tcx, ty::BareFnTy<'tcx>> {
-        let unsafety = try!(self.unsafeties(a.unsafety, b.unsafety));
-        let abi = try!(self.abi(a.abi, b.abi));
-        let sig = try!(self.binders(&a.sig, &b.sig));
-        Ok(ty::BareFnTy {unsafety: unsafety,
-                         abi: abi,
-                         sig: sig})
-    }
-
-    fn fn_sigs(&self, a: &ty::FnSig<'tcx>, b: &ty::FnSig<'tcx>) -> cres<'tcx, ty::FnSig<'tcx>> {
-        if a.variadic != b.variadic {
-            return Err(ty::terr_variadic_mismatch(expected_found(self, a.variadic, b.variadic)));
-        }
-
-        let inputs = try!(argvecs(self,
-                                  &a.inputs,
-                                  &b.inputs));
-
-        let output = try!(match (a.output, b.output) {
-            (ty::FnConverging(a_ty), ty::FnConverging(b_ty)) =>
-                Ok(ty::FnConverging(try!(self.tys(a_ty, b_ty)))),
-            (ty::FnDiverging, ty::FnDiverging) =>
-                Ok(ty::FnDiverging),
-            (a, b) =>
-                Err(ty::terr_convergence_mismatch(
-                    expected_found(self, a != ty::FnDiverging, b != ty::FnDiverging))),
-        });
-
-        return Ok(ty::FnSig {inputs: inputs,
-                             output: output,
-                             variadic: a.variadic});
-
-
-        fn argvecs<'tcx, C>(combiner: &C,
-                            a_args: &[Ty<'tcx>],
-                            b_args: &[Ty<'tcx>])
-                            -> cres<'tcx, Vec<Ty<'tcx>>>
-                            where C: Combine<'tcx> {
-            if a_args.len() == b_args.len() {
-                a_args.iter().zip(b_args.iter())
-                    .map(|(a, b)| combiner.args(*a, *b)).collect()
-            } else {
-                Err(ty::terr_arg_count)
-            }
-        }
-    }
-
-    fn args(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> cres<'tcx, Ty<'tcx>> {
-        self.tys_with_variance(ty::Contravariant, a, b).and_then(|t| Ok(t))
-    }
-
-    fn unsafeties(&self, a: Unsafety, b: Unsafety) -> cres<'tcx, Unsafety> {
-        if a != b {
-            Err(ty::terr_unsafety_mismatch(expected_found(self, a, b)))
-        } else {
-            Ok(a)
-        }
-    }
-
-    fn abi(&self, a: abi::Abi, b: abi::Abi) -> cres<'tcx, abi::Abi> {
-        if a == b {
-            Ok(a)
-        } else {
-            Err(ty::terr_abi_mismatch(expected_found(self, a, b)))
-        }
-    }
-
-    fn projection_tys(&self,
-                      a: &ty::ProjectionTy<'tcx>,
-                      b: &ty::ProjectionTy<'tcx>)
-                      -> cres<'tcx, ty::ProjectionTy<'tcx>>
-    {
-        if a.item_name != b.item_name {
-            Err(ty::terr_projection_name_mismatched(
-                expected_found(self, a.item_name, b.item_name)))
-        } else {
-            let trait_ref = try!(self.trait_refs(&*a.trait_ref, &*b.trait_ref));
-            Ok(ty::ProjectionTy { trait_ref: Rc::new(trait_ref), item_name: a.item_name })
-        }
-    }
-
-    fn projection_predicates(&self,
-                             a: &ty::ProjectionPredicate<'tcx>,
-                             b: &ty::ProjectionPredicate<'tcx>)
-                             -> cres<'tcx, ty::ProjectionPredicate<'tcx>>
-    {
-        let projection_ty = try!(self.projection_tys(&a.projection_ty, &b.projection_ty));
-        let ty = try!(self.tys(a.ty, b.ty));
-        Ok(ty::ProjectionPredicate { projection_ty: projection_ty, ty: ty })
-    }
-
-    fn projection_bounds(&self,
-                         a: &Vec<ty::PolyProjectionPredicate<'tcx>>,
-                         b: &Vec<ty::PolyProjectionPredicate<'tcx>>)
-                         -> cres<'tcx, Vec<ty::PolyProjectionPredicate<'tcx>>>
-    {
-        // To be compatible, `a` and `b` must be for precisely the
-        // same set of traits and item names. We always require that
-        // projection bounds lists are sorted by trait-def-id and item-name,
-        // so we can just iterate through the lists pairwise, so long as they are the
-        // same length.
-        if a.len() != b.len() {
-            Err(ty::terr_projection_bounds_length(expected_found(self, a.len(), b.len())))
-        } else {
-            a.iter()
-                .zip(b.iter())
-                .map(|(a, b)| self.binders(a, b))
-                .collect()
-        }
-    }
-
-    fn existential_bounds(&self,
-                          a: &ty::ExistentialBounds<'tcx>,
-                          b: &ty::ExistentialBounds<'tcx>)
-                          -> cres<'tcx, ty::ExistentialBounds<'tcx>>
-    {
-        let r = try!(self.regions_with_variance(ty::Contravariant, a.region_bound, b.region_bound));
-        let nb = try!(self.builtin_bounds(a.builtin_bounds, b.builtin_bounds));
-        let pb = try!(self.projection_bounds(&a.projection_bounds, &b.projection_bounds));
-        Ok(ty::ExistentialBounds { region_bound: r,
-                                   builtin_bounds: nb,
-                                   projection_bounds: pb })
-    }
-
-    fn builtin_bounds(&self,
-                      a: BuiltinBounds,
-                      b: BuiltinBounds)
-                      -> cres<'tcx, BuiltinBounds>
-    {
-        // Two sets of builtin bounds are only relatable if they are
-        // precisely the same (but see the coercion code).
-        if a != b {
-            Err(ty::terr_builtin_bounds(expected_found(self, a, b)))
-        } else {
-            Ok(a)
-        }
-    }
-
-    fn trait_refs(&self,
-                  a: &ty::TraitRef<'tcx>,
-                  b: &ty::TraitRef<'tcx>)
-                  -> cres<'tcx, ty::TraitRef<'tcx>>
-    {
-        // Different traits cannot be related
-        if a.def_id != b.def_id {
-            Err(ty::terr_traits(expected_found(self, a.def_id, b.def_id)))
-        } else {
-            let substs = try!(self.substs(a.def_id, a.substs, b.substs));
-            Ok(ty::TraitRef { def_id: a.def_id, substs: self.tcx().mk_substs(substs) })
-        }
-    }
-
-    fn binders<T>(&self, a: &ty::Binder<T>, b: &ty::Binder<T>) -> cres<'tcx, ty::Binder<T>>
-        where T : Combineable<'tcx>;
-    // this must be overridden to do correctly, so as to account for higher-ranked
-    // behavior
-}
-
-pub trait Combineable<'tcx> : Repr<'tcx> + TypeFoldable<'tcx> {
-    fn combine<C:Combine<'tcx>>(combiner: &C, a: &Self, b: &Self) -> cres<'tcx, Self>;
-}
-
-impl<'tcx,T> Combineable<'tcx> for Rc<T>
-    where T : Combineable<'tcx>
-{
-    fn combine<C>(combiner: &C,
-                  a: &Rc<T>,
-                  b: &Rc<T>)
-                  -> cres<'tcx, Rc<T>>
-                  where C: Combine<'tcx> {
-        Ok(Rc::new(try!(Combineable::combine(combiner, &**a, &**b))))
-    }
-}
-
-impl<'tcx> Combineable<'tcx> for ty::TraitRef<'tcx> {
-    fn combine<C>(combiner: &C,
-                  a: &ty::TraitRef<'tcx>,
-                  b: &ty::TraitRef<'tcx>)
-                  -> cres<'tcx, ty::TraitRef<'tcx>>
-                  where C: Combine<'tcx> {
-        combiner.trait_refs(a, b)
-    }
-}
-
-impl<'tcx> Combineable<'tcx> for Ty<'tcx> {
-    fn combine<C>(combiner: &C,
-                  a: &Ty<'tcx>,
-                  b: &Ty<'tcx>)
-                  -> cres<'tcx, Ty<'tcx>>
-                  where C: Combine<'tcx> {
-        combiner.tys(*a, *b)
-    }
-}
-
-impl<'tcx> Combineable<'tcx> for ty::ProjectionPredicate<'tcx> {
-    fn combine<C>(combiner: &C,
-                  a: &ty::ProjectionPredicate<'tcx>,
-                  b: &ty::ProjectionPredicate<'tcx>)
-                  -> cres<'tcx, ty::ProjectionPredicate<'tcx>>
-                  where C: Combine<'tcx> {
-        combiner.projection_predicates(a, b)
-    }
-}
-
-impl<'tcx> Combineable<'tcx> for ty::FnSig<'tcx> {
-    fn combine<C>(combiner: &C,
-                  a: &ty::FnSig<'tcx>,
-                  b: &ty::FnSig<'tcx>)
-                  -> cres<'tcx, ty::FnSig<'tcx>>
-                  where C: Combine<'tcx> {
-        combiner.fn_sigs(a, b)
-    }
-}
-
 #[derive(Clone)]
 pub struct CombineFields<'a, 'tcx: 'a> {
     pub infcx: &'a InferCtxt<'a, 'tcx>,
@@ -433,234 +59,133 @@ pub struct CombineFields<'a, 'tcx: 'a> {
     pub trace: TypeTrace<'tcx>,
 }
 
-pub fn expected_found<'tcx, C, T>(this: &C,
-                                  a: T,
-                                  b: T)
-                                  -> ty::expected_found<T>
-                                  where C: Combine<'tcx> {
-    if this.a_is_expected() {
-        ty::expected_found {expected: a, found: b}
-    } else {
-        ty::expected_found {expected: b, found: a}
-    }
-}
-
-pub fn super_tys<'tcx, C>(this: &C,
-                          a: Ty<'tcx>,
-                          b: Ty<'tcx>)
-                          -> cres<'tcx, Ty<'tcx>>
-                          where C: Combine<'tcx> {
-    let tcx = this.infcx().tcx;
-    let a_sty = &a.sty;
-    let b_sty = &b.sty;
-    debug!("super_tys: a_sty={:?} b_sty={:?}", a_sty, b_sty);
-    return match (a_sty, b_sty) {
-        // The "subtype" ought to be handling cases involving var:
-        (&ty::ty_infer(TyVar(_)), _)
-        | (_, &ty::ty_infer(TyVar(_))) =>
-            tcx.sess.bug(
-                &format!("{}: bot and var types should have been handled ({},{})",
-                this.tag(),
-                a.repr(this.infcx().tcx),
-                b.repr(this.infcx().tcx))),
-
-        (&ty::ty_err, _) | (_, &ty::ty_err) => Ok(tcx.types.err),
+pub fn super_combine_tys<'a,'tcx:'a,R>(infcx: &InferCtxt<'a, 'tcx>,
+                                       relation: &mut R,
+                                       a: Ty<'tcx>,
+                                       b: Ty<'tcx>)
+                                       -> RelateResult<'tcx, Ty<'tcx>>
+    where R: TypeRelation<'a,'tcx>
+{
+    let a_is_expected = relation.a_is_expected();
 
+    match (&a.sty, &b.sty) {
         // Relate integral variables to other types
-        (&ty::ty_infer(IntVar(a_id)), &ty::ty_infer(IntVar(b_id))) => {
-            try!(this.infcx().simple_vars(this.a_is_expected(),
-                                            a_id, b_id));
+        (&ty::ty_infer(ty::IntVar(a_id)), &ty::ty_infer(ty::IntVar(b_id))) => {
+            try!(infcx.int_unification_table
+                      .borrow_mut()
+                      .unify_var_var(a_id, b_id)
+                      .map_err(|e| int_unification_error(a_is_expected, e)));
             Ok(a)
         }
-        (&ty::ty_infer(IntVar(v_id)), &ty::ty_int(v)) => {
-            unify_integral_variable(this, this.a_is_expected(),
-                                    v_id, IntType(v))
+        (&ty::ty_infer(ty::IntVar(v_id)), &ty::ty_int(v)) => {
+            unify_integral_variable(infcx, a_is_expected, v_id, IntType(v))
         }
-        (&ty::ty_int(v), &ty::ty_infer(IntVar(v_id))) => {
-            unify_integral_variable(this, !this.a_is_expected(),
-                                    v_id, IntType(v))
+        (&ty::ty_int(v), &ty::ty_infer(ty::IntVar(v_id))) => {
+            unify_integral_variable(infcx, !a_is_expected, v_id, IntType(v))
         }
-        (&ty::ty_infer(IntVar(v_id)), &ty::ty_uint(v)) => {
-            unify_integral_variable(this, this.a_is_expected(),
-                                    v_id, UintType(v))
+        (&ty::ty_infer(ty::IntVar(v_id)), &ty::ty_uint(v)) => {
+            unify_integral_variable(infcx, a_is_expected, v_id, UintType(v))
         }
-        (&ty::ty_uint(v), &ty::ty_infer(IntVar(v_id))) => {
-            unify_integral_variable(this, !this.a_is_expected(),
-                                    v_id, UintType(v))
+        (&ty::ty_uint(v), &ty::ty_infer(ty::IntVar(v_id))) => {
+            unify_integral_variable(infcx, !a_is_expected, v_id, UintType(v))
         }
 
         // Relate floating-point variables to other types
-        (&ty::ty_infer(FloatVar(a_id)), &ty::ty_infer(FloatVar(b_id))) => {
-            try!(this.infcx().simple_vars(this.a_is_expected(), a_id, b_id));
+        (&ty::ty_infer(ty::FloatVar(a_id)), &ty::ty_infer(ty::FloatVar(b_id))) => {
+            try!(infcx.float_unification_table
+                      .borrow_mut()
+                      .unify_var_var(a_id, b_id)
+                      .map_err(|e| float_unification_error(relation.a_is_expected(), e)));
             Ok(a)
         }
-        (&ty::ty_infer(FloatVar(v_id)), &ty::ty_float(v)) => {
-            unify_float_variable(this, this.a_is_expected(), v_id, v)
-        }
-        (&ty::ty_float(v), &ty::ty_infer(FloatVar(v_id))) => {
-            unify_float_variable(this, !this.a_is_expected(), v_id, v)
+        (&ty::ty_infer(ty::FloatVar(v_id)), &ty::ty_float(v)) => {
+            unify_float_variable(infcx, a_is_expected, v_id, v)
         }
-
-        (&ty::ty_char, _)
-        | (&ty::ty_bool, _)
-        | (&ty::ty_int(_), _)
-        | (&ty::ty_uint(_), _)
-        | (&ty::ty_float(_), _) => {
-            if a == b {
-                Ok(a)
-            } else {
-                Err(ty::terr_sorts(expected_found(this, a, b)))
-            }
+        (&ty::ty_float(v), &ty::ty_infer(ty::FloatVar(v_id))) => {
+            unify_float_variable(infcx, !a_is_expected, v_id, v)
         }
 
-        (&ty::ty_param(ref a_p), &ty::ty_param(ref b_p)) if
-          a_p.idx == b_p.idx && a_p.space == b_p.space => Ok(a),
-
-        (&ty::ty_enum(a_id, a_substs), &ty::ty_enum(b_id, b_substs))
-          if a_id == b_id => {
-            let substs = try!(this.substs(a_id, a_substs, b_substs));
-            Ok(ty::mk_enum(tcx, a_id, tcx.mk_substs(substs)))
+        // All other cases of inference are errors
+        (&ty::ty_infer(_), _) |
+        (_, &ty::ty_infer(_)) => {
+            Err(ty::terr_sorts(ty_relate::expected_found(relation, &a, &b)))
         }
 
-        (&ty::ty_trait(ref a_), &ty::ty_trait(ref b_)) => {
-            debug!("Trying to match traits {:?} and {:?}", a, b);
-            let principal = try!(this.binders(&a_.principal, &b_.principal));
-            let bounds = try!(this.existential_bounds(&a_.bounds, &b_.bounds));
-            Ok(ty::mk_trait(tcx, principal, bounds))
-        }
-
-        (&ty::ty_struct(a_id, a_substs), &ty::ty_struct(b_id, b_substs))
-          if a_id == b_id => {
-            let substs = try!(this.substs(a_id, a_substs, b_substs));
-            Ok(ty::mk_struct(tcx, a_id, tcx.mk_substs(substs)))
-        }
-
-        (&ty::ty_closure(a_id, a_substs),
-         &ty::ty_closure(b_id, b_substs))
-          if a_id == b_id => {
-            // All ty_closure types with the same id represent
-            // the (anonymous) type of the same closure expression. So
-            // all of their regions should be equated.
-            let substs = try!(this.substs_variances(None, a_substs, b_substs));
-            Ok(ty::mk_closure(tcx, a_id, tcx.mk_substs(substs)))
-        }
 
-        (&ty::ty_uniq(a_inner), &ty::ty_uniq(b_inner)) => {
-            let typ = try!(this.tys(a_inner, b_inner));
-            Ok(ty::mk_uniq(tcx, typ))
-        }
-
-        (&ty::ty_ptr(ref a_mt), &ty::ty_ptr(ref b_mt)) => {
-            let mt = try!(this.mts(a_mt, b_mt));
-            Ok(ty::mk_ptr(tcx, mt))
-        }
-
-        (&ty::ty_rptr(a_r, ref a_mt), &ty::ty_rptr(b_r, ref b_mt)) => {
-            let r = try!(this.regions_with_variance(ty::Contravariant, *a_r, *b_r));
-            let mt = try!(this.mts(a_mt, b_mt));
-            Ok(ty::mk_rptr(tcx, tcx.mk_region(r), mt))
-        }
-
-        (&ty::ty_vec(a_t, Some(sz_a)), &ty::ty_vec(b_t, Some(sz_b))) => {
-            this.tys(a_t, b_t).and_then(|t| {
-                if sz_a == sz_b {
-                    Ok(ty::mk_vec(tcx, t, Some(sz_a)))
-                } else {
-                    Err(ty::terr_fixed_array_size(expected_found(this, sz_a, sz_b)))
-                }
-            })
-        }
-
-        (&ty::ty_vec(a_t, sz_a), &ty::ty_vec(b_t, sz_b)) => {
-            this.tys(a_t, b_t).and_then(|t| {
-                if sz_a == sz_b {
-                    Ok(ty::mk_vec(tcx, t, sz_a))
-                } else {
-                    Err(ty::terr_sorts(expected_found(this, a, b)))
-                }
-            })
-        }
-
-        (&ty::ty_str, &ty::ty_str) => Ok(ty::mk_str(tcx)),
-
-        (&ty::ty_tup(ref as_), &ty::ty_tup(ref bs)) => {
-            if as_.len() == bs.len() {
-                as_.iter().zip(bs.iter())
-                   .map(|(a, b)| this.tys(*a, *b))
-                   .collect::<Result<_, _>>()
-                   .map(|ts| ty::mk_tup(tcx, ts))
-            } else if as_.len() != 0 && bs.len() != 0 {
-                Err(ty::terr_tuple_size(
-                    expected_found(this, as_.len(), bs.len())))
-            } else {
-                Err(ty::terr_sorts(expected_found(this, a, b)))
-            }
-        }
-
-        (&ty::ty_bare_fn(a_opt_def_id, a_fty), &ty::ty_bare_fn(b_opt_def_id, b_fty))
-            if a_opt_def_id == b_opt_def_id =>
-        {
-            let fty = try!(this.bare_fn_tys(a_fty, b_fty));
-            Ok(ty::mk_bare_fn(tcx, a_opt_def_id, tcx.mk_bare_fn(fty)))
-        }
-
-        (&ty::ty_projection(ref a_data), &ty::ty_projection(ref b_data)) => {
-            let projection_ty = try!(this.projection_tys(a_data, b_data));
-            Ok(ty::mk_projection(tcx, projection_ty.trait_ref, projection_ty.item_name))
-        }
-
-        _ => Err(ty::terr_sorts(expected_found(this, a, b))),
-    };
-
-    fn unify_integral_variable<'tcx, C>(this: &C,
-                                        vid_is_expected: bool,
-                                        vid: ty::IntVid,
-                                        val: ty::IntVarValue)
-                                        -> cres<'tcx, Ty<'tcx>>
-                                        where C: Combine<'tcx> {
-        try!(this.infcx().simple_var_t(vid_is_expected, vid, val));
-        match val {
-            IntType(v) => Ok(ty::mk_mach_int(this.tcx(), v)),
-            UintType(v) => Ok(ty::mk_mach_uint(this.tcx(), v)),
+        _ => {
+            ty_relate::super_relate_tys(relation, a, b)
         }
     }
+}
 
-    fn unify_float_variable<'tcx, C>(this: &C,
-                                     vid_is_expected: bool,
-                                     vid: ty::FloatVid,
-                                     val: ast::FloatTy)
-                                     -> cres<'tcx, Ty<'tcx>>
-                                     where C: Combine<'tcx> {
-        try!(this.infcx().simple_var_t(vid_is_expected, vid, val));
-        Ok(ty::mk_mach_float(this.tcx(), val))
+fn unify_integral_variable<'a,'tcx>(infcx: &InferCtxt<'a,'tcx>,
+                                    vid_is_expected: bool,
+                                    vid: ty::IntVid,
+                                    val: ty::IntVarValue)
+                                    -> RelateResult<'tcx, Ty<'tcx>>
+{
+    try!(infcx
+         .int_unification_table
+         .borrow_mut()
+         .unify_var_value(vid, val)
+         .map_err(|e| int_unification_error(vid_is_expected, e)));
+    match val {
+        IntType(v) => Ok(ty::mk_mach_int(infcx.tcx, v)),
+        UintType(v) => Ok(ty::mk_mach_uint(infcx.tcx, v)),
     }
 }
 
-impl<'f, 'tcx> CombineFields<'f, 'tcx> {
-    pub fn switch_expected(&self) -> CombineFields<'f, 'tcx> {
+fn unify_float_variable<'a,'tcx>(infcx: &InferCtxt<'a,'tcx>,
+                                 vid_is_expected: bool,
+                                 vid: ty::FloatVid,
+                                 val: ast::FloatTy)
+                                 -> RelateResult<'tcx, Ty<'tcx>>
+{
+    try!(infcx
+         .float_unification_table
+         .borrow_mut()
+         .unify_var_value(vid, val)
+         .map_err(|e| float_unification_error(vid_is_expected, e)));
+    Ok(ty::mk_mach_float(infcx.tcx, val))
+}
+
+impl<'a, 'tcx> CombineFields<'a, 'tcx> {
+    pub fn tcx(&self) -> &'a ty::ctxt<'tcx> {
+        self.infcx.tcx
+    }
+
+    pub fn switch_expected(&self) -> CombineFields<'a, 'tcx> {
         CombineFields {
             a_is_expected: !self.a_is_expected,
             ..(*self).clone()
         }
     }
 
-    fn equate(&self) -> Equate<'f, 'tcx> {
-        Equate((*self).clone())
+    pub fn equate(&self) -> Equate<'a, 'tcx> {
+        Equate::new(self.clone())
+    }
+
+    pub fn bivariate(&self) -> Bivariate<'a, 'tcx> {
+        Bivariate::new(self.clone())
+    }
+
+    pub fn sub(&self) -> Sub<'a, 'tcx> {
+        Sub::new(self.clone())
     }
 
-    fn bivariate(&self) -> Bivariate<'f, 'tcx> {
-        Bivariate((*self).clone())
+    pub fn lub(&self) -> Lub<'a, 'tcx> {
+        Lub::new(self.clone())
     }
 
-    fn sub(&self) -> Sub<'f, 'tcx> {
-        Sub((*self).clone())
+    pub fn glb(&self) -> Glb<'a, 'tcx> {
+        Glb::new(self.clone())
     }
 
     pub fn instantiate(&self,
                        a_ty: Ty<'tcx>,
                        dir: RelationDir,
                        b_vid: ty::TyVid)
-                       -> cres<'tcx, ()>
+                       -> RelateResult<'tcx, ()>
     {
         let tcx = self.infcx.tcx;
         let mut stack = Vec::new();
@@ -724,15 +249,12 @@ impl<'f, 'tcx> CombineFields<'f, 'tcx> {
             // relations wind up attributed to the same spans. We need
             // to associate causes/spans with each of the relations in
             // the stack to get this right.
-            match dir {
-                BiTo => try!(self.bivariate().tys(a_ty, b_ty)),
-
-                EqTo => try!(self.equate().tys(a_ty, b_ty)),
-
-                SubtypeOf => try!(self.sub().tys(a_ty, b_ty)),
-
-                SupertypeOf => try!(self.sub().tys_with_variance(ty::Contravariant, a_ty, b_ty)),
-            };
+            try!(match dir {
+                BiTo => self.bivariate().relate(&a_ty, &b_ty),
+                EqTo => self.equate().relate(&a_ty, &b_ty),
+                SubtypeOf => self.sub().relate(&a_ty, &b_ty),
+                SupertypeOf => self.sub().relate_with_variance(ty::Contravariant, &a_ty, &b_ty),
+            });
         }
 
         Ok(())
@@ -746,7 +268,7 @@ impl<'f, 'tcx> CombineFields<'f, 'tcx> {
                   ty: Ty<'tcx>,
                   for_vid: ty::TyVid,
                   make_region_vars: bool)
-                  -> cres<'tcx, Ty<'tcx>>
+                  -> RelateResult<'tcx, Ty<'tcx>>
     {
         let mut generalize = Generalizer {
             infcx: self.infcx,
@@ -839,3 +361,37 @@ impl<'cx, 'tcx> ty_fold::TypeFolder<'tcx> for Generalizer<'cx, 'tcx> {
         self.infcx.next_region_var(MiscVariable(self.span))
     }
 }
+
+pub trait RelateResultCompare<'tcx, T> {
+    fn compare<F>(&self, t: T, f: F) -> RelateResult<'tcx, T> where
+        F: FnOnce() -> ty::type_err<'tcx>;
+}
+
+impl<'tcx, T:Clone + PartialEq> RelateResultCompare<'tcx, T> for RelateResult<'tcx, T> {
+    fn compare<F>(&self, t: T, f: F) -> RelateResult<'tcx, T> where
+        F: FnOnce() -> ty::type_err<'tcx>,
+    {
+        self.clone().and_then(|s| {
+            if s == t {
+                self.clone()
+            } else {
+                Err(f())
+            }
+        })
+    }
+}
+
+fn int_unification_error<'tcx>(a_is_expected: bool, v: (ty::IntVarValue, ty::IntVarValue))
+                               -> ty::type_err<'tcx>
+{
+    let (a, b) = v;
+    ty::terr_int_mismatch(ty_relate::expected_found_bool(a_is_expected, &a, &b))
+}
+
+fn float_unification_error<'tcx>(a_is_expected: bool,
+                                 v: (ast::FloatTy, ast::FloatTy))
+                                 -> ty::type_err<'tcx>
+{
+    let (a, b) = v;
+    ty::terr_float_mismatch(ty_relate::expected_found_bool(a_is_expected, &a, &b))
+}
diff --git a/src/librustc/middle/infer/equate.rs b/src/librustc/middle/infer/equate.rs
index 59ed2dfd24f25..2003f459d89b4 100644
--- a/src/librustc/middle/infer/equate.rs
+++ b/src/librustc/middle/infer/equate.rs
@@ -8,51 +8,43 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use super::combine::{self, CombineFields};
+use super::higher_ranked::HigherRankedRelations;
+use super::{Subtype};
+use super::type_variable::{EqTo};
+
 use middle::ty::{self, Ty};
 use middle::ty::TyVar;
-use middle::infer::combine::*;
-use middle::infer::cres;
-use middle::infer::Subtype;
-use middle::infer::type_variable::EqTo;
-use util::ppaux::Repr;
+use middle::ty_relate::{Relate, RelateResult, TypeRelation};
+use util::ppaux::{Repr};
 
-pub struct Equate<'f, 'tcx: 'f> {
-    fields: CombineFields<'f, 'tcx>
+pub struct Equate<'a, 'tcx: 'a> {
+    fields: CombineFields<'a, 'tcx>
 }
 
-#[allow(non_snake_case)]
-pub fn Equate<'f, 'tcx>(cf: CombineFields<'f, 'tcx>) -> Equate<'f, 'tcx> {
-    Equate { fields: cf }
+impl<'a, 'tcx> Equate<'a, 'tcx> {
+    pub fn new(fields: CombineFields<'a, 'tcx>) -> Equate<'a, 'tcx> {
+        Equate { fields: fields }
+    }
 }
 
-impl<'f, 'tcx> Combine<'tcx> for Equate<'f, 'tcx> {
-    fn tag(&self) -> String { "Equate".to_string() }
-    fn fields<'a>(&'a self) -> &'a CombineFields<'a, 'tcx> { &self.fields }
+impl<'a, 'tcx> TypeRelation<'a,'tcx> for Equate<'a, 'tcx> {
+    fn tag(&self) -> &'static str { "Equate" }
 
-    fn tys_with_variance(&self, _: ty::Variance, a: Ty<'tcx>, b: Ty<'tcx>)
-                         -> cres<'tcx, Ty<'tcx>>
-    {
-        // Once we're equating, it doesn't matter what the variance is.
-        self.tys(a, b)
-    }
+    fn tcx(&self) -> &'a ty::ctxt<'tcx> { self.fields.tcx() }
 
-    fn regions_with_variance(&self, _: ty::Variance, a: ty::Region, b: ty::Region)
-                             -> cres<'tcx, ty::Region>
-    {
-        // Once we're equating, it doesn't matter what the variance is.
-        self.regions(a, b)
-    }
+    fn a_is_expected(&self) -> bool { self.fields.a_is_expected }
 
-    fn regions(&self, a: ty::Region, b: ty::Region) -> cres<'tcx, ty::Region> {
-        debug!("{}.regions({}, {})",
-               self.tag(),
-               a.repr(self.fields.infcx.tcx),
-               b.repr(self.fields.infcx.tcx));
-        self.infcx().region_vars.make_eqregion(Subtype(self.trace()), a, b);
-        Ok(a)
+    fn relate_with_variance<T:Relate<'a,'tcx>>(&mut self,
+                                               _: ty::Variance,
+                                               a: &T,
+                                               b: &T)
+                                               -> RelateResult<'tcx, T>
+    {
+        self.relate(a, b)
     }
 
-    fn tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> cres<'tcx, Ty<'tcx>> {
+    fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
         debug!("{}.tys({}, {})", self.tag(),
                a.repr(self.fields.infcx.tcx), b.repr(self.fields.infcx.tcx));
         if a == b { return Ok(a); }
@@ -77,15 +69,26 @@ impl<'f, 'tcx> Combine<'tcx> for Equate<'f, 'tcx> {
             }
 
             _ => {
-                super_tys(self, a, b)
+                combine::super_combine_tys(self.fields.infcx, self, a, b)
             }
         }
     }
 
-    fn binders<T>(&self, a: &ty::Binder<T>, b: &ty::Binder<T>) -> cres<'tcx, ty::Binder<T>>
-        where T : Combineable<'tcx>
+    fn regions(&mut self, a: ty::Region, b: ty::Region) -> RelateResult<'tcx, ty::Region> {
+        debug!("{}.regions({}, {})",
+               self.tag(),
+               a.repr(self.fields.infcx.tcx),
+               b.repr(self.fields.infcx.tcx));
+        let origin = Subtype(self.fields.trace.clone());
+        self.fields.infcx.region_vars.make_eqregion(origin, a, b);
+        Ok(a)
+    }
+
+    fn binders<T>(&mut self, a: &ty::Binder<T>, b: &ty::Binder<T>)
+                  -> RelateResult<'tcx, ty::Binder<T>>
+        where T: Relate<'a, 'tcx>
     {
-        try!(self.sub().binders(a, b));
-        self.sub().binders(b, a)
+        try!(self.fields.higher_ranked_sub(a, b));
+        self.fields.higher_ranked_sub(b, a)
     }
 }
diff --git a/src/librustc/middle/infer/freshen.rs b/src/librustc/middle/infer/freshen.rs
index e41b949d5df1d..29f74d12ea3e8 100644
--- a/src/librustc/middle/infer/freshen.rs
+++ b/src/librustc/middle/infer/freshen.rs
@@ -37,7 +37,7 @@ use middle::ty_fold::TypeFolder;
 use std::collections::hash_map::{self, Entry};
 
 use super::InferCtxt;
-use super::unify::InferCtxtMethodsForSimplyUnifiableTypes;
+use super::unify::ToType;
 
 pub struct TypeFreshener<'a, 'tcx:'a> {
     infcx: &'a InferCtxt<'a, 'tcx>,
@@ -104,29 +104,38 @@ impl<'a, 'tcx> TypeFolder<'tcx> for TypeFreshener<'a, 'tcx> {
     }
 
     fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
+        let tcx = self.infcx.tcx;
+
         match t.sty {
             ty::ty_infer(ty::TyVar(v)) => {
-                self.freshen(self.infcx.type_variables.borrow().probe(v),
-                               ty::TyVar(v),
-                               ty::FreshTy)
+                self.freshen(
+                    self.infcx.type_variables.borrow().probe(v),
+                    ty::TyVar(v),
+                    ty::FreshTy)
             }
 
             ty::ty_infer(ty::IntVar(v)) => {
-                self.freshen(self.infcx.probe_var(v),
-                             ty::IntVar(v),
-                             ty::FreshIntTy)
+                self.freshen(
+                    self.infcx.int_unification_table.borrow_mut()
+                                                    .probe(v)
+                                                    .map(|v| v.to_type(tcx)),
+                    ty::IntVar(v),
+                    ty::FreshIntTy)
             }
 
             ty::ty_infer(ty::FloatVar(v)) => {
-                self.freshen(self.infcx.probe_var(v),
-                             ty::FloatVar(v),
-                             ty::FreshIntTy)
+                self.freshen(
+                    self.infcx.float_unification_table.borrow_mut()
+                                                      .probe(v)
+                                                      .map(|v| v.to_type(tcx)),
+                    ty::FloatVar(v),
+                    ty::FreshIntTy)
             }
 
             ty::ty_infer(ty::FreshTy(c)) |
             ty::ty_infer(ty::FreshIntTy(c)) => {
                 if c >= self.freshen_count {
-                    self.tcx().sess.bug(
+                    tcx.sess.bug(
                         &format!("Encountered a freshend type with id {} \
                                   but our counter is only at {}",
                                  c,
diff --git a/src/librustc/middle/infer/glb.rs b/src/librustc/middle/infer/glb.rs
index 3b83d37f58234..5822fb0f2d432 100644
--- a/src/librustc/middle/infer/glb.rs
+++ b/src/librustc/middle/infer/glb.rs
@@ -8,67 +8,79 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-use super::combine::*;
-use super::lattice::*;
+use super::combine::CombineFields;
 use super::higher_ranked::HigherRankedRelations;
-use super::cres;
+use super::InferCtxt;
+use super::lattice::{self, LatticeDir};
 use super::Subtype;
 
 use middle::ty::{self, Ty};
+use middle::ty_relate::{Relate, RelateResult, TypeRelation};
 use util::ppaux::Repr;
 
 /// "Greatest lower bound" (common subtype)
-pub struct Glb<'f, 'tcx: 'f> {
-    fields: CombineFields<'f, 'tcx>
+pub struct Glb<'a, 'tcx: 'a> {
+    fields: CombineFields<'a, 'tcx>
 }
 
-#[allow(non_snake_case)]
-pub fn Glb<'f, 'tcx>(cf: CombineFields<'f, 'tcx>) -> Glb<'f, 'tcx> {
-    Glb { fields: cf }
+impl<'a, 'tcx> Glb<'a, 'tcx> {
+    pub fn new(fields: CombineFields<'a, 'tcx>) -> Glb<'a, 'tcx> {
+        Glb { fields: fields }
+    }
 }
 
-impl<'f, 'tcx> Combine<'tcx> for Glb<'f, 'tcx> {
-    fn tag(&self) -> String { "Glb".to_string() }
-    fn fields<'a>(&'a self) -> &'a CombineFields<'a, 'tcx> { &self.fields }
+impl<'a, 'tcx> TypeRelation<'a, 'tcx> for Glb<'a, 'tcx> {
+    fn tag(&self) -> &'static str { "Glb" }
+
+    fn tcx(&self) -> &'a ty::ctxt<'tcx> { self.fields.tcx() }
+
+    fn a_is_expected(&self) -> bool { self.fields.a_is_expected }
 
-    fn tys_with_variance(&self, v: ty::Variance, a: Ty<'tcx>, b: Ty<'tcx>)
-                         -> cres<'tcx, Ty<'tcx>>
+    fn relate_with_variance<T:Relate<'a,'tcx>>(&mut self,
+                                               variance: ty::Variance,
+                                               a: &T,
+                                               b: &T)
+                                               -> RelateResult<'tcx, T>
     {
-        match v {
-            ty::Invariant => self.equate().tys(a, b),
-            ty::Covariant => self.tys(a, b),
-            ty::Bivariant => self.bivariate().tys(a, b),
-            ty::Contravariant => self.lub().tys(a, b),
+        match variance {
+            ty::Invariant => self.fields.equate().relate(a, b),
+            ty::Covariant => self.relate(a, b),
+            ty::Bivariant => self.fields.bivariate().relate(a, b),
+            ty::Contravariant => self.fields.lub().relate(a, b),
         }
     }
 
-    fn regions_with_variance(&self, v: ty::Variance, a: ty::Region, b: ty::Region)
-                             -> cres<'tcx, ty::Region>
-    {
-        match v {
-            ty::Invariant => self.equate().regions(a, b),
-            ty::Covariant => self.regions(a, b),
-            ty::Bivariant => self.bivariate().regions(a, b),
-            ty::Contravariant => self.lub().regions(a, b),
-        }
+    fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
+        lattice::super_lattice_tys(self, a, b)
     }
 
-    fn regions(&self, a: ty::Region, b: ty::Region) -> cres<'tcx, ty::Region> {
+    fn regions(&mut self, a: ty::Region, b: ty::Region) -> RelateResult<'tcx, ty::Region> {
         debug!("{}.regions({}, {})",
                self.tag(),
                a.repr(self.fields.infcx.tcx),
                b.repr(self.fields.infcx.tcx));
 
-        Ok(self.fields.infcx.region_vars.glb_regions(Subtype(self.trace()), a, b))
+        let origin = Subtype(self.fields.trace.clone());
+        Ok(self.fields.infcx.region_vars.glb_regions(origin, a, b))
     }
 
-    fn tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> cres<'tcx, Ty<'tcx>> {
-        super_lattice_tys(self, a, b)
+    fn binders<T>(&mut self, a: &ty::Binder<T>, b: &ty::Binder<T>)
+                  -> RelateResult<'tcx, ty::Binder<T>>
+        where T: Relate<'a, 'tcx>
+    {
+        self.fields.higher_ranked_glb(a, b)
     }
+}
 
-    fn binders<T>(&self, a: &ty::Binder<T>, b: &ty::Binder<T>) -> cres<'tcx, ty::Binder<T>>
-        where T : Combineable<'tcx>
-    {
-        self.higher_ranked_glb(a, b)
+impl<'a, 'tcx> LatticeDir<'a,'tcx> for Glb<'a, 'tcx> {
+    fn infcx(&self) -> &'a InferCtxt<'a,'tcx> {
+        self.fields.infcx
+    }
+
+    fn relate_bound(&self, v: Ty<'tcx>, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, ()> {
+        let mut sub = self.fields.sub();
+        try!(sub.relate(&v, &a));
+        try!(sub.relate(&v, &b));
+        Ok(())
     }
 }
diff --git a/src/librustc/middle/infer/higher_ranked/mod.rs b/src/librustc/middle/infer/higher_ranked/mod.rs
index 16b387330b9ef..f347d28b93c2b 100644
--- a/src/librustc/middle/infer/higher_ranked/mod.rs
+++ b/src/librustc/middle/infer/higher_ranked/mod.rs
@@ -11,25 +11,26 @@
 //! Helper routines for higher-ranked things. See the `doc` module at
 //! the end of the file for details.
 
-use super::{CombinedSnapshot, cres, InferCtxt, HigherRankedType, SkolemizationMap};
-use super::combine::{Combine, Combineable};
+use super::{CombinedSnapshot, InferCtxt, HigherRankedType, SkolemizationMap};
+use super::combine::CombineFields;
 
 use middle::subst;
 use middle::ty::{self, Binder};
 use middle::ty_fold::{self, TypeFoldable};
+use middle::ty_relate::{Relate, RelateResult, TypeRelation};
 use syntax::codemap::Span;
 use util::nodemap::{FnvHashMap, FnvHashSet};
 use util::ppaux::Repr;
 
-pub trait HigherRankedRelations<'tcx> {
-    fn higher_ranked_sub<T>(&self, a: &Binder<T>, b: &Binder<T>) -> cres<'tcx, Binder<T>>
-        where T : Combineable<'tcx>;
+pub trait HigherRankedRelations<'a,'tcx> {
+    fn higher_ranked_sub<T>(&self, a: &Binder<T>, b: &Binder<T>) -> RelateResult<'tcx, Binder<T>>
+        where T: Relate<'a,'tcx>;
 
-    fn higher_ranked_lub<T>(&self, a: &Binder<T>, b: &Binder<T>) -> cres<'tcx, Binder<T>>
-        where T : Combineable<'tcx>;
+    fn higher_ranked_lub<T>(&self, a: &Binder<T>, b: &Binder<T>) -> RelateResult<'tcx, Binder<T>>
+        where T: Relate<'a,'tcx>;
 
-    fn higher_ranked_glb<T>(&self, a: &Binder<T>, b: &Binder<T>) -> cres<'tcx, Binder<T>>
-        where T : Combineable<'tcx>;
+    fn higher_ranked_glb<T>(&self, a: &Binder<T>, b: &Binder<T>) -> RelateResult<'tcx, Binder<T>>
+        where T: Relate<'a,'tcx>;
 }
 
 trait InferCtxtExt {
@@ -40,15 +41,15 @@ trait InferCtxtExt {
                                         -> Vec<ty::RegionVid>;
 }
 
-impl<'tcx,C> HigherRankedRelations<'tcx> for C
-    where C : Combine<'tcx>
-{
+impl<'a,'tcx> HigherRankedRelations<'a,'tcx> for CombineFields<'a,'tcx> {
     fn higher_ranked_sub<T>(&self, a: &Binder<T>, b: &Binder<T>)
-                            -> cres<'tcx, Binder<T>>
-        where T : Combineable<'tcx>
+                            -> RelateResult<'tcx, Binder<T>>
+        where T: Relate<'a,'tcx>
     {
+        let tcx = self.infcx.tcx;
+
         debug!("higher_ranked_sub(a={}, b={})",
-               a.repr(self.tcx()), b.repr(self.tcx()));
+               a.repr(tcx), b.repr(tcx));
 
         // Rather than checking the subtype relationship between `a` and `b`
         // as-is, we need to do some extra work here in order to make sure
@@ -60,32 +61,32 @@ impl<'tcx,C> HigherRankedRelations<'tcx> for C
 
         // Start a snapshot so we can examine "all bindings that were
         // created as part of this type comparison".
-        return self.infcx().try(|snapshot| {
+        return self.infcx.commit_if_ok(|snapshot| {
             // First, we instantiate each bound region in the subtype with a fresh
             // region variable.
             let (a_prime, _) =
-                self.infcx().replace_late_bound_regions_with_fresh_var(
-                    self.trace().origin.span(),
+                self.infcx.replace_late_bound_regions_with_fresh_var(
+                    self.trace.origin.span(),
                     HigherRankedType,
                     a);
 
             // Second, we instantiate each bound region in the supertype with a
             // fresh concrete region.
             let (b_prime, skol_map) =
-                self.infcx().skolemize_late_bound_regions(b, snapshot);
+                self.infcx.skolemize_late_bound_regions(b, snapshot);
 
-            debug!("a_prime={}", a_prime.repr(self.tcx()));
-            debug!("b_prime={}", b_prime.repr(self.tcx()));
+            debug!("a_prime={}", a_prime.repr(tcx));
+            debug!("b_prime={}", b_prime.repr(tcx));
 
             // Compare types now that bound regions have been replaced.
-            let result = try!(Combineable::combine(self, &a_prime, &b_prime));
+            let result = try!(self.sub().relate(&a_prime, &b_prime));
 
             // Presuming type comparison succeeds, we need to check
             // that the skolemized regions do not "leak".
-            match leak_check(self.infcx(), &skol_map, snapshot) {
+            match leak_check(self.infcx, &skol_map, snapshot) {
                 Ok(()) => { }
                 Err((skol_br, tainted_region)) => {
-                    if self.a_is_expected() {
+                    if self.a_is_expected {
                         debug!("Not as polymorphic!");
                         return Err(ty::terr_regions_insufficiently_polymorphic(skol_br,
                                                                                tainted_region));
@@ -98,42 +99,42 @@ impl<'tcx,C> HigherRankedRelations<'tcx> for C
             }
 
             debug!("higher_ranked_sub: OK result={}",
-                   result.repr(self.tcx()));
+                   result.repr(tcx));
 
             Ok(ty::Binder(result))
         });
     }
 
-    fn higher_ranked_lub<T>(&self, a: &Binder<T>, b: &Binder<T>) -> cres<'tcx, Binder<T>>
-        where T : Combineable<'tcx>
+    fn higher_ranked_lub<T>(&self, a: &Binder<T>, b: &Binder<T>) -> RelateResult<'tcx, Binder<T>>
+        where T: Relate<'a,'tcx>
     {
         // Start a snapshot so we can examine "all bindings that were
         // created as part of this type comparison".
-        return self.infcx().try(|snapshot| {
+        return self.infcx.commit_if_ok(|snapshot| {
             // Instantiate each bound region with a fresh region variable.
-            let span = self.trace().origin.span();
+            let span = self.trace.origin.span();
             let (a_with_fresh, a_map) =
-                self.infcx().replace_late_bound_regions_with_fresh_var(
+                self.infcx.replace_late_bound_regions_with_fresh_var(
                     span, HigherRankedType, a);
             let (b_with_fresh, _) =
-                self.infcx().replace_late_bound_regions_with_fresh_var(
+                self.infcx.replace_late_bound_regions_with_fresh_var(
                     span, HigherRankedType, b);
 
             // Collect constraints.
             let result0 =
-                try!(Combineable::combine(self, &a_with_fresh, &b_with_fresh));
+                try!(self.lub().relate(&a_with_fresh, &b_with_fresh));
             let result0 =
-                self.infcx().resolve_type_vars_if_possible(&result0);
+                self.infcx.resolve_type_vars_if_possible(&result0);
             debug!("lub result0 = {}", result0.repr(self.tcx()));
 
             // Generalize the regions appearing in result0 if possible
-            let new_vars = self.infcx().region_vars_confined_to_snapshot(snapshot);
-            let span = self.trace().origin.span();
+            let new_vars = self.infcx.region_vars_confined_to_snapshot(snapshot);
+            let span = self.trace.origin.span();
             let result1 =
                 fold_regions_in(
                     self.tcx(),
                     &result0,
-                    |r, debruijn| generalize_region(self.infcx(), span, snapshot, debruijn,
+                    |r, debruijn| generalize_region(self.infcx, span, snapshot, debruijn,
                                                     &new_vars, &a_map, r));
 
             debug!("lub({},{}) = {}",
@@ -194,40 +195,40 @@ impl<'tcx,C> HigherRankedRelations<'tcx> for C
         }
     }
 
-    fn higher_ranked_glb<T>(&self, a: &Binder<T>, b: &Binder<T>) -> cres<'tcx, Binder<T>>
-        where T : Combineable<'tcx>
+    fn higher_ranked_glb<T>(&self, a: &Binder<T>, b: &Binder<T>) -> RelateResult<'tcx, Binder<T>>
+        where T: Relate<'a,'tcx>
     {
-        debug!("{}.higher_ranked_glb({}, {})",
-               self.tag(), a.repr(self.tcx()), b.repr(self.tcx()));
+        debug!("higher_ranked_glb({}, {})",
+               a.repr(self.tcx()), b.repr(self.tcx()));
 
         // Make a snapshot so we can examine "all bindings that were
         // created as part of this type comparison".
-        return self.infcx().try(|snapshot| {
+        return self.infcx.commit_if_ok(|snapshot| {
             // Instantiate each bound region with a fresh region variable.
             let (a_with_fresh, a_map) =
-                self.infcx().replace_late_bound_regions_with_fresh_var(
-                    self.trace().origin.span(), HigherRankedType, a);
+                self.infcx.replace_late_bound_regions_with_fresh_var(
+                    self.trace.origin.span(), HigherRankedType, a);
             let (b_with_fresh, b_map) =
-                self.infcx().replace_late_bound_regions_with_fresh_var(
-                    self.trace().origin.span(), HigherRankedType, b);
+                self.infcx.replace_late_bound_regions_with_fresh_var(
+                    self.trace.origin.span(), HigherRankedType, b);
             let a_vars = var_ids(self, &a_map);
             let b_vars = var_ids(self, &b_map);
 
             // Collect constraints.
             let result0 =
-                try!(Combineable::combine(self, &a_with_fresh, &b_with_fresh));
+                try!(self.glb().relate(&a_with_fresh, &b_with_fresh));
             let result0 =
-                self.infcx().resolve_type_vars_if_possible(&result0);
+                self.infcx.resolve_type_vars_if_possible(&result0);
             debug!("glb result0 = {}", result0.repr(self.tcx()));
 
             // Generalize the regions appearing in result0 if possible
-            let new_vars = self.infcx().region_vars_confined_to_snapshot(snapshot);
-            let span = self.trace().origin.span();
+            let new_vars = self.infcx.region_vars_confined_to_snapshot(snapshot);
+            let span = self.trace.origin.span();
             let result1 =
                 fold_regions_in(
                     self.tcx(),
                     &result0,
-                    |r, debruijn| generalize_region(self.infcx(), span, snapshot, debruijn,
+                    |r, debruijn| generalize_region(self.infcx, span, snapshot, debruijn,
                                                     &new_vars,
                                                     &a_map, &a_vars, &b_vars,
                                                     r));
@@ -332,17 +333,19 @@ impl<'tcx,C> HigherRankedRelations<'tcx> for C
     }
 }
 
-fn var_ids<'tcx, T: Combine<'tcx>>(combiner: &T,
-                                   map: &FnvHashMap<ty::BoundRegion, ty::Region>)
-                                   -> Vec<ty::RegionVid> {
-    map.iter().map(|(_, r)| match *r {
-            ty::ReInfer(ty::ReVar(r)) => { r }
-            r => {
-                combiner.infcx().tcx.sess.span_bug(
-                    combiner.trace().origin.span(),
-                    &format!("found non-region-vid: {:?}", r));
-            }
-        }).collect()
+fn var_ids<'a, 'tcx>(fields: &CombineFields<'a, 'tcx>,
+                      map: &FnvHashMap<ty::BoundRegion, ty::Region>)
+                     -> Vec<ty::RegionVid> {
+    map.iter()
+       .map(|(_, r)| match *r {
+           ty::ReInfer(ty::ReVar(r)) => { r }
+           r => {
+               fields.tcx().sess.span_bug(
+                   fields.trace.origin.span(),
+                   &format!("found non-region-vid: {:?}", r));
+           }
+       })
+       .collect()
 }
 
 fn is_var_in_set(new_vars: &[ty::RegionVid], r: ty::Region) -> bool {
@@ -356,8 +359,8 @@ fn fold_regions_in<'tcx, T, F>(tcx: &ty::ctxt<'tcx>,
                                unbound_value: &T,
                                mut fldr: F)
                                -> T
-    where T : Combineable<'tcx>,
-          F : FnMut(ty::Region, ty::DebruijnIndex) -> ty::Region,
+    where T: TypeFoldable<'tcx>,
+          F: FnMut(ty::Region, ty::DebruijnIndex) -> ty::Region,
 {
     unbound_value.fold_with(&mut ty_fold::RegionFolder::new(tcx, &mut |region, current_depth| {
         // we should only be encountering "escaping" late-bound regions here,
diff --git a/src/librustc/middle/infer/lattice.rs b/src/librustc/middle/infer/lattice.rs
index 9c764628c14f8..57001083b03e2 100644
--- a/src/librustc/middle/infer/lattice.rs
+++ b/src/librustc/middle/infer/lattice.rs
@@ -29,48 +29,32 @@
 //! over a `LatticeValue`, which is a value defined with respect to
 //! a lattice.
 
-use super::*;
-use super::combine::*;
-use super::glb::Glb;
-use super::lub::Lub;
+use super::combine;
+use super::InferCtxt;
 
 use middle::ty::TyVar;
 use middle::ty::{self, Ty};
+use middle::ty_relate::{RelateResult, TypeRelation};
 use util::ppaux::Repr;
 
-pub trait LatticeDir<'tcx> {
+pub trait LatticeDir<'f,'tcx> : TypeRelation<'f,'tcx> {
+    fn infcx(&self) -> &'f InferCtxt<'f, 'tcx>;
+
     // Relates the type `v` to `a` and `b` such that `v` represents
     // the LUB/GLB of `a` and `b` as appropriate.
-    fn relate_bound(&self, v: Ty<'tcx>, a: Ty<'tcx>, b: Ty<'tcx>) -> cres<'tcx, ()>;
-}
-
-impl<'a, 'tcx> LatticeDir<'tcx> for Lub<'a, 'tcx> {
-    fn relate_bound(&self, v: Ty<'tcx>, a: Ty<'tcx>, b: Ty<'tcx>) -> cres<'tcx, ()> {
-        let sub = self.sub();
-        try!(sub.tys(a, v));
-        try!(sub.tys(b, v));
-        Ok(())
-    }
-}
-
-impl<'a, 'tcx> LatticeDir<'tcx> for Glb<'a, 'tcx> {
-    fn relate_bound(&self, v: Ty<'tcx>, a: Ty<'tcx>, b: Ty<'tcx>) -> cres<'tcx, ()> {
-        let sub = self.sub();
-        try!(sub.tys(v, a));
-        try!(sub.tys(v, b));
-        Ok(())
-    }
+    fn relate_bound(&self, v: Ty<'tcx>, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, ()>;
 }
 
-pub fn super_lattice_tys<'tcx, L:LatticeDir<'tcx>+Combine<'tcx>>(this: &L,
-                                                                 a: Ty<'tcx>,
-                                                                 b: Ty<'tcx>)
-                                                                 -> cres<'tcx, Ty<'tcx>>
+pub fn super_lattice_tys<'a,'tcx,L:LatticeDir<'a,'tcx>>(this: &mut L,
+                                                        a: Ty<'tcx>,
+                                                        b: Ty<'tcx>)
+                                                        -> RelateResult<'tcx, Ty<'tcx>>
+    where 'tcx: 'a
 {
     debug!("{}.lattice_tys({}, {})",
            this.tag(),
-           a.repr(this.infcx().tcx),
-           b.repr(this.infcx().tcx));
+           a.repr(this.tcx()),
+           b.repr(this.tcx()));
 
     if a == b {
         return Ok(a);
@@ -95,7 +79,7 @@ pub fn super_lattice_tys<'tcx, L:LatticeDir<'tcx>+Combine<'tcx>>(this: &L,
         }
 
         _ => {
-            super_tys(this, a, b)
+            combine::super_combine_tys(this.infcx(), this, a, b)
         }
     }
 }
diff --git a/src/librustc/middle/infer/lub.rs b/src/librustc/middle/infer/lub.rs
index 5000ab32ff671..f456687be13ac 100644
--- a/src/librustc/middle/infer/lub.rs
+++ b/src/librustc/middle/infer/lub.rs
@@ -8,67 +8,80 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-use super::combine::*;
+use super::combine::CombineFields;
 use super::higher_ranked::HigherRankedRelations;
-use super::lattice::*;
-use super::cres;
+use super::InferCtxt;
+use super::lattice::{self, LatticeDir};
 use super::Subtype;
 
 use middle::ty::{self, Ty};
+use middle::ty_relate::{Relate, RelateResult, TypeRelation};
 use util::ppaux::Repr;
 
 /// "Least upper bound" (common supertype)
-pub struct Lub<'f, 'tcx: 'f> {
-    fields: CombineFields<'f, 'tcx>
+pub struct Lub<'a, 'tcx: 'a> {
+    fields: CombineFields<'a, 'tcx>
 }
 
-#[allow(non_snake_case)]
-pub fn Lub<'f, 'tcx>(cf: CombineFields<'f, 'tcx>) -> Lub<'f, 'tcx> {
-    Lub { fields: cf }
+impl<'a, 'tcx> Lub<'a, 'tcx> {
+    pub fn new(fields: CombineFields<'a, 'tcx>) -> Lub<'a, 'tcx> {
+        Lub { fields: fields }
+    }
 }
 
-impl<'f, 'tcx> Combine<'tcx> for Lub<'f, 'tcx> {
-    fn tag(&self) -> String { "Lub".to_string() }
-    fn fields<'a>(&'a self) -> &'a CombineFields<'a, 'tcx> { &self.fields }
+impl<'a, 'tcx> TypeRelation<'a, 'tcx> for Lub<'a, 'tcx> {
+    fn tag(&self) -> &'static str { "Lub" }
+
+    fn tcx(&self) -> &'a ty::ctxt<'tcx> { self.fields.tcx() }
+
+    fn a_is_expected(&self) -> bool { self.fields.a_is_expected }
 
-    fn tys_with_variance(&self, v: ty::Variance, a: Ty<'tcx>, b: Ty<'tcx>)
-                         -> cres<'tcx, Ty<'tcx>>
+    fn relate_with_variance<T:Relate<'a,'tcx>>(&mut self,
+                                               variance: ty::Variance,
+                                               a: &T,
+                                               b: &T)
+                                               -> RelateResult<'tcx, T>
     {
-        match v {
-            ty::Invariant => self.equate().tys(a, b),
-            ty::Covariant => self.tys(a, b),
-            ty::Bivariant => self.bivariate().tys(a, b),
-            ty::Contravariant => self.glb().tys(a, b),
+        match variance {
+            ty::Invariant => self.fields.equate().relate(a, b),
+            ty::Covariant => self.relate(a, b),
+            ty::Bivariant => self.fields.bivariate().relate(a, b),
+            ty::Contravariant => self.fields.glb().relate(a, b),
         }
     }
 
-    fn regions_with_variance(&self, v: ty::Variance, a: ty::Region, b: ty::Region)
-                             -> cres<'tcx, ty::Region>
-    {
-        match v {
-            ty::Invariant => self.equate().regions(a, b),
-            ty::Covariant => self.regions(a, b),
-            ty::Bivariant => self.bivariate().regions(a, b),
-            ty::Contravariant => self.glb().regions(a, b),
-        }
+    fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
+        lattice::super_lattice_tys(self, a, b)
     }
 
-    fn regions(&self, a: ty::Region, b: ty::Region) -> cres<'tcx, ty::Region> {
+    fn regions(&mut self, a: ty::Region, b: ty::Region) -> RelateResult<'tcx, ty::Region> {
         debug!("{}.regions({}, {})",
                self.tag(),
                a.repr(self.tcx()),
                b.repr(self.tcx()));
 
-        Ok(self.infcx().region_vars.lub_regions(Subtype(self.trace()), a, b))
+        let origin = Subtype(self.fields.trace.clone());
+        Ok(self.fields.infcx.region_vars.lub_regions(origin, a, b))
     }
 
-    fn tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> cres<'tcx, Ty<'tcx>> {
-        super_lattice_tys(self, a, b)
+    fn binders<T>(&mut self, a: &ty::Binder<T>, b: &ty::Binder<T>)
+                  -> RelateResult<'tcx, ty::Binder<T>>
+        where T: Relate<'a, 'tcx>
+    {
+        self.fields.higher_ranked_lub(a, b)
     }
+}
 
-    fn binders<T>(&self, a: &ty::Binder<T>, b: &ty::Binder<T>) -> cres<'tcx, ty::Binder<T>>
-        where T : Combineable<'tcx>
-    {
-        self.higher_ranked_lub(a, b)
+impl<'a, 'tcx> LatticeDir<'a,'tcx> for Lub<'a, 'tcx> {
+    fn infcx(&self) -> &'a InferCtxt<'a,'tcx> {
+        self.fields.infcx
+    }
+
+    fn relate_bound(&self, v: Ty<'tcx>, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, ()> {
+        let mut sub = self.fields.sub();
+        try!(sub.relate(&a, &v));
+        try!(sub.relate(&b, &v));
+        Ok(())
     }
 }
+
diff --git a/src/librustc/middle/infer/mod.rs b/src/librustc/middle/infer/mod.rs
index 4cc9b65c2dab3..da811c354578d 100644
--- a/src/librustc/middle/infer/mod.rs
+++ b/src/librustc/middle/infer/mod.rs
@@ -28,7 +28,8 @@ use middle::ty::{TyVid, IntVid, FloatVid, RegionVid, UnconstrainedNumeric};
 use middle::ty::replace_late_bound_regions;
 use middle::ty::{self, Ty};
 use middle::ty_fold::{TypeFolder, TypeFoldable};
-use std::cell::RefCell;
+use middle::ty_relate::{Relate, RelateResult, TypeRelation};
+use std::cell::{RefCell};
 use std::fmt;
 use std::rc::Rc;
 use syntax::ast;
@@ -38,12 +39,9 @@ use util::nodemap::FnvHashMap;
 use util::ppaux::ty_to_string;
 use util::ppaux::{Repr, UserString};
 
-use self::combine::{Combine, Combineable, CombineFields};
+use self::combine::CombineFields;
 use self::region_inference::{RegionVarBindings, RegionSnapshot};
-use self::equate::Equate;
-use self::sub::Sub;
-use self::lub::Lub;
-use self::unify::{UnificationTable, InferCtxtMethodsForSimplyUnifiableTypes};
+use self::unify::{ToType, UnificationTable};
 use self::error_reporting::ErrorReporting;
 
 pub mod bivariate;
@@ -62,9 +60,7 @@ pub mod type_variable;
 pub mod unify;
 
 pub type Bound<T> = Option<T>;
-
-pub type cres<'tcx, T> = Result<T,ty::type_err<'tcx>>; // "combine result"
-pub type ures<'tcx> = cres<'tcx, ()>; // "unify result"
+pub type UnitResult<'tcx> = RelateResult<'tcx, ()>; // "unify result"
 pub type fres<T> = Result<T, fixup_err>; // "fixup result"
 
 pub struct InferCtxt<'a, 'tcx: 'a> {
@@ -265,7 +261,7 @@ pub enum LateBoundRegionConversionTime {
 ///
 /// See `error_reporting.rs` for more details
 #[derive(Clone, Debug)]
-pub enum RegionVariableOrigin<'tcx> {
+pub enum RegionVariableOrigin {
     // Region variables created for ill-categorized reasons,
     // mostly indicates places in need of refactoring
     MiscVariable(Span),
@@ -280,7 +276,7 @@ pub enum RegionVariableOrigin<'tcx> {
     Autoref(Span),
 
     // Regions created as part of an automatic coercion
-    Coercion(TypeTrace<'tcx>),
+    Coercion(Span),
 
     // Region variables created as the values for early-bound regions
     EarlyBoundRegion(Span, ast::Name),
@@ -343,8 +339,7 @@ pub fn common_supertype<'a, 'tcx>(cx: &InferCtxt<'a, 'tcx>,
         values: Types(expected_found(a_is_expected, a, b))
     };
 
-    let result =
-        cx.commit_if_ok(|| cx.lub(a_is_expected, trace.clone()).tys(a, b));
+    let result = cx.commit_if_ok(|_| cx.lub(a_is_expected, trace.clone()).relate(&a, &b));
     match result {
         Ok(t) => t,
         Err(ref err) => {
@@ -359,29 +354,28 @@ pub fn mk_subty<'a, 'tcx>(cx: &InferCtxt<'a, 'tcx>,
                           origin: TypeOrigin,
                           a: Ty<'tcx>,
                           b: Ty<'tcx>)
-                          -> ures<'tcx>
+                          -> UnitResult<'tcx>
 {
     debug!("mk_subty({} <: {})", a.repr(cx.tcx), b.repr(cx.tcx));
-    cx.commit_if_ok(|| {
-        cx.sub_types(a_is_expected, origin, a, b)
-    })
+    cx.sub_types(a_is_expected, origin, a, b)
 }
 
 pub fn can_mk_subty<'a, 'tcx>(cx: &InferCtxt<'a, 'tcx>,
                               a: Ty<'tcx>,
                               b: Ty<'tcx>)
-                              -> ures<'tcx> {
+                              -> UnitResult<'tcx> {
     debug!("can_mk_subty({} <: {})", a.repr(cx.tcx), b.repr(cx.tcx));
     cx.probe(|_| {
         let trace = TypeTrace {
             origin: Misc(codemap::DUMMY_SP),
             values: Types(expected_found(true, a, b))
         };
-        cx.sub(true, trace).tys(a, b).to_ures()
+        cx.sub(true, trace).relate(&a, &b).map(|_| ())
     })
 }
 
-pub fn can_mk_eqty<'a, 'tcx>(cx: &InferCtxt<'a, 'tcx>, a: Ty<'tcx>, b: Ty<'tcx>) -> ures<'tcx>
+pub fn can_mk_eqty<'a, 'tcx>(cx: &InferCtxt<'a, 'tcx>, a: Ty<'tcx>, b: Ty<'tcx>)
+                             -> UnitResult<'tcx>
 {
     cx.can_equate(&a, &b)
 }
@@ -401,11 +395,10 @@ pub fn mk_eqty<'a, 'tcx>(cx: &InferCtxt<'a, 'tcx>,
                          origin: TypeOrigin,
                          a: Ty<'tcx>,
                          b: Ty<'tcx>)
-                         -> ures<'tcx>
+                         -> UnitResult<'tcx>
 {
     debug!("mk_eqty({} <: {})", a.repr(cx.tcx), b.repr(cx.tcx));
-    cx.commit_if_ok(
-        || cx.eq_types(a_is_expected, origin, a, b))
+    cx.commit_if_ok(|_| cx.eq_types(a_is_expected, origin, a, b))
 }
 
 pub fn mk_sub_poly_trait_refs<'a, 'tcx>(cx: &InferCtxt<'a, 'tcx>,
@@ -413,12 +406,11 @@ pub fn mk_sub_poly_trait_refs<'a, 'tcx>(cx: &InferCtxt<'a, 'tcx>,
                                    origin: TypeOrigin,
                                    a: ty::PolyTraitRef<'tcx>,
                                    b: ty::PolyTraitRef<'tcx>)
-                                   -> ures<'tcx>
+                                   -> UnitResult<'tcx>
 {
     debug!("mk_sub_trait_refs({} <: {})",
            a.repr(cx.tcx), b.repr(cx.tcx));
-    cx.commit_if_ok(
-        || cx.sub_poly_trait_refs(a_is_expected, origin, a.clone(), b.clone()))
+    cx.commit_if_ok(|_| cx.sub_poly_trait_refs(a_is_expected, origin, a.clone(), b.clone()))
 }
 
 fn expected_found<T>(a_is_expected: bool,
@@ -433,57 +425,6 @@ fn expected_found<T>(a_is_expected: bool,
     }
 }
 
-trait then<'tcx> {
-    fn then<T, F>(&self, f: F) -> Result<T, ty::type_err<'tcx>> where
-        T: Clone,
-        F: FnOnce() -> Result<T, ty::type_err<'tcx>>;
-}
-
-impl<'tcx> then<'tcx> for ures<'tcx> {
-    fn then<T, F>(&self, f: F) -> Result<T, ty::type_err<'tcx>> where
-        T: Clone,
-        F: FnOnce() -> Result<T, ty::type_err<'tcx>>,
-    {
-        self.and_then(move |_| f())
-    }
-}
-
-trait ToUres<'tcx> {
-    fn to_ures(&self) -> ures<'tcx>;
-}
-
-impl<'tcx, T> ToUres<'tcx> for cres<'tcx, T> {
-    fn to_ures(&self) -> ures<'tcx> {
-        match *self {
-          Ok(ref _v) => Ok(()),
-          Err(ref e) => Err((*e))
-        }
-    }
-}
-
-trait CresCompare<'tcx, T> {
-    fn compare<F>(&self, t: T, f: F) -> cres<'tcx, T> where
-        F: FnOnce() -> ty::type_err<'tcx>;
-}
-
-impl<'tcx, T:Clone + PartialEq> CresCompare<'tcx, T> for cres<'tcx, T> {
-    fn compare<F>(&self, t: T, f: F) -> cres<'tcx, T> where
-        F: FnOnce() -> ty::type_err<'tcx>,
-    {
-        (*self).clone().and_then(move |s| {
-            if s == t {
-                (*self).clone()
-            } else {
-                Err(f())
-            }
-        })
-    }
-}
-
-pub fn uok<'tcx>() -> ures<'tcx> {
-    Ok(())
-}
-
 #[must_use = "once you start a snapshot, you should always consume it"]
 pub struct CombinedSnapshot {
     type_snapshot: type_variable::Snapshot,
@@ -512,41 +453,56 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         use middle::ty::UnconstrainedNumeric::{Neither, UnconstrainedInt, UnconstrainedFloat};
         match ty.sty {
             ty::ty_infer(ty::IntVar(vid)) => {
-                match self.int_unification_table.borrow_mut().get(self.tcx, vid).value {
-                    None => UnconstrainedInt,
-                    _ => Neither,
+                if self.int_unification_table.borrow_mut().has_value(vid) {
+                    Neither
+                } else {
+                    UnconstrainedInt
                 }
             },
             ty::ty_infer(ty::FloatVar(vid)) => {
-                match self.float_unification_table.borrow_mut().get(self.tcx, vid).value {
-                    None => return UnconstrainedFloat,
-                    _ => Neither,
+                if self.float_unification_table.borrow_mut().has_value(vid) {
+                    Neither
+                } else {
+                    UnconstrainedFloat
                 }
             },
             _ => Neither,
         }
     }
 
-    pub fn combine_fields<'b>(&'b self, a_is_expected: bool, trace: TypeTrace<'tcx>)
-                              -> CombineFields<'b, 'tcx> {
+    fn combine_fields(&'a self, a_is_expected: bool, trace: TypeTrace<'tcx>)
+                      -> CombineFields<'a, 'tcx> {
         CombineFields {infcx: self,
                        a_is_expected: a_is_expected,
                        trace: trace}
     }
 
-    pub fn equate<'b>(&'b self, a_is_expected: bool, trace: TypeTrace<'tcx>)
-                      -> Equate<'b, 'tcx> {
-        Equate(self.combine_fields(a_is_expected, trace))
+    // public so that it can be used from the rustc_driver unit tests
+    pub fn equate(&'a self, a_is_expected: bool, trace: TypeTrace<'tcx>)
+              -> equate::Equate<'a, 'tcx>
+    {
+        self.combine_fields(a_is_expected, trace).equate()
     }
 
-    pub fn sub<'b>(&'b self, a_is_expected: bool, trace: TypeTrace<'tcx>)
-                   -> Sub<'b, 'tcx> {
-        Sub(self.combine_fields(a_is_expected, trace))
+    // public so that it can be used from the rustc_driver unit tests
+    pub fn sub(&'a self, a_is_expected: bool, trace: TypeTrace<'tcx>)
+               -> sub::Sub<'a, 'tcx>
+    {
+        self.combine_fields(a_is_expected, trace).sub()
     }
 
-    pub fn lub<'b>(&'b self, a_is_expected: bool, trace: TypeTrace<'tcx>)
-                   -> Lub<'b, 'tcx> {
-        Lub(self.combine_fields(a_is_expected, trace))
+    // public so that it can be used from the rustc_driver unit tests
+    pub fn lub(&'a self, a_is_expected: bool, trace: TypeTrace<'tcx>)
+               -> lub::Lub<'a, 'tcx>
+    {
+        self.combine_fields(a_is_expected, trace).lub()
+    }
+
+    // public so that it can be used from the rustc_driver unit tests
+    pub fn glb(&'a self, a_is_expected: bool, trace: TypeTrace<'tcx>)
+               -> glb::Glb<'a, 'tcx>
+    {
+        self.combine_fields(a_is_expected, trace).glb()
     }
 
     fn start_snapshot(&self) -> CombinedSnapshot {
@@ -609,11 +565,19 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         r
     }
 
-    /// Execute `f` and commit the bindings if successful
+    /// Execute `f` and commit the bindings if closure `f` returns `Ok(_)`
     pub fn commit_if_ok<T, E, F>(&self, f: F) -> Result<T, E> where
-        F: FnOnce() -> Result<T, E>
+        F: FnOnce(&CombinedSnapshot) -> Result<T, E>
     {
-        self.commit_unconditionally(move || self.try(move |_| f()))
+        debug!("commit_if_ok()");
+        let snapshot = self.start_snapshot();
+        let r = f(&snapshot);
+        debug!("commit_if_ok() -- r.is_ok() = {}", r.is_ok());
+        match r {
+            Ok(_) => { self.commit_from(snapshot); }
+            Err(_) => { self.rollback_to(snapshot); }
+        }
+        r
     }
 
     /// Execute `f` and commit only the region bindings if successful.
@@ -628,7 +592,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                                float_snapshot,
                                region_vars_snapshot } = self.start_snapshot();
 
-        let r = self.try(move |_| f());
+        let r = self.commit_if_ok(|_| f());
 
         // Roll back any non-region bindings - they should be resolved
         // inside `f`, with, e.g. `resolve_type_vars_if_possible`.
@@ -649,25 +613,6 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         r
     }
 
-    /// Execute `f`, unroll bindings on panic
-    pub fn try<T, E, F>(&self, f: F) -> Result<T, E> where
-        F: FnOnce(&CombinedSnapshot) -> Result<T, E>
-    {
-        debug!("try()");
-        let snapshot = self.start_snapshot();
-        let r = f(&snapshot);
-        debug!("try() -- r.is_ok() = {}", r.is_ok());
-        match r {
-            Ok(_) => {
-                self.commit_from(snapshot);
-            }
-            Err(_) => {
-                self.rollback_to(snapshot);
-            }
-        }
-        r
-    }
-
     /// Execute `f` then unroll any bindings it creates
     pub fn probe<R, F>(&self, f: F) -> R where
         F: FnOnce(&CombinedSnapshot) -> R,
@@ -691,12 +636,12 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                      origin: TypeOrigin,
                      a: Ty<'tcx>,
                      b: Ty<'tcx>)
-                     -> ures<'tcx>
+                     -> UnitResult<'tcx>
     {
         debug!("sub_types({} <: {})", a.repr(self.tcx), b.repr(self.tcx));
-        self.commit_if_ok(|| {
+        self.commit_if_ok(|_| {
             let trace = TypeTrace::types(origin, a_is_expected, a, b);
-            self.sub(a_is_expected, trace).tys(a, b).to_ures()
+            self.sub(a_is_expected, trace).relate(&a, &b).map(|_| ())
         })
     }
 
@@ -705,11 +650,11 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                     origin: TypeOrigin,
                     a: Ty<'tcx>,
                     b: Ty<'tcx>)
-                    -> ures<'tcx>
+                    -> UnitResult<'tcx>
     {
-        self.commit_if_ok(|| {
+        self.commit_if_ok(|_| {
             let trace = TypeTrace::types(origin, a_is_expected, a, b);
-            self.equate(a_is_expected, trace).tys(a, b).to_ures()
+            self.equate(a_is_expected, trace).relate(&a, &b).map(|_| ())
         })
     }
 
@@ -718,17 +663,17 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                           origin: TypeOrigin,
                           a: Rc<ty::TraitRef<'tcx>>,
                           b: Rc<ty::TraitRef<'tcx>>)
-                          -> ures<'tcx>
+                          -> UnitResult<'tcx>
     {
         debug!("sub_trait_refs({} <: {})",
                a.repr(self.tcx),
                b.repr(self.tcx));
-        self.commit_if_ok(|| {
+        self.commit_if_ok(|_| {
             let trace = TypeTrace {
                 origin: origin,
                 values: TraitRefs(expected_found(a_is_expected, a.clone(), b.clone()))
             };
-            self.sub(a_is_expected, trace).trait_refs(&*a, &*b).to_ures()
+            self.sub(a_is_expected, trace).relate(&*a, &*b).map(|_| ())
         })
     }
 
@@ -737,17 +682,17 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                                origin: TypeOrigin,
                                a: ty::PolyTraitRef<'tcx>,
                                b: ty::PolyTraitRef<'tcx>)
-                               -> ures<'tcx>
+                               -> UnitResult<'tcx>
     {
         debug!("sub_poly_trait_refs({} <: {})",
                a.repr(self.tcx),
                b.repr(self.tcx));
-        self.commit_if_ok(|| {
+        self.commit_if_ok(|_| {
             let trace = TypeTrace {
                 origin: origin,
                 values: PolyTraitRefs(expected_found(a_is_expected, a.clone(), b.clone()))
             };
-            self.sub(a_is_expected, trace).binders(&a, &b).to_ures()
+            self.sub(a_is_expected, trace).relate(&a, &b).map(|_| ())
         })
     }
 
@@ -774,7 +719,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     pub fn leak_check(&self,
                       skol_map: &SkolemizationMap,
                       snapshot: &CombinedSnapshot)
-                      -> ures<'tcx>
+                      -> UnitResult<'tcx>
     {
         /*! See `higher_ranked::leak_check` */
 
@@ -799,8 +744,8 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     pub fn equality_predicate(&self,
                               span: Span,
                               predicate: &ty::PolyEquatePredicate<'tcx>)
-                              -> ures<'tcx> {
-        self.try(|snapshot| {
+                              -> UnitResult<'tcx> {
+        self.commit_if_ok(|snapshot| {
             let (ty::EquatePredicate(a, b), skol_map) =
                 self.skolemize_late_bound_regions(predicate, snapshot);
             let origin = EquatePredicate(span);
@@ -812,8 +757,8 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     pub fn region_outlives_predicate(&self,
                                      span: Span,
                                      predicate: &ty::PolyRegionOutlivesPredicate)
-                                     -> ures<'tcx> {
-        self.try(|snapshot| {
+                                     -> UnitResult<'tcx> {
+        self.commit_if_ok(|snapshot| {
             let (ty::OutlivesPredicate(r_a, r_b), skol_map) =
                 self.skolemize_late_bound_regions(predicate, snapshot);
             let origin = RelateRegionParamBound(span);
@@ -852,7 +797,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             .new_key(None)
     }
 
-    pub fn next_region_var(&self, origin: RegionVariableOrigin<'tcx>) -> ty::Region {
+    pub fn next_region_var(&self, origin: RegionVariableOrigin) -> ty::Region {
         ty::ReInfer(ty::ReVar(self.region_vars.new_region_var(origin)))
     }
 
@@ -948,12 +893,18 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             }
 
             ty::ty_infer(ty::IntVar(v)) => {
-                self.probe_var(v)
+                self.int_unification_table
+                    .borrow_mut()
+                    .probe(v)
+                    .map(|v| v.to_type(self.tcx))
                     .unwrap_or(typ)
             }
 
             ty::ty_infer(ty::FloatVar(v)) => {
-                self.probe_var(v)
+                self.float_unification_table
+                    .borrow_mut()
+                    .probe(v)
+                    .map(|v| v.to_type(self.tcx))
                     .unwrap_or(typ)
             }
 
@@ -1104,8 +1055,8 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         self.region_vars.verify_generic_bound(origin, kind, a, bs);
     }
 
-    pub fn can_equate<T>(&self, a: &T, b: &T) -> ures<'tcx>
-        where T : Combineable<'tcx> + Repr<'tcx>
+    pub fn can_equate<'b,T>(&'b self, a: &T, b: &T) -> UnitResult<'tcx>
+        where T: Relate<'b,'tcx> + Repr<'tcx>
     {
         debug!("can_equate({}, {})", a.repr(self.tcx), b.repr(self.tcx));
         self.probe(|_| {
@@ -1116,9 +1067,8 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             let e = self.tcx.types.err;
             let trace = TypeTrace { origin: Misc(codemap::DUMMY_SP),
                                     values: Types(expected_found(true, e, e)) };
-            let eq = self.equate(true, trace);
-            Combineable::combine(&eq, a, b)
-        }).to_ures()
+            self.equate(true, trace).relate(a, b)
+        }).map(|_| ())
     }
 }
 
@@ -1304,14 +1254,14 @@ impl<'tcx> Repr<'tcx> for SubregionOrigin<'tcx> {
     }
 }
 
-impl<'tcx> RegionVariableOrigin<'tcx> {
+impl RegionVariableOrigin {
     pub fn span(&self) -> Span {
         match *self {
             MiscVariable(a) => a,
             PatternRegion(a) => a,
             AddrOfRegion(a) => a,
             Autoref(a) => a,
-            Coercion(ref a) => a.span(),
+            Coercion(a) => a,
             EarlyBoundRegion(a, _) => a,
             LateBoundRegion(a, _, _) => a,
             BoundRegionInCoherence(_) => codemap::DUMMY_SP,
@@ -1320,7 +1270,7 @@ impl<'tcx> RegionVariableOrigin<'tcx> {
     }
 }
 
-impl<'tcx> Repr<'tcx> for RegionVariableOrigin<'tcx> {
+impl<'tcx> Repr<'tcx> for RegionVariableOrigin {
     fn repr(&self, tcx: &ty::ctxt<'tcx>) -> String {
         match *self {
             MiscVariable(a) => {
@@ -1333,7 +1283,7 @@ impl<'tcx> Repr<'tcx> for RegionVariableOrigin<'tcx> {
                 format!("AddrOfRegion({})", a.repr(tcx))
             }
             Autoref(a) => format!("Autoref({})", a.repr(tcx)),
-            Coercion(ref a) => format!("Coercion({})", a.repr(tcx)),
+            Coercion(a) => format!("Coercion({})", a.repr(tcx)),
             EarlyBoundRegion(a, b) => {
                 format!("EarlyBoundRegion({},{})", a.repr(tcx), b.repr(tcx))
             }
diff --git a/src/librustc/middle/infer/region_inference/mod.rs b/src/librustc/middle/infer/region_inference/mod.rs
index b539dded12e8a..98347e97e09c3 100644
--- a/src/librustc/middle/infer/region_inference/mod.rs
+++ b/src/librustc/middle/infer/region_inference/mod.rs
@@ -18,7 +18,6 @@ pub use self::RegionResolutionError::*;
 pub use self::VarValue::*;
 use self::Classification::*;
 
-use super::cres;
 use super::{RegionVariableOrigin, SubregionOrigin, TypeTrace, MiscVariable};
 
 use middle::region;
@@ -26,6 +25,7 @@ use middle::ty::{self, Ty};
 use middle::ty::{BoundRegion, FreeRegion, Region, RegionVid};
 use middle::ty::{ReEmpty, ReStatic, ReInfer, ReFree, ReEarlyBound};
 use middle::ty::{ReLateBound, ReScope, ReVar, ReSkolemized, BrFresh};
+use middle::ty_relate::RelateResult;
 use middle::graph;
 use middle::graph::{Direction, NodeIndex};
 use util::common::indenter;
@@ -115,7 +115,7 @@ pub enum RegionResolutionError<'tcx> {
     /// Could not infer a value for `v` because `sub_r <= v` (due to
     /// `sub_origin`) but `v <= sup_r` (due to `sup_origin`) and
     /// `sub_r <= sup_r` does not hold.
-    SubSupConflict(RegionVariableOrigin<'tcx>,
+    SubSupConflict(RegionVariableOrigin,
                    SubregionOrigin<'tcx>, Region,
                    SubregionOrigin<'tcx>, Region),
 
@@ -124,7 +124,7 @@ pub enum RegionResolutionError<'tcx> {
     /// Could not infer a value for `v` because `v <= r1` (due to
     /// `origin1`) and `v <= r2` (due to `origin2`) and
     /// `r1` and `r2` have no intersection.
-    SupSupConflict(RegionVariableOrigin<'tcx>,
+    SupSupConflict(RegionVariableOrigin,
                    SubregionOrigin<'tcx>, Region,
                    SubregionOrigin<'tcx>, Region),
 
@@ -132,7 +132,7 @@ pub enum RegionResolutionError<'tcx> {
     /// more specific errors message by suggesting to the user where they
     /// should put a lifetime. In those cases we process and put those errors
     /// into `ProcessedErrors` before we do any reporting.
-    ProcessedErrors(Vec<RegionVariableOrigin<'tcx>>,
+    ProcessedErrors(Vec<RegionVariableOrigin>,
                     Vec<(TypeTrace<'tcx>, ty::type_err<'tcx>)>,
                     Vec<SameRegions>),
 }
@@ -168,7 +168,7 @@ pub type CombineMap = FnvHashMap<TwoRegions, RegionVid>;
 
 pub struct RegionVarBindings<'a, 'tcx: 'a> {
     tcx: &'a ty::ctxt<'tcx>,
-    var_origins: RefCell<Vec<RegionVariableOrigin<'tcx>>>,
+    var_origins: RefCell<Vec<RegionVariableOrigin>>,
 
     // Constraints of the form `A <= B` introduced by the region
     // checker.  Here at least one of `A` and `B` must be a region
@@ -316,7 +316,7 @@ impl<'a, 'tcx> RegionVarBindings<'a, 'tcx> {
         len as u32
     }
 
-    pub fn new_region_var(&self, origin: RegionVariableOrigin<'tcx>) -> RegionVid {
+    pub fn new_region_var(&self, origin: RegionVariableOrigin) -> RegionVid {
         let id = self.num_vars();
         self.var_origins.borrow_mut().push(origin.clone());
         let vid = RegionVid { index: id };
@@ -798,7 +798,8 @@ impl<'a, 'tcx> RegionVarBindings<'a, 'tcx> {
     /// regions are given as argument, in any order, a consistent result is returned.
     fn lub_free_regions(&self,
                         a: &FreeRegion,
-                        b: &FreeRegion) -> ty::Region
+                        b: &FreeRegion)
+                        -> ty::Region
     {
         return match a.cmp(b) {
             Less => helper(self, a, b),
@@ -823,7 +824,8 @@ impl<'a, 'tcx> RegionVarBindings<'a, 'tcx> {
     fn glb_concrete_regions(&self,
                             a: Region,
                             b: Region)
-                         -> cres<'tcx, Region> {
+                            -> RelateResult<'tcx, Region>
+    {
         debug!("glb_concrete_regions({:?}, {:?})", a, b);
         match (a, b) {
             (ReLateBound(..), _) |
@@ -898,7 +900,8 @@ impl<'a, 'tcx> RegionVarBindings<'a, 'tcx> {
     /// returned.
     fn glb_free_regions(&self,
                         a: &FreeRegion,
-                        b: &FreeRegion) -> cres<'tcx, ty::Region>
+                        b: &FreeRegion)
+                        -> RelateResult<'tcx, ty::Region>
     {
         return match a.cmp(b) {
             Less => helper(self, a, b),
@@ -908,7 +911,7 @@ impl<'a, 'tcx> RegionVarBindings<'a, 'tcx> {
 
         fn helper<'a, 'tcx>(this: &RegionVarBindings<'a, 'tcx>,
                             a: &FreeRegion,
-                            b: &FreeRegion) -> cres<'tcx, ty::Region>
+                            b: &FreeRegion) -> RelateResult<'tcx, ty::Region>
         {
             if this.tcx.region_maps.sub_free_region(*a, *b) {
                 Ok(ty::ReFree(*a))
@@ -926,7 +929,8 @@ impl<'a, 'tcx> RegionVarBindings<'a, 'tcx> {
                         region_a: ty::Region,
                         region_b: ty::Region,
                         scope_a: region::CodeExtent,
-                        scope_b: region::CodeExtent) -> cres<'tcx, Region>
+                        scope_b: region::CodeExtent)
+                        -> RelateResult<'tcx, Region>
     {
         // We want to generate the intersection of two
         // scopes or two free regions.  So, if one of
diff --git a/src/librustc/middle/infer/sub.rs b/src/librustc/middle/infer/sub.rs
index 5d23fe3f1348d..31b654a5b3fd3 100644
--- a/src/librustc/middle/infer/sub.rs
+++ b/src/librustc/middle/infer/sub.rs
@@ -8,64 +8,49 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-use super::combine::*;
-use super::cres;
+use super::combine::{self, CombineFields};
 use super::higher_ranked::HigherRankedRelations;
 use super::Subtype;
 use super::type_variable::{SubtypeOf, SupertypeOf};
 
 use middle::ty::{self, Ty};
 use middle::ty::TyVar;
-use util::ppaux::Repr;
+use middle::ty_relate::{Relate, RelateResult, TypeRelation};
+use util::ppaux::{Repr};
 
 /// "Greatest lower bound" (common subtype)
-pub struct Sub<'f, 'tcx: 'f> {
-    fields: CombineFields<'f, 'tcx>
+pub struct Sub<'a, 'tcx: 'a> {
+    fields: CombineFields<'a, 'tcx>
 }
 
-#[allow(non_snake_case)]
-pub fn Sub<'f, 'tcx>(cf: CombineFields<'f, 'tcx>) -> Sub<'f, 'tcx> {
-    Sub { fields: cf }
+impl<'a, 'tcx> Sub<'a, 'tcx> {
+    pub fn new(f: CombineFields<'a, 'tcx>) -> Sub<'a, 'tcx> {
+        Sub { fields: f }
+    }
 }
 
-impl<'f, 'tcx> Combine<'tcx> for Sub<'f, 'tcx> {
-    fn tag(&self) -> String { "Sub".to_string() }
-    fn fields<'a>(&'a self) -> &'a CombineFields<'a, 'tcx> { &self.fields }
-
-    fn tys_with_variance(&self, v: ty::Variance, a: Ty<'tcx>, b: Ty<'tcx>)
-                         -> cres<'tcx, Ty<'tcx>>
-    {
-        match v {
-            ty::Invariant => self.equate().tys(a, b),
-            ty::Covariant => self.tys(a, b),
-            ty::Bivariant => self.bivariate().tys(a, b),
-            ty::Contravariant => Sub(self.fields.switch_expected()).tys(b, a),
-        }
-    }
+impl<'a, 'tcx> TypeRelation<'a, 'tcx> for Sub<'a, 'tcx> {
+    fn tag(&self) -> &'static str { "Sub" }
+    fn tcx(&self) -> &'a ty::ctxt<'tcx> { self.fields.infcx.tcx }
+    fn a_is_expected(&self) -> bool { self.fields.a_is_expected }
 
-    fn regions_with_variance(&self, v: ty::Variance, a: ty::Region, b: ty::Region)
-                             -> cres<'tcx, ty::Region>
+    fn relate_with_variance<T:Relate<'a,'tcx>>(&mut self,
+                                               variance: ty::Variance,
+                                               a: &T,
+                                               b: &T)
+                                               -> RelateResult<'tcx, T>
     {
-        match v {
-            ty::Invariant => self.equate().regions(a, b),
-            ty::Covariant => self.regions(a, b),
-            ty::Bivariant => self.bivariate().regions(a, b),
-            ty::Contravariant => Sub(self.fields.switch_expected()).regions(b, a),
+        match variance {
+            ty::Invariant => self.fields.equate().relate(a, b),
+            ty::Covariant => self.relate(a, b),
+            ty::Bivariant => self.fields.bivariate().relate(a, b),
+            ty::Contravariant => self.fields.switch_expected().sub().relate(b, a),
         }
     }
 
-    fn regions(&self, a: ty::Region, b: ty::Region) -> cres<'tcx, ty::Region> {
-        debug!("{}.regions({}, {})",
-               self.tag(),
-               a.repr(self.tcx()),
-               b.repr(self.tcx()));
-        self.infcx().region_vars.make_subregion(Subtype(self.trace()), a, b);
-        Ok(a)
-    }
+    fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
+        debug!("{}.tys({}, {})", self.tag(), a.repr(self.tcx()), b.repr(self.tcx()));
 
-    fn tys(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> cres<'tcx, Ty<'tcx>> {
-        debug!("{}.tys({}, {})", self.tag(),
-               a.repr(self.tcx()), b.repr(self.tcx()));
         if a == b { return Ok(a); }
 
         let infcx = self.fields.infcx;
@@ -80,8 +65,8 @@ impl<'f, 'tcx> Combine<'tcx> for Sub<'f, 'tcx> {
             }
             (&ty::ty_infer(TyVar(a_id)), _) => {
                 try!(self.fields
-                       .switch_expected()
-                       .instantiate(b, SupertypeOf, a_id));
+                         .switch_expected()
+                         .instantiate(b, SupertypeOf, a_id));
                 Ok(a)
             }
             (_, &ty::ty_infer(TyVar(b_id))) => {
@@ -94,14 +79,25 @@ impl<'f, 'tcx> Combine<'tcx> for Sub<'f, 'tcx> {
             }
 
             _ => {
-                super_tys(self, a, b)
+                combine::super_combine_tys(self.fields.infcx, self, a, b)
             }
         }
     }
 
-    fn binders<T>(&self, a: &ty::Binder<T>, b: &ty::Binder<T>) -> cres<'tcx, ty::Binder<T>>
-        where T : Combineable<'tcx>
+    fn regions(&mut self, a: ty::Region, b: ty::Region) -> RelateResult<'tcx, ty::Region> {
+        debug!("{}.regions({}, {})",
+               self.tag(),
+               a.repr(self.tcx()),
+               b.repr(self.tcx()));
+        let origin = Subtype(self.fields.trace.clone());
+        self.fields.infcx.region_vars.make_subregion(origin, a, b);
+        Ok(a)
+    }
+
+    fn binders<T>(&mut self, a: &ty::Binder<T>, b: &ty::Binder<T>)
+                  -> RelateResult<'tcx, ty::Binder<T>>
+        where T: Relate<'a,'tcx>
     {
-        self.higher_ranked_sub(a, b)
+        self.fields.higher_ranked_sub(a, b)
     }
 }
diff --git a/src/librustc/middle/infer/unify.rs b/src/librustc/middle/infer/unify.rs
index 8a736d47b5d89..39271d4cdc597 100644
--- a/src/librustc/middle/infer/unify.rs
+++ b/src/librustc/middle/infer/unify.rs
@@ -12,11 +12,8 @@ pub use self::VarValue::*;
 
 use std::marker;
 
-use middle::ty::{expected_found, IntVarValue};
+use middle::ty::{IntVarValue};
 use middle::ty::{self, Ty};
-use middle::infer::{uok, ures};
-use middle::infer::InferCtxt;
-use std::cell::RefCell;
 use std::fmt::Debug;
 use std::marker::PhantomData;
 use syntax::ast;
@@ -35,14 +32,9 @@ use util::snapshot_vec as sv;
 pub trait UnifyKey : Clone + Debug + PartialEq {
     type Value : UnifyValue;
 
-    fn index(&self) -> usize;
+    fn index(&self) -> u32;
 
-    fn from_index(u: usize) -> Self;
-
-    // Given an inference context, returns the unification table
-    // appropriate to this key type.
-    fn unification_table<'v>(infcx: &'v InferCtxt)
-                             -> &'v RefCell<UnificationTable<Self>>;
+    fn from_index(u: u32) -> Self;
 
     fn tag(k: Option<Self>) -> &'static str;
 }
@@ -130,21 +122,25 @@ impl<K:UnifyKey> UnificationTable<K> {
 
     pub fn new_key(&mut self, value: K::Value) -> K {
         let index = self.values.push(Root(value, 0));
-        let k = UnifyKey::from_index(index);
+        let k = UnifyKey::from_index(index as u32);
         debug!("{}: created new key: {:?}",
                UnifyKey::tag(None::<K>),
                k);
         k
     }
 
-    /// Find the root node for `vid`. This uses the standard union-find algorithm with path
-    /// compression: http://en.wikipedia.org/wiki/Disjoint-set_data_structure
-    pub fn get(&mut self, tcx: &ty::ctxt, vid: K) -> Node<K> {
-        let index = vid.index();
+    /// Find the root node for `vid`. This uses the standard
+    /// union-find algorithm with path compression:
+    /// <http://en.wikipedia.org/wiki/Disjoint-set_data_structure>.
+    ///
+    /// NB. This is a building-block operation and you would probably
+    /// prefer to call `probe` below.
+    fn get(&mut self, vid: K) -> Node<K> {
+        let index = vid.index() as usize;
         let value = (*self.values.get(index)).clone();
         match value {
             Redirect(redirect) => {
-                let node: Node<K> = self.get(tcx, redirect.clone());
+                let node: Node<K> = self.get(redirect.clone());
                 if node.key != redirect {
                     // Path compression
                     self.values.set(index, Redirect(node.key.clone()));
@@ -158,58 +154,58 @@ impl<K:UnifyKey> UnificationTable<K> {
     }
 
     fn is_root(&self, key: &K) -> bool {
-        match *self.values.get(key.index()) {
+        let index = key.index() as usize;
+        match *self.values.get(index) {
             Redirect(..) => false,
             Root(..) => true,
         }
     }
 
-    /// Sets the value for `vid` to `new_value`. `vid` MUST be a root node! Also, we must be in the
-    /// middle of a snapshot.
-    pub fn set<'tcx>(&mut self,
-                     _tcx: &ty::ctxt<'tcx>,
-                     key: K,
-                     new_value: VarValue<K>)
-    {
+    /// Sets the value for `vid` to `new_value`. `vid` MUST be a root
+    /// node! This is an internal operation used to impl other things.
+    fn set(&mut self, key: K, new_value: VarValue<K>) {
         assert!(self.is_root(&key));
 
         debug!("Updating variable {:?} to {:?}",
                key, new_value);
 
-        self.values.set(key.index(), new_value);
+        let index = key.index() as usize;
+        self.values.set(index, new_value);
     }
 
-    /// Either redirects node_a to node_b or vice versa, depending on the relative rank. Returns
-    /// the new root and rank. You should then update the value of the new root to something
-    /// suitable.
-    pub fn unify<'tcx>(&mut self,
-                       tcx: &ty::ctxt<'tcx>,
-                       node_a: &Node<K>,
-                       node_b: &Node<K>)
-                       -> (K, usize)
-    {
+    /// Either redirects `node_a` to `node_b` or vice versa, depending
+    /// on the relative rank. The value associated with the new root
+    /// will be `new_value`.
+    ///
+    /// NB: This is the "union" operation of "union-find". It is
+    /// really more of a building block. If the values associated with
+    /// your key are non-trivial, you would probably prefer to call
+    /// `unify_var_var` below.
+    fn unify(&mut self, node_a: &Node<K>, node_b: &Node<K>, new_value: K::Value) {
         debug!("unify(node_a(id={:?}, rank={:?}), node_b(id={:?}, rank={:?}))",
                node_a.key,
                node_a.rank,
                node_b.key,
                node_b.rank);
 
-        if node_a.rank > node_b.rank {
+        let (new_root, new_rank) = if node_a.rank > node_b.rank {
             // a has greater rank, so a should become b's parent,
             // i.e., b should redirect to a.
-            self.set(tcx, node_b.key.clone(), Redirect(node_a.key.clone()));
+            self.set(node_b.key.clone(), Redirect(node_a.key.clone()));
             (node_a.key.clone(), node_a.rank)
         } else if node_a.rank < node_b.rank {
             // b has greater rank, so a should redirect to b.
-            self.set(tcx, node_a.key.clone(), Redirect(node_b.key.clone()));
+            self.set(node_a.key.clone(), Redirect(node_b.key.clone()));
             (node_b.key.clone(), node_b.rank)
         } else {
             // If equal, redirect one to the other and increment the
             // other's rank.
             assert_eq!(node_a.rank, node_b.rank);
-            self.set(tcx, node_b.key.clone(), Redirect(node_a.key.clone()));
+            self.set(node_b.key.clone(), Redirect(node_a.key.clone()));
             (node_a.key.clone(), node_a.rank + 1)
-        }
+        };
+
+        self.set(new_root, Root(new_value, new_rank));
     }
 }
 
@@ -223,69 +219,26 @@ impl<K:UnifyKey> sv::SnapshotVecDelegate for Delegate<K> {
 }
 
 ///////////////////////////////////////////////////////////////////////////
-// Code to handle simple keys like ints, floats---anything that
-// doesn't have a subtyping relationship we need to worry about.
-
-/// Indicates a type that does not have any kind of subtyping
-/// relationship.
-pub trait SimplyUnifiable<'tcx> : Clone + PartialEq + Debug {
-    fn to_type(&self, tcx: &ty::ctxt<'tcx>) -> Ty<'tcx>;
-    fn to_type_err(expected_found<Self>) -> ty::type_err<'tcx>;
-}
-
-pub fn err<'tcx, V:SimplyUnifiable<'tcx>>(a_is_expected: bool,
-                                          a_t: V,
-                                          b_t: V)
-                                          -> ures<'tcx> {
-    if a_is_expected {
-        Err(SimplyUnifiable::to_type_err(
-            ty::expected_found {expected: a_t, found: b_t}))
-    } else {
-        Err(SimplyUnifiable::to_type_err(
-            ty::expected_found {expected: b_t, found: a_t}))
-    }
-}
-
-pub trait InferCtxtMethodsForSimplyUnifiableTypes<'tcx,K,V>
-    where K : UnifyKey<Value=Option<V>>,
-          V : SimplyUnifiable<'tcx>,
-          Option<V> : UnifyValue,
-{
-    fn simple_vars(&self,
-                   a_is_expected: bool,
-                   a_id: K,
-                   b_id: K)
-                   -> ures<'tcx>;
-    fn simple_var_t(&self,
-                    a_is_expected: bool,
-                    a_id: K,
-                    b: V)
-                    -> ures<'tcx>;
-    fn probe_var(&self, a_id: K) -> Option<Ty<'tcx>>;
-}
-
-impl<'a,'tcx,V,K> InferCtxtMethodsForSimplyUnifiableTypes<'tcx,K,V> for InferCtxt<'a,'tcx>
-    where K : UnifyKey<Value=Option<V>>,
-          V : SimplyUnifiable<'tcx>,
-          Option<V> : UnifyValue,
+// Code to handle keys which carry a value, like ints,
+// floats---anything that doesn't have a subtyping relationship we
+// need to worry about.
+
+impl<'tcx,K,V> UnificationTable<K>
+    where K: UnifyKey<Value=Option<V>>,
+          V: Clone+PartialEq,
+          Option<V>: UnifyValue,
 {
-    /// Unifies two simple keys. Because simple keys do not have any subtyping relationships, if
-    /// both keys have already been associated with a value, then those two values must be the
-    /// same.
-    fn simple_vars(&self,
-                   a_is_expected: bool,
-                   a_id: K,
-                   b_id: K)
-                   -> ures<'tcx>
+    pub fn unify_var_var(&mut self,
+                         a_id: K,
+                         b_id: K)
+                         -> Result<(),(V,V)>
     {
-        let tcx = self.tcx;
-        let table = UnifyKey::unification_table(self);
-        let node_a: Node<K> = table.borrow_mut().get(tcx, a_id);
-        let node_b: Node<K> = table.borrow_mut().get(tcx, b_id);
+        let node_a = self.get(a_id);
+        let node_b = self.get(b_id);
         let a_id = node_a.key.clone();
         let b_id = node_b.key.clone();
 
-        if a_id == b_id { return uok(); }
+        if a_id == b_id { return Ok(()); }
 
         let combined = {
             match (&node_a.value, &node_b.value) {
@@ -293,61 +246,52 @@ impl<'a,'tcx,V,K> InferCtxtMethodsForSimplyUnifiableTypes<'tcx,K,V> for InferCtx
                     None
                 }
                 (&Some(ref v), &None) | (&None, &Some(ref v)) => {
-                    Some((*v).clone())
+                    Some(v.clone())
                 }
                 (&Some(ref v1), &Some(ref v2)) => {
                     if *v1 != *v2 {
-                        return err(a_is_expected, (*v1).clone(), (*v2).clone())
+                        return Err((v1.clone(), v2.clone()));
                     }
-                    Some((*v1).clone())
+                    Some(v1.clone())
                 }
             }
         };
 
-        let (new_root, new_rank) = table.borrow_mut().unify(tcx,
-                                                            &node_a,
-                                                            &node_b);
-        table.borrow_mut().set(tcx, new_root, Root(combined, new_rank));
-        return Ok(())
+        Ok(self.unify(&node_a, &node_b, combined))
     }
 
     /// Sets the value of the key `a_id` to `b`. Because simple keys do not have any subtyping
     /// relationships, if `a_id` already has a value, it must be the same as `b`.
-    fn simple_var_t(&self,
-                    a_is_expected: bool,
-                    a_id: K,
-                    b: V)
-                    -> ures<'tcx>
+    pub fn unify_var_value(&mut self,
+                           a_id: K,
+                           b: V)
+                           -> Result<(),(V,V)>
     {
-        let tcx = self.tcx;
-        let table = UnifyKey::unification_table(self);
-        let node_a = table.borrow_mut().get(tcx, a_id);
+        let node_a = self.get(a_id);
         let a_id = node_a.key.clone();
 
         match node_a.value {
             None => {
-                table.borrow_mut().set(tcx, a_id, Root(Some(b), node_a.rank));
-                return Ok(());
+                self.set(a_id, Root(Some(b), node_a.rank));
+                Ok(())
             }
 
             Some(ref a_t) => {
                 if *a_t == b {
-                    return Ok(());
+                    Ok(())
                 } else {
-                    return err(a_is_expected, (*a_t).clone(), b);
+                    Err((a_t.clone(), b))
                 }
             }
         }
     }
 
-    fn probe_var(&self, a_id: K) -> Option<Ty<'tcx>> {
-        let tcx = self.tcx;
-        let table = UnifyKey::unification_table(self);
-        let node_a = table.borrow_mut().get(tcx, a_id);
-        match node_a.value {
-            None => None,
-            Some(ref a_t) => Some(a_t.to_type(tcx))
-        }
+    pub fn has_value(&mut self, id: K) -> bool {
+        self.get(id).value.is_some()
+    }
+
+    pub fn probe(&mut self, a_id: K) -> Option<V> {
+        self.get(a_id).value.clone()
     }
 }
 
@@ -355,33 +299,24 @@ impl<'a,'tcx,V,K> InferCtxtMethodsForSimplyUnifiableTypes<'tcx,K,V> for InferCtx
 
 // Integral type keys
 
+pub trait ToType<'tcx> {
+    fn to_type(&self, tcx: &ty::ctxt<'tcx>) -> Ty<'tcx>;
+}
+
 impl UnifyKey for ty::IntVid {
     type Value = Option<IntVarValue>;
-
-    fn index(&self) -> usize { self.index as usize }
-
-    fn from_index(i: usize) -> ty::IntVid { ty::IntVid { index: i as u32 } }
-
-    fn unification_table<'v>(infcx: &'v InferCtxt) -> &'v RefCell<UnificationTable<ty::IntVid>> {
-        return &infcx.int_unification_table;
-    }
-
-    fn tag(_: Option<ty::IntVid>) -> &'static str {
-        "IntVid"
-    }
+    fn index(&self) -> u32 { self.index }
+    fn from_index(i: u32) -> ty::IntVid { ty::IntVid { index: i } }
+    fn tag(_: Option<ty::IntVid>) -> &'static str { "IntVid" }
 }
 
-impl<'tcx> SimplyUnifiable<'tcx> for IntVarValue {
+impl<'tcx> ToType<'tcx> for IntVarValue {
     fn to_type(&self, tcx: &ty::ctxt<'tcx>) -> Ty<'tcx> {
         match *self {
             ty::IntType(i) => ty::mk_mach_int(tcx, i),
             ty::UintType(i) => ty::mk_mach_uint(tcx, i),
         }
     }
-
-    fn to_type_err(err: expected_found<IntVarValue>) -> ty::type_err<'tcx> {
-        return ty::terr_int_mismatch(err);
-    }
 }
 
 impl UnifyValue for Option<IntVarValue> { }
@@ -390,29 +325,16 @@ impl UnifyValue for Option<IntVarValue> { }
 
 impl UnifyKey for ty::FloatVid {
     type Value = Option<ast::FloatTy>;
-
-    fn index(&self) -> usize { self.index as usize }
-
-    fn from_index(i: usize) -> ty::FloatVid { ty::FloatVid { index: i as u32 } }
-
-    fn unification_table<'v>(infcx: &'v InferCtxt) -> &'v RefCell<UnificationTable<ty::FloatVid>> {
-        return &infcx.float_unification_table;
-    }
-
-    fn tag(_: Option<ty::FloatVid>) -> &'static str {
-        "FloatVid"
-    }
+    fn index(&self) -> u32 { self.index }
+    fn from_index(i: u32) -> ty::FloatVid { ty::FloatVid { index: i } }
+    fn tag(_: Option<ty::FloatVid>) -> &'static str { "FloatVid" }
 }
 
 impl UnifyValue for Option<ast::FloatTy> {
 }
 
-impl<'tcx> SimplyUnifiable<'tcx> for ast::FloatTy {
+impl<'tcx> ToType<'tcx> for ast::FloatTy {
     fn to_type(&self, tcx: &ty::ctxt<'tcx>) -> Ty<'tcx> {
         ty::mk_mach_float(tcx, *self)
     }
-
-    fn to_type_err(err: expected_found<ast::FloatTy>) -> ty::type_err<'tcx> {
-        ty::terr_float_mismatch(err)
-    }
 }
diff --git a/src/librustc/middle/traits/project.rs b/src/librustc/middle/traits/project.rs
index 1594d8b2e0d04..7488b8f046e74 100644
--- a/src/librustc/middle/traits/project.rs
+++ b/src/librustc/middle/traits/project.rs
@@ -81,7 +81,7 @@ pub fn poly_project_and_unify_type<'cx,'tcx>(
            obligation.repr(selcx.tcx()));
 
     let infcx = selcx.infcx();
-    infcx.try(|snapshot| {
+    infcx.commit_if_ok(|snapshot| {
         let (skol_predicate, skol_map) =
             infcx.skolemize_late_bound_regions(&obligation.predicate, snapshot);
 
@@ -291,6 +291,7 @@ impl<'a,'b,'tcx> TypeFolder<'tcx> for AssociatedTypeNormalizer<'a,'b,'tcx> {
     }
 }
 
+#[derive(Clone)]
 pub struct Normalized<'tcx,T> {
     pub value: T,
     pub obligations: Vec<PredicateObligation<'tcx>>,
diff --git a/src/librustc/middle/traits/select.rs b/src/librustc/middle/traits/select.rs
index cb9d90744a446..bb2d37c944c86 100644
--- a/src/librustc/middle/traits/select.rs
+++ b/src/librustc/middle/traits/select.rs
@@ -39,11 +39,13 @@ use middle::ty::{self, RegionEscape, ToPolyTraitRef, Ty};
 use middle::infer;
 use middle::infer::{InferCtxt, TypeFreshener};
 use middle::ty_fold::TypeFoldable;
+use middle::ty_match;
+use middle::ty_relate::TypeRelation;
 use std::cell::RefCell;
-use std::collections::hash_map::HashMap;
 use std::rc::Rc;
 use syntax::{abi, ast};
 use util::common::ErrorReported;
+use util::nodemap::FnvHashMap;
 use util::ppaux::Repr;
 
 pub struct SelectionContext<'cx, 'tcx:'cx> {
@@ -87,8 +89,8 @@ struct TraitObligationStack<'prev, 'tcx: 'prev> {
 
 #[derive(Clone)]
 pub struct SelectionCache<'tcx> {
-    hashmap: RefCell<HashMap<Rc<ty::TraitRef<'tcx>>,
-                             SelectionResult<'tcx, SelectionCandidate<'tcx>>>>,
+    hashmap: RefCell<FnvHashMap<Rc<ty::TraitRef<'tcx>>,
+                                SelectionResult<'tcx, SelectionCandidate<'tcx>>>>,
 }
 
 pub enum MethodMatchResult {
@@ -474,7 +476,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             unbound_input_types &&
              (self.intercrate ||
               stack.iter().skip(1).any(
-                  |prev| stack.fresh_trait_ref.def_id() == prev.fresh_trait_ref.def_id()))
+                  |prev| self.match_fresh_trait_refs(&stack.fresh_trait_ref,
+                                                     &prev.fresh_trait_ref)))
         {
             debug!("evaluate_stack({}) --> unbound argument, recursion -->  ambiguous",
                    stack.fresh_trait_ref.repr(self.tcx()));
@@ -1271,7 +1274,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             return;
         }
 
-        self.infcx.try(|snapshot| {
+        self.infcx.commit_if_ok(|snapshot| {
             let bound_self_ty =
                 self.infcx.resolve_type_vars_if_possible(&obligation.self_ty());
             let (self_ty, _) =
@@ -1808,7 +1811,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
         // For each type, produce a vector of resulting obligations
         let obligations: Result<Vec<Vec<_>>, _> = bound_types.iter().map(|nested_ty| {
-            self.infcx.try(|snapshot| {
+            self.infcx.commit_if_ok(|snapshot| {
                 let (skol_ty, skol_map) =
                     self.infcx().skolemize_late_bound_regions(nested_ty, snapshot);
                 let Normalized { value: normalized_ty, mut obligations } =
@@ -1918,7 +1921,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                                     obligation: &TraitObligation<'tcx>)
     {
         let _: Result<(),()> =
-            self.infcx.try(|snapshot| {
+            self.infcx.commit_if_ok(|snapshot| {
                 let result =
                     self.match_projection_obligation_against_bounds_from_trait(obligation,
                                                                                snapshot);
@@ -2073,7 +2076,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                                                                 trait_def_id,
                                                                 nested);
 
-        let trait_obligations: Result<VecPerParamSpace<_>,()> = self.infcx.try(|snapshot| {
+        let trait_obligations: Result<VecPerParamSpace<_>,()> = self.infcx.commit_if_ok(|snapshot| {
             let poly_trait_ref = obligation.predicate.to_poly_trait_ref();
             let (trait_ref, skol_map) =
                 self.infcx().skolemize_late_bound_regions(&poly_trait_ref, snapshot);
@@ -2107,7 +2110,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
         // First, create the substitutions by matching the impl again,
         // this time not in a probe.
-        self.infcx.try(|snapshot| {
+        self.infcx.commit_if_ok(|snapshot| {
             let (skol_obligation_trait_ref, skol_map) =
                 self.infcx().skolemize_late_bound_regions(&obligation.predicate, snapshot);
             let substs =
@@ -2505,6 +2508,15 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     ///////////////////////////////////////////////////////////////////////////
     // Miscellany
 
+    fn match_fresh_trait_refs(&self,
+                              previous: &ty::PolyTraitRef<'tcx>,
+                              current: &ty::PolyTraitRef<'tcx>)
+                              -> bool
+    {
+        let mut matcher = ty_match::Match::new(self.tcx());
+        matcher.relate(previous, current).is_ok()
+    }
+
     fn push_stack<'o,'s:'o>(&mut self,
                             previous_stack: TraitObligationStackList<'s, 'tcx>,
                             obligation: &'o TraitObligation<'tcx>)
@@ -2664,7 +2676,7 @@ impl<'tcx> Repr<'tcx> for SelectionCandidate<'tcx> {
 impl<'tcx> SelectionCache<'tcx> {
     pub fn new() -> SelectionCache<'tcx> {
         SelectionCache {
-            hashmap: RefCell::new(HashMap::new())
+            hashmap: RefCell::new(FnvHashMap())
         }
     }
 }
diff --git a/src/librustc/middle/ty_fold.rs b/src/librustc/middle/ty_fold.rs
index f17ba78007bb2..5f77574f65ed4 100644
--- a/src/librustc/middle/ty_fold.rs
+++ b/src/librustc/middle/ty_fold.rs
@@ -39,6 +39,8 @@ use middle::subst::VecPerParamSpace;
 use middle::ty::{self, Ty};
 use middle::traits;
 use std::rc::Rc;
+use syntax::abi;
+use syntax::ast;
 use syntax::owned_slice::OwnedSlice;
 use util::ppaux::Repr;
 
@@ -47,7 +49,7 @@ use util::ppaux::Repr;
 
 /// The TypeFoldable trait is implemented for every type that can be folded.
 /// Basically, every type that has a corresponding method in TypeFolder.
-pub trait TypeFoldable<'tcx> {
+pub trait TypeFoldable<'tcx>: Repr<'tcx> + Clone {
     fn fold_with<F: TypeFolder<'tcx>>(&self, folder: &mut F) -> Self;
 }
 
@@ -149,12 +151,20 @@ pub trait TypeFolder<'tcx> : Sized {
 // can easily refactor the folding into the TypeFolder trait as
 // needed.
 
-impl<'tcx> TypeFoldable<'tcx> for () {
-    fn fold_with<F:TypeFolder<'tcx>>(&self, _: &mut F) -> () {
-        ()
+macro_rules! CopyImpls {
+    ($($ty:ty),+) => {
+        $(
+            impl<'tcx> TypeFoldable<'tcx> for $ty {
+                fn fold_with<F:TypeFolder<'tcx>>(&self, _: &mut F) -> $ty {
+                    *self
+                }
+            }
+        )+
     }
 }
 
+CopyImpls! { (), ast::Unsafety, abi::Abi }
+
 impl<'tcx, T:TypeFoldable<'tcx>, U:TypeFoldable<'tcx>> TypeFoldable<'tcx> for (T, U) {
     fn fold_with<F:TypeFolder<'tcx>>(&self, folder: &mut F) -> (T, U) {
         (self.0.fold_with(folder), self.1.fold_with(folder))
diff --git a/src/librustc/middle/ty_match.rs b/src/librustc/middle/ty_match.rs
new file mode 100644
index 0000000000000..bb00fadc39c95
--- /dev/null
+++ b/src/librustc/middle/ty_match.rs
@@ -0,0 +1,95 @@
+// Copyright 2012 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use middle::ty::{self, Ty};
+use middle::ty_relate::{self, Relate, TypeRelation, RelateResult};
+use util::ppaux::Repr;
+
+/// A type "A" *matches* "B" if the fresh types in B could be
+/// substituted with values so as to make it equal to A. Matching is
+/// intended to be used only on freshened types, and it basically
+/// indicates if the non-freshened versions of A and B could have been
+/// unified.
+///
+/// It is only an approximation. If it yields false, unification would
+/// definitely fail, but a true result doesn't mean unification would
+/// succeed. This is because we don't track the "side-constraints" on
+/// type variables, nor do we track if the same freshened type appears
+/// more than once. To some extent these approximations could be
+/// fixed, given effort.
+///
+/// Like subtyping, matching is really a binary relation, so the only
+/// important thing about the result is Ok/Err. Also, matching never
+/// affects any type variables or unification state.
+pub struct Match<'a, 'tcx: 'a> {
+    tcx: &'a ty::ctxt<'tcx>
+}
+
+impl<'a, 'tcx> Match<'a, 'tcx> {
+    pub fn new(tcx: &'a ty::ctxt<'tcx>) -> Match<'a, 'tcx> {
+        Match { tcx: tcx }
+    }
+}
+
+impl<'a, 'tcx> TypeRelation<'a, 'tcx> for Match<'a, 'tcx> {
+    fn tag(&self) -> &'static str { "Match" }
+    fn tcx(&self) -> &'a ty::ctxt<'tcx> { self.tcx }
+    fn a_is_expected(&self) -> bool { true } // irrelevant
+
+    fn relate_with_variance<T:Relate<'a,'tcx>>(&mut self,
+                                               _: ty::Variance,
+                                               a: &T,
+                                               b: &T)
+                                               -> RelateResult<'tcx, T>
+    {
+        self.relate(a, b)
+    }
+
+    fn regions(&mut self, a: ty::Region, b: ty::Region) -> RelateResult<'tcx, ty::Region> {
+        debug!("{}.regions({}, {})",
+               self.tag(),
+               a.repr(self.tcx()),
+               b.repr(self.tcx()));
+        Ok(a)
+    }
+
+    fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
+        debug!("{}.tys({}, {})", self.tag(),
+               a.repr(self.tcx()), b.repr(self.tcx()));
+        if a == b { return Ok(a); }
+
+        match (&a.sty, &b.sty) {
+            (_, &ty::ty_infer(ty::FreshTy(_))) |
+            (_, &ty::ty_infer(ty::FreshIntTy(_))) => {
+                Ok(a)
+            }
+
+            (&ty::ty_infer(_), _) |
+            (_, &ty::ty_infer(_)) => {
+                Err(ty::terr_sorts(ty_relate::expected_found(self, &a, &b)))
+            }
+
+            (&ty::ty_err, _) | (_, &ty::ty_err) => {
+                Ok(self.tcx().types.err)
+            }
+
+            _ => {
+                ty_relate::super_relate_tys(self, a, b)
+            }
+        }
+    }
+
+    fn binders<T>(&mut self, a: &ty::Binder<T>, b: &ty::Binder<T>)
+                  -> RelateResult<'tcx, ty::Binder<T>>
+        where T: Relate<'a,'tcx>
+    {
+        Ok(ty::Binder(try!(self.relate(a.skip_binder(), b.skip_binder()))))
+    }
+}
diff --git a/src/librustc/middle/ty_relate/mod.rs b/src/librustc/middle/ty_relate/mod.rs
new file mode 100644
index 0000000000000..1205b7d957930
--- /dev/null
+++ b/src/librustc/middle/ty_relate/mod.rs
@@ -0,0 +1,655 @@
+// Copyright 2012-2013 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! Generalized type relating mechanism. A type relation R relates a
+//! pair of values (A, B). A and B are usually types or regions but
+//! can be other things. Examples of type relations are subtyping,
+//! type equality, etc.
+
+use middle::subst::{ErasedRegions, NonerasedRegions, ParamSpace, Substs};
+use middle::ty::{self, Ty};
+use middle::ty_fold::TypeFoldable;
+use std::rc::Rc;
+use syntax::abi;
+use syntax::ast;
+use util::ppaux::Repr;
+
+pub type RelateResult<'tcx, T> = Result<T, ty::type_err<'tcx>>;
+
+pub trait TypeRelation<'a,'tcx> : Sized {
+    fn tcx(&self) -> &'a ty::ctxt<'tcx>;
+
+    /// Returns a static string we can use for printouts.
+    fn tag(&self) -> &'static str;
+
+    /// Returns true if the value `a` is the "expected" type in the
+    /// relation. Just affects error messages.
+    fn a_is_expected(&self) -> bool;
+
+    /// Generic relation routine suitable for most anything.
+    fn relate<T:Relate<'a,'tcx>>(&mut self, a: &T, b: &T) -> RelateResult<'tcx, T> {
+        Relate::relate(self, a, b)
+    }
+
+    /// Switch variance for the purpose of relating `a` and `b`.
+    fn relate_with_variance<T:Relate<'a,'tcx>>(&mut self,
+                                               variance: ty::Variance,
+                                               a: &T,
+                                               b: &T)
+                                               -> RelateResult<'tcx, T>;
+
+    // Overrideable relations. You shouldn't typically call these
+    // directly, instead call `relate()`, which in turn calls
+    // these. This is both more uniform but also allows us to add
+    // additional hooks for other types in the future if needed
+    // without making older code, which called `relate`, obsolete.
+
+    fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>)
+           -> RelateResult<'tcx, Ty<'tcx>>;
+
+    fn regions(&mut self, a: ty::Region, b: ty::Region)
+               -> RelateResult<'tcx, ty::Region>;
+
+    fn binders<T>(&mut self, a: &ty::Binder<T>, b: &ty::Binder<T>)
+                  -> RelateResult<'tcx, ty::Binder<T>>
+        where T: Relate<'a,'tcx>;
+}
+
+pub trait Relate<'a,'tcx>: TypeFoldable<'tcx> {
+    fn relate<R:TypeRelation<'a,'tcx>>(relation: &mut R,
+                                       a: &Self,
+                                       b: &Self)
+                                       -> RelateResult<'tcx, Self>;
+}
+
+///////////////////////////////////////////////////////////////////////////
+// Relate impls
+
+impl<'a,'tcx:'a> Relate<'a,'tcx> for ty::mt<'tcx> {
+    fn relate<R>(relation: &mut R,
+                 a: &ty::mt<'tcx>,
+                 b: &ty::mt<'tcx>)
+                 -> RelateResult<'tcx, ty::mt<'tcx>>
+        where R: TypeRelation<'a,'tcx>
+    {
+        debug!("{}.mts({}, {})",
+               relation.tag(),
+               a.repr(relation.tcx()),
+               b.repr(relation.tcx()));
+        if a.mutbl != b.mutbl {
+            Err(ty::terr_mutability)
+        } else {
+            let mutbl = a.mutbl;
+            let variance = match mutbl {
+                ast::MutImmutable => ty::Covariant,
+                ast::MutMutable => ty::Invariant,
+            };
+            let ty = try!(relation.relate_with_variance(variance, &a.ty, &b.ty));
+            Ok(ty::mt {ty: ty, mutbl: mutbl})
+        }
+    }
+}
+
+// substitutions are not themselves relatable without more context,
+// but they is an important subroutine for things that ARE relatable,
+// like traits etc.
+fn relate_item_substs<'a,'tcx:'a,R>(relation: &mut R,
+                                    item_def_id: ast::DefId,
+                                    a_subst: &Substs<'tcx>,
+                                    b_subst: &Substs<'tcx>)
+                                    -> RelateResult<'tcx, Substs<'tcx>>
+    where R: TypeRelation<'a,'tcx>
+{
+    debug!("substs: item_def_id={} a_subst={} b_subst={}",
+           item_def_id.repr(relation.tcx()),
+           a_subst.repr(relation.tcx()),
+           b_subst.repr(relation.tcx()));
+
+    let variances;
+    let opt_variances = if relation.tcx().variance_computed.get() {
+        variances = ty::item_variances(relation.tcx(), item_def_id);
+        Some(&*variances)
+    } else {
+        None
+    };
+    relate_substs(relation, opt_variances, a_subst, b_subst)
+}
+
+fn relate_substs<'a,'tcx,R>(relation: &mut R,
+                            variances: Option<&ty::ItemVariances>,
+                            a_subst: &Substs<'tcx>,
+                            b_subst: &Substs<'tcx>)
+                            -> RelateResult<'tcx, Substs<'tcx>>
+    where R: TypeRelation<'a,'tcx>
+{
+    let mut substs = Substs::empty();
+
+    for &space in &ParamSpace::all() {
+        let a_tps = a_subst.types.get_slice(space);
+        let b_tps = b_subst.types.get_slice(space);
+        let t_variances = variances.map(|v| v.types.get_slice(space));
+        let tps = try!(relate_type_params(relation, t_variances, a_tps, b_tps));
+        substs.types.replace(space, tps);
+    }
+
+    match (&a_subst.regions, &b_subst.regions) {
+        (&ErasedRegions, _) | (_, &ErasedRegions) => {
+            substs.regions = ErasedRegions;
+        }
+
+        (&NonerasedRegions(ref a), &NonerasedRegions(ref b)) => {
+            for &space in &ParamSpace::all() {
+                let a_regions = a.get_slice(space);
+                let b_regions = b.get_slice(space);
+                let r_variances = variances.map(|v| v.regions.get_slice(space));
+                let regions = try!(relate_region_params(relation,
+                                                        r_variances,
+                                                        a_regions,
+                                                        b_regions));
+                substs.mut_regions().replace(space, regions);
+            }
+        }
+    }
+
+    Ok(substs)
+}
+
+fn relate_type_params<'a,'tcx,R>(relation: &mut R,
+                                 variances: Option<&[ty::Variance]>,
+                                 a_tys: &[Ty<'tcx>],
+                                 b_tys: &[Ty<'tcx>])
+                                 -> RelateResult<'tcx, Vec<Ty<'tcx>>>
+    where R: TypeRelation<'a,'tcx>
+{
+    if a_tys.len() != b_tys.len() {
+        return Err(ty::terr_ty_param_size(expected_found(relation,
+                                                         &a_tys.len(),
+                                                         &b_tys.len())));
+    }
+
+    (0 .. a_tys.len())
+        .map(|i| {
+            let a_ty = a_tys[i];
+            let b_ty = b_tys[i];
+            let v = variances.map_or(ty::Invariant, |v| v[i]);
+            relation.relate_with_variance(v, &a_ty, &b_ty)
+        })
+        .collect()
+}
+
+fn relate_region_params<'a,'tcx:'a,R>(relation: &mut R,
+                                      variances: Option<&[ty::Variance]>,
+                                      a_rs: &[ty::Region],
+                                      b_rs: &[ty::Region])
+                                      -> RelateResult<'tcx, Vec<ty::Region>>
+    where R: TypeRelation<'a,'tcx>
+{
+    let tcx = relation.tcx();
+    let num_region_params = a_rs.len();
+
+    debug!("relate_region_params(a_rs={}, \
+            b_rs={}, variances={})",
+           a_rs.repr(tcx),
+           b_rs.repr(tcx),
+           variances.repr(tcx));
+
+    assert_eq!(num_region_params,
+               variances.map_or(num_region_params,
+                                |v| v.len()));
+
+    assert_eq!(num_region_params, b_rs.len());
+
+    (0..a_rs.len())
+        .map(|i| {
+            let a_r = a_rs[i];
+            let b_r = b_rs[i];
+            let variance = variances.map_or(ty::Invariant, |v| v[i]);
+            relation.relate_with_variance(variance, &a_r, &b_r)
+        })
+        .collect()
+}
+
+impl<'a,'tcx:'a> Relate<'a,'tcx> for ty::BareFnTy<'tcx> {
+    fn relate<R>(relation: &mut R,
+                 a: &ty::BareFnTy<'tcx>,
+                 b: &ty::BareFnTy<'tcx>)
+                 -> RelateResult<'tcx, ty::BareFnTy<'tcx>>
+        where R: TypeRelation<'a,'tcx>
+    {
+        let unsafety = try!(relation.relate(&a.unsafety, &b.unsafety));
+        let abi = try!(relation.relate(&a.abi, &b.abi));
+        let sig = try!(relation.relate(&a.sig, &b.sig));
+        Ok(ty::BareFnTy {unsafety: unsafety,
+                         abi: abi,
+                         sig: sig})
+    }
+}
+
+impl<'a,'tcx:'a> Relate<'a,'tcx> for ty::FnSig<'tcx> {
+    fn relate<R>(relation: &mut R,
+                 a: &ty::FnSig<'tcx>,
+                 b: &ty::FnSig<'tcx>)
+                 -> RelateResult<'tcx, ty::FnSig<'tcx>>
+        where R: TypeRelation<'a,'tcx>
+    {
+        if a.variadic != b.variadic {
+            return Err(ty::terr_variadic_mismatch(
+                expected_found(relation, &a.variadic, &b.variadic)));
+        }
+
+        let inputs = try!(relate_arg_vecs(relation,
+                                          &a.inputs,
+                                          &b.inputs));
+
+        let output = try!(match (a.output, b.output) {
+            (ty::FnConverging(a_ty), ty::FnConverging(b_ty)) =>
+                Ok(ty::FnConverging(try!(relation.relate(&a_ty, &b_ty)))),
+            (ty::FnDiverging, ty::FnDiverging) =>
+                Ok(ty::FnDiverging),
+            (a, b) =>
+                Err(ty::terr_convergence_mismatch(
+                    expected_found(relation, &(a != ty::FnDiverging), &(b != ty::FnDiverging)))),
+        });
+
+        return Ok(ty::FnSig {inputs: inputs,
+                             output: output,
+                             variadic: a.variadic});
+    }
+}
+
+fn relate_arg_vecs<'a,'tcx,R>(relation: &mut R,
+                              a_args: &[Ty<'tcx>],
+                              b_args: &[Ty<'tcx>])
+                              -> RelateResult<'tcx, Vec<Ty<'tcx>>>
+    where R: TypeRelation<'a,'tcx>
+{
+    if a_args.len() != b_args.len() {
+        return Err(ty::terr_arg_count);
+    }
+
+    a_args.iter()
+          .zip(b_args.iter())
+          .map(|(a, b)| relation.relate_with_variance(ty::Contravariant, a, b))
+          .collect()
+}
+
+impl<'a,'tcx:'a> Relate<'a,'tcx> for ast::Unsafety {
+    fn relate<R>(relation: &mut R,
+                 a: &ast::Unsafety,
+                 b: &ast::Unsafety)
+                 -> RelateResult<'tcx, ast::Unsafety>
+        where R: TypeRelation<'a,'tcx>
+    {
+        if a != b {
+            Err(ty::terr_unsafety_mismatch(expected_found(relation, a, b)))
+        } else {
+            Ok(*a)
+        }
+    }
+}
+
+impl<'a,'tcx:'a> Relate<'a,'tcx> for abi::Abi {
+    fn relate<R>(relation: &mut R,
+                 a: &abi::Abi,
+                 b: &abi::Abi)
+                 -> RelateResult<'tcx, abi::Abi>
+        where R: TypeRelation<'a,'tcx>
+    {
+        if a == b {
+            Ok(*a)
+        } else {
+            Err(ty::terr_abi_mismatch(expected_found(relation, a, b)))
+        }
+    }
+}
+
+impl<'a,'tcx:'a> Relate<'a,'tcx> for ty::ProjectionTy<'tcx> {
+    fn relate<R>(relation: &mut R,
+                 a: &ty::ProjectionTy<'tcx>,
+                 b: &ty::ProjectionTy<'tcx>)
+                 -> RelateResult<'tcx, ty::ProjectionTy<'tcx>>
+        where R: TypeRelation<'a,'tcx>
+    {
+        if a.item_name != b.item_name {
+            Err(ty::terr_projection_name_mismatched(
+                expected_found(relation, &a.item_name, &b.item_name)))
+        } else {
+            let trait_ref = try!(relation.relate(&*a.trait_ref, &*b.trait_ref));
+            Ok(ty::ProjectionTy { trait_ref: Rc::new(trait_ref), item_name: a.item_name })
+        }
+    }
+}
+
+impl<'a,'tcx:'a> Relate<'a,'tcx> for ty::ProjectionPredicate<'tcx> {
+    fn relate<R>(relation: &mut R,
+                 a: &ty::ProjectionPredicate<'tcx>,
+                 b: &ty::ProjectionPredicate<'tcx>)
+                 -> RelateResult<'tcx, ty::ProjectionPredicate<'tcx>>
+        where R: TypeRelation<'a,'tcx>
+    {
+        let projection_ty = try!(relation.relate(&a.projection_ty, &b.projection_ty));
+        let ty = try!(relation.relate(&a.ty, &b.ty));
+        Ok(ty::ProjectionPredicate { projection_ty: projection_ty, ty: ty })
+    }
+}
+
+impl<'a,'tcx:'a> Relate<'a,'tcx> for Vec<ty::PolyProjectionPredicate<'tcx>> {
+    fn relate<R>(relation: &mut R,
+                 a: &Vec<ty::PolyProjectionPredicate<'tcx>>,
+                 b: &Vec<ty::PolyProjectionPredicate<'tcx>>)
+                 -> RelateResult<'tcx, Vec<ty::PolyProjectionPredicate<'tcx>>>
+        where R: TypeRelation<'a,'tcx>
+    {
+        // To be compatible, `a` and `b` must be for precisely the
+        // same set of traits and item names. We always require that
+        // projection bounds lists are sorted by trait-def-id and item-name,
+        // so we can just iterate through the lists pairwise, so long as they are the
+        // same length.
+        if a.len() != b.len() {
+            Err(ty::terr_projection_bounds_length(expected_found(relation, &a.len(), &b.len())))
+        } else {
+            a.iter()
+                .zip(b.iter())
+                .map(|(a, b)| relation.relate(a, b))
+                .collect()
+        }
+    }
+}
+
+impl<'a,'tcx:'a> Relate<'a,'tcx> for ty::ExistentialBounds<'tcx> {
+    fn relate<R>(relation: &mut R,
+                 a: &ty::ExistentialBounds<'tcx>,
+                 b: &ty::ExistentialBounds<'tcx>)
+                 -> RelateResult<'tcx, ty::ExistentialBounds<'tcx>>
+        where R: TypeRelation<'a,'tcx>
+    {
+        let r = try!(relation.relate_with_variance(ty::Contravariant,
+                                                   &a.region_bound,
+                                                   &b.region_bound));
+        let nb = try!(relation.relate(&a.builtin_bounds, &b.builtin_bounds));
+        let pb = try!(relation.relate(&a.projection_bounds, &b.projection_bounds));
+        Ok(ty::ExistentialBounds { region_bound: r,
+                                   builtin_bounds: nb,
+                                   projection_bounds: pb })
+    }
+}
+
+impl<'a,'tcx:'a> Relate<'a,'tcx> for ty::BuiltinBounds {
+    fn relate<R>(relation: &mut R,
+                 a: &ty::BuiltinBounds,
+                 b: &ty::BuiltinBounds)
+                 -> RelateResult<'tcx, ty::BuiltinBounds>
+        where R: TypeRelation<'a,'tcx>
+    {
+        // Two sets of builtin bounds are only relatable if they are
+        // precisely the same (but see the coercion code).
+        if a != b {
+            Err(ty::terr_builtin_bounds(expected_found(relation, a, b)))
+        } else {
+            Ok(*a)
+        }
+    }
+}
+
+impl<'a,'tcx:'a> Relate<'a,'tcx> for ty::TraitRef<'tcx> {
+    fn relate<R>(relation: &mut R,
+                 a: &ty::TraitRef<'tcx>,
+                 b: &ty::TraitRef<'tcx>)
+                 -> RelateResult<'tcx, ty::TraitRef<'tcx>>
+        where R: TypeRelation<'a,'tcx>
+    {
+        // Different traits cannot be related
+        if a.def_id != b.def_id {
+            Err(ty::terr_traits(expected_found(relation, &a.def_id, &b.def_id)))
+        } else {
+            let substs = try!(relate_item_substs(relation, a.def_id, a.substs, b.substs));
+            Ok(ty::TraitRef { def_id: a.def_id, substs: relation.tcx().mk_substs(substs) })
+        }
+    }
+}
+
+impl<'a,'tcx:'a> Relate<'a,'tcx> for Ty<'tcx> {
+    fn relate<R>(relation: &mut R,
+                 a: &Ty<'tcx>,
+                 b: &Ty<'tcx>)
+                 -> RelateResult<'tcx, Ty<'tcx>>
+        where R: TypeRelation<'a,'tcx>
+    {
+        relation.tys(a, b)
+    }
+}
+
+/// The main "type relation" routine. Note that this does not handle
+/// inference artifacts, so you should filter those out before calling
+/// it.
+pub fn super_relate_tys<'a,'tcx:'a,R>(relation: &mut R,
+                                      a: Ty<'tcx>,
+                                      b: Ty<'tcx>)
+                                      -> RelateResult<'tcx, Ty<'tcx>>
+    where R: TypeRelation<'a,'tcx>
+{
+    let tcx = relation.tcx();
+    let a_sty = &a.sty;
+    let b_sty = &b.sty;
+    debug!("super_tys: a_sty={:?} b_sty={:?}", a_sty, b_sty);
+    match (a_sty, b_sty) {
+        (&ty::ty_infer(_), _) |
+        (_, &ty::ty_infer(_)) =>
+        {
+            // The caller should handle these cases!
+            tcx.sess.bug("var types encountered in super_relate_tys")
+        }
+
+        (&ty::ty_err, _) | (_, &ty::ty_err) =>
+        {
+            Ok(tcx.types.err)
+        }
+
+        (&ty::ty_char, _) |
+        (&ty::ty_bool, _) |
+        (&ty::ty_int(_), _) |
+        (&ty::ty_uint(_), _) |
+        (&ty::ty_float(_), _) |
+        (&ty::ty_str, _)
+            if a == b =>
+        {
+            Ok(a)
+        }
+
+        (&ty::ty_param(ref a_p), &ty::ty_param(ref b_p))
+            if a_p.idx == b_p.idx && a_p.space == b_p.space =>
+        {
+            Ok(a)
+        }
+
+        (&ty::ty_enum(a_id, a_substs), &ty::ty_enum(b_id, b_substs))
+            if a_id == b_id =>
+        {
+            let substs = try!(relate_item_substs(relation, a_id, a_substs, b_substs));
+            Ok(ty::mk_enum(tcx, a_id, tcx.mk_substs(substs)))
+        }
+
+        (&ty::ty_trait(ref a_), &ty::ty_trait(ref b_)) =>
+        {
+            let principal = try!(relation.relate(&a_.principal, &b_.principal));
+            let bounds = try!(relation.relate(&a_.bounds, &b_.bounds));
+            Ok(ty::mk_trait(tcx, principal, bounds))
+        }
+
+        (&ty::ty_struct(a_id, a_substs), &ty::ty_struct(b_id, b_substs))
+            if a_id == b_id =>
+        {
+            let substs = try!(relate_item_substs(relation, a_id, a_substs, b_substs));
+            Ok(ty::mk_struct(tcx, a_id, tcx.mk_substs(substs)))
+        }
+
+        (&ty::ty_closure(a_id, a_substs),
+         &ty::ty_closure(b_id, b_substs))
+            if a_id == b_id =>
+        {
+            // All ty_closure types with the same id represent
+            // the (anonymous) type of the same closure expression. So
+            // all of their regions should be equated.
+            let substs = try!(relate_substs(relation, None, a_substs, b_substs));
+            Ok(ty::mk_closure(tcx, a_id, tcx.mk_substs(substs)))
+        }
+
+        (&ty::ty_uniq(a_inner), &ty::ty_uniq(b_inner)) =>
+        {
+            let typ = try!(relation.relate(&a_inner, &b_inner));
+            Ok(ty::mk_uniq(tcx, typ))
+        }
+
+        (&ty::ty_ptr(ref a_mt), &ty::ty_ptr(ref b_mt)) =>
+        {
+            let mt = try!(relation.relate(a_mt, b_mt));
+            Ok(ty::mk_ptr(tcx, mt))
+        }
+
+        (&ty::ty_rptr(a_r, ref a_mt), &ty::ty_rptr(b_r, ref b_mt)) =>
+        {
+            let r = try!(relation.relate_with_variance(ty::Contravariant, a_r, b_r));
+            let mt = try!(relation.relate(a_mt, b_mt));
+            Ok(ty::mk_rptr(tcx, tcx.mk_region(r), mt))
+        }
+
+        (&ty::ty_vec(a_t, Some(sz_a)), &ty::ty_vec(b_t, Some(sz_b))) =>
+        {
+            let t = try!(relation.relate(&a_t, &b_t));
+            if sz_a == sz_b {
+                Ok(ty::mk_vec(tcx, t, Some(sz_a)))
+            } else {
+                Err(ty::terr_fixed_array_size(expected_found(relation, &sz_a, &sz_b)))
+            }
+        }
+
+        (&ty::ty_vec(a_t, None), &ty::ty_vec(b_t, None)) =>
+        {
+            let t = try!(relation.relate(&a_t, &b_t));
+            Ok(ty::mk_vec(tcx, t, None))
+        }
+
+        (&ty::ty_tup(ref as_), &ty::ty_tup(ref bs)) =>
+        {
+            if as_.len() == bs.len() {
+                let ts = try!(as_.iter()
+                                 .zip(bs.iter())
+                                 .map(|(a, b)| relation.relate(a, b))
+                                 .collect::<Result<_, _>>());
+                Ok(ty::mk_tup(tcx, ts))
+            } else if as_.len() != 0 && bs.len() != 0 {
+                Err(ty::terr_tuple_size(
+                    expected_found(relation, &as_.len(), &bs.len())))
+            } else {
+                Err(ty::terr_sorts(expected_found(relation, &a, &b)))
+            }
+        }
+
+        (&ty::ty_bare_fn(a_opt_def_id, a_fty), &ty::ty_bare_fn(b_opt_def_id, b_fty))
+            if a_opt_def_id == b_opt_def_id =>
+        {
+            let fty = try!(relation.relate(a_fty, b_fty));
+            Ok(ty::mk_bare_fn(tcx, a_opt_def_id, tcx.mk_bare_fn(fty)))
+        }
+
+        (&ty::ty_projection(ref a_data), &ty::ty_projection(ref b_data)) =>
+        {
+            let projection_ty = try!(relation.relate(a_data, b_data));
+            Ok(ty::mk_projection(tcx, projection_ty.trait_ref, projection_ty.item_name))
+        }
+
+        _ =>
+        {
+            Err(ty::terr_sorts(expected_found(relation, &a, &b)))
+        }
+    }
+}
+
+impl<'a,'tcx:'a> Relate<'a,'tcx> for ty::Region {
+    fn relate<R>(relation: &mut R,
+                 a: &ty::Region,
+                 b: &ty::Region)
+                 -> RelateResult<'tcx, ty::Region>
+        where R: TypeRelation<'a,'tcx>
+    {
+        relation.regions(*a, *b)
+    }
+}
+
+impl<'a,'tcx:'a,T> Relate<'a,'tcx> for ty::Binder<T>
+    where T: Relate<'a,'tcx>
+{
+    fn relate<R>(relation: &mut R,
+                 a: &ty::Binder<T>,
+                 b: &ty::Binder<T>)
+                 -> RelateResult<'tcx, ty::Binder<T>>
+        where R: TypeRelation<'a,'tcx>
+    {
+        relation.binders(a, b)
+    }
+}
+
+impl<'a,'tcx:'a,T> Relate<'a,'tcx> for Rc<T>
+    where T: Relate<'a,'tcx>
+{
+    fn relate<R>(relation: &mut R,
+                 a: &Rc<T>,
+                 b: &Rc<T>)
+                 -> RelateResult<'tcx, Rc<T>>
+        where R: TypeRelation<'a,'tcx>
+    {
+        let a: &T = a;
+        let b: &T = b;
+        Ok(Rc::new(try!(relation.relate(a, b))))
+    }
+}
+
+impl<'a,'tcx:'a,T> Relate<'a,'tcx> for Box<T>
+    where T: Relate<'a,'tcx>
+{
+    fn relate<R>(relation: &mut R,
+                 a: &Box<T>,
+                 b: &Box<T>)
+                 -> RelateResult<'tcx, Box<T>>
+        where R: TypeRelation<'a,'tcx>
+    {
+        let a: &T = a;
+        let b: &T = b;
+        Ok(Box::new(try!(relation.relate(a, b))))
+    }
+}
+
+///////////////////////////////////////////////////////////////////////////
+// Error handling
+
+pub fn expected_found<'a,'tcx,R,T>(relation: &mut R,
+                                   a: &T,
+                                   b: &T)
+                                   -> ty::expected_found<T>
+    where R: TypeRelation<'a,'tcx>, T: Clone
+{
+    expected_found_bool(relation.a_is_expected(), a, b)
+}
+
+pub fn expected_found_bool<T>(a_is_expected: bool,
+                              a: &T,
+                              b: &T)
+                              -> ty::expected_found<T>
+    where T: Clone
+{
+    let a = a.clone();
+    let b = b.clone();
+    if a_is_expected {
+        ty::expected_found {expected: a, found: b}
+    } else {
+        ty::expected_found {expected: b, found: a}
+    }
+}
+
diff --git a/src/librustc/util/ppaux.rs b/src/librustc/util/ppaux.rs
index 4405a9d75ee62..60b422b3769d5 100644
--- a/src/librustc/util/ppaux.rs
+++ b/src/librustc/util/ppaux.rs
@@ -1526,3 +1526,9 @@ impl<'tcx> UserString<'tcx> for ty::Predicate<'tcx> {
         }
     }
 }
+
+impl<'tcx> Repr<'tcx> for ast::Unsafety {
+    fn repr(&self, _: &ctxt<'tcx>) -> String {
+        format!("{:?}", *self)
+    }
+}
diff --git a/src/librustc_driver/test.rs b/src/librustc_driver/test.rs
index ed4d30f300c78..f9be71561e384 100644
--- a/src/librustc_driver/test.rs
+++ b/src/librustc_driver/test.rs
@@ -22,7 +22,7 @@ use rustc_typeck::middle::stability;
 use rustc_typeck::middle::subst;
 use rustc_typeck::middle::subst::Subst;
 use rustc_typeck::middle::ty::{self, Ty};
-use rustc_typeck::middle::infer::combine::Combine;
+use rustc_typeck::middle::ty_relate::TypeRelation;
 use rustc_typeck::middle::infer;
 use rustc_typeck::middle::infer::lub::Lub;
 use rustc_typeck::middle::infer::glb::Glb;
@@ -350,21 +350,21 @@ impl<'a, 'tcx> Env<'a, 'tcx> {
 
     pub fn sub(&self) -> Sub<'a, 'tcx> {
         let trace = self.dummy_type_trace();
-        Sub(self.infcx.combine_fields(true, trace))
+        self.infcx.sub(true, trace)
     }
 
     pub fn lub(&self) -> Lub<'a, 'tcx> {
         let trace = self.dummy_type_trace();
-        Lub(self.infcx.combine_fields(true, trace))
+        self.infcx.lub(true, trace)
     }
 
     pub fn glb(&self) -> Glb<'a, 'tcx> {
         let trace = self.dummy_type_trace();
-        Glb(self.infcx.combine_fields(true, trace))
+        self.infcx.glb(true, trace)
     }
 
     pub fn make_lub_ty(&self, t1: Ty<'tcx>, t2: Ty<'tcx>) -> Ty<'tcx> {
-        match self.lub().tys(t1, t2) {
+        match self.lub().relate(&t1, &t2) {
             Ok(t) => t,
             Err(ref e) => panic!("unexpected error computing LUB: {}",
                                 ty::type_err_to_str(self.infcx.tcx, e))
@@ -374,7 +374,7 @@ impl<'a, 'tcx> Env<'a, 'tcx> {
     /// Checks that `t1 <: t2` is true (this may register additional
     /// region checks).
     pub fn check_sub(&self, t1: Ty<'tcx>, t2: Ty<'tcx>) {
-        match self.sub().tys(t1, t2) {
+        match self.sub().relate(&t1, &t2) {
             Ok(_) => { }
             Err(ref e) => {
                 panic!("unexpected error computing sub({},{}): {}",
@@ -388,7 +388,7 @@ impl<'a, 'tcx> Env<'a, 'tcx> {
     /// Checks that `t1 <: t2` is false (this may register additional
     /// region checks).
     pub fn check_not_sub(&self, t1: Ty<'tcx>, t2: Ty<'tcx>) {
-        match self.sub().tys(t1, t2) {
+        match self.sub().relate(&t1, &t2) {
             Err(_) => { }
             Ok(_) => {
                 panic!("unexpected success computing sub({},{})",
@@ -400,7 +400,7 @@ impl<'a, 'tcx> Env<'a, 'tcx> {
 
     /// Checks that `LUB(t1,t2) == t_lub`
     pub fn check_lub(&self, t1: Ty<'tcx>, t2: Ty<'tcx>, t_lub: Ty<'tcx>) {
-        match self.lub().tys(t1, t2) {
+        match self.lub().relate(&t1, &t2) {
             Ok(t) => {
                 self.assert_eq(t, t_lub);
             }
@@ -417,7 +417,7 @@ impl<'a, 'tcx> Env<'a, 'tcx> {
                self.ty_to_string(t1),
                self.ty_to_string(t2),
                self.ty_to_string(t_glb));
-        match self.glb().tys(t1, t2) {
+        match self.glb().relate(&t1, &t2) {
             Err(e) => {
                 panic!("unexpected error computing LUB: {:?}", e)
             }
diff --git a/src/librustc_typeck/check/callee.rs b/src/librustc_typeck/check/callee.rs
index 31ac0a57ba0e1..3f9c14e0afe39 100644
--- a/src/librustc_typeck/check/callee.rs
+++ b/src/librustc_typeck/check/callee.rs
@@ -83,9 +83,7 @@ pub fn check_call<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
                   UnresolvedTypeAction::Error,
                   LvaluePreference::NoPreference,
                   |adj_ty, idx| {
-                      let autoderefref = ty::AutoDerefRef { autoderefs: idx, autoref: None };
-                      try_overloaded_call_step(fcx, call_expr, callee_expr,
-                                               adj_ty, autoderefref)
+                      try_overloaded_call_step(fcx, call_expr, callee_expr, adj_ty, idx)
                   });
 
     match result {
@@ -119,13 +117,15 @@ fn try_overloaded_call_step<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
                                       call_expr: &'tcx ast::Expr,
                                       callee_expr: &'tcx ast::Expr,
                                       adjusted_ty: Ty<'tcx>,
-                                      autoderefref: ty::AutoDerefRef<'tcx>)
+                                      autoderefs: usize)
                                       -> Option<CallStep<'tcx>>
 {
-    debug!("try_overloaded_call_step(call_expr={}, adjusted_ty={}, autoderefref={})",
+    debug!("try_overloaded_call_step(call_expr={}, adjusted_ty={}, autoderefs={})",
            call_expr.repr(fcx.tcx()),
            adjusted_ty.repr(fcx.tcx()),
-           autoderefref.repr(fcx.tcx()));
+           autoderefs);
+
+    let autoderefref = ty::AutoDerefRef { autoderefs: autoderefs, autoref: None };
 
     // If the callee is a bare function or a closure, then we're all set.
     match structurally_resolved_type(fcx, callee_expr.span, adjusted_ty).sty {
@@ -161,6 +161,18 @@ fn try_overloaded_call_step<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
             }
         }
 
+        // Hack: we know that there are traits implementing Fn for &F
+        // where F:Fn and so forth. In the particular case of types
+        // like `x: &mut FnMut()`, if there is a call `x()`, we would
+        // normally translate to `FnMut::call_mut(&mut x, ())`, but
+        // that winds up requiring `mut x: &mut FnMut()`. A little
+        // over the top. The simplest fix by far is to just ignore
+        // this case and deref again, so we wind up with
+        // `FnMut::call_mut(&mut *x, ())`.
+        ty::ty_rptr(..) if autoderefs == 0 => {
+            return None;
+        }
+
         _ => {}
     }
 
diff --git a/src/librustc_typeck/check/coercion.rs b/src/librustc_typeck/check/coercion.rs
index ae1dbbb1b00ad..ced6cec3ef0dc 100644
--- a/src/librustc_typeck/check/coercion.rs
+++ b/src/librustc_typeck/check/coercion.rs
@@ -62,12 +62,11 @@
 
 use check::{autoderef, FnCtxt, NoPreference, PreferMutLvalue, UnresolvedTypeAction};
 
-use middle::infer::{self, cres, Coercion, TypeTrace};
-use middle::infer::combine::Combine;
-use middle::infer::sub::Sub;
+use middle::infer::{self, Coercion};
 use middle::subst;
 use middle::ty::{AutoPtr, AutoDerefRef, AdjustDerefRef, AutoUnsize, AutoUnsafe};
 use middle::ty::{self, mt, Ty};
+use middle::ty_relate::RelateResult;
 use util::common::indent;
 use util::ppaux;
 use util::ppaux::Repr;
@@ -76,10 +75,10 @@ use syntax::ast;
 
 struct Coerce<'a, 'tcx: 'a> {
     fcx: &'a FnCtxt<'a, 'tcx>,
-    trace: TypeTrace<'tcx>
+    origin: infer::TypeOrigin,
 }
 
-type CoerceResult<'tcx> = cres<'tcx, Option<ty::AutoAdjustment<'tcx>>>;
+type CoerceResult<'tcx> = RelateResult<'tcx, Option<ty::AutoAdjustment<'tcx>>>;
 
 impl<'f, 'tcx> Coerce<'f, 'tcx> {
     fn tcx(&self) -> &ty::ctxt<'tcx> {
@@ -87,14 +86,16 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
     }
 
     fn subtype(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
-        let sub = Sub(self.fcx.infcx().combine_fields(false, self.trace.clone()));
-        try!(sub.tys(a, b));
+        try!(self.fcx.infcx().sub_types(false, self.origin.clone(), a, b));
         Ok(None) // No coercion required.
     }
 
-    fn outlives(&self, a: ty::Region, b: ty::Region) -> cres<'tcx, ()> {
-        let sub = Sub(self.fcx.infcx().combine_fields(false, self.trace.clone()));
-        try!(sub.regions(b, a));
+    fn outlives(&self,
+                origin: infer::SubregionOrigin<'tcx>,
+                a: ty::Region,
+                b: ty::Region)
+                -> RelateResult<'tcx, ()> {
+        infer::mk_subr(self.fcx.infcx(), origin, b, a);
         Ok(())
     }
 
@@ -190,7 +191,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
             _ => return self.subtype(a, b)
         }
 
-        let coercion = Coercion(self.trace.clone());
+        let coercion = Coercion(self.origin.span());
         let r_borrow = self.fcx.infcx().next_region_var(coercion);
         let autoref = Some(AutoPtr(r_borrow, mutbl_b, None));
 
@@ -214,7 +215,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
             }
             let ty = ty::mk_rptr(self.tcx(), r_borrow,
                                  mt {ty: inner_ty, mutbl: mutbl_b});
-            if let Err(err) = self.fcx.infcx().try(|_| self.subtype(ty, b)) {
+            if let Err(err) = self.subtype(ty, b) {
                 if first_error.is_none() {
                     first_error = Some(err);
                 }
@@ -264,12 +265,12 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
                             return Err(ty::terr_mutability);
                         }
 
-                        let coercion = Coercion(self.trace.clone());
+                        let coercion = Coercion(self.origin.span());
                         let r_borrow = self.fcx.infcx().next_region_var(coercion);
                         let ty = ty::mk_rptr(self.tcx(),
                                              self.tcx().mk_region(r_borrow),
                                              ty::mt{ty: ty, mutbl: mt_b.mutbl});
-                        try!(self.fcx.infcx().try(|_| self.subtype(ty, b)));
+                        try!(self.subtype(ty, b));
                         debug!("Success, coerced with AutoDerefRef(1, \
                                 AutoPtr(AutoUnsize({:?})))", kind);
                         Ok(Some(AdjustDerefRef(AutoDerefRef {
@@ -290,7 +291,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
 
                         let ty = ty::mk_ptr(self.tcx(),
                                              ty::mt{ty: ty, mutbl: mt_b.mutbl});
-                        try!(self.fcx.infcx().try(|_| self.subtype(ty, b)));
+                        try!(self.subtype(ty, b));
                         debug!("Success, coerced with AutoDerefRef(1, \
                                 AutoPtr(AutoUnsize({:?})))", kind);
                         Ok(Some(AdjustDerefRef(AutoDerefRef {
@@ -306,7 +307,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
                 match self.unsize_ty(t_a, t_b) {
                     Some((ty, kind)) => {
                         let ty = ty::mk_uniq(self.tcx(), ty);
-                        try!(self.fcx.infcx().try(|_| self.subtype(ty, b)));
+                        try!(self.subtype(ty, b));
                         debug!("Success, coerced with AutoDerefRef(1, \
                                 AutoUnsizeUniq({:?}))", kind);
                         Ok(Some(AdjustDerefRef(AutoDerefRef {
@@ -365,9 +366,10 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
                             let ty_a1 = ty::mk_trait(tcx, data_a.principal.clone(), bounds_a1);
 
                             // relate `a1` to `b`
-                            let result = self.fcx.infcx().try(|_| {
+                            let result = self.fcx.infcx().commit_if_ok(|_| {
                                 // it's ok to upcast from Foo+'a to Foo+'b so long as 'a : 'b
-                                try!(self.outlives(data_a.bounds.region_bound,
+                                try!(self.outlives(infer::RelateObjectBound(self.origin.span()),
+                                                   data_a.bounds.region_bound,
                                                    data_b.bounds.region_bound));
                                 self.subtype(ty_a1, ty_b)
                             });
@@ -399,7 +401,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
                         let mut result = None;
                         let tps = ty_substs_a.iter().zip(ty_substs_b.iter()).enumerate();
                         for (i, (tp_a, tp_b)) in tps {
-                            if self.fcx.infcx().try(|_| self.subtype(*tp_a, *tp_b)).is_ok() {
+                            if self.subtype(*tp_a, *tp_b).is_ok() {
                                 continue;
                             }
                             match self.unsize_ty(*tp_a, *tp_b) {
@@ -408,7 +410,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
                                     let mut new_substs = substs_a.clone();
                                     new_substs.types.get_mut_slice(subst::TypeSpace)[i] = new_tp;
                                     let ty = ty::mk_struct(tcx, did_a, tcx.mk_substs(new_substs));
-                                    if self.fcx.infcx().try(|_| self.subtype(ty, ty_b)).is_err() {
+                                    if self.subtype(ty, ty_b).is_err() {
                                         debug!("Unsized type parameter '{}', but still \
                                                 could not match types {} and {}",
                                                ppaux::ty_to_string(tcx, *tp_a),
@@ -534,14 +536,13 @@ pub fn mk_assignty<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
                              expr: &ast::Expr,
                              a: Ty<'tcx>,
                              b: Ty<'tcx>)
-                             -> cres<'tcx, ()> {
+                             -> RelateResult<'tcx, ()> {
     debug!("mk_assignty({} -> {})", a.repr(fcx.tcx()), b.repr(fcx.tcx()));
     let adjustment = try!(indent(|| {
-        fcx.infcx().commit_if_ok(|| {
-            let origin = infer::ExprAssignable(expr.span);
+        fcx.infcx().commit_if_ok(|_| {
             Coerce {
                 fcx: fcx,
-                trace: infer::TypeTrace::types(origin, false, a, b)
+                origin: infer::ExprAssignable(expr.span),
             }.coerce(expr, a, b)
         })
     }));
diff --git a/src/librustc_typeck/check/compare_method.rs b/src/librustc_typeck/check/compare_method.rs
index 1c5f2c5607857..532277d75b2e0 100644
--- a/src/librustc_typeck/check/compare_method.rs
+++ b/src/librustc_typeck/check/compare_method.rs
@@ -282,7 +282,7 @@ pub fn compare_impl_method<'tcx>(tcx: &ty::ctxt<'tcx>,
     let trait_fty = ty::mk_bare_fn(tcx, None, tcx.mk_bare_fn(trait_m.fty.clone()));
     let trait_fty = trait_fty.subst(tcx, &trait_to_skol_substs);
 
-    let err = infcx.try(|snapshot| {
+    let err = infcx.commit_if_ok(|snapshot| {
         let origin = infer::MethodCompatCheck(impl_m_span);
 
         let (impl_sig, _) =
diff --git a/src/librustc_typeck/check/dropck.rs b/src/librustc_typeck/check/dropck.rs
index 49f4399b2c7b4..2f7e0073e1751 100644
--- a/src/librustc_typeck/check/dropck.rs
+++ b/src/librustc_typeck/check/dropck.rs
@@ -95,7 +95,7 @@ fn ensure_drop_params_and_item_params_correspond<'tcx>(
         ty::lookup_item_type(tcx, self_type_did);
 
     let infcx = infer::new_infer_ctxt(tcx);
-    infcx.try(|snapshot| {
+    infcx.commit_if_ok(|snapshot| {
         let (named_type_to_skolem, skol_map) =
             infcx.construct_skolemized_subst(named_type_generics, snapshot);
         let named_type_skolem = named_type.subst(tcx, &named_type_to_skolem);
diff --git a/src/librustc_typeck/check/method/probe.rs b/src/librustc_typeck/check/method/probe.rs
index 6349ea57f2ff1..e203019bd0638 100644
--- a/src/librustc_typeck/check/method/probe.rs
+++ b/src/librustc_typeck/check/method/probe.rs
@@ -1130,7 +1130,7 @@ impl<'a,'tcx> ProbeContext<'a,'tcx> {
     ///////////////////////////////////////////////////////////////////////////
     // MISCELLANY
 
-    fn make_sub_ty(&self, sub: Ty<'tcx>, sup: Ty<'tcx>) -> infer::ures<'tcx> {
+    fn make_sub_ty(&self, sub: Ty<'tcx>, sup: Ty<'tcx>) -> infer::UnitResult<'tcx> {
         self.infcx().sub_types(false, infer::Misc(DUMMY_SP), sub, sup)
     }
 
diff --git a/src/librustc_typeck/check/regionck.rs b/src/librustc_typeck/check/regionck.rs
index 3edea6d300444..9171367468026 100644
--- a/src/librustc_typeck/check/regionck.rs
+++ b/src/librustc_typeck/check/regionck.rs
@@ -1542,7 +1542,7 @@ fn projection_bounds<'a,'tcx>(rcx: &Rcx<'a, 'tcx>,
             debug!("projection_bounds: outlives={} (2)",
                    outlives.repr(tcx));
 
-            let region_result = infcx.try(|_| {
+            let region_result = infcx.commit_if_ok(|_| {
                 let (outlives, _) =
                     infcx.replace_late_bound_regions_with_fresh_var(
                         span,
diff --git a/src/librustc_typeck/coherence/mod.rs b/src/librustc_typeck/coherence/mod.rs
index eaf07a3ef13a7..51d0c18872dc4 100644
--- a/src/librustc_typeck/coherence/mod.rs
+++ b/src/librustc_typeck/coherence/mod.rs
@@ -30,7 +30,6 @@ use middle::ty::{ty_uint, ty_closure, ty_uniq, ty_bare_fn};
 use middle::ty::ty_projection;
 use middle::ty;
 use CrateCtxt;
-use middle::infer::combine::Combine;
 use middle::infer::InferCtxt;
 use middle::infer::new_infer_ctxt;
 use std::collections::HashSet;
diff --git a/src/test/compile-fail/dst-bad-coerce1.rs b/src/test/compile-fail/dst-bad-coerce1.rs
index ddc929017718d..2d87345db2245 100644
--- a/src/test/compile-fail/dst-bad-coerce1.rs
+++ b/src/test/compile-fail/dst-bad-coerce1.rs
@@ -23,10 +23,6 @@ pub fn main() {
     let f2: &Fat<[isize; 3]> = &f1;
     let f3: &Fat<[usize]> = f2;
     //~^ ERROR mismatched types
-    //~| expected `&Fat<[usize]>`
-    //~| found `&Fat<[isize; 3]>`
-    //~| expected usize
-    //~| found isize
 
     // With a trait.
     let f1 = Fat { ptr: Foo };
diff --git a/src/test/compile-fail/object-lifetime-default-elision.rs b/src/test/compile-fail/object-lifetime-default-elision.rs
index 0077d10e6ca82..4fba45e2a66c5 100644
--- a/src/test/compile-fail/object-lifetime-default-elision.rs
+++ b/src/test/compile-fail/object-lifetime-default-elision.rs
@@ -81,8 +81,8 @@ fn load3<'a,'b>(ss: &'a SomeTrait) -> &'b SomeTrait {
     // which fails to type check.
 
     ss
-        //~^ ERROR cannot infer
-        //~| ERROR mismatched types
+        //~^ ERROR lifetime of the source pointer does not outlive lifetime bound
+        //~| ERROR cannot infer
 }
 
 fn main() {
diff --git a/src/test/compile-fail/object-lifetime-default-from-box-error.rs b/src/test/compile-fail/object-lifetime-default-from-box-error.rs
index 70752cbfda19f..7fae530984f89 100644
--- a/src/test/compile-fail/object-lifetime-default-from-box-error.rs
+++ b/src/test/compile-fail/object-lifetime-default-from-box-error.rs
@@ -25,7 +25,7 @@ fn load(ss: &mut SomeStruct) -> Box<SomeTrait> {
     // `Box<SomeTrait>` defaults to a `'static` bound, so this return
     // is illegal.
 
-    ss.r //~ ERROR mismatched types
+    ss.r //~ ERROR lifetime of the source pointer does not outlive lifetime bound
 }
 
 fn store(ss: &mut SomeStruct, b: Box<SomeTrait>) {
@@ -38,7 +38,7 @@ fn store(ss: &mut SomeStruct, b: Box<SomeTrait>) {
 fn store1<'b>(ss: &mut SomeStruct, b: Box<SomeTrait+'b>) {
     // Here we override the lifetimes explicitly, and so naturally we get an error.
 
-    ss.r = b; //~ ERROR mismatched types
+    ss.r = b; //~ ERROR lifetime of the source pointer does not outlive lifetime bound
 }
 
 fn main() {
diff --git a/src/test/compile-fail/regions-close-over-type-parameter-multiple.rs b/src/test/compile-fail/regions-close-over-type-parameter-multiple.rs
index 0f8bc6d684f12..10b883d4dc830 100644
--- a/src/test/compile-fail/regions-close-over-type-parameter-multiple.rs
+++ b/src/test/compile-fail/regions-close-over-type-parameter-multiple.rs
@@ -27,7 +27,7 @@ fn make_object_good2<'a,'b,A:SomeTrait+'a+'b>(v: A) -> Box<SomeTrait+'b> {
 
 fn make_object_bad<'a,'b,'c,A:SomeTrait+'a+'b>(v: A) -> Box<SomeTrait+'c> {
     // A outlives 'a AND 'b...but not 'c.
-    box v as Box<SomeTrait+'a> //~ ERROR mismatched types
+    box v as Box<SomeTrait+'a> //~ ERROR lifetime of the source pointer does not outlive
 }
 
 fn main() {
diff --git a/src/test/compile-fail/regions-trait-object-subtyping.rs b/src/test/compile-fail/regions-trait-object-subtyping.rs
index 8d05cb67e77b1..f3722690ef895 100644
--- a/src/test/compile-fail/regions-trait-object-subtyping.rs
+++ b/src/test/compile-fail/regions-trait-object-subtyping.rs
@@ -22,7 +22,7 @@ fn foo2<'a:'b,'b>(x: &'b mut (Dummy+'a)) -> &'b mut (Dummy+'b) {
 
 fn foo3<'a,'b>(x: &'a mut Dummy) -> &'b mut Dummy {
     // Without knowing 'a:'b, we can't coerce
-    x //~ ERROR mismatched types
+    x //~ ERROR lifetime of the source pointer does not outlive
      //~^ ERROR cannot infer
 }
 
diff --git a/src/test/run-pass/unboxed-closures-blanket-fn-mut.rs b/src/test/run-pass/unboxed-closures-blanket-fn-mut.rs
new file mode 100644
index 0000000000000..37dccca1e2245
--- /dev/null
+++ b/src/test/run-pass/unboxed-closures-blanket-fn-mut.rs
@@ -0,0 +1,37 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Test that you can supply `&F` where `F: FnMut()`.
+
+// pretty-expanded FIXME #23616
+
+#![feature(lang_items, unboxed_closures)]
+
+fn a<F:FnMut() -> i32>(mut f: F) -> i32 {
+    f()
+}
+
+fn b(f: &mut FnMut() -> i32) -> i32 {
+    a(f)
+}
+
+fn c<F:FnMut() -> i32>(f: &mut F) -> i32 {
+    a(f)
+}
+
+fn main() {
+    let z: isize = 7;
+
+    let x = b(&mut || 22);
+    assert_eq!(x, 22);
+
+    let x = c(&mut || 22);
+    assert_eq!(x, 22);
+}
diff --git a/src/test/run-pass/unboxed-closures-blanket-fn.rs b/src/test/run-pass/unboxed-closures-blanket-fn.rs
new file mode 100644
index 0000000000000..0f93966077bc3
--- /dev/null
+++ b/src/test/run-pass/unboxed-closures-blanket-fn.rs
@@ -0,0 +1,37 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Test that you can supply `&F` where `F: Fn()`.
+
+// pretty-expanded FIXME #23616
+
+#![feature(lang_items, unboxed_closures)]
+
+fn a<F:Fn() -> i32>(f: F) -> i32 {
+    f()
+}
+
+fn b(f: &Fn() -> i32) -> i32 {
+    a(f)
+}
+
+fn c<F:Fn() -> i32>(f: &F) -> i32 {
+    a(f)
+}
+
+fn main() {
+    let z: isize = 7;
+
+    let x = b(&|| 22);
+    assert_eq!(x, 22);
+
+    let x = c(&|| 22);
+    assert_eq!(x, 22);
+}