two phase borrows: ensure reservations are limited to assignments to temps

[pnkfelix]

Spawned off of #46537 (comment)

@arielb1 put its simply:

However, it's probably a wise idea to limit reservations to assignments to temporaries. I don't think
MIR construction ever does anything else, but you shouldn't be able to do this:

let mut a = 0;
let mut b = 0;
let mut x: &mut u32 = &mut a;
let y = &mut x;
*y = &mut b; // this borrow should be active
// look at me, no use of `*y`, but:
use(x, &b); // should be illegal

[pnkfelix]

My thinking here is that we should add an assert! ensuring the (believed current invariant) holds.

(We could perhaps go further and try to make the two-phase borrow system force an immediate activation when it sees an assignment to a non-temporary, if we discover that the supposed invariant does not hold.)

[pnkfelix]

cc #46037

[pnkfelix]

Hmm it seems according to https://github.com/rust-lang/rust/blob/master/src/test/compile-fail/borrowck/borrowck-issue-14498.rs that the proposed invariant does not hold. Good thing we attempted to check it!

Compiler error (once I added a relatively narrow assertion to the MIR borrowck):

error: internal compiler error: /home/pnkfelix/Dev/Mozilla/rust-mirborrowck/src/librustc_mir/borrow_check/mod.rs:342: mutable borrows must be assigned to locals
  --> /home/pnkfelix/Dev/Mozilla/rust-mirborrowck/src/test/compile-fail/borrowck/borrowck-issue-14498.rs:27:25
   |
27 |     let y: Box<_> = box &mut x;
   |                         ^^^^^^

note: the compiler unexpectedly panicked. this is a bug.