MIR-borrowck: paths off of statics should not be borrow-checked

[pnkfelix]

Paths off of statics should not be borrow-checked, as they already require unsafe. But currently MIR-borrowck is flagging errors for them, while AST-borrowck lets them through without complaint.

(Spawned off of #44985 (review))

[KiChjang]

I'm taking a look at this.

[nikomatsakis]

Example test:

struct Foo { x: usize }

static mut SFOO : Foo = Foo{x: 23 }; 

impl Foo {
    fn x(&mut self) -> &mut usize { &mut self.x }
}

fn main() {
    unsafe {
        let x = SFOO.x();
        SFOO.x += 1; // ?
        *x += 1; 
    }
}

This does indeed pass (for better or worse...) in AST-based borrowck.

[nikomatsakis]

I think we should at least warn about this scenario. We could do this by looking at the borrow path during the access checks to see if it was a borrow of a static.

[nikomatsakis]

After some discussion on gitter, decided that we should leave warning for later consideration, and just match AST borrowck in this case for now. This is not a bug fix to AST-borrowck, no need to deviate right now.

[pnkfelix]

Note for those reading this bug in future: the example test written in above comment is actually UB, because it is creating two simultaneous &mut references that point to the same location.