fix: only generate trait bound for associated types in field types

[lowr]

Given the following definitions:

trait Trait {
    type A;
    type B;
    type C;
}

#[derive(Clone)]
struct S<T: Trait>
where
    T::A: Send,
{
    qualified: <T as Trait>::B,
    shorthand: T::C,
}

we currently expand the derive macro to:

impl<T> Clone for S<T>
where
    T: Trait + Clone,
    T::A: Clone,
    T::B: Clone,
    T::C: Clone,
{ /* ... */ }

This does not match how rustc expands it. Specifically, Clone bounds for T::A and T::B should not be generated.

The criteria for associated types to get bound seem to be 1) the associated type appears as part of field types AND 2) it's written in the shorthand form. I have no idea why rustc doesn't consider qualified associated types (there's even a comment that suggests they should be considered; see rust-lang/rust#50730), but it's important to follow rustc.

[lowr]

This should fix #14954, but I'd rather have the reporter confirm it (hence no issue linking) since I couldn't reproduce the ! part due to the lack of knowledge of other types involved.

[HKalbasi]

@bors r+

[bors]

📌 Commit 4f0c6fa has been approved by HKalbasi

It is now in the queue for this repository.

[bors]

⌛ Testing commit 4f0c6fa with merge 085a311...

[bors]

☀️ Test successful - checks-actions
Approved by: HKalbasi
Pushing 085a311 to master...