Tracking Issue for const_ptr_as_ref

[lilasta]

Feature gate: #![feature(const_ptr_as_ref)]

Public API

impl<T: ?Sized> NonNull<T> {
    pub const unsafe fn as_mut<'a>(&mut self) -> &'a mut T;
}

Steps / History

• Implementation: Make PTR::as_ref and similar methods const. #91823
• Implement ptr_as_ref_unchecked #122492
• move some const fn out of the const_ptr_as_ref feature #130164
• Final comment period (FCP)
• Stabilization PR

Unresolved Questions

• None yet.

[GrigorenkoPV]

Hello!

Now that #122492 has been merged, 3 new methods are also const-gated behind this flag

• <*const T>::as_ref_unchecked
• <*mut T>::as_ref_unchecked
• <*mut T>::as_mut_unchecked

Should they be added to the OP?

[lilasta]

I have added them to the list.

[RalfJung]

@rust-lang/libs-api I propose we const-stabilize the subset of the methods above that are stable for non-const calls:

// core::ptr
impl<T: ?Sized> *const T {
    pub const unsafe fn as_ref<'a>(self) -> Option<&'a T>;
}

impl<T: ?Sized> *mut T {
    pub const unsafe fn as_ref<'a>(self) -> Option<&'a T>;
    pub const unsafe fn as_mut<'a>(self) -> Option<&'a mut T>;
}

impl<T: ?Sized> NonNull<T> {
    pub const unsafe fn as_mut<'a>(&mut self) -> &'a mut T;
}

The rest should be moved to the feature gate tracking their general stability, no separate const-feature-gate is needed. (EDIT: that is happening in #130164)

[RalfJung]

Ah wait, the Option-returning ones are non-trivial since they require the is_null test... they should be moved under the const_ptr_is_null feature gate (#74939).

So then only one function is left to be stabilized here: NonNull::as_mut.

[dtolnay]

@rfcbot fcp merge

Subsequent to #129195, we are ready to stabilize const on NonNull::as_mut:

rust/library/core/src/ptr/non_null.rs

Lines 400 to 404 in d7522d8

	pub const unsafe fn as_mut<'a>(&mut self) -> &'a mut T {
	// SAFETY: the caller must guarantee that `self` meets all the
	// requirements for a mutable reference.
	unsafe { &mut *self.as_ptr() }
	}

This function is the mut analog of NonNull::as_ref for which we stabilized const in #102198.

rust/library/core/src/ptr/non_null.rs

Lines 362 to 367 in d7522d8

	pub const unsafe fn as_ref<'a>(&self) -> &'a T {
	// SAFETY: the caller must guarantee that `self` meets all the
	// requirements for a reference.
	// `cast_const` avoids a mutable raw pointer deref.
	unsafe { &*self.as_ptr().cast_const() }
	}

[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.