Stabilize ptr::{from_ref, from_mut}

[WaffleLapkin]

I propose to stabilize the following APIs:

// mod core::ptr

pub const fn from_ref<T: ?Sized>(r: &T) -> *const T;
pub const fn from_mut<T: ?Sized>(r: &mut T) -> *mut T;

Tracking issue: #106116

---

@RalfJung what do you think we should do with from_mut? Stabilize it as const, given that you can't call it anyway (no way to get &mut in const fn)? Defer stabilizing it as const leaving the same issue/feature? Change issue/feature? Change issue/feature to the "&mut in const fn" one?

[rustbot]

r? @Mark-Simulacrum

(rustbot has picked a reviewer for you, use r? to override)

[RalfJung]

what do you think we should do with from_mut? Stabilize it as const, given that you can't call it anyway (no way to get &mut in const fn)? Defer stabilizing it as const leaving the same issue/feature? Change issue/feature? Change issue/feature to the "&mut in const fn" one?

Given that one can already transmute &mut T to *mut T in const fn, I see no issue with stabilizing this.
Cc @rust-lang/wg-const-eval

[Mark-Simulacrum]

r? dtolnay

[dtolnay]

@rust-lang/libs-api:
@rfcbot fcp merge

Notes from the tracking issue:

#106116 (comment) is worth considering: currently you can take a &mut T and use ptr::from_mut to cast to *mut T then coerce to *const T, or you can take &mut T and coerce to &T followed by ptr::from_ref to cast to *const T. These are easy to confuse and appear to accomplish the same thing, but they do not. One of them produces a pointer that is okay to write to (after casting back to *mut) while the other produces a pointer that is UB to write to.

use core::ptr;

fn main() {
    let m: &mut i32 = &mut 0;
    
    let p1: *const i32 = ptr::from_ref(m);
    let p2: *const i32 = ptr::from_mut(m);
}

#106116 (comment) regarding fn from_ptr(Option<&T>) -> *const T — I think it's fine for callers to express this as opt_reference.map_or_else(ptr::null, ptr::from_ref).

[rfcbot]

Team member @dtolnay has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Amanieu
• @BurntSushi
• @dtolnay
• @joshtriplett
• @m-ou-se

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[rfcbot]

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

[rfcbot]

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.

[dtolnay]

Thanks!

[dtolnay]

• Rebased to resolve conflicts with Implement thread parking for xous #116839
  and Some cleanup and improvement for invalid ref casting impl #118909

[dtolnay]

@bors r+

[bors]

📌 Commit b863e9b has been approved by dtolnay

It is now in the queue for this repository.