Alloc features cleanup

[m-ou-se]

This sorts and categorizes the #![features] in alloc and removes unused ones.

This is part of #87766

The following feature attributes were unnecessary and are removed:

// Library features:
-#![feature(cow_is_borrowed)]
-#![feature(maybe_uninit_uninit_array)]
-#![feature(slice_partition_dedup)]

// Language features:
-#![feature(arbitrary_self_types)]
-#![feature(auto_traits)]
-#![feature(box_patterns)]
-#![feature(decl_macro)]
-#![feature(nll)]

[rust-highfive]

r? @yaahc

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[bjorn3]

-#![feature(nll)]

These exist to test full nll mode. #66498 (comment)

[m-ou-se]

If we really need to keep that in alloc and/or core, we should put a clear comment why it's there. Is there any documentation of what enabling this feature will change?

[bjorn3]

NLL is currently in migration mode by default, which still uses the old AST based regionck. #![feature(nll)] completes the transition to full NLL mode. #57895 is the tracking issue for completing the transition. One of the last remaining blockers for completing the transition is #86700 I think. By the way libstd used to require it at least at some point: #66494 (comment)

[yaahc]

LGTM once the NLL bit is sorted.

[bors]

☔ The latest upstream changes (presumably #86808) made this pull request unmergeable. Please resolve the merge conflicts.

[dtolnay]

@bors delegate+

Looks good after a rebase and putting back #![feature(nll)].

[bors]

✌️ @m-ou-se can now approve this pull request

[m-ou-se]

@bors r=yaahc,dtolnay

[bors]

📌 Commit 6fdcedc has been approved by yaahc,dtolnay