Return tasks from spawn and spawn_stream

Also, eliminate the background spawning methods. We can spawn futures
on the executor and then spawn those on the app if we need to wait for
the result of running one.

gpui/src/executor.rs

@@ -1,6 +1,6 @@
-// #[cfg(not(test))]
 use anyhow::{anyhow, Result};
 use async_task::Runnable;
+use pin_project::pin_project;
 use smol::prelude::*;
 use smol::{channel, Executor};
 use std::rc::Rc;
@@ -17,9 +17,24 @@ pub enum Foreground {
     Test(smol::LocalExecutor<'static>),
 }
 
+#[pin_project(project = ForegroundTaskProject)]
 pub enum ForegroundTask<T> {
-    Platform(async_task::Task<T>),
-    Test(smol::Task<T>),
+    Platform(#[pin] async_task::Task<T>),
+    Test(#[pin] smol::Task<T>),
+}
+
+impl<T> Future for ForegroundTask<T> {
+    type Output = T;
+
+    fn poll(
+        self: std::pin::Pin<&mut Self>,
+        ctx: &mut std::task::Context<'_>,
+    ) -> std::task::Poll<Self::Output> {
+        match self.project() {
+            ForegroundTaskProject::Platform(task) => task.poll(ctx),
+            ForegroundTaskProject::Test(task) => task.poll(ctx),
+        }
+    }
 }
 
 pub struct Background {
@@ -27,6 +42,7 @@ pub struct Background {
     _stop: channel::Sender<()>,
 }
 
+#[must_use]
 pub type BackgroundTask<T> = smol::Task<T>;
 
 impl Foreground {
@@ -69,6 +85,7 @@ impl Foreground {
     }
 }
 
+#[must_use]
 impl<T> ForegroundTask<T> {
     pub fn detach(self) {
         match self {