casting trait methods to FnOnce(&mut dyn Trait) is not ergonomic

[programmerjake]

In the following code, I'm trying to cast Trait::f to FnOnce(&mut dyn Trait) which should be possible, but the obvious method doesn't work. None of the combinations I tried got it to work (I could just use a lambda that called f, but that defeats the purpose).

trait Trait {
    fn f(&mut self);
}

fn g<F: FnOnce(&mut dyn Trait)>(f: F) {}

fn f() {
    g(Trait::f)
}

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
error[E0631]: type mismatch in function arguments
 --> src/lib.rs:8:7
  |
2 |     fn f(&mut self);
  |     ---------------- found signature of `for<'r> fn(&'r mut _) -> _`
...
5 | fn g<F: FnOnce(&mut dyn Trait)>(f: F) {}
  |    -    ---------------------- required by this bound in `g`
...
8 |     g(Trait::f)
  |       ^^^^^^^^ expected signature of `for<'r> fn(&'r mut (dyn Trait + 'r)) -> _`

error[E0271]: type mismatch resolving `for<'r> <for<'s> fn(&'s mut _) {<_ as Trait>::f} as std::ops::FnOnce<(&'r mut (dyn Trait + 'r),)>>::Output == ()`
 --> src/lib.rs:8:5
  |
5 | fn g<F: FnOnce(&mut dyn Trait)>(f: F) {}
  |    -    ---------------------- required by this bound in `g`
...
8 |     g(Trait::f)
  |     ^ expected bound lifetime parameter, found concrete lifetime

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0271`.
error: could not compile `playground`.

To learn more, run the command again with --verbose.

[programmerjake]

I'm reporting this here because I consider it a UI bug.

[estebank]

You can make this work by introducing an extra type parameter: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=3b4a9c25c6acae74ef2da4bc9f137163

The error happens because Self is not dyn Trait.

[programmerjake]

You can make this work by introducing an extra type parameter: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=3b4a9c25c6acae74ef2da4bc9f137163

The error happens because Self is not dyn Trait.

I don't think that will work for my particular usecase, since I have a dyn Trait that I pass to another function along with the particular trait method to call:

trait Trait {
    fn f(&mut self);
}

fn g<F: FnOnce(&mut dyn Trait)>(f: F, v: &mut dyn Trait) {
    f(v)
}

fn f(v: &mut dyn Trait) {
    g(Trait::f, v)
}

[estebank]

It can work if your type param T doesn't require it to be Sized. #57744 covers guiding people in that direction.