Rollup merge of #122195 - jieyouxu:impl-return-note, r=fmease

Note that the caller chooses a type for type param

```
error[E0308]: mismatched types
  --> $DIR/return-impl-trait.rs:23:5
   |
LL | fn other_bounds<T>() -> T
   |                 -       -
   |                 |       |
   |                 |       expected `T` because of return type
   |                 |       help: consider using an impl return type: `impl Trait`
   |                 expected this type parameter
...
LL |     ()
   |     ^^ expected type parameter `T`, found `()`
   |
   = note: expected type parameter `T`
                   found unit type `()`
   = note: the caller chooses the type of T which can be different from ()
```

Tried to see if "expected this type parameter" can be replaced, but that goes all the way to `rustc_infer` so seems not worth the effort and can affect other diagnostics.

Revives #112088 and #104755.

compiler/rustc_hir_typeck/messages.ftl

@@ -103,6 +103,8 @@ hir_typeck_no_associated_item = no {$item_kind} named `{$item_name}` found for {
     *[other] {" "}in the current scope
 }
 
+hir_typeck_note_caller_chooses_ty_for_ty_param = the caller chooses a type for `{$ty_param_name}` which can be different from `{$found_ty}`
+
 hir_typeck_note_edition_guide = for more on editions, read https://doc.rust-lang.org/edition-guide
 
 hir_typeck_option_result_asref = use `{$def_path}::as_ref` to convert `{$expected_ty}` to `{$expr_ty}`

compiler/rustc_hir_typeck/src/errors.rs

@@ -11,7 +11,7 @@ use rustc_middle::ty::Ty;
 use rustc_span::{
     edition::{Edition, LATEST_STABLE_EDITION},
     symbol::Ident,
-    Span,
+    Span, Symbol,
 };
 
 #[derive(Diagnostic)]
@@ -614,3 +614,10 @@ pub struct SuggestConvertViaMethod<'tcx> {
     pub expected: Ty<'tcx>,
     pub found: Ty<'tcx>,
 }
+
+#[derive(Subdiagnostic)]
+#[note(hir_typeck_note_caller_chooses_ty_for_ty_param)]
+pub struct NoteCallerChoosesTyForTyParam<'tcx> {
+    pub ty_param_name: Symbol,
+    pub found_ty: Ty<'tcx>,
+}

compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

@@ -889,7 +889,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                             self.dcx(),
                             errors::ExpectedReturnTypeLabel::Other { span: hir_ty.span, expected },
                         );
-                        self.try_suggest_return_impl_trait(err, expected, ty, fn_id);
+                        self.try_suggest_return_impl_trait(err, expected, found, fn_id);
+                        self.note_caller_chooses_ty_for_ty_param(err, expected, found);
                         return true;
                     }
                 }
@@ -899,6 +900,23 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         false
     }
 
+    fn note_caller_chooses_ty_for_ty_param(
+        &self,
+        diag: &mut Diag<'_>,
+        expected: Ty<'tcx>,
+        found: Ty<'tcx>,
+    ) {
+        if let ty::Param(expected_ty_as_param) = expected.kind() {
+            diag.subdiagnostic(
+                self.dcx(),
+                errors::NoteCallerChoosesTyForTyParam {
+                    ty_param_name: expected_ty_as_param.name,
+                    found_ty: found,
+                },
+            );
+        }
+    }
+
     /// check whether the return type is a generic type with a trait bound
     /// only suggest this if the generic param is not present in the arguments
     /// if this is true, hint them towards changing the return type to `impl Trait`