Does not infer associated type in another associated type

[edwin0cheng]

It is related to #2592:

struct Bar;
struct Foo;

trait A {
    type OutputA;
}

impl A for Bar {
    type OutputA = Foo;
}

trait B {
    type Output;    
    fn foo() -> Self::Output;
}

impl<T:A> B for T {
    type Output = T::OutputA;
    fn foo() -> Self::Output { loop {} }
}

fn main() {
    let b = Bar::foo();  // <- b is unknown    
}

[flodiebold]

I think this is a slightly different issue than the one in #2592. This one I think is probably related to rust-lang/chalk#234; the one in #2592 is more complicated, because Chalk would need to know that integer variables can only be one of a few different types for that one.

[flodiebold]

Another case possibly related to that Chalk issue:

trait T {
    type O;
}

impl T for () {
    type O = ();
}

fn f<X, F>(_v: F)
where
    X: T,
    F: FnOnce(X::O),
{
}

fn main() {
    f::<(), _>(|_z| {});
}

[flodiebold]

Another simple case:

    let mut action_contexts: Vec<i32> = vec![1, 2, 3];
    for v in action_contexts.iter_mut() {
        dbg!(v);
    }

[flodiebold]

Example from #3548:

    let x = vec![Foo {i: 1}, Foo {i: 2}];
    if let Some(found) = x.iter().find(|x| x.i == 1) {
        found.<CURSOR_HERE>
    }