Introduce AsyncAppContext and simplify spawning

Now when you call spawn in various context, you pass an FnOnce that is called with an AsyncAppContext and returns a static future. This allows you to write async code similar to how our tests work, borrowing the guts of the AsyncAppContext when needed to interact, but using normal async await semantics instead of callbacks.

Co-Authored-By: Max Brunsfeld <maxbrunsfeld@gmail.com>

zed/src/editor/buffer/mod.rs

@@ -4,11 +4,9 @@ mod selection;
 mod text;
 
 pub use anchor::*;
-use futures_core::future::LocalBoxFuture;
 pub use point::*;
 use seahash::SeaHasher;
 pub use selection::*;
-use smol::future::FutureExt;
 pub use text::*;
 
 use crate::{
@@ -19,7 +17,7 @@ use crate::{
     worktree::FileHandle,
 };
 use anyhow::{anyhow, Result};
-use gpui::{Entity, ModelContext};
+use gpui::{Entity, ModelContext, Task};
 use lazy_static::lazy_static;
 use rand::prelude::*;
 use std::{
@@ -475,21 +473,23 @@ impl Buffer {
         &mut self,
         new_file: Option<FileHandle>,
         ctx: &mut ModelContext<Self>,
-    ) -> LocalBoxFuture<'static, Result<()>> {
+    ) -> Task<Result<()>> {
         let snapshot = self.snapshot();
         let version = self.version.clone();
-        if let Some(file) = new_file.as_ref().or(self.file.as_ref()) {
-            let save_task = file.save(snapshot, ctx.as_ref());
-            ctx.spawn(save_task, |me, save_result, ctx| {
-                if save_result.is_ok() {
-                    me.did_save(version, new_file, ctx);
+        let file = self.file.clone();
+        let handle = ctx.handle();
+
+        ctx.spawn(|mut ctx| async move {
+            if let Some(file) = new_file.as_ref().or(file.as_ref()) {
+                let result = ctx.read(|ctx| file.save(snapshot, ctx.as_ref())).await;
+                if result.is_ok() {
+                    handle.update(&mut ctx, |me, ctx| me.did_save(version, new_file, ctx));
                 }
-                save_result
-            })
-            .boxed_local()
-        } else {
-            async { Ok(()) }.boxed_local()
-        }
+                result
+            } else {
+                Ok(())
+            }
+        })
     }
 
     fn did_save(