E0434 suggests changing the fn item of a trait impl to a closure

Code

trait T {
    fn t() -> impl FnOnce() -> ();
}

fn f(x: String) {
    struct S;
    impl T for S {
        fn t() -> impl FnOnce() -> () {
            move || {let _ = x;}
        }
    }
}

Current output

error[E0434]: can't capture dynamic environment in a fn item
 --> src/lib.rs:9:30
  |
9 |             move || {let _ = x;}
  |                              ^
  |
  = help: use the `|| { ... }` closure form instead

Desired output

error[E0434]: can't capture dynamic environment in a fn item
 --> src/lib.rs:9:30
  |
9 |             move || {let _ = x;}
  |                              ^
  |

Rationale and extra context

I don't think it's possible to implement a trait without using a fn item. A more detailed hint that refers to the exect fn keyword would also be nice (see https://github.com/rust-lang/rust/issues/112845).

The suggestion is wrong in this case.

Other cases

Rust Version

rustc 1.93.1 (01f6ddf75 2026-02-11)
binary: rustc
commit-hash: 01f6ddf7588f42ae2d7eb0a2f21d44e8e96674cf
commit-date: 2026-02-11
host: x86_64-unknown-linux-gnu
release: 1.93.1
LLVM version: 21.1.8

Anything else?

No response

Reproduce E0434 with the Rust snippet in src/lib.rs and inspect the compiler diagnostic handling for the suggestion to use a closure. The work is done when this trait-impl case no longer emits the incorrect suggestion while the diagnostic remains correct for applicable cases.