rust/tests/ui/unboxed-closures/fn-traits-hrtb-coercion.rs

//! Test for issue <github.com/rust-lang/rust/issues/30904>
//! Related to higher-ranked lifetime inference with unboxed closures and FnOnce.

#![feature(fn_traits, unboxed_closures)]

fn test<F: for<'x> FnOnce<(&'x str,)>>(_: F) {}

struct Compose<F, G>(F, G);

impl<T, F, G> FnOnce<(T,)> for Compose<F, G>
where
    F: FnOnce<(T,)>,
    G: FnOnce<(F::Output,)>,
{
    type Output = G::Output;
    extern "rust-call" fn call_once(self, (x,): (T,)) -> G::Output {
        (self.1)((self.0)(x))
    }
}

struct Str<'a>(&'a str);

fn mk_str<'a>(s: &'a str) -> Str<'a> {
    Str(s)
}

fn main() {
    let _: for<'a> fn(&'a str) -> Str<'a> = mk_str;
    let _: for<'a> fn(&'a str) -> Str<'a> = Str;
    //~^ ERROR: mismatched types

    test(|_: &str| {});
    test(mk_str);
    test(Str);

    test(Compose(|_: &str| {}, |_| {}));
    test(Compose(mk_str, |_| {}));
    test(Compose(Str, |_| {}));
}