Note that type param is chosen by caller when suggesting return impl …

…Trait

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`

tests/ui/return/return-impl-trait-bad.stderr

@@ -10,6 +10,7 @@ LL |     "this should not suggest impl Trait"
    |
    = note: expected type parameter `T`
                    found reference `&'static str`
+   = note: the caller chooses a type for `T` which can be different from `&'static str`
 
 error[E0308]: mismatched types
   --> $DIR/return-impl-trait-bad.rs:9:5
@@ -23,6 +24,7 @@ LL |     "this will not suggest it, because that would probably be wrong"
    |
    = note: expected type parameter `T`
                    found reference `&'static str`
+   = note: the caller chooses a type for `T` which can be different from `&'static str`
 
 error[E0308]: mismatched types
   --> $DIR/return-impl-trait-bad.rs:17:5
@@ -37,6 +39,7 @@ LL |     "don't suggest this, because Option<T> places additional constraints"
    |
    = note: expected type parameter `T`
                    found reference `&'static str`
+   = note: the caller chooses a type for `T` which can be different from `&'static str`
 
 error[E0308]: mismatched types
   --> $DIR/return-impl-trait-bad.rs:28:5
@@ -53,6 +56,7 @@ LL |     "don't suggest this, because the generic param is used in the bound."
    |
    = note: expected type parameter `T`
                    found reference `&'static str`
+   = note: the caller chooses a type for `T` which can be different from `&'static str`
 
 error: aborting due to 4 previous errors
 

tests/ui/return/return-impl-trait.stderr

@@ -12,6 +12,7 @@ LL |     ()
    |
    = note: expected type parameter `T`
                    found unit type `()`
+   = note: the caller chooses a type for `T` which can be different from `()`
 
 error[E0308]: mismatched types
   --> $DIR/return-impl-trait.rs:23:5
@@ -28,6 +29,7 @@ LL |     ()
    |
    = note: expected type parameter `T`
                    found unit type `()`
+   = note: the caller chooses a type for `T` which can be different from `()`
 
 error: aborting due to 2 previous errors
 

tests/ui/return/return-ty-mismatch-note.rs

@@ -0,0 +1,21 @@
+// Checks existence of a note for "a caller chooses ty for ty param" upon return ty mismatch.
+
+fn f<T>() -> (T,) {
+    (0,) //~ ERROR mismatched types
+}
+
+fn g<U, V>() -> (U, V) {
+    (0, "foo")
+    //~^ ERROR mismatched types
+    //~| ERROR mismatched types
+}
+
+fn h() -> u8 {
+    0u8
+}
+
+fn main() {
+    f::<()>();
+    g::<(), ()>;
+    let _ = h();
+}

tests/ui/return/return-ty-mismatch-note.stderr

@@ -0,0 +1,36 @@
+error[E0308]: mismatched types
+  --> $DIR/return-ty-mismatch-note.rs:4:6
+   |
+LL | fn f<T>() -> (T,) {
+   |      - expected this type parameter
+LL |     (0,)
+   |      ^ expected type parameter `T`, found integer
+   |
+   = note: expected type parameter `T`
+                        found type `{integer}`
+
+error[E0308]: mismatched types
+  --> $DIR/return-ty-mismatch-note.rs:8:6
+   |
+LL | fn g<U, V>() -> (U, V) {
+   |      - expected this type parameter
+LL |     (0, "foo")
+   |      ^ expected type parameter `U`, found integer
+   |
+   = note: expected type parameter `U`
+                        found type `{integer}`
+
+error[E0308]: mismatched types
+  --> $DIR/return-ty-mismatch-note.rs:8:9
+   |
+LL | fn g<U, V>() -> (U, V) {
+   |         - expected this type parameter
+LL |     (0, "foo")
+   |         ^^^^^ expected type parameter `V`, found `&str`
+   |
+   = note: expected type parameter `V`
+                   found reference `&'static str`
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0308`.