noop_waker_ref is unsound

[goffrie]

Demo which segfaults in playground:

use futures::task::noop_waker_ref;
fn main() {
    let waker = std::thread::spawn(|| noop_waker_ref()).join().unwrap();
    waker.wake_by_ref();
}

noop_waker_ref() returns a 'static reference into TLS, but Waker is Sync so &'static Waker is Send, allowing the reference to outlive the thread it came from.

It should use something more like a lazy_static.

[Nemo157]

@goffrie thanks, opened #2095 which should fix this.