Fix invalid "ref mut mut" sugestion

[estebank]

Change output from:

error: cannot borrow immutable local variable `x` as mutable
  --> <anon>:12:23
   |
11 |         TestEnum::Item(ref mut x) => {
   |                        --------- use `ref mut mut x` here to make mutable
12 |             test(&mut x);
   |                       ^ cannot borrow mutably

to

error: cannot borrow immutable local variable `x` as mutable
  --> <anon>:12:23
   |
12 |             test(&mut x);
   |                       ^
   |                       |
   |                       cannot reborrow mutably
   |                       try removing `&mut` here

Fixes #37139, #34337, #34126

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @pnkfelix (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[estebank]

r? @jonathandturner

[sophiajt]

I need to see the full example, but this:

error: cannot borrow immutable local variable `x` as mutable
  --> <anon>:12:23
   |
12 |             test(&mut x);
   |                       ^
   |                       |
   |                       cannot reborrow mutably
   |                       try removing `&mut` here

Seems like it might not be correct. If you remove &mut and the function expects a borrow, won't it still error?

[estebank]

@jonathandturner the original code was (from #37139):

enum TestEnum {
    Item(i32),
}

fn test(x: &mut i32) {
    println!("hi! {:?}", x);
}

fn main() {
    let mut x = TestEnum::Item(10);
    match x {
        TestEnum::Item(ref mut x) => {
            test(&mut x);
        }
    }
}

When compiling it fails saying that x cannot be borrowed as mutable, while the same code without &mut works correctly, as x is already of type &mut i32.

[alexcrichton]

Oh dear that is indeed a confusing error message (ref mut mut)! Perhaps we could test if a reference is needed and assuage @jonathandturner's concerns by not mentioning this change when it would just lead to another compile error?

(although that may just end up being too convoluted)

[estebank]

@alexcrichton @jonathandturner I haven't been able to come up with a case where a wrong suggestion would be given. In the cases where it'd be wrong to remove the &mut the compiler throws a type error, never reaching this NOTE.

[sophiajt]

This at least seems like a net improvement, especially if @estebank 's comment that another error will catch the bad case for us.

@bors r+

[bors]

📌 Commit 1a5456b has been approved by jonathandturner

[bors]

⌛ Testing commit 1a5456b with merge 026add5...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-cargotest, auto-linux-64-cross-freebsd, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-opt-rustbuild, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-opt, auto-mac-64-opt-rustbuild, auto-mac-cross-ios-opt, auto-win-gnu-32-opt, auto-win-gnu-32-opt-rustbuild, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-cargotest, auto-win-msvc-64-opt, auto-win-msvc-64-opt-rustbuild
Approved by: jonathandturner
Pushing 026add5 to master...