Remove invalid suggestion involving Fn trait bound

[FabianWolff]

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:

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.

[rust-highfive]

r? @LeSeulArtichaut

(rust-highfive has picked a reviewer for you, use r? to override)

[LeSeulArtichaut]

Looks good to me, but I don't feel too comfortable approving, so I'll delegate to @estebank. Left two potential nitpicks

[LeSeulArtichaut]

r? @estebank

[bors]

☔ The latest upstream changes (presumably #86338) made this pull request unmergeable. Please resolve the merge conflicts.

[estebank]

r=me after addressing @LeSeulArtichaut's comments.

[LeSeulArtichaut]

@bors r=estebank

[bors]

📌 Commit f8c10ff has been approved by estebank

[bors]

⌛ Testing commit f8c10ff with merge a6ece56...

[bors]

☀️ Test successful - checks-actions
Approved by: estebank
Pushing a6ece56 to master...