Comment by armchairhacker

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);
    }