redundant_closure doesn't handle Fn

[mladedav]

Summary

When a closure |val| fun(val) is used where fun is Fn, the suggestion is to use it directly as fun. However, that may not compile and the correct suggestion is &fun.

Lint Name

redundat_closure

Reproducer

I tried this code:

#![warn(clippy::redundant_closure)]
#![allow(dead_code)]

fn foo<F: Fn(u32) -> u32 + 'static>(f: F) -> Option<u32> {
    let x = Box::new(move || None.map(|x| f(x)));

    x()
}

I saw this happen:

    Checking playground v0.0.1 (/playground)
warning: redundant closure
 --> src/lib.rs:5:39
  |
5 |     let x = Box::new(move || None.map(|x| f(x)));
  |                                       ^^^^^^^^ help: replace the closure with the function itself: `f`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#redundant_closure
note: the lint level is defined here
 --> src/lib.rs:1:9
  |
1 | #![warn(clippy::redundant_closure)]
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^

warning: `playground` (lib) generated 1 warning (run `cargo clippy --fix --lib -p playground` to apply 1 suggestion)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.62s

and if I apply that fix, I get:

error[E0525]: expected a closure that implements the `Fn` trait, but this closure only implements `FnOnce`
 --> src/lib.rs:5:22
  |
5 |     let x = Box::new(move || None.map(f));
  |                      ^^^^^^^          - closure is `FnOnce` because it moves the variable `f` out of its environment
  |                      |
  |                      this closure implements `FnOnce`, not `Fn`
6 |
7 |     x()
  |     --- the requirement to implement `Fn` derives from here
  |
  = note: required for `Box<{closure@src/lib.rs:5:22: 5:29}>` to implement `Fn<()>`

For more information about this error, try `rustc --explain E0525`.
error: could not compile `playground` (lib) due to 1 previous error

I expected to see this happen (note the ampersand, that's what's missing):

...

5 |     let x = Box::new(move || None.map(|x| f(x)));
  |                                       ^^^^^^^^ help: replace the closure with the function itself: `&f`
...

Version

1.78.0 (playground)

Additional Labels

@rustbot label +I-suggestion-causes-error

[J-ZhengLi]

looks like another adjustment problem?

@rustbot claim