assigning to immutable argument produces a confusing error

[nikomatsakis]

This program:

fn foo(_x: u32) {
    _x = 4;
}

fn main() {}

produces this error:

error[E0384]: re-assignment of immutable variable `_x`
 --> src/main.rs:2:5
  |
1 | fn foo(_x: u32) {
  |        -- first assignment to `_x`
2 |     _x = 4;
  |     ^^^^^^ re-assignment of immutable variable

I consider this a somewhat confusing error. I think it should rather be:

error[E0384]: assignment of immutable argument `_x`
 --> src/main.rs:2:5
  |
1 | fn foo(_x: u32) {
  |        -- `_x` not declared as `mut`
2 |     _x = 4;
  |     ^^^^^^ assignment of immutable argument `_x`

perhaps with a suggestion or something.

This error is issued by the borrow checker, so to do this right we should also modify the MIR-based borrow checker.

[nikomatsakis]

cc @estebank

[nikomatsakis]

Looks like this was fixed by #47261

[estebank]

Only for MIR, that's why I didn't close the ticket, but if that's not a problem let's leave it closed.