The Y combinator causes hanging

[Nugine]

rust-analyzer version: 0add6e9 2021-12-20 dev

rustc version: rustc 1.59.0-nightly (efec54529 2021-12-04)

/// The Y combinator
pub fn y<'a, A, O>(f: impl Fn(&dyn Fn(A) -> O, A) -> O + 'a) -> impl Fn(A) -> O + 'a {
    fn wrap<A, O>(f: &impl Fn(&dyn Fn(A) -> O, A) -> O) -> impl Fn(A) -> O + Copy + '_ {
        move |a| f(&wrap(f), a)
    }
    move |a| wrap(&f)(a)
}

pub fn fib(n: u64) -> u64 {
    y(|f, n| match n {
        0 => 0,
        1 => 1,
        n => f(n - 1) + f(n - 2),
    })(n)
}

pub fn factorial(n: u64) -> u64 {
    y(|f, n| match n {
        0 => 1,
        n => n * f(n - 1),
    })(n)
}

[flodiebold]

Possibly rust-lang/chalk#688 again.

[flodiebold]

Can still reproduce.

[lowr]

Marking as unactionable as there seems to be nothing we can do in our repo.

Minimal test case:

//- minicore: fn, sized
fn y<P, R, F: FnOnce(P, &dyn FnOnce(P) -> R) -> R>(_: F) {}

fn test() {
    y(|_: usize, _| -> () {});
}

[lbfalvy]

Is there any workaround? The only way I'd found to rephrase affected types was to wrap the function reference in a tuple struct, but since Fn trait impls aren't stable this generates a bit of syntax clutter that I'd like to avoid.

[lbfalvy]

This issue is an absolute pain to track down and google in a codebase of any substantial size because the problematic types are pretty much always inferred. Would it be possible to detect this situation and abort with a descriptive message so the user knows what they need to avoid?