Infinite recursion with a defaulted, contravariant trait

[nikomatsakis]

This test fails to compile:

unsafe trait MySend {
    fn dummy(&self) { }
}

unsafe impl MySend for .. { }

unsafe impl<T: MySend + ?Sized> MySend for Unique<T> { }

pub struct Unique<T:?Sized> {
    pointer: *const T,
}

pub struct Node<V> {
    vals: V,
    edges: Unique<Node<V>>,
}

fn is_send<T: MySend>() {}

fn main() {
    is_send::<Node<&'static ()>>();
}

the output is of the form:

...
/home/nmatsakis/tmp/22655b.rs:21:5: 21:33 note: required because of the requirements on the impl of `MySend` for `Node<&()>`
/home/nmatsakis/tmp/22655b.rs:21     is_send::<Node<&'static ()>>();
                                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/nmatsakis/tmp/22655b.rs:21:5: 21:33 note: required because of the requirements on the impl of `MySend` for `Node<&()>`
/home/nmatsakis/tmp/22655b.rs:21     is_send::<Node<&'static ()>>();
                                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/nmatsakis/tmp/22655b.rs:21:5: 21:33 note: required because of the requirements on the impl of `MySend` for `Node<&()>`
/home/nmatsakis/tmp/22655b.rs:21     is_send::<Node<&'static ()>>();
                                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/nmatsakis/tmp/22655b.rs:21:5: 21:33 note: required because of the requirements on the impl of `MySend` for `Node<&()>`
/home/nmatsakis/tmp/22655b.rs:21     is_send::<Node<&'static ()>>();
                                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/nmatsakis/tmp/22655b.rs:21:5: 21:33 note: required because of the requirements on the impl of `MySend` for `Node<&()>`
/home/nmatsakis/tmp/22655b.rs:21     is_send::<Node<&'static ()>>();
                                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/nmatsakis/tmp/22655b.rs:21:5: 21:33 note: required because of the requirements on the impl of `MySend` for `Node<&()>`
/home/nmatsakis/tmp/22655b.rs:21     is_send::<Node<&'static ()>>();
                                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/nmatsakis/tmp/22655b.rs:21:5: 21:33 note: required because of the requirements on the impl of `MySend` for `Node<&()>`
/home/nmatsakis/tmp/22655b.rs:21     is_send::<Node<&'static ()>>();
                                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/nmatsakis/tmp/22655b.rs:21:5: 21:33 note: required because of the requirements on the impl of `MySend` for `Node<&()>`
/home/nmatsakis/tmp/22655b.rs:21     is_send::<Node<&'static ()>>();
                                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/nmatsakis/tmp/22655b.rs:21:5: 21:33 note: required because of the requirements on the impl of `MySend` for `Node<&()>`
/home/nmatsakis/tmp/22655b.rs:21     is_send::<Node<&'static ()>>();
                                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/nmatsakis/tmp/22655b.rs:21:5: 21:33 note: required because of the requirements on the impl of `MySend` for `Node<&()>`
/home/nmatsakis/tmp/22655b.rs:21     is_send::<Node<&'static ()>>();
                                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/nmatsakis/tmp/22655b.rs:21:5: 21:33 note: required because of the requirements on the impl of `MySend` for `Node<&()>`
/home/nmatsakis/tmp/22655b.rs:21     is_send::<Node<&'static ()>>();
                                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/nmatsakis/tmp/22655b.rs:21:5: 21:33 note: required because of the requirements on the impl of `MySend` for `Node<&()>`
/home/nmatsakis/tmp/22655b.rs:21     is_send::<Node<&'static ()>>();
                                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/nmatsakis/tmp/22655b.rs:21:5: 21:33 note: required because of the requirements on the impl of `MySend` for `Node<&()>`
/home/nmatsakis/tmp/22655b.rs:21     is_send::<Node<&'static ()>>();
                                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/nmatsakis/tmp/22655b.rs:21:5: 21:33 note: required because of the requirements on the impl of `MySend` for `Node<&'static ()>`
/home/nmatsakis/tmp/22655b.rs:21     is_send::<Node<&'static ()>>();
                                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/nmatsakis/tmp/22655b.rs:21:5: 21:33 note: required by `is_send`
/home/nmatsakis/tmp/22655b.rs:21     is_send::<Node<&'static ()>>();

This bug is branched off of #22655. It represents the underlying problem that caused #22655.

[nikomatsakis]

Fixing this bug would allow MarkerTrait to be made contravariant again, as designated by a FIXME in the source.