Cleanup: Use `#![feature(associated_type_bounds)]` where applicable

[Centril]

We can now write bounds of form:

T: MyTrait<AssociatedType: OtherTrait>,

When you find bounds of form:

T: MyTrait,
T::AssociatedType: OtherTrait,
// or, but less commonly:
<T as MyTrait>::AssociatedType: OtherTrait,

You can rewrite them to the first form (but you'll need to add the feature gate...).

One regex you can try is: ::\w+: \w+ (doesn't cover lifetimes).

This is relevant both for the standard library and throughout the compiler.
Clippy may also want to do this and possibly also add an experimental lint? cc @oli-obk

cc #52662

This issue has been assigned to @iluuu1994 via this comment.

[cuviper]

Shouldn't this wait for the feature to reach beta?
(This would be an annoying thing to alternate on #[cfg(bootstrap)].)

[Centril]

Either works for me.

[iluuu1994]

@rustbot claim

I'll implement this now, you can decide whether you want to merge it now or wait until it's stabilized. We could also make an issue to remove the #![feature(associated_type_bounds)] once the feature is stabilized.

[iluuu1994]

Do you guys prefer:

impl<T> Clone for ...<T>
    where T: IntoIterator<IntoIter: Clone>

or

impl<T: IntoIterator<IntoIter: Clone>> Clone for ...<T>

I like for former better.

[Centril]

@iluuu1994 The first one should be:

impl<T> Clone for ...<T>
where
    T: IntoIterator<IntoIter: Clone>,

per rustfmt.

which one you use is situational I think; I'd use the latter if it cleanly fits on one line and there are not a lot of bounds and the former otherwise.