#[may_dangle] attribute

src/librustc/ty/mod.rs

@@ -742,6 +742,14 @@ impl<'tcx> Generics<'tcx> {
     pub fn count(&self) -> usize {
         self.parent_count() + self.own_count()
     }
+
+    pub fn region_param(&self, param: &EarlyBoundRegion) -> &RegionParameterDef<'tcx> {
+        &self.regions[param.index as usize - self.has_self as usize]
+    }
+
+    pub fn type_param(&self, param: &ParamTy) -> &TypeParameterDef<'tcx> {
+        &self.types[param.idx as usize - self.has_self as usize - self.regions.len()]
+    }
 }
 
 /// Bounds on generics.

src/librustc_typeck/check/dropck.rs

@@ -556,20 +556,10 @@ fn has_dtor_of_interest<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
             } else {
                 return DropckKind::BorrowedDataMustStrictlyOutliveSelf;
             };
+
             let method = tcx.impl_or_trait_item(dtor_method);
-            let substs = Substs::for_item(tcx,
-                                          method.container().id(),
-                                          |def, _| if def.pure_wrt_drop {
-                                              tcx.mk_region(ty::ReStatic)
-                                          } else {
-                                              substs.region_for_def(def)
-                                          },
-                                          |def, _| if def.pure_wrt_drop {
-                                              tcx.mk_nil()
-                                          } else {
-                                              substs.type_for_def(def)
-                                          });
-            let revised_ty = tcx.mk_adt(adt_def, &substs);
+            let impl_id: DefId = method.container().id();
+            let revised_ty = revise_self_ty(tcx, adt_def, impl_id, substs);
             return DropckKind::RevisedSelf(revised_ty);
         }
         ty::TyTrait(..) | ty::TyProjection(..) | ty::TyAnon(..) => {
@@ -581,3 +571,77 @@ fn has_dtor_of_interest<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
         }
     }
 }
+
+// Constructs new Ty just like the type defined by `adt_def` coupled
+// with `substs`, except each type and lifetime parameter marked as
+// `#[may_dangle]` in the Drop impl (identified by `impl_id`) is
+// respectively mapped to `()` or `'static`.
+//
+// For example: If the `adt_def` maps to:
+//
+//   enum Foo<'a, X, Y> { ... }
+//
+// and the `impl_id` maps to:
+//
+//   impl<#[may_dangle] 'a, X, #[may_dangle] Y> Drop for Foo<'a, X, Y> { ... }
+//
+// then revises input: `Foo<'r,i64,&'r i64>` to: `Foo<'static,i64,()>`
+fn revise_self_ty<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
+                                  adt_def: ty::AdtDef<'tcx>,
+                                  impl_id: DefId,
+                                  substs: &Substs<'tcx>) -> Ty<'tcx> {
+    // Get generics for `impl Drop` to query for `#[may_dangle]` attr.
+    let impl_bindings = tcx.lookup_generics(impl_id);
+
+    // Get Substs attached to Self on `impl Drop`; process in parallel
+    // with `substs`, replacing dangling entries as appropriate.
+    let self_substs = {
+        let impl_self_ty: Ty<'tcx> = tcx.lookup_item_type(impl_id).ty;
+        if let ty::TyAdt(self_adt_def, self_substs) = impl_self_ty.sty {
+            assert_eq!(adt_def, self_adt_def);
+            self_substs
+        } else {
+            bug!("Self in `impl Drop for _` must be an Adt.");
+        }
+    };
+
+    // Walk `substs` + `self_substs`, build new substs appropriate for
+    // `adt_def`; each non-dangling param reuses entry from `substs`.
+    let substs = Substs::for_item(
+        tcx,
+        adt_def.did,
+        |def, _| {
+            let r_orig = substs.region_for_def(def);
+            let impl_self_orig = self_substs.region_for_def(def);
+            let r = if let ty::Region::ReEarlyBound(ref ebr) = *impl_self_orig {
+                if impl_bindings.region_param(ebr).pure_wrt_drop {
+                    tcx.mk_region(ty::ReStatic)
+                } else {
+                    r_orig
+                }
+            } else {
+                bug!("substs for an impl must map regions to ReEarlyBound");
+            };
+            debug!("has_dtor_of_interest mapping def {:?} orig {:?} to {:?}",
+                   def, r_orig, r);
+            r
+        },
+        |def, _| {
+            let t_orig = substs.type_for_def(def);
+            let impl_self_orig = self_substs.type_for_def(def);
+            let t = if let ty::TypeVariants::TyParam(ref pt) = impl_self_orig.sty {
+                if impl_bindings.type_param(pt).pure_wrt_drop {
+                    tcx.mk_nil()
+                } else {
+                    t_orig
+                }
+            } else {
+                bug!("substs for an impl must map types to TyParam");
+            };
+            debug!("has_dtor_of_interest mapping def {:?} orig {:?} {:?} to {:?} {:?}",
+                   def, t_orig, t_orig.sty, t, t.sty);
+            t
+        });
+
+    return tcx.mk_adt(adt_def, &substs);
+}

src/test/compile-fail/dropck-eyepatch-reorder.rs

@@ -0,0 +1,73 @@
+// Copyright 2016 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.
+
+#![feature(generic_param_attrs)]
+#![feature(dropck_eyepatch)]
+
+// The point of this test is to test uses of `#[may_dangle]` attribute
+// where the formal declaration order (in the impl generics) does not
+// match the actual usage order (in the type instantiation).
+//
+// See also dropck-eyepatch.rs for more information about the general
+// structure of the test.
+
+use std::fmt;
+
+struct Dt<A: fmt::Debug>(&'static str, A);
+struct Dr<'a, B:'a+fmt::Debug>(&'static str, &'a B);
+struct Pt<A: fmt::Debug, B: fmt::Debug>(&'static str, A, B);
+struct Pr<'a, 'b, B:'a+'b+fmt::Debug>(&'static str, &'a B, &'b B);
+struct St<A: fmt::Debug>(&'static str, A);
+struct Sr<'a, B:'a+fmt::Debug>(&'static str, &'a B);
+
+impl<A: fmt::Debug> Drop for Dt<A> {
+    fn drop(&mut self) { println!("drop {} {:?}", self.0, self.1); }
+}
+impl<'a, B: fmt::Debug> Drop for Dr<'a, B> {
+    fn drop(&mut self) { println!("drop {} {:?}", self.0, self.1); }
+}
+unsafe impl<B: fmt::Debug, #[may_dangle] A: fmt::Debug> Drop for Pt<A, B> {
+    // (unsafe to access self.1  due to #[may_dangle] on A)
+    fn drop(&mut self) { println!("drop {} {:?}", self.0, self.2); }
+}
+unsafe impl<'b, #[may_dangle] 'a, B: fmt::Debug> Drop for Pr<'a, 'b, B> {
+    // (unsafe to access self.1 due to #[may_dangle] on 'a)
+    fn drop(&mut self) { println!("drop {} {:?}", self.0, self.2); }
+}
+
+fn main() {
+    use std::cell::Cell;
+    let c_long;
+    let (c, mut dt, mut dr, mut pt, mut pr, st, sr)
+        : (Cell<_>, Dt<_>, Dr<_>, Pt<_, _>, Pr<_>, St<_>, Sr<_>);
+    c_long = Cell::new(1);
+    c = Cell::new(1);
+
+    // No error: sufficiently long-lived state can be referenced in dtors
+    dt = Dt("dt", &c_long);
+    dr = Dr("dr", &c_long);
+    // Error: destructor order imprecisely modelled
+    dt = Dt("dt", &c); //~ ERROR `c` does not live long enough
+    dr = Dr("dr", &c); //~ ERROR `c` does not live long enough
+
+    // No error: Drop impl asserts .1 (A and &'a _) are not accessed
+    pt = Pt("pt", &c, &c_long);
+    pr = Pr("pr", &c, &c_long);
+
+    // Error: Drop impl's assertion does not apply to `B` nor `&'b _`
+    pt = Pt("pt", &c_long, &c); //~ ERROR `c` does not live long enough
+    pr = Pr("pr", &c_long, &c); //~ ERROR `c` does not live long enough
+
+    // No error: St and Sr have no destructor.
+    st = St("st", &c);
+    sr = Sr("sr", &c);
+
+    println!("{:?}", (dt.0, dr.0, pt.0, pr.0, st.0, sr.0));
+}

src/test/run-pass/dropck-eyepatch-reorder.rs

@@ -0,0 +1,89 @@
+// Copyright 2016 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.
+
+#![feature(generic_param_attrs)]
+#![feature(dropck_eyepatch)]
+
+// The point of this test is to test uses of `#[may_dangle]` attribute
+// where the formal declaration order (in the impl generics) does not
+// match the actual usage order (in the type instantiation).
+//
+// See also dropck-eyepatch.rs for more information about the general
+// structure of the test.
+
+trait Foo { fn foo(&self, _: &str); }
+
+struct Dt<A: Foo>(&'static str, A);
+struct Dr<'a, B:'a+Foo>(&'static str, &'a B);
+struct Pt<A: Foo, B: Foo>(&'static str, A, B);
+struct Pr<'a, 'b, B:'a+'b+Foo>(&'static str, &'a B, &'b B);
+struct St<A: Foo>(&'static str, A);
+struct Sr<'a, B:'a+Foo>(&'static str, &'a B);
+
+impl<A: Foo> Drop for Dt<A> {
+    fn drop(&mut self) { println!("drop {}", self.0); self.1.foo(self.0); }
+}
+impl<'a, B: Foo> Drop for Dr<'a, B> {
+    fn drop(&mut self) { println!("drop {}", self.0); self.1.foo(self.0); }
+}
+unsafe impl<B: Foo, #[may_dangle] A: Foo> Drop for Pt<A, B> {
+    // (unsafe to access self.1  due to #[may_dangle] on A)
+    fn drop(&mut self) { println!("drop {}", self.0); self.2.foo(self.0); }
+}
+unsafe impl<'b, #[may_dangle] 'a, B: Foo> Drop for Pr<'a, 'b, B> {
+    // (unsafe to access self.1 due to #[may_dangle] on 'a)
+    fn drop(&mut self) { println!("drop {}", self.0); self.2.foo(self.0); }
+}
+
+fn main() {
+    use std::cell::RefCell;
+
+    impl Foo for RefCell<String> {
+        fn foo(&self, s: &str) {
+            let s2 = format!("{}|{}", *self.borrow(), s);
+            *self.borrow_mut() = s2;
+        }
+    }
+
+    impl<'a, T:Foo> Foo for &'a T {
+        fn foo(&self, s: &str) {
+            (*self).foo(s);
+        }
+    }
+
+    struct CheckOnDrop(RefCell<String>, &'static str);
+    impl Drop for CheckOnDrop {
+        fn drop(&mut self) { assert_eq!(*self.0.borrow(), self.1); }
+    }
+
+    let c_long;
+    let (c, dt, dr, pt, pr, st, sr)
+        : (CheckOnDrop, Dt<_>, Dr<_>, Pt<_, _>, Pr<_>, St<_>, Sr<_>);
+    c_long = CheckOnDrop(RefCell::new("c_long".to_string()),
+                         "c_long|pr|pt|dr|dt");
+    c = CheckOnDrop(RefCell::new("c".to_string()),
+                    "c");
+
+    // No error: sufficiently long-lived state can be referenced in dtors
+    dt = Dt("dt", &c_long.0);
+    dr = Dr("dr", &c_long.0);
+
+    // No error: Drop impl asserts .1 (A and &'a _) are not accessed
+    pt = Pt("pt", &c.0, &c_long.0);
+    pr = Pr("pr", &c.0, &c_long.0);
+
+    // No error: St and Sr have no destructor.
+    st = St("st", &c.0);
+    sr = Sr("sr", &c.0);
+
+    println!("{:?}", (dt.0, dr.0, pt.0, pr.0, st.0, sr.0));
+    assert_eq!(*c_long.0.borrow(), "c_long");
+    assert_eq!(*c.0.borrow(), "c");
+}