You most definitely can.

    fn foo<F, T>(f: F)
    where
        F: Fn(T),
        T: ToString,
    {
        f("Hello World")
    }

Or did I not understand what you meant?

Something like this isn’t possible

    fn foo(f: impl for<T: ToString> Fn(T)) {
        f(“Hello World”);
        f(3.14);
    }

    fn main() {
        f(|x| println!("{}", x.to_string());
    }

The workaround:

    trait FnToString {
        fn call(&self, x: impl ToString);
    }

    fn foo(f: impl FnToString) {
        f.call("Hello World");
        f.call(3.14);
    }

    struct AnFnToString;
    impl FnToString for AnFnToString {
        fn call(&self, x: impl ToString) {
            println!("{}", x.to_string());
        }
    }

    fn main() {
        foo(AnFnToString);
    }

Ha, yes, I see what you mean now. That's not really the closure's fault but monomorphization of the foo function. The specific thing you want to do would require boxing the value, or do more involved typing.

Do you have an example of this? I'm not sure I follow it exactly.