rustc suggests to use impl trait in closure parameter which doesn't work

[pacak]

Code

let a = |x: dyn Fn()| x();

Current output

--> src/lib.rs:14:17
   |
14 |     let a = |x: dyn Fn()| x();
   |                 ^^^^^^^^ doesn't have a size known at compile-time
   |
   = help: the trait `Sized` is not implemented for `(dyn Fn() + 'static)`
help: you can use `impl Trait` as the argument type
   |
14 |     let a = |x: impl Fn()| x();
   |                 ~~~~
help: function arguments must have a statically known size, borrowed types always have a known size
   |
14 |     let a = |x: &dyn Fn()| x();
   |                 +

Desired output

help: function arguments must have a statically known size, borrowed types always have a known size
   |
14 |     let a = |x: &dyn Fn()| x();
   |                 +

Rationale and extra context

We start with some obviously invalid code, rustc makes two suggestions - pass &dyn Trait by reference - this works and use impl Trait which doesn't work.

error[E0562]: `impl Trait` is not allowed in closure parameters
  --> src/lib.rs:15:17
   |
15 |     let b = |x: impl Fn()| x();
   |                 ^^^^^^^^^
   |
   = note: `impl Trait` is only allowed in arguments and return types of functions and methods

For more information about this error, try `rustc --explain E0562`.

Usually following compiler suggestions you get closer to having working code, but here this leads to a dead end.

Other cases

Rust Version

rustc 1.85.0 (4d91de4e4 2025-02-17)
binary: rustc
commit-hash: 4d91de4e48198da2e33413efdcd9cd2cc0c46688
commit-date: 2025-02-17
host: x86_64-unknown-linux-gnu
release: 1.85.0
LLVM version: 19.1.7

Anything else?

Nightly makes a similar suggestion, also broken.

[rperier]

@rustbot claim