Fix UB due to missing 'static on task::waker

[Darksonn]

The futures::task::waker function currently allows you to transmute an Arc<T> into a Waker, even if T is not 'static. This can cause an use-after-free.

use std::sync::Arc;
use futures::task::{waker, ArcWake};

struct MyRef<'a> {
    a: &'a str,
}

impl<'a> ArcWake for MyRef<'a> {
    fn wake_by_ref(arc_self: &Arc<Self>) {
        println!("{}", arc_self.a);
    }
}

fn main() {
    let string = "Hello World!".to_string();
    let waker = waker(Arc::new(MyRef { a: &string }));
    drop(string);
    waker.wake(); // This causes an use-after-free of the string.
}

playground

[taiki-e]

Thanks!