message: async-fn test: make run_fut more general and entirely safe
diff: #![feature(never_type)]
  -use std::{future::Future, pin::Pin, task::Poll};
  -use std::task::{Wake, Waker, Context};
  -use std::sync::Arc;
  +use std::future::Future;
  // See if we can run a basic `async fn`
  pub async fn foo(x: &u32, y: u32) -> u32 {
  @@ -47,7 +45,10 @@ async fn partial_init(x: u32) -> u32 {
  let _x: (String, !) = (String::new(), return async { x + x }.await);
  }
  -fn run_fut(mut fut: impl Future<Output=u32>, output: u32) {
  +fn run_fut<T>(fut: impl Future<Output = T>) -> T {
  + use std::sync::Arc;
  + use std::task::{Context, Poll, Wake, Waker};
  +
  struct MyWaker;
  impl Wake for MyWaker {
  fn wake(self: Arc<Self>) {
  @@ -57,16 +58,20 @@ fn run_fut(mut fut: impl Future<Output=u32>, output: u32) {
  let waker = Waker::from(Arc::new(MyWaker));
  let mut context = Context::from_waker(&waker);
  - assert_eq!(unsafe { Pin::new_unchecked(&mut fut) }.poll(&mut context), Poll::Ready(output));
  +
  + let mut pinned = Box::pin(fut);
  + loop {
  + match pinned.as_mut().poll(&mut context) {
  + Poll::Pending => continue,
  + Poll::Ready(v) => return v,
  + }
  + }
  }
  fn main() {
  let x = 5;
  - run_fut(foo(&x, 7), 31);
  -
  - run_fut(build_aggregate(1, 2, 3, 4), 10);
  -
  - run_fut(includes_never(false, 4), 16);
  -
  - run_fut(partial_init(4), 8);
  + assert_eq!(run_fut(foo(&x, 7)), 31);
  + assert_eq!(run_fut(build_aggregate(1, 2, 3, 4)), 10);
  + assert_eq!(run_fut(includes_never(false, 4)), 16);
  + assert_eq!(run_fut(partial_init(4)), 8);
  }
  