MIR-borrowck: assignment to static mut shouldn't be an error

[arielb1]

MIR borrowck errors out on assignments to mutable statics, which should be allowed (in unsafe code) e.g.

static mut Y: u32 = 0;

unsafe fn should_ok() {
    Y = 1;
}

fn main() {}

which causes:

$ rustc x.rs -Z borrowck-mir
error: cannot assign to immutable static item Y (Mir)
 --> x.rs:5:5
  |
5 |     Y = 1;
  |     ^^^^^

error: aborting due to previous error

[nikomatsakis]

Seems like this ought to fall out from permissions checking (#44837), right?

[arielb1]

Nope, still broken in 88a28ff (which includes #44837)

[KiChjang]

Looking at this as well. I believe the error comes from the improper static check in check_if_reassignment_to_immutable_state.