mark sys_common::once::generic::Once::new const-stable

[krasimirgg]

Attempt to address #103191 by marking the impl const-stable.
Picked the declaration from the callsite:

rust/library/std/src/sync/once.rs

Line 67 in 21b2465

#[rustc_const_stable(feature = "const_once_new", since = "1.32.0")]

This is similar to #98457.

With this in, python3 x.py build library/std --target x86_64-unknown-none succeeds.

[rust-highfive]

r? @joshtriplett

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[RalfJung]

https://doc.rust-lang.org/nightly/std/sync/struct.Once.html#method.new has been const fn since forever so this is probably just an internal change?
r? @m-ou-se

[Amanieu]

This seems to be necessary because the generic implementation uses ptr::invalid_mut, which is still unstable.

@bors r+ rollup

[bors]

📌 Commit df5d035 has been approved by Amanieu

It is now in the queue for this repository.

[Manishearth]

This seems to be necessary because the generic implementation uses ptr::invalid_mut, which is still unstable.

cc @estebank @oli-obk

might be worth looking at showing proximal cause in the diagnostics for stuff like this, fwiw

[estebank]

@Manishearth can we have an minimal repro for this? I'm not as involved with the const fn space so I don't have the context for how the current state is bad.

[Manishearth]

@estebank tricky thing is that because it has to do with const_stable it needs to be reprod in std.

I can try but I don't fully understand what's happening here either, that's why I cc'd oli

[oli-obk]

I think this is a hole in our const stability checks. Basically functions without stability attributes are usable from stable const fn even if they internally call unstable const fn