Confusing error when trying to re-borrow a function argument

[JaniM]

It is a common mistake, especially for beginners, to attempt to reborrow function arguments when calling other functions.

I tried this code:

struct T;
fn foo(x: &mut T) {}
fn bar(x: &mut T) {
    foo(&mut x)
}

I expected to see this happen: Compile error following what the above code does if foo(&x) is attempted instead.

error[Exxxx]: mismatched types
 --> src/lib.rs:6:9
  |
6 |     foo(&mut x)
  |         ^^ expected `&mut T`, found `&mut &mut T`
  |
  = note; you don't need to use `&mut` when you already have a mutable reference

Instead, this happened: Misleading error.

error[E0596]: cannot borrow `x` as mutable, as it is not declared as mutable
 --> src/lib.rs:6:9
  |
5 | fn bar(x: &mut T) {
  |        - help: consider changing this to be mutable: `mut x`
6 |     foo(&mut x)
  |         ^^^^^^ cannot borrow as mutable

Meta

1.45.2 srable & 1.47.0 nightly on the playground

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=768e5a2d690393f89aed38be3fa5dee6

[pickfire]

Seemed similar to #71389 except that is *x = xxx but this is f(x). I believe it may even be possible to have a fix for this like Maybe try removing '&mut '?

[estebank]

It not incorrect, just stylistically suspect given the circumstances. &mut x will reborrow, and it is what to use if you were calling foo multiple times. Only in this case, where you only need to pass x around once is the suggestion not what you want to do.