"cannot borrow a constant which may contain interior mutability" for local variable inside constant evaluation

[comex]

This code (playground link):

#![feature(const_raw_ptr_deref)]
use std::cell::UnsafeCell;
const FOO: u32 = {
    let x = UnsafeCell::new(42);
    unsafe { *x.get() }
};

produces on latest nightly:

error[E0492]: cannot borrow a constant which may contain interior mutability, create a static instead

However, following the advice to create a static simply results in a "constants cannot refer to statics" error.

The limitation seems unnecessary. We're not really borrowing a constant; x only exists during constant evaluation and will not be emitted into the binary, so there's no reason it would be problematic to mutate it.

This issue affects memoffset.

[oli-obk]

This issue exists since 1.33: https://rust.godbolt.org/z/iktZhE

Simpler example:

use std::cell::Cell;
const FOO: () = {
    let x = Cell::new(42);
    let y = &x;
};

I think we can lift this restriction entirely if the borrow is of a named local or isn't in a return expression.

[oli-obk]

cc @rust-lang/wg-const-eval