I'm experimenting with impls of my own traits for closures; my trait has an associated type for the closure's output.
This works fine, with the associated type in Fn's return position:
trait MyFn<Input> {
type State;
fn call(&self, state: &mut Self::State, input: Input);
}
impl<Input, State, F> MyFn<Input> for F where F: Fn(Input) -> State {
type State = State;
fn call(&self, state: &mut State, input: Input) {
*state = self(input);
}
}
But I can't get the same associated type to work in the Fn's parameter position:
impl<Input, State, F> MyFn<Input> for F where F: Fn(&mut State, Input) {
type State = State;
fn call(&self, state: &mut State, input: Input) {
self(state, input)
}
}
error[E0207]: the type parameter `State` is not constrained by the impl trait, self type, or predicates
--> src/lib.rs:15:13
|
15 | impl<Input, State, F> MyFn<Input> for F where F: Fn(&mut State, Input) {
| ^^^^^ unconstrained type parameter
Surely the type parameter State is constrained by the where predicate?