Here's one that caught me off guard:

    #[derive(Debug)]
    struct Foo<T> { pub bar: T }

    fn hmm<T>(x: T) -> Foo<T> {
        Foo { bar: x }
    }

    fn main() {
        let f:Foo<f32> = hmm(1.23);
        print!("got: {:?}\n", f);
    }

So maybe that's not crossing function boundaries, but the type for the generic "hmm" function as the rvalue is being inferred from the explicitly declared type of the lvalue. I know it's pointless to try and change anyone's mind about this, but I personally find it unsettling :-)