Debug impls of ExtractIf have inconsistent trait bounds

[dtolnay]

std::vec::ExtractIf has:

impl<T, F> Debug for ExtractIf<'_, T, F>
where
    T: Debug,
    F: Debug;

std::collections::linked_list::ExtractIf has:

impl<T, F> Debug for ExtractIf<'_, T, F>
where
    T: Debug;

std::collections::hash_set::ExtractIf has:

impl<T, F> Debug for ExtractIf<'_, T, F>
where
    F: FnMut(&T) -> bool;

std::collections::btree_set::ExtractIf has:

impl<T, F> Debug for ExtractIf<'_, T, F>
where
    T: Debug,
    F: FnMut(&T) -> bool;

	Vec	LinkedList	HashSet	BTreeSet
T: Debug	☑️	☑️		☑️
F: Debug	☑️
F: FnMut			☑️	☑️

There should not need to be 4 different permutations for these 4 effectively-identical interfaces. We should be able to decide:

• Should collection elements be printed or not.
• Should the predicate be printed or not. Almost certainly not, given that closures do not implement Debug.
• Should the Debug impl ever need to invoke the predicate. Almost certainly not.

[dtolnay]

This is time-sensitive for Rust 1.87 if we want to change any of the impls that were just stabilized in nightly.

[m-ou-se]

The ExtractIf types for Vec, LinkedList and HashSet have been stabilized in 1.87, which is still in nightly.

The one for BTreeSet is still unstable.

[m-ou-se]

There should not need to be 4 different permutations for these 4 effectively-identical interfaces. We should be able to decide:

* Should collection elements be printed or not.

* Should the predicate be printed or not. Almost certainly not, given that closures do not implement Debug.

* Should the Debug impl ever need to invoke the predicate. Almost certainly not.

It sounds to me like it should in all cases be T: Debug only, like the one of LinkedList.

[BurntSushi]

I agree. Just T: Debug only seems like the right call here. Nice catch @dtolnay!

[dtolnay]

• Hashbrown PR: Drop FnMut trait bounds from ExtractIf data structures hashbrown#616
• Standard library PR: Consistent trait bounds for ExtractIf Debug impls #139764 (requires hashbrown release + update)
• Beta revert: [beta] Delay hash_extract_if stabilization from 1.87 to 1.88 #139765