How do I solve the error “the precise format of `Fn`-family traits' type parameters is subject to change”?

Question

I have written a problem solver in Rust which as a subroutine needs to make calls to a function which is given as a black box (essentially I would like to give an argument o

Answer 1

The direct answer is to do exactly as the error message says:

Use parenthetical notation instead

That is, instead of Fn<(A, B)>, use Fn(A, B)

The real problem is that you are not allowed to implement the Fn* family of traits yourself in stable Rust.

The real question you are asking is harder to be sure of because you haven't provided a MCVE, so we are reduced to guessing. I'd say you should flip it around the other way; create a new trait, implement it for closures and your type:

trait Solve {
    type Output;
    fn solve(&mut self) -> Self::Output;
}

impl<F, T> Solve for F
where
    F: FnMut() -> T,
{
    type Output = T;

    fn solve(&mut self) -> Self::Output {
        (self)()
    }
}

struct Test;
impl Solve for Test {
    // interesting things
}

fn main() {}