Functional Programming: allow `impl Trait` in `Fn() -> impl Trait`


Hi, I don’t know if this is possible, or maybe this is hard to implement.

I think we need to allow impl Trait in Fn() -> impl Trait,

here the reason for it.

What do you think?


I think this is rust-lang/rust#45994, although that’s a very short issue.

As far as I know this case should be equivalent to other traits associated types, which work already (playground), but for some reason the Fn type sugar disallows this. The linked issue mentions HRTB, so maybe that’s why it wouldn’t be as straightforward for Fn types, but I’m not sure exactly how that interacts with it.

fn foo() -> impl Iterator<Item = impl std::fmt::Debug> {

fn main() {
    for i in foo() {
        println!("{:?}", i);