Stabilise as_array in [_] and *const [_]; stabilise as_mut_array in [_] and *mut [_].

[bjoernager]

Tracking issue: #133508

This PR stabilises the as_array and as_mut_array associated functions from the slice_as_array feature gate:

impl<T> [T] {
    pub const fn as_array<const N: usize>(&self) -> Option<&[T; N]>;

    pub const fn as_mut_array<const N: usize>(&mut self) -> Option<&mut [T; N]>;
}

impl<T> *const [T] {
    pub const fn as_array<const N: usize>(self) -> Option<*const [T; N]>;
}

impl<T> *mut [T] {
    pub const fn as_mut_array<const N: usize>(self) -> Option<*mut [T; N]>;
}

FCP missing.

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[bjoernager]

@rustbot label needs-fcp

[zachs18]

tidy [features]: /checkout/library/alloc/src/sync.rs:1229: feature gate slice_as_array has inconsistent `issue`: "133508" mismatches the previous `issue` of "none"

The specific tidy error is about Arc::into_array IIUC.

I think since this is a partial stabilization of feature(slice_as_array) (because it does not stabilize {Box, Rc, Arc}::into_array), it will need to change the feature name of either the newly-stable or the remaining-unstable functions to a different name, so they do not conflict.

IIUC, the intended workflow is to rename the feature on the being-stabilized functions, and leave the feature name the same on the remaining-unstable functions, like was done in #94640 (it stablized NonNull::len, and changed its feature from slice_ptr_len to slice_ptr_len_nonnull, while leaving slice_ptr_len unstable)

If you want to cherry-pick zachs18@e801f3e it should fix the tidy error I think.

[bjoernager]

Oh, I wasn't aware. Thank you!