Rollup merge of #153643 - TaKO8Ki:issue-153539, r=Kivooeo

Avoid projection-only suggestions for inherent assoc types

Fixes #153539.

Type mismatch diagnostics in `note_and_explain_type_err` currently route both `ty::Projectio`n and `ty::Inherent`
through the same associated-type suggestion path. For inherent associated types, that path can reach helpers that
are only valid for trait projections.

Author: JonathanBrouwer

compiler/rustc_trait_selection/src/error_reporting/infer/note_and_explain.rs

@@ -272,18 +272,21 @@ impl<T> Trait<T> for X {
                                 values.found, values.expected,
                             )
                         };
-                        if !(self.suggest_constraining_opaque_associated_type(
-                            diag,
-                            msg,
-                            proj_ty,
-                            values.expected,
-                        ) || self.suggest_constraint(
-                            diag,
-                            &msg,
-                            body_owner_def_id,
-                            proj_ty,
-                            values.expected,
-                        )) {
+                        let suggested_projection_constraint = proj_ty.kind(tcx)
+                            == ty::AliasTyKind::Projection
+                            && (self.suggest_constraining_opaque_associated_type(
+                                diag,
+                                msg,
+                                proj_ty,
+                                values.expected,
+                            ) || self.suggest_constraint(
+                                diag,
+                                &msg,
+                                body_owner_def_id,
+                                proj_ty,
+                                values.expected,
+                            ));
+                        if !suggested_projection_constraint {
                             diag.help(msg());
                             diag.note(
                                 "for more information, visit \

tests/ui/associated-inherent-types/inherent-assoc-ty-mismatch-issue-153539.rs

@@ -0,0 +1,21 @@
+//@ compile-flags: -Znext-solver=globally
+#![feature(inherent_associated_types)]
+#![expect(incomplete_features)]
+
+// Regression test for https://github.com/rust-lang/rust/issues/153539:
+
+struct S<'a>(&'a ());
+
+impl<X> S<'_> {
+    //~^ ERROR the type parameter `X` is not constrained by the impl trait, self type, or predicates
+    type P = ();
+}
+
+fn ret_ref_local<'e>() -> &'e i32 {
+    let f: for<'a> fn(&'a i32) -> S<'a>::P = |x| _ = x;
+
+    f(&1)
+    //~^ ERROR mismatched types
+}
+
+fn main() {}

tests/ui/associated-inherent-types/inherent-assoc-ty-mismatch-issue-153539.stderr

@@ -0,0 +1,22 @@
+error[E0207]: the type parameter `X` is not constrained by the impl trait, self type, or predicates
+  --> $DIR/inherent-assoc-ty-mismatch-issue-153539.rs:9:6
+   |
+LL | impl<X> S<'_> {
+   |      ^ unconstrained type parameter
+
+error[E0308]: mismatched types
+  --> $DIR/inherent-assoc-ty-mismatch-issue-153539.rs:17:5
+   |
+LL | fn ret_ref_local<'e>() -> &'e i32 {
+   |                           ------- expected `&'e i32` because of return type
+...
+LL |     f(&1)
+   |     ^^^^^ expected `&i32`, found associated type
+   |
+   = help: consider constraining the associated type `S<'_>::P` to `&'e i32`
+   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0207, E0308.
+For more information about an error, try `rustc --explain E0207`.