TAITs are suggestable

compiler/rustc_middle/src/ty/diagnostics.rs

@@ -3,8 +3,8 @@
 use crate::ty::subst::{GenericArg, GenericArgKind};
 use crate::ty::TyKind::*;
 use crate::ty::{
-    ConstKind, ExistentialPredicate, ExistentialProjection, ExistentialTraitRef, InferTy,
+    ConstKind, DefIdTree, ExistentialPredicate, ExistentialProjection, ExistentialTraitRef,
-    ProjectionTy, Term, Ty, TyCtxt, TypeAndMut,
+    InferTy, ProjectionTy, Term, Ty, TyCtxt, TypeAndMut,
 };
 
 use rustc_data_structures::fx::FxHashMap;
@@ -74,10 +74,10 @@ impl<'tcx> Ty<'tcx> {
     }
 
     /// Whether the type can be safely suggested during error recovery.
-    pub fn is_suggestable(self) -> bool {
+    pub fn is_suggestable(self, tcx: TyCtxt<'tcx>) -> bool {
-        fn generic_arg_is_suggestible(arg: GenericArg<'_>) -> bool {
+        fn generic_arg_is_suggestible<'tcx>(arg: GenericArg<'tcx>, tcx: TyCtxt<'tcx>) -> bool {
             match arg.unpack() {
-                GenericArgKind::Type(ty) => ty.is_suggestable(),
+                GenericArgKind::Type(ty) => ty.is_suggestable(tcx),
                 GenericArgKind::Const(c) => const_is_suggestable(c.val()),
                 _ => true,
             }
@@ -99,8 +99,7 @@ impl<'tcx> Ty<'tcx> {
         // temporary, so I'll leave this as a fixme.
 
         match self.kind() {
-            Opaque(..)
+            FnDef(..)
-            | FnDef(..)
             | Closure(..)
             | Infer(..)
             | Generator(..)
@@ -108,27 +107,38 @@ impl<'tcx> Ty<'tcx> {
             | Bound(_, _)
             | Placeholder(_)
             | Error(_) => false,
+            Opaque(did, substs) => {
+                let parent = tcx.parent(*did).expect("opaque types always have a parent");
+                if let hir::def::DefKind::TyAlias | hir::def::DefKind::AssocTy = tcx.def_kind(parent)
+                    && let Opaque(parent_did, _) = tcx.type_of(parent).kind()
+                    && parent_did == did
+                {
+                    substs.iter().all(|a| generic_arg_is_suggestible(a, tcx))
+                } else {
+                    false
+                }
+            }
             Dynamic(dty, _) => dty.iter().all(|pred| match pred.skip_binder() {
                 ExistentialPredicate::Trait(ExistentialTraitRef { substs, .. }) => {
-                    substs.iter().all(generic_arg_is_suggestible)
+                    substs.iter().all(|a| generic_arg_is_suggestible(a, tcx))
                 }
                 ExistentialPredicate::Projection(ExistentialProjection {
                     substs, term, ..
                 }) => {
                     let term_is_suggestable = match term {
-                        Term::Ty(ty) => ty.is_suggestable(),
+                        Term::Ty(ty) => ty.is_suggestable(tcx),
                         Term::Const(c) => const_is_suggestable(c.val()),
                     };
-                    term_is_suggestable && substs.iter().all(generic_arg_is_suggestible)
+                    term_is_suggestable && substs.iter().all(|a| generic_arg_is_suggestible(a, tcx))
                 }
                 _ => true,
             }),
             Projection(ProjectionTy { substs: args, .. }) | Adt(_, args) => {
-                args.iter().all(generic_arg_is_suggestible)
+                args.iter().all(|a| generic_arg_is_suggestible(a, tcx))
             }
-            Tuple(args) => args.iter().all(|ty| ty.is_suggestable()),
+            Tuple(args) => args.iter().all(|ty| ty.is_suggestable(tcx)),
-            Slice(ty) | RawPtr(TypeAndMut { ty, .. }) | Ref(_, ty, _) => ty.is_suggestable(),
+            Slice(ty) | RawPtr(TypeAndMut { ty, .. }) | Ref(_, ty, _) => ty.is_suggestable(tcx),
-            Array(ty, c) => ty.is_suggestable() && const_is_suggestable(c.val()),
+            Array(ty, c) => ty.is_suggestable(tcx) && const_is_suggestable(c.val()),
             _ => true,
         }
     }

compiler/rustc_typeck/src/astconv/generics.rs

@@ -86,7 +86,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                         let param_type = tcx.infer_ctxt().enter(|infcx| {
                             infcx.resolve_numeric_literals_with_default(tcx.type_of(param.def_id))
                         });
-                        if param_type.is_suggestable() {
+                        if param_type.is_suggestable(tcx) {
                             err.span_suggestion(
                                 tcx.def_span(src_def_id),
                                 "consider changing this type parameter to be a `const` generic",

compiler/rustc_typeck/src/astconv/mod.rs

@@ -2466,7 +2466,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                     span,
                     ty,
                     opt_sugg: Some((span, Applicability::MachineApplicable))
-                        .filter(|_| ty.is_suggestable()),
+                        .filter(|_| ty.is_suggestable(tcx)),
                 });
 
                 ty

compiler/rustc_typeck/src/check/fn_ctxt/suggestions.rs

@@ -525,7 +525,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             self.resolve_numeric_literals_with_default(self.resolve_vars_if_possible(found));
         // Only suggest changing the return type for methods that
         // haven't set a return type at all (and aren't `fn main()` or an impl).
-        match (&fn_decl.output, found.is_suggestable(), can_suggest, expected.is_unit()) {
+        match (&fn_decl.output, found.is_suggestable(self.tcx), can_suggest, expected.is_unit()) {
             (&hir::FnRetTy::DefaultReturn(span), true, true, true) => {
                 err.span_suggestion(
                     span,

compiler/rustc_typeck/src/collect.rs

@@ -2004,7 +2004,7 @@ 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.skip_binder().output();
-            if ret_ty.is_suggestable() {
+            if ret_ty.is_suggestable(tcx) {
                 diag.span_suggestion(
                     ty.span,
                     "replace with the correct return type",
@@ -2013,7 +2013,7 @@ fn infer_return_ty_for_fn_sig<'tcx>(
                 );
             } 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()) {
+                if fn_sig.skip_binder().inputs_and_output.iter().all(|t| t.is_suggestable(tcx)) {
                     diag.span_suggestion(
                         ty.span,
                         "replace with the correct return type",

src/test/ui/type-alias-impl-trait/issue-77179.stderr

@@ -2,7 +2,10 @@ error[E0121]: the placeholder `_` is not allowed within types on item signatures
   --> $DIR/issue-77179.rs:7:22
    |
 LL | fn test() -> Pointer<_> {
-   |                      ^ not allowed in type signatures
+   |              --------^-
+   |              |       |
+   |              |       not allowed in type signatures
+   |              help: replace with the correct return type: `Pointer<i32>`
 
 error: aborting due to previous error