Multiple errors for the same missing mut

[RalfJung]

For the following program

fn main() {
    let v = Vec::new();
    v.push(0);
    v.push(0);
}

rustc shows two errors:

error[E0596]: cannot borrow immutable local variable `v` as mutable
 --> src/main.rs:3:5
  |
2 |     let v = Vec::new();
  |         - consider changing this to `mut v`
3 |     v.push(0);
  |     ^ cannot borrow mutably

error[E0596]: cannot borrow immutable local variable `v` as mutable
 --> src/main.rs:4:5
  |
2 |     let v = Vec::new();
  |         - consider changing this to `mut v`
3 |     v.push(0);
4 |     v.push(0);
  |     ^ cannot borrow mutably

error: aborting due to 2 previous errors

These are not exactly the same, but still pretty much a duplicate.

it would be nice if rustc could avoid complaining about the same thing twice here.

[estebank]

We should collect borrow errors by definition span and emit a single diagnostic:

error[E0596]: cannot borrow immutable local variable `v` as mutable
 --> src/main.rs:3:5
  |
2 |     let v = Vec::new();
  |         - consider changing this to `mut v`
3 |     v.push(0);
  |     ^ cannot borrow mutably
4 |     v.push(0);
  |     ^ cannot borrow mutably

error: aborting due to previous error

Also, we probably should bound how many lines of incorrect mutable borrow attempts are displayed, show only up to three(?) primary spans, and if more, display:

error[E0596]: cannot borrow immutable local variable `v` as mutable
 --> src/main.rs:3:5
  |
2 |     let v = Vec::new();
  |         - consider changing this to `mut v`
3 |     v.push(0);
  |     ^ cannot borrow mutably
4 |     v.push(0);
  |     ^ cannot borrow mutably
5 |     v.push(0);
  |     ^ cannot borrow mutably
  = note: ...and more

error: aborting due to previous error

After all, once the binding is fixed all of those errors would go away, so we're not hiding useful information from the user.