Auto merge of #86400 - FabianWolff:issue-85735, r=estebank

Remove invalid suggestion involving `Fn` trait bound

This pull request closes #85735. The actual issue is a duplicate of #21974, but #85735 contains a further problem, which is an invalid suggestion if `Fn`/`FnMut`/`FnOnce` trait bounds are involved: The suggestion code checks whether the trait bound ends with `>` to determine whether it has any generic arguments, but the `Fn*` traits have a special syntax for generic arguments that doesn't involve angle brackets. The example given in #85735:
```rust
trait Foo {}
impl<'a, 'b, T> Foo for T
where
    T: FnMut(&'a ()),
    T: FnMut(&'b ()), {

    }
```
currently produces:
```
error[E0283]: type annotations needed
   --> src/lib.rs:4:8
    |
4   |       T: FnMut(&'a ()),
    |          ^^^^^^^^^^^^^ cannot infer type for type parameter `T`
    |
    = note: cannot satisfy `T: FnMut<(&'a (),)>`
help: consider specifying the type arguments in the function call
    |
4   |     T: FnMut(&'a ())::<Self, Args>,
    |                     ^^^^^^^^^^^^^^

error: aborting due to previous error
```
which is incorrect, because there is no function call, and applying the suggestion would lead to a parse error. With my changes, I get:
```
error[E0283]: type annotations needed
   --> test.rs:4:8
    |
4   |     T: FnMut(&'a ()),
    |        ^^^^^^^^^^^^^ cannot infer type for type parameter `T`
    |
   ::: [...]/library/core/src/ops/function.rs:147:1
    |
147 | pub trait FnMut<Args>: FnOnce<Args> {
    | ----------------------------------- required by this bound in `FnMut`
    |
    = note: cannot satisfy `T: FnMut<(&'a (),)>`

error: aborting due to previous error
```
i.e. I have added a check to prevent the invalid suggestion from being issued for `Fn*` bounds, while the underlying issue #21974 remains for now.

0 files changed, 0 insertions, 0 deletions