E0106: help diagonistic suggests invalid syntax `mut T`

[das-buh]

Code

fn foo(_f: impl Fn() -> &mut ()) {}

Current output

error[E0106]: missing lifetime specifier
 --> bad.rs:1:25
  |
1 | fn foo(_f: impl Fn() -> &mut ()) {}
  |                         ^ expected named lifetime parameter
  |
  = help: this function's return type contains a borrowed value, but there is no value for it to be borrowed from
help: consider using the `'static` lifetime, but this is uncommon unless you're returning a borrowed value from a `const` or a `static`
  |
1 | fn foo(_f: impl Fn() -> &'static mut ()) {}
  |                          +++++++
help: instead, you are more likely to want to return an owned value
  |
1 - fn foo(_f: impl Fn() -> &mut ()) {}
1 + fn foo(_f: impl Fn() -> mut ()) {}
  |

Desired output

error[E0106]: missing lifetime specifier
 --> bad.rs:1:25
  |
1 | fn foo(_f: impl Fn() -> &mut ()) {}
  |                         ^ expected named lifetime parameter
  |
  = help: this function's return type contains a borrowed value, but there is no value for it to be borrowed from
help: consider using the `'static` lifetime, but this is uncommon unless you're returning a borrowed value from a `const` or a `static`
  |
1 | fn foo(_f: impl Fn() -> &'static mut ()) {}
  |                          +++++++
help: instead, you are more likely to want to return an owned value
  |
1 - fn foo(_f: impl Fn() -> &mut ()) {}
1 + fn foo(_f: impl Fn() -> ()) {}
  |

Rationale and extra context

In type annotations for closure return types, the help diagnostic suggests returning an owned value instead of a reference. However, it suggests the invalid syntax mut T, only removing the & instead of the full &mut. It should most likely suggest changing &mut T into T instead.

Other cases

Rust Version

rustc 1.92.0 (ded5c06cf 2025-12-08)
binary: rustc
commit-hash: ded5c06cf21d2b93bffd5d884aa6e96934ee4234
commit-date: 2025-12-08
host: aarch64-pc-windows-msvc
release: 1.92.0
LLVM version: 21.1.3

Anything else?

No response

[reddevilmidzy]

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