Make this work, and StreamingIterator will be able to do a lot.

twice-nothing.rs

pub trait Nothing<'a> {
    fn nothing(&'a mut self) -> ();

    fn twice_nothing(&'a mut self) -> () {
        { self.nothing(); }
        { self.nothing(); }
    }
}

// This works.

pub trait Nothing2 {
    fn nothing(&mut self) -> ();

    fn twice_nothing(&mut self) -> () {
        { self.nothing(); }
        { self.nothing(); }
    }
}

// Here's the actual type we're working with. This was found via much trial and error.

pub trait StreamingIterator<'a, T> {
    /// Return either the next item in the sequence, or `None` if all items
    /// have been consumed.
    fn next(&'a mut self) -> Option<T>;
}