Incorrectly positioned, partially hidden error message

[dtolnay]

use std::collections::BTreeMap;

fn main() {
    let mut map = BTreeMap::new();
    map.entry("e").or_insert(0) += 1;
}

rustc 1.18.0 (03fc9d6 2017-06-06)

error[E0368]: binary assignment operation `+=` cannot be applied to type `&mut {integer}`
 --> src/main.rs:5:5
  |
5 |     map.entry("e").or_insert(0) += 1;
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot use `+=` on type `&mut {integer}`

rustc 1.19.0-beta.1 (a879841 2017-06-06) and rustc 1.19.0-nightly (4bf5c99 2017-06-10)

error[E0368]: binary assignment operation `+=` cannot be applied to type `&mut {integer}`
 --> src/main.rs:5:5
  |
5 |     map.entry("e").or_insert(0) += 1;
  |     ---------------------------^^^^^ot use `+=` on type `&mut {integer}`

[dtolnay]

cc @estebank who recently touched this code.