Tracking issue for const slice::from_raw_parts_mut (const_slice_from_raw_parts_mut)

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)

Implementation PR for ptr::slice_from_raw_parts: #67462

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

Rollup merge of rust-lang#67462 - DutchGhost:const_slice_from_raw_par…

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

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.

@DutchGhost

@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

🔔 This is now entering its final comment period, as per the review above. 🔔

Weird. My box was ticked, but FCP didn't get triggered. I unticked and re-ticked my box.

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

Rollup merge of rust-lang#130403 - eduardosm:stabilize-const_slice_fr…

Auto merge of rust-lang#130403 - eduardosm:stabilize-const_slice_from…

Rollup merge of rust-lang#130403 - eduardosm:stabilize-const_slice_fr…

Unrolled build for rust-lang#130403

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Tracking issue for const slice::from_raw_parts_mut (const_slice_from_raw_parts_mut) #67456

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Tracking issue for const slice::from_raw_parts_mut (const_slice_from_raw_parts_mut) #67456

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DoumanAsh commented on Apr 22, 2020

DutchGhost commented on Apr 23, 2020

josephlr commented on Apr 24, 2020

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