Compiler incorrectly suggests &mut x instead of &(mut x) in a pattern for Copy types. #122415
Description
Code
fn mutate(_y: &mut i32) {}
fn foo(&x: &i32) {
mutate(&mut x);
}Current output
Compiling playground v0.0.1 (/playground)
error[E0596]: cannot borrow `x` as mutable, as it is not declared as mutable
--> src/lib.rs:4:12
|
4 | mutate(&mut x);
| ^^^^^^ cannot borrow as mutable
|
help: consider changing this to be mutable
|
3 | fn foo(&mut x: &i32) {
| +++
For more information about this error, try `rustc --explain E0596`.
error: could not compile `playground` (lib) due to 1 previous errorDesired output
Instead of suggesting &mut x: &i32, which doesn't compile, the compiler should suggest &(mut x): &i32, which does compile.
Rationale and extra context
This incorrect suggestion occurs when attempting to use a &x pattern to copy a Copy type (i32 in the reproduction code) from a reference, and then mutate the copy. The correct thing to do is to dereference with a & pattern, and assign it to a mut binding, which is written as &(mut x). The compiler incorrectly suggests &mut x, which means something else, and doesn't work.
The original use case that led to this issue had a one-line closure instead of the foo function, which makes doing something like let mut x = x undesirable.
Other cases
No response
Rust Version
1.78.0-nightly (2024-03-11 4a0cc881dcc4d800f106) on the playgroundAnything else?
No response