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.

9 files changed, 68 insertions, 0 deletions

diff --git a/tests/ui/return/return-impl-trait-bad.stderr b/tests/ui/return/return-impl-trait-bad.stderr
index a015b9f53af..6277c5feb20 100644
--- a/tests/ui/return/return-impl-trait-bad.stderr
+++ b/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
 
diff --git a/tests/ui/return/return-impl-trait.stderr b/tests/ui/return/return-impl-trait.stderr
index 707f014a16f..114ae0c2445 100644
--- a/tests/ui/return/return-impl-trait.stderr
+++ b/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
 
diff --git a/tests/ui/return/return-ty-mismatch-note.rs b/tests/ui/return/return-ty-mismatch-note.rs
new file mode 100644
index 00000000000..352bc2a1637
--- /dev/null
+++ b/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();
+}
diff --git a/tests/ui/return/return-ty-mismatch-note.stderr b/tests/ui/return/return-ty-mismatch-note.stderr
new file mode 100644
index 00000000000..135903da5c2
--- /dev/null
+++ b/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`.
diff --git a/tests/ui/suggestions/clone-on-unconstrained-borrowed-type-param.stderr b/tests/ui/suggestions/clone-on-unconstrained-borrowed-type-param.stderr
index afbb9c32d51..2c4be26a82b 100644
--- a/tests/ui/suggestions/clone-on-unconstrained-borrowed-type-param.stderr
+++ b/tests/ui/suggestions/clone-on-unconstrained-borrowed-type-param.stderr
@@ -10,6 +10,7 @@ LL |     t.clone()
    |
    = note: expected type parameter `_`
                    found reference `&_`
+   = note: the caller chooses a type for `T` which can be different from `&T`
 note: `T` does not implement `Clone`, so `&T` was cloned instead
   --> $DIR/clone-on-unconstrained-borrowed-type-param.rs:3:5
    |
diff --git a/tests/ui/trait-bounds/restrict-assoc-type-of-generic-bound.stderr b/tests/ui/trait-bounds/restrict-assoc-type-of-generic-bound.stderr
index 5024ad21892..7aa32557af2 100644
--- a/tests/ui/trait-bounds/restrict-assoc-type-of-generic-bound.stderr
+++ b/tests/ui/trait-bounds/restrict-assoc-type-of-generic-bound.stderr
@@ -10,6 +10,7 @@ LL |     return a.bar();
    |
    = note: expected type parameter `B`
              found associated type `<A as MyTrait>::T`
+   = note: the caller chooses a type for `B` which can be different from `<A as MyTrait>::T`
 help: consider further restricting this bound
    |
 LL | pub fn foo<A: MyTrait<T = B>, B>(a: A) -> B {
diff --git a/tests/ui/type/type-parameter-names.stderr b/tests/ui/type/type-parameter-names.stderr
index 8e3e2388c6c..be9000a99e4 100644
--- a/tests/ui/type/type-parameter-names.stderr
+++ b/tests/ui/type/type-parameter-names.stderr
@@ -13,6 +13,7 @@ LL |     x
               found type parameter `Foo`
    = note: a type parameter was expected, but a different one was found; you might be missing a type parameter or trait bound
    = note: for more information, visit https://doc.rust-lang.org/book/ch10-02-traits.html#traits-as-parameters
+   = note: the caller chooses a type for `Bar` which can be different from `Foo`
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/type/type-params-in-different-spaces-3.stderr b/tests/ui/type/type-params-in-different-spaces-3.stderr
index 58783fe1ff0..3c0c022f112 100644
--- a/tests/ui/type/type-params-in-different-spaces-3.stderr
+++ b/tests/ui/type/type-params-in-different-spaces-3.stderr
@@ -14,6 +14,7 @@ LL |         u
               found type parameter `X`
    = note: a type parameter was expected, but a different one was found; you might be missing a type parameter or trait bound
    = note: for more information, visit https://doc.rust-lang.org/book/ch10-02-traits.html#traits-as-parameters
+   = note: the caller chooses a type for `Self` which can be different from `X`
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/typeck/issue-13853.stderr b/tests/ui/typeck/issue-13853.stderr
index 0683c782933..45363c87d29 100644
--- a/tests/ui/typeck/issue-13853.stderr
+++ b/tests/ui/typeck/issue-13853.stderr
@@ -9,6 +9,7 @@ LL |         self.iter()
    |
    = note: expected type parameter `I`
                       found struct `std::slice::Iter<'_, N>`
+   = note: the caller chooses a type for `I` which can be different from `std::slice::Iter<'_, N>`
 
 error[E0599]: no method named `iter` found for reference `&G` in the current scope
   --> $DIR/issue-13853.rs:27:23