make ptr metadata functions callable from stable const fn

[RalfJung]

So far this was done with a bunch of rustc_allow_const_fn_unstable. But those should be the exception, not the norm. If we are confident we can expose the ptr metadata APIs indirectly in stable const fn, we should just mark them as rustc_const_stable. And we better be confident we can do that since it's already been done a while ago. ;)

In particular this marks two intrinsics as const-stable: aggregate_raw_ptr, ptr_metadata. This should be uncontroversial, they are trivial to implement in the interpreter.
Cc @rust-lang/wg-const-eval @rust-lang/lang

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[RalfJung]

FWIW this is "reversible" in the sense that the PR doesn't stabilize anything new. Exposing this intrinsic on stable has already happened.

[workingjubilee]

Currently the in-flight PR #130403 has a rustc_allow_const_fn_unstable for this.

But already in the codebase we have these for intrinsics::ptr_metadata:

• rust/library/core/src/slice/mod.rs

  Lines 114 to 117 in 42ff2ee

  	#[rustc_allow_const_fn_unstable(ptr_metadata)]
  	#[inline]
  	#[must_use]
  	pub const fn len(&self) -> usize {

• rust/library/core/src/ptr/mut_ptr.rs

  Lines 1807 to 1808 in 42ff2ee

  	#[rustc_allow_const_fn_unstable(ptr_metadata)]
  	pub const fn len(self) -> usize {

And these for intrinsics::aggregate_raw_ptr:

• rust/library/core/src/ptr/mod.rs

  Lines 602 to 604 in 42ff2ee

  	#[rustc_allow_const_fn_unstable(ptr_metadata)]
  	#[rustc_diagnostic_item = "ptr_null"]
  	pub const fn null<T: ?Sized + Thin>() -> *const T {

• rust/library/core/src/ptr/mod.rs

  Lines 628 to 630 in 42ff2ee

  	#[rustc_allow_const_fn_unstable(ptr_metadata)]
  	#[rustc_diagnostic_item = "ptr_null_mut"]
  	pub const fn null_mut<T: ?Sized + Thin>() -> *mut T {

• rust/library/core/src/ptr/mod.rs

  Lines 952 to 954 in 42ff2ee

  	#[rustc_allow_const_fn_unstable(ptr_metadata)]
  	#[rustc_diagnostic_item = "ptr_slice_from_raw_parts"]
  	pub const fn slice_from_raw_parts<T>(data: *const T, len: usize) -> *const [T] {

[RalfJung]

Indeed, they show up in the diff for this PR as these lines are being removed here.

[scottmcm]

Making the intrinsics const-stable seems clearly good to me.

The stabilization-track stuff is less obvious to me. But if you're confident that's ok -- it won't show const-stable in the rustdocs, for example -- then sure, r=me.

[scottmcm]

Hmm, seems odd to mark this as const-stable when it's not normal-stable? I don't remember use doing that elsewhere?

[RalfJung]

There is precedent for that: the raw_vec_internals_const feature.

[slanterns]

Another example: #128228 (comment). tbh it also feels surprising to me 😅

[RalfJung]

Yeah I think we should redo how we do const stability. See #129815.

But for now this is the system that we got.

[RalfJung]

But if you're confident that's ok -- it won't show const-stable in the rustdocs, for example

Well, I am confident this is how const stability is supposed to be used. There might be bugs, of course...

I'll check if we have a test to ensure that such functions are not callable on stable, and add a test if we don't have one already.

[RalfJung]

The stabilization-track stuff is less obvious to me. But if you're confident that's ok -- it won't show const-stable in the rustdocs, for example -- then sure, r=me.

I added a test so I am confident in the most important part, that this is not callable on stable. You can see here how this looks in rustdoc: it has the "This is a nightly-only experimental API" banner. It also shows "const: 1.82.0", which maybe we should suggest they hide. But IMO it's still pretty clear that this is an unstable function.

@bors r=scottmcm

[bors]

📌 Commit 74977fd has been approved by scottmcm

It is now in the queue for this repository.

[RalfJung]

@bors r=scottmcm

[bors]

📌 Commit 7650307 has been approved by scottmcm

It is now in the queue for this repository.