On stable, we prevent users from creating an &mut that points to memory inside a const by forbidding the creation of mutable references during const-eval. This limitation is only temporary, see #57349. We have a feature flag, const_mut_refs, that allows users to create mutable references, but no attempt is made to prevent &mut from escaping into the final value of a const like so:
#![feature(const_mut_refs)]
const FOO: &mut i32 = &mut 4;
fn main() {
*FOO = 2;
}
This errors on the current nightly, and if there were a feature gate that allowed it, we would get an ICE:
error: internal compiler error: src/librustc_mir/interpret/intern.rs:238: const qualif failed to prevent mutable references
I think we've not yet settled on the semantics we want. We're allowing them in const fn and relying on the borrow checker to prevent references from escaping.
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