Ensure 'static lifetime for future output

Protect against rust-lang/rust#84366

Author: timokroeger

src/backend/async_task.rs

@@ -24,7 +24,11 @@ pub fn dispatch(msg: &MSG) -> bool {
     }
 }
 
-pub fn spawn<F: Future + 'static>(future: F) -> JoinHandle<F> {
+pub fn spawn<F>(future: F) -> JoinHandle<F>
+where
+    F: Future + 'static,
+    F::Output: 'static,
+{
     // Its important to get the current thread id *outside* of the `schedule`
     // closure which can run from a different.
     let thread_id = unsafe { GetCurrentThreadId() };
@@ -48,10 +52,12 @@ pub fn spawn<F: Future + 'static>(future: F) -> JoinHandle<F> {
     }
 }
 
-fn spawn_local<F: Future + 'static>(
-    future: F,
-    schedule: impl Schedule + Send + Sync + 'static,
-) -> (Runnable, async_task::Task<F::Output>) {
+fn spawn_local<F, S>(future: F, schedule: S) -> (Runnable, async_task::Task<F::Output>)
+where
+    F: Future + 'static,
+    F::Output: 'static,
+    S: Schedule + Send + Sync + 'static,
+{
     // SAFETY: The `future` does not need to be `Send` because the thread that
     // receives the runnable is our own. All other safety properties are ensured
     // by the function signature.

src/backend/window.rs

@@ -83,7 +83,11 @@ impl<F: Future> Future for JoinHandle<F> {
     }
 }
 
-pub fn spawn<F: Future + 'static>(future: F) -> JoinHandle<F> {
+pub fn spawn<F>(future: F) -> JoinHandle<F>
+where
+    F: Future + 'static,
+    F::Output: 'static,
+{
     // Create a message only window to run the tasks.
     let window = Window::new_reentrant(true, (), |_, msg| {
         if msg.msg == MSG_ID_WAKE {

src/lib.rs

@@ -29,7 +29,11 @@ use windows_sys::Win32::{
 ///
 /// Panics when the message loops is running already. This happens when
 /// `block_on` or `run` is called from async tasks running on this executor.
-pub fn block_on<T: 'static>(future: impl Future<Output = T> + 'static) -> T {
+pub fn block_on<F>(future: F) -> F::Output
+where
+    F: Future + 'static,
+    F::Output: 'static,
+{
     // Wrap the future so it quits the message loop when finished.
     let task = backend::spawn(async move {
         let result = future.await;
@@ -116,6 +120,10 @@ pub type JoinHandle<F> = backend::JoinHandle<F>;
 /// This function may be used to spawn tasks when the message loop is not
 /// running. The provided future will start running once the message loop
 /// is entered with [`MessageLoop::block_on()`] or [`MessageLoop::run()`].
-pub fn spawn<F: Future + 'static>(future: F) -> JoinHandle<F> {
+pub fn spawn<F>(future: F) -> JoinHandle<F>
+where
+    F: Future + 'static,
+    F::Output: 'static,
+{
     backend::spawn(future)
 }