Confusing error message with closures

[est31]

struct Foo {
    u :u32,
}
impl Foo {
    fn foo(&mut self) {
        || {
            let r = &mut self;
            r.u += 1;
        };
    }
}

gives

error[E0595]: closure cannot assign to immutable argument `self`
 --> src/main.rs:6:9
  |
6 |         || {
  |         ^^
  |         |
  |         cannot reborrow mutably
  |         try removing `&mut` here

error: aborting due to previous error

error: Could not compile `playground`.

Which is very cryptic and doesn't really show what's going on.

[est31]

cc @estebank

[estebank]

Just created a PR to address part of the wording. The full fix would be to have the span pointing at the reborrow instead of the closure. Even when changing that, the diagnostic should still point at the current span to provide the context of the closure.

The MIR output is slightly better in that we get an error pointing at the reborrow, but it is still not great:

% rustc file.rs -Zborrowck=compare
error[E0596]: cannot borrow immutable item `self` as mutable (Mir)
 --> file.rs:7:21
  |
7 |             let r = &mut self;
  |                     ^^^^^^^^^ cannot borrow as mutable
  |
  = note: Value not mutable causing this error: `self`

error[E0595]: closure cannot assign to immutable argument `self` (Ast)
 --> file.rs:6:9
  |
5 |     fn foo(&mut self) {
  |            --------- given this existing mutable borrow...
6 |         || {
  |         ^^ cannot reborrow immutable argument `self` mutably in this closure

error[E0596]: cannot borrow immutable item `self` as mutable (Mir)
 --> file.rs:6:9
  |
6 | /         || {
7 | |             let r = &mut self;
8 | |             r.u += 1;
9 | |         };
  | |_________^ cannot borrow as mutable

error: aborting due to 3 previous errors

[crlf0710]

Just suggesting changing &mut self to &mut *self for this pattern is enough, i believe...

[estebank]

Why was this ticket closed (and locked)? The subpar diagnostics are still being emitted...