how to save a function pointer with dyn parameters in a structure

Question

I'm getting

error[E0308]: mismatched types
  --> src/main.rs:38:39
   |
37 |     fn set_source_outcome<S: Source, O: Outcome>(&mut self, selector: fn(s: &S) -> &O) {
   |                           - this type parameter
38 |         self.selector = Some(Box::new(selector));
   |                                       ^^^^^^^^ expected trait object `dyn Source`, found type parameter `S`
   |
   = note: expected fn pointer `for<'r> fn(&'r (dyn Source + 'r)) -> &'r (dyn Outcome + 'r)`
              found fn pointer `for<'r> fn(&'r S) -> &'r O`
   = help: type parameters must be constrained to match other types
   = note: for more information, visit https://doc.rust-lang.org/book/ch10-02-traits.html#traits-as-parameters

when trying to save a function with dyn parameters in a structure, like this:

struct X {
    selector: Option<Box<fn(s: &dyn Source) -> &dyn Outcome>>,
}

impl X {
    fn set_source_outcome<S: Source, O: Outcome>(&mut self, selector: fn(s: &S) -> &O) {
        self.selector = Some(Box::new(selector));
    }
}

Is there any way to do so? playground: here

Answer 1

ok, looks somewhat like but still works playground

a more straightforward solution is most welcome