error[E0308]: mismatched types
  --> $DIR/name-mentioning-macro.rs:8:9
   |
LL | fn foo(x: impl Foo<bar!()>) {
   |           ---------------- expected this type parameter
LL |     let () = x;
   |         ^^   - this expression has type `impl Foo<bar!()>`
   |         |
   |         expected type parameter `impl Foo<bar!()>`, found `()`
   |
   = note: expected type parameter `impl Foo<bar!()>`
                   found unit type `()`

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0308`.