Make const/fn return params more suggestable

compiler/rustc_hir_analysis/src/astconv/mod.rs

@@ -2945,12 +2945,12 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                     if r.is_erased() { tcx.lifetimes.re_static } else { r }
                 });
                 let span = ast_ty.span;
-                tcx.sess.emit_err(TypeofReservedKeywordUsed {
-                    span,
-                    ty,
-                    opt_sugg: Some((span, Applicability::MachineApplicable))
-                        .filter(|_| ty.is_suggestable(tcx, false)),
-                });
+                let (ty, opt_sugg) = if let Some(ty) = ty.make_suggestable(tcx, false) {
+                    (ty, Some((span, Applicability::MachineApplicable)))
+                } else {
+                    (ty, None)
+                };
+                tcx.sess.emit_err(TypeofReservedKeywordUsed { span, ty, opt_sugg });
 
                 ty
             }

compiler/rustc_hir_analysis/src/collect.rs

@@ -1199,28 +1199,22 @@ fn infer_return_ty_for_fn_sig<'tcx>(
             visitor.visit_ty(ty);
             let mut diag = bad_placeholder(tcx, visitor.0, "return type");
             let ret_ty = fn_sig.output();
-            if ret_ty.is_suggestable(tcx, false) {
+            if let Some(ret_ty) = ret_ty.make_suggestable(tcx, false) {
                 diag.span_suggestion(
                     ty.span,
                     "replace with the correct return type",
                     ret_ty,
                     Applicability::MachineApplicable,
                 );
-            } else if matches!(ret_ty.kind(), ty::FnDef(..)) {
-                let fn_sig = ret_ty.fn_sig(tcx);
-                if fn_sig
-                    .skip_binder()
-                    .inputs_and_output
-                    .iter()
-                    .all(|t| t.is_suggestable(tcx, false))
-                {
-                    diag.span_suggestion(
-                        ty.span,
-                        "replace with the correct return type",
-                        fn_sig,
-                        Applicability::MachineApplicable,
-                    );
-                }
+            } else if matches!(ret_ty.kind(), ty::FnDef(..))
+                && let Some(fn_sig) = ret_ty.fn_sig(tcx).make_suggestable(tcx, false)
+            {
+                diag.span_suggestion(
+                    ty.span,
+                    "replace with the correct return type",
+                    fn_sig,
+                    Applicability::MachineApplicable,
+                );
             } else if let Some(sugg) = suggest_impl_trait(tcx, ret_ty, ty.span, hir_id, def_id) {
                 diag.span_suggestion(
                     ty.span,
@@ -1280,9 +1274,7 @@ fn suggest_impl_trait<'tcx>(
             let trait_name = tcx.item_name(trait_def_id);
             let args_tuple = substs.type_at(1);
             let ty::Tuple(types) = *args_tuple.kind() else { return None; };
-            if !types.is_suggestable(tcx, false) {
-                return None;
-            }
+            let types = types.make_suggestable(tcx, false)?;
             let maybe_ret =
                 if item_ty.is_unit() { String::new() } else { format!(" -> {item_ty}") };
             Some(format!(
@@ -1337,7 +1329,7 @@ fn suggest_impl_trait<'tcx>(
         // FIXME(compiler-errors): We may benefit from resolving regions here.
         if ocx.select_where_possible().is_empty()
             && let item_ty = infcx.resolve_vars_if_possible(item_ty)
-            && item_ty.is_suggestable(tcx, false)
+            && let Some(item_ty) = item_ty.make_suggestable(tcx, false)
             && let Some(sugg) = formatter(tcx, infcx.resolve_vars_if_possible(substs), trait_def_id, assoc_item_def_id, item_ty)
         {
             return Some(sugg);

compiler/rustc_hir_analysis/src/collect/type_of.rs

@@ -8,7 +8,9 @@ use rustc_middle::hir::nested_filter;
 use rustc_middle::ty::print::with_forced_trimmed_paths;
 use rustc_middle::ty::subst::InternalSubsts;
 use rustc_middle::ty::util::IntTypeExt;
-use rustc_middle::ty::{self, DefIdTree, Ty, TyCtxt, TypeFolder, TypeSuperFoldable, TypeVisitable};
+use rustc_middle::ty::{
+    self, DefIdTree, IsSuggestable, Ty, TyCtxt, TypeFolder, TypeSuperFoldable, TypeVisitable,
+};
 use rustc_span::symbol::Ident;
 use rustc_span::{Span, DUMMY_SP};
 
@@ -845,37 +847,23 @@ fn infer_placeholder_type<'a>(
 ) -> Ty<'a> {
     // Attempts to make the type nameable by turning FnDefs into FnPtrs.
     struct MakeNameable<'tcx> {
-        success: bool,
         tcx: TyCtxt<'tcx>,
     }
 
-    impl<'tcx> MakeNameable<'tcx> {
-        fn new(tcx: TyCtxt<'tcx>) -> Self {
-            MakeNameable { success: true, tcx }
-        }
-    }
-
     impl<'tcx> TypeFolder<'tcx> for MakeNameable<'tcx> {
         fn tcx(&self) -> TyCtxt<'tcx> {
             self.tcx
         }
 
         fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
-            if !self.success {
-                return ty;
-            }
-
-            match ty.kind() {
+            let ty = match *ty.kind() {
                 ty::FnDef(def_id, substs) => {
-                    self.tcx.mk_fn_ptr(self.tcx.fn_sig(*def_id).subst(self.tcx, substs))
+                    self.tcx.mk_fn_ptr(self.tcx.fn_sig(def_id).subst(self.tcx, substs))
                 }
-                // FIXME: non-capturing closures should also suggest a function pointer
-                ty::Closure(..) | ty::Generator(..) => {
-                    self.success = false;
-                    ty
-                }
-                _ => ty.super_fold_with(self),
-            }
+                _ => ty,
+            };
+
+            ty.super_fold_with(self)
         }
     }
 
@@ -898,15 +886,11 @@ fn infer_placeholder_type<'a>(
                     suggestions.clear();
                 }
 
-                // Suggesting unnameable types won't help.
-                let mut mk_nameable = MakeNameable::new(tcx);
-                let ty = mk_nameable.fold_ty(ty);
-                let sugg_ty = if mk_nameable.success { Some(ty) } else { None };
-                if let Some(sugg_ty) = sugg_ty {
+                if let Some(ty) = ty.make_suggestable(tcx, false) {
                     err.span_suggestion(
                         span,
                         &format!("provide a type for the {item}", item = kind),
-                        format!("{colon} {sugg_ty}"),
+                        format!("{colon} {ty}"),
                         Applicability::MachineApplicable,
                     );
                 } else {
@@ -923,15 +907,12 @@ fn infer_placeholder_type<'a>(
             let mut diag = bad_placeholder(tcx, vec![span], kind);
 
             if !ty.references_error() {
-                let mut mk_nameable = MakeNameable::new(tcx);
-                let ty = mk_nameable.fold_ty(ty);
-                let sugg_ty = if mk_nameable.success { Some(ty) } else { None };
-                if let Some(sugg_ty) = sugg_ty {
+                if let Some(ty) = ty.make_suggestable(tcx, false) {
                     diag.span_suggestion(
                         span,
                         "replace with the correct type",
-                        sugg_ty,
-                        Applicability::MaybeIncorrect,
+                        ty,
+                        Applicability::MachineApplicable,
                     );
                 } else {
                     with_forced_trimmed_paths!(diag.span_note(

compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

@@ -687,7 +687,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 return true;
             }
             &hir::FnRetTy::DefaultReturn(span) if expected.is_unit() => {
-                if found.is_suggestable(self.tcx, false) {
+                if let Some(found) = found.make_suggestable(self.tcx, false) {
                     err.subdiagnostic(AddReturnTypeSuggestion::Add { span, found: found.to_string() });
                     return true;
                 } else if let ty::Closure(_, substs) = found.kind()

compiler/rustc_hir_typeck/src/op.rs

@@ -490,9 +490,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                                             if let Some(output_def_id) = output_def_id
                                                 && let Some(trait_def_id) = trait_def_id
                                                 && self.tcx.parent(output_def_id) == trait_def_id
-                                                && output_ty.is_suggestable(self.tcx, false)
+                                                && let Some(output_ty) = output_ty.make_suggestable(self.tcx, false)
                                             {
-                                                Some(("Output", *output_ty))
+                                                Some(("Output", output_ty))
                                             } else {
                                                 None
                                             }

compiler/rustc_middle/src/ty/diagnostics.rs

@@ -3,8 +3,9 @@
 use std::ops::ControlFlow;
 
 use crate::ty::{
-    visit::TypeVisitable, AliasTy, Const, ConstKind, DefIdTree, InferConst, InferTy, Opaque,
-    PolyTraitPredicate, Projection, Ty, TyCtxt, TypeSuperVisitable, TypeVisitor,
+    visit::TypeVisitable, AliasTy, Const, ConstKind, DefIdTree, FallibleTypeFolder, InferConst,
+    InferTy, Opaque, PolyTraitPredicate, Projection, Ty, TyCtxt, TypeFoldable, TypeSuperFoldable,
+    TypeSuperVisitable, TypeVisitor,
 };
 
 use rustc_data_structures::fx::FxHashMap;
@@ -76,7 +77,7 @@ impl<'tcx> Ty<'tcx> {
     }
 }
 
-pub trait IsSuggestable<'tcx> {
+pub trait IsSuggestable<'tcx>: Sized {
     /// Whether this makes sense to suggest in a diagnostic.
     ///
     /// We filter out certain types and constants since they don't provide
@@ -87,15 +88,21 @@ pub trait IsSuggestable<'tcx> {
     /// Only if `infer_suggestable` is true, we consider type and const
     /// inference variables to be suggestable.
     fn is_suggestable(self, tcx: TyCtxt<'tcx>, infer_suggestable: bool) -> bool;
+
+    fn make_suggestable(self, tcx: TyCtxt<'tcx>, infer_suggestable: bool) -> Option<Self>;
 }
 
 impl<'tcx, T> IsSuggestable<'tcx> for T
 where
-    T: TypeVisitable<'tcx>,
+    T: TypeVisitable<'tcx> + TypeFoldable<'tcx>,
 {
     fn is_suggestable(self, tcx: TyCtxt<'tcx>, infer_suggestable: bool) -> bool {
         self.visit_with(&mut IsSuggestableVisitor { tcx, infer_suggestable }).is_continue()
     }
+
+    fn make_suggestable(self, tcx: TyCtxt<'tcx>, infer_suggestable: bool) -> Option<T> {
+        self.try_fold_with(&mut MakeSuggestableFolder { tcx, infer_suggestable }).ok()
+    }
 }
 
 pub fn suggest_arbitrary_trait_bound<'tcx>(
@@ -509,3 +516,83 @@ impl<'tcx> TypeVisitor<'tcx> for IsSuggestableVisitor<'tcx> {
         c.super_visit_with(self)
     }
 }
+
+pub struct MakeSuggestableFolder<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    infer_suggestable: bool,
+}
+
+impl<'tcx> FallibleTypeFolder<'tcx> for MakeSuggestableFolder<'tcx> {
+    type Error = ();
+
+    fn tcx(&self) -> TyCtxt<'tcx> {
+        self.tcx
+    }
+
+    fn try_fold_ty(&mut self, t: Ty<'tcx>) -> Result<Ty<'tcx>, Self::Error> {
+        let t = match *t.kind() {
+            Infer(InferTy::TyVar(_)) if self.infer_suggestable => t,
+
+            FnDef(def_id, substs) => {
+                self.tcx.mk_fn_ptr(self.tcx.fn_sig(def_id).subst(self.tcx, substs))
+            }
+
+            // FIXME(compiler-errors): We could replace these with infer, I guess.
+            Closure(..)
+            | Infer(..)
+            | Generator(..)
+            | GeneratorWitness(..)
+            | Bound(_, _)
+            | Placeholder(_)
+            | Error(_) => {
+                return Err(());
+            }
+
+            Alias(Opaque, AliasTy { def_id, .. }) => {
+                let parent = self.tcx.parent(def_id);
+                if let hir::def::DefKind::TyAlias | hir::def::DefKind::AssocTy = self.tcx.def_kind(parent)
+                    && let Alias(Opaque, AliasTy { def_id: parent_opaque_def_id, .. }) = *self.tcx.type_of(parent).kind()
+                    && parent_opaque_def_id == def_id
+                {
+                    t
+                } else {
+                    return Err(());
+                }
+            }
+
+            Param(param) => {
+                // FIXME: It would be nice to make this not use string manipulation,
+                // but it's pretty hard to do this, since `ty::ParamTy` is missing
+                // sufficient info to determine if it is synthetic, and we don't
+                // always have a convenient way of getting `ty::Generics` at the call
+                // sites we invoke `IsSuggestable::is_suggestable`.
+                if param.name.as_str().starts_with("impl ") {
+                    return Err(());
+                }
+
+                t
+            }
+
+            _ => t,
+        };
+
+        t.try_super_fold_with(self)
+    }
+
+    fn try_fold_const(&mut self, c: Const<'tcx>) -> Result<Const<'tcx>, ()> {
+        let c = match c.kind() {
+            ConstKind::Infer(InferConst::Var(_)) if self.infer_suggestable => c,
+
+            ConstKind::Infer(..)
+            | ConstKind::Bound(..)
+            | ConstKind::Placeholder(..)
+            | ConstKind::Error(..) => {
+                return Err(());
+            }
+
+            _ => c,
+        };
+
+        c.try_super_fold_with(self)
+    }
+}

compiler/rustc_middle/src/ty/fold.rs

@@ -105,7 +105,7 @@ pub trait TypeSuperFoldable<'tcx>: TypeFoldable<'tcx> {
 /// the infallible methods of this trait to ensure that the two APIs
 /// are coherent.
 pub trait TypeFolder<'tcx>: FallibleTypeFolder<'tcx, Error = !> {
-    fn tcx<'a>(&'a self) -> TyCtxt<'tcx>;
+    fn tcx(&self) -> TyCtxt<'tcx>;
 
     fn fold_binder<T>(&mut self, t: Binder<'tcx, T>) -> Binder<'tcx, T>
     where

tests/ui/suggestions/suggest-fn-ptr-for-fn-item-in-fn-ret.fixed

@@ -0,0 +1,12 @@
+// run-rustfix
+
+#![allow(unused)]
+
+struct Wrapper<T>(T);
+
+fn bar() -> Wrapper<fn()> { Wrapper(foo) }
+//~^ ERROR the placeholder `_` is not allowed within types on item signatures for return types
+
+fn foo() {}
+
+fn main() {}

tests/ui/suggestions/suggest-fn-ptr-for-fn-item-in-fn-ret.rs

@@ -0,0 +1,12 @@
+// run-rustfix
+
+#![allow(unused)]
+
+struct Wrapper<T>(T);
+
+fn bar() -> _ { Wrapper(foo) }
+//~^ ERROR the placeholder `_` is not allowed within types on item signatures for return types
+
+fn foo() {}
+
+fn main() {}

tests/ui/suggestions/suggest-fn-ptr-for-fn-item-in-fn-ret.stderr

@@ -0,0 +1,12 @@
+error[E0121]: the placeholder `_` is not allowed within types on item signatures for return types
+  --> $DIR/suggest-fn-ptr-for-fn-item-in-fn-ret.rs:7:13
+   |
+LL | fn bar() -> _ { Wrapper(foo) }
+   |             ^
+   |             |
+   |             not allowed in type signatures
+   |             help: replace with the correct return type: `Wrapper<fn()>`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0121`.