Don't suggest types whose inner type is erroneous

Currently, we check if the returned type equals to `tcx.ty_error()` not to emit
erroneous types, but this has a pitfall; for example,
`Option<[type error]> != tcx.ty_error()` holds.

Author: hkmatsumoto

compiler/rustc_typeck/src/collect.rs

@@ -41,7 +41,7 @@ use rustc_middle::ty::subst::InternalSubsts;
 use rustc_middle::ty::util::Discr;
 use rustc_middle::ty::util::IntTypeExt;
 use rustc_middle::ty::{self, AdtKind, Const, DefIdTree, Ty, TyCtxt};
-use rustc_middle::ty::{ReprOptions, ToPredicate, WithConstness};
+use rustc_middle::ty::{ReprOptions, ToPredicate, TypeFoldable, WithConstness};
 use rustc_session::lint;
 use rustc_session::parse::feature_err;
 use rustc_span::symbol::{kw, sym, Ident, Symbol};
@@ -1779,7 +1779,7 @@ fn fn_sig(tcx: TyCtxt<'_>, def_id: DefId) -> ty::PolyFnSig<'_> {
                     visitor.visit_ty(ty);
                     let mut diag = bad_placeholder_type(tcx, visitor.0, "return type");
                     let ret_ty = fn_sig.skip_binder().output();
-                    if ret_ty != tcx.ty_error() {
+                    if !ret_ty.references_error() {
                         if !ret_ty.is_closure() {
                             let ret_ty_str = match ret_ty.kind() {
                                 // Suggest a function pointer return type instead of a unique function definition

src/test/ui/typeck/issue-91450-inner-ty-error.rs

@@ -0,0 +1,7 @@
+// Regression test for #91450.
+// This test ensures that the compiler does not suggest `Foo<[type error]>` in diagnostic messages.
+
+fn foo() -> Option<_> {} //~ ERROR: [E0308]
+//~^ ERROR: the type placeholder `_` is not allowed
+
+fn main() {}

src/test/ui/typeck/issue-91450-inner-ty-error.stderr

@@ -0,0 +1,21 @@
+error[E0308]: mismatched types
+  --> $DIR/issue-91450-inner-ty-error.rs:4:13
+   |
+LL | fn foo() -> Option<_> {}
+   |    ---      ^^^^^^^^^ expected enum `Option`, found `()`
+   |    |
+   |    implicitly returns `()` as its body has no tail or `return` expression
+   |
+   = note:   expected enum `Option<_>`
+           found unit type `()`
+
+error[E0121]: the type placeholder `_` is not allowed within types on item signatures for return types
+  --> $DIR/issue-91450-inner-ty-error.rs:4:20
+   |
+LL | fn foo() -> Option<_> {}
+   |                    ^ not allowed in type signatures
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0121, E0308.
+For more information about an error, try `rustc --explain E0121`.