Migrate libraries from ptr::slice_from_raw_parts to .cast_slice

[Paladynee]

Tracking issue for #![feature(ptr_cast_slice)]: #149103
This commit updates most callsites of ptr::slice_from_raw_parts within ./library to the more concise but unstable postfix .cast_slice() method on raw pointers, using the ptr_cast_slice feature. Some tests in alloctests have also been updated.

No functional change, debatably improved readability.

[rustbot]

stdarch is developed in its own repository. If possible, consider making this change to rust-lang/stdarch instead.

cc @Amanieu, @folkertdev, @sayantn

[rustbot]

r? @jhpratt

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[Paladynee]

some notable places like core::slice::from_raw_parts have not been updated, because i felt like changing that would harm readability than improving it.

[Paladynee]

hmm wait let me revert that doc change, apparently it is exactly the same in std::ptr::slice_from_raw_parts. this is all so confusing since all the names are super similar lol

[xtqqczze]

Could &*ptr::slice_from_raw_parts be written as slice::from_raw_parts?

[jhpratt]

stdarch is developed in its own repository. If possible, consider making this change to rust-lang/stdarch instead.

Aside from this, r=me

[jhpratt]

@rustbot author

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[Paladynee]

Could &*ptr::slice_from_raw_parts be written as slice::from_raw_parts?

i believe they are equivalent other than the debug UB assertion, yes

[Paladynee]

I'm not touching anything in;

• coretests
• core::slice
• any ptr::slice_from_raw_parts{_mut} call in stable documentations
• believe i can revert the stdarch thing since it seems too complicated for me to change for now

with that, i think this pr is ready

@rustbot ready

[xtqqczze]

Could &*ptr::slice_from_raw_parts be written as slice::from_raw_parts?

The library/core/src/slice/sort/shared/smallsort.rs changes should be reverted as they conflict with #156192.

[Paladynee]

Could &*ptr::slice_from_raw_parts be written as slice::from_raw_parts?

The library/core/src/slice/sort/shared/smallsort.rs changes should be reverted as they conflict with #156192.

done