Polonius borrow checker regression in recent nightlies

[robsmith11]

The following code was working with -Zpolonius up until rustc 1.33.0-nightly (c0bbc3927 2019-01-03). Since that nightly, the borrow checker fails.

Was this an intentional change? The two borrows are disjoint, so I think it should have been fine.

struct St {
    x:u64,
    v:Vec<u64>,
}

fn main() {
    let mut s = St { x:0, v:Vec::new() };
    s.v.extend((0..1).map(|a| a + s.x));
}

error[E0502]: cannot borrow `s.v` as mutable because it is also borrowed as immutable
 --> src/main.rs:8:5
  |                                                                                                                   
  8 |     s.v.extend((0..1).map(|a| a + s.x));
  |     ^^^^------^^^^^^^^^^^^---^^^^^-^^^^
  |     |   |                 |       |
  |     |   |                 |       first borrow occurs due to use of `s` in closure
  |     |   |                 immutable borrow occurs here
  |     |   immutable borrow later used by call
  |     mutable borrow occurs here

error: aborting due to previous error

[matthewjasper]

Yes, this is intentional. The second borrow is of s (closures can only capture whole variables) and overlaps with the first borrow.

[jonas-schievink]

To clarify some more: There's an accepted RFC that would make the closure borrow s.x instead of the entire s, tracked in #53488