Promotion of non-ZST mutable references is incompatible with const_mut_refs

[RalfJung]

Consider the following code:

#![feature(const_mut_refs)]

static mut TEST: () = {
    let x = &mut [1,2,3];
    x[0] += 1;
};

This should just work. But instead it throws an error:

error[E0080]: could not evaluate static initializer
 --> src/lib.rs:5:5
  |
5 |     x[0] += 1;
  |     ^^^^^^^^^ modifying a static's initial value from another static's initializer

The reason for this is that &mut [1,2,3] gets lifetime extended via promotion, so this reference now points to a separate mutable static -- but mutable statics cannot be mutated during CTFE.

I see no way to fix this, other than stopping to promote mutable references -- which we should IMO do anyway, they are a very strange and unprincipled special case. Just imagine doing this in a loop, suddenly all these different and mutable allocations share the same address...

Cc @rust-lang/wg-const-eval

[eddyb]

Ugh, that's an ancient rule (believe it or not, it's a workaround for not being able to write static mut Foo: [T; _] = ...; and have the length be inferred!) and it was never meant to get this far, I guess I didn't think it through properly when adding the "outermost scope" rule, which is the only position where allowing it makes sense.

In the current implementation, it should've never been promoted, only allowed, my bad!