Avoid using bare Fn* trait syntax in error messages

Code

fn foo(_: impl FnOnce(&())) {}

fn main() {
    foo(std::mem::drop);
}

Available at this playground.

Current output

Compiling playground v0.0.1 (/playground)
error: implementation of `FnOnce` is not general enough
 --> src/main.rs:6:5
  |
6 |     foo(std::mem::drop);
  |     ^^^^^^^^^^^^^^^^^^^ implementation of `FnOnce` is not general enough
  |
  = note: `fn(&'2 ()) {std::mem::drop::<&'2 ()>}` must implement `FnOnce<(&'1 (),)>`, for any lifetime `'1`...
  = note: ...but it actually implements `FnOnce<(&'2 (),)>`, for some specific lifetime `'2`

Desired output

Compiling playground v0.0.1 (/playground)
error: implementation of `FnOnce` is not general enough
 --> src/main.rs:6:5
  |
6 |     foo(std::mem::drop);
  |     ^^^^^^^^^^^^^^^^^^^ implementation of `FnOnce` is not general enough
  |
  = note: `fn(&'2 ()) {std::mem::drop::<&'2 ()>}` must implement `FnOnce(&'1 ())`, for any lifetime `'1`...
  = note: ...but it actually implements `FnOnce(&'2 ())`, for some specific lifetime `'2`

Rationale and extra context

Would generally rather not see FnOnce<(T,)> syntax in user-facing messages, especially since it's unstable.

Rust Version

rustc 1.90.0 (1159e78c4 2025-09-14)
binary: rustc
commit-hash: 1159e78c4747b02ef996e55082b704c09b970588
commit-date: 2025-09-14
host: x86_64-unknown-linux-gnu
release: 1.90.0
LLVM version: 20.1.8

Anything else?

No response

Reproduce the reported Rust example from the issue and inspect the compiler diagnostic path that formats the FnOnce bounds. Compare the current and desired messages, then add coverage for avoiding bare Fn* trait syntax while preserving the lifetime details.