Constants and interior mutability

[RalfJung]

According to @oli-obk, there are currently some checks somewhere to rule out interior mutability in constants. However, this is accepted:

use std::sync::atomic::AtomicUsize;
const FOO : AtomicUsize = AtomicUsize::new(3);

It would be great if the reasons for this check could be spelled out, ideally somewhere in this repo :D

[oli-obk]

Interior mutability is ruled out in statics, not in constants. Constants are annotated internally so statics using them know about it

[RalfJung]

This works as well:

use std::sync::atomic::AtomicUsize;
static FOO : AtomicUsize = AtomicUsize::new(3);

And indeed why would it not...

[oli-obk]

discussion continued in rust-lang/rust#53819