Stabilize maybe_uninit_write_slice

[thaliaarchi]

Stabilize feature maybe_uninit_write_slice (tracked in #79995).

Note that this also const-stabilizes <[MaybeUninit<_>]>::write_copy_of_slice. That method depends on <[_]>::copy_from_slice, which is already const-stable, and the unstable <[MaybeUninit<_>]>::assume_init_mut, which is just casts.

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[thaliaarchi]

Note that this feature is one day into the FCP, so this is almost, but not quite, ready to merge.

[hkBst]

lgtm

View changes since this review

[thaliaarchi]

The FCP passed, so I believe this is now ready to merge

[clarfonthey]

Is this actually required? I thought we stopped needing most of these after the latest bootstrap reworks…

[thaliaarchi]

It appears necessary. With that line removed:

error[E0658]: use of unstable library feature `maybe_uninit_write_slice`
   --> compiler/rustc_proc_macro/../../library/proc_macro/src/bridge/arena.rs:105:27
    |
105 |         let bytes = alloc.write_copy_of_slice(string.as_bytes());
    |                           ^^^^^^^^^^^^^^^^^^^

[clarfonthey]

Hmm, that's extra strange considering the weird relative library path, but it is what it is. Thanks for verifying.

[thaliaarchi]

@jieyouxu Is this behavior expected?

[jieyouxu]

Yes, proc-macros can be one of those special cases, because see

rust/compiler/rustc_proc_macro/Cargo.toml

Lines 1 to 5 in 6e41e61

	# We need to use a separate crate including library/proc_macro as opposed to a
	# direct path dependency on library/proc_macro because doing the latter will
	# cause two copies of libproc_macro.rlib to end up in the sysroot, breaking
	# proc-macro crates. In addition it confuses the workspace_members function of
	# bootstrap.

So I believe library/proc_macro can still need cfg-bootstrap bits, but other parts of library should (generally) not need it anymore.