Error message improvement: Manual reborrowing with `&mut *variable`

[mstange]

In this testcase, I wanted to reborrow outer, but accidentally wrote &mut outer instead of &mut *outer. However, the error's suggestion was try removing `&mut` here which wouldn't have worked.
It would be better to have the suggestion try adding `*` here in this particular scenario.

fn test(outer: &mut Option<i32>) {
    match (&mut outer, 23) {
        (Some(inner), _) => {
            *inner = 17;
        },
        _ => {
            *outer = Some(2);
        },
    }
}

   Compiling playground v0.0.1 (/playground)
error[E0596]: cannot borrow `outer` as mutable, as it is not declared as mutable
 --> src/lib.rs:2:12
  |
2 |     match (&mut outer, 23) {
  |            ^^^^^^^^^^
  |            |
  |            cannot borrow as mutable
  |            try removing `&mut` here

error: aborting due to previous error

[mbartlett21]

The mutable reference does not seem to be reborrowed when it is used in a pattern, but is still when passed to a function that explicitly takes a mutable reference. The code below works:

fn takes_mut<T>(v: &mut T) -> &mut T {
    v
}

fn test(outer: &mut Option<i32>) {
    match (takes_mut(outer), 23) {
        (Some(inner), _) => {
            *inner = 17;
        },
        _ => {
            *outer = Some(2);
        },
    }
}

[estebank]

Current output:

error[E0596]: cannot borrow `outer` as mutable, as it is not declared as mutable
 --> src/lib.rs:2:12
  |
2 |     match (&mut outer, 23) {
  |            ^^^^^^^^^^ cannot borrow as mutable
  |
note: the binding is already a mutable borrow
 --> src/lib.rs:1:16
  |
1 | fn test(outer: &mut Option<i32>) {
  |                ^^^^^^^^^^^^^^^^
help: try removing `&mut` here
  |
2 -     match (&mut outer, 23) {
2 +     match (outer, 23) {
  |

Applying the suggestion will lead to another compile error:

error[E0382]: use of moved value: `outer`
 --> src/lib.rs:7:13
  |
1 | fn test(outer: &mut Option<i32>) {
  |         ----- move occurs because `outer` has type `&mut Option<i32>`, which does not implement the `Copy` trait
2 |     match (outer, 23) {
  |            ----- value moved here
...
7 |             *outer = Some(2);
  |             ^^^^^^^^^^^^^^^^ value used here after move

Changing the binding to be fn test(mut outer: &mut Option<i32>) instead would make the original code compile.

I don't know if we can perform any advanced enough analysis to detect this case and suggest mut outer correctly only in these cases. We might have to do the best next thing which is to suggest both removing &mut and making the binding mut. :-/

[estebank]

Current output, we suggest both options (the correct one first):

error[E0596]: cannot borrow `outer` as mutable, as it is not declared as mutable
 --> src/lib.rs:2:12
  |
2 |     match (&mut outer, 23) {
  |            ^^^^^^^^^^ cannot borrow as mutable
  |
note: the binding is already a mutable borrow
 --> src/lib.rs:1:16
  |
1 | fn test(outer: &mut Option<i32>) {
  |                ^^^^^^^^^^^^^^^^
help: consider making the binding mutable if you need to reborrow multiple times
  |
1 | fn test(mut outer: &mut Option<i32>) {
  |         +++
help: if there is only one mutable reborrow, remove the `&mut`
  |
2 -     match (&mut outer, 23) {
2 +     match (outer, 23) {
  |

When applying the second suggestion the error has not changed (it'd be nice if it suggested borrowing.

If immutably borrowing, we don't provide enough context of where the immutability is coming from:

error[E0594]: cannot assign to `*inner`, which is behind a `&` reference
 --> src/lib.rs:4:13
  |
4 |             *inner = 17;
  |             ^^^^^^^^^^^ `inner` is a `&` reference, so it cannot be written to