Tracking Issue for #![feature(ptr_cast_slice)]

Feature gate: #![feature(ptr_cast_slice)]

This is a tracking issue for rust-lang/libs-team#693

This feature adds method calls on raw pointers that call into the respective ptr::slice_from_raw_parts{_mut} method for the types *const T, *mut T and NonNull<T> to make using raw pointers more ergonomic in Rust by supporting postfix method chain syntax while creating slices.

Public API

// core::ptr::const_ptr
impl<T> *const T {
    pub const fn cast_slice(self, len: usize) -> *const [T];
}

// core::ptr::mut_ptr
impl<T> *mut T {
    pub const fn cast_slice(self, len: usize) -> *mut [T];
}

// core::ptr::non_null
impl<T> NonNull<T> {
    pub const fn cast_slice(self, len: usize) -> NonNull<[T]>;
}

Steps / History

(Remember to update the S-tracking-* label when checking boxes.)

ACP: rust-lang/libs-team#693
Implementation: Implement cast_slice for raw pointer types #149110
Final comment period (FCP)¹
Stabilization PR

Unresolved Questions

~~the inference problem~~
- ~~infers: let null10: *const [u8] = ptr::slice_from_raw_parts(ptr::null(), 10);~~
- ~~does not infer: let null10: *const [u8] = ptr::null().cast_slice(10);~~
- deemed okay to have Tracking Issue for #![feature(ptr_cast_slice)] #149103 (comment)

https://std-dev-guide.rust-lang.org/feature-lifecycle/stabilization.html ↩

Rollup merge of rust-lang#149110 - Paladynee:acp/ptr_cast_slice, r=jh…

Unrolled build for #149110

Rollup merge of rust-lang#156109 - Paladynee:lib/cast-slice-improveme…

Auto merge of #156109 - Paladynee:lib/cast-slice-improvements, r=jhpratt

Rollup merge of rust-lang#156109 - Paladynee:lib/cast-slice-improveme…

Unrolled build for #156109

Rollup merge of #156109 - Paladynee:lib/cast-slice-improvements, r=jh…

@rustbot

status update:

sorry for the late update on this feature, #156109 migrated std to use ptr.cast_slice on most places in std other than a few places i arbitrarily deemed "too dangerous to change" etc.

i have a hunch that since this is just a "wrap another function" feature it'll be quite straightforward to push this slowly towards stabilization.

a few questions that still remain are:

A) the inference problem
- infers: let null10: *const [u8] = ptr::slice_from_raw_parts(ptr::null(), 10);
- does not infer: let null10: *const [u8] = ptr::null().cast_slice(10);
B) whether the feature can be stabilized as const as it's stabilized normally
C) the impl block impl<T> vs impl<T: PointeeSized> (carried over from ptr::slice_from_raw_parts)
D) whether the docs should redirect to ptr::slice_from_raw_parts{_mut} instead of manually inlining them (like pointer::copy_from_nonoverlapping)

libs-api: questions B, C, D
@rustbot label +I-libs-api-nominated

The inference failure is odd because this works.

fn main() {
    struct Woof<T: ?Sized>(std::marker::PhantomData<T>);
    const fn null<T: ?Sized>() -> Woof<T> { Woof(std::marker::PhantomData) }
    impl<T> Woof<T> {
        const fn cast_slice(self, _len: usize) -> Woof<[T]> {
            null()
        }
    }
    
    let _: Woof<[u8]> = null().cast_slice(3);
}

This also infers correctly: let _: *const [u8] = <*const _>::cast_slice(std::ptr::null(), 3);

as mentioned on Zulip the inference failure when calling methods on a pointer to a deduced type is intentional: #46906

per today's libs-api meeting: (a) we're okay with the bad inference, (b) this should be const, (c) impl<T> since [T] requires T: Sized, (d) dealer's choice, we don't have strong opinions on this. thanks!

recent policy is to do FCPs on ~~implementation~~ stabilisation PRs, so feel free to send one and ping me (or someone else on libs) there ^^

Rollup merge of rust-lang#162927 - malezjaa:stabilize-ptr-cast-slice,…

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Tracking Issue for `#![feature(ptr_cast_slice)]` #149103

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Tracking Issue for #![feature(ptr_cast_slice)] #149103

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Paladynee commented on Jul 11, 2026

ds84182 commented on Jul 12, 2026

ds84182 commented on Jul 12, 2026

programmerjake commented on Jul 12, 2026

nia-e commented on Jul 14, 2026

malezjaa commented on Sep 17, 2026

nia-e commented on Sep 17, 2026

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Tracking Issue for `#![feature(ptr_cast_slice)]` #149103