Tracking issue for const slice::from_raw_parts_mut (const_slice_from_raw_parts_mut)

[DutchGhost]

Make slice::from_raw_parts and slice::from_raw_parts_mut a const fn available under a feature flag.
This would require a change in the ptr module as well, as slice module just forwards to it.

slice::from_raw_parts[mut] is used in alot of places (e.g slice::from_ref[mut], which would get one step closer into constification if slice::from_raw_parts[mut] is a const fn.

Here is a little playground to show it's possible:
https://play.rust-lang.org/?version=nightly&mode=release&edition=2018&gist=dd5c506a3e082c619f557d972e9956ff

In order to get this working, the following functions and functionalities need to be constified:

• ptr::slice_from_raw_parts (Make ptr::slice_from_raw_parts a const fn available under a feature flag #67462)

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Implementation PR for ptr::slice_from_raw_parts: #67462

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Partially stabilized in #97522; remaining unstable functions:

// core::ptr
pub const fn slice_from_raw_parts_mut<T>(data: *mut T, len: usize) -> *mut [T];

// core::slice
pub const unsafe fn from_raw_parts_mut<'a, T>(data: *mut T, len: usize) -> &'a mut [T];

// core::ptr::NonNull
pub const fn slice_from_raw_parts(data: NonNull<T>, len: usize) -> Self

[DoumanAsh]

Is there particular reason why it remains gated by feature flag instead of just being const fn as it is?

[DutchGhost]

Is there particular reason why it remains gated by feature flag instead of just being const fn as it is?

The constant versions of function's and methods in libcore/libstd need a period of time under a feature flag before they can be stabilized as a const function/method. Const functions are 'relatively' new, and the ability of what one can do inside isn't fully determined yet I think.
One of those abilities is how to handle raw pointers, which slice::from_raw_parts[mut] needs.

[josephlr]

@DutchGhost would it make sense for the slice::{from_raw_parts[_mut], from_ref, from_mut} to be constified behind this feature as well? The only thing that seems blocking that is this debug_assert which calls is_aligned_and_not_null.

Checking the alignment will need support from the MIR interpreter, see #62420