async_trait

Author: sakex

src/SUMMARY.md

@@ -241,8 +241,9 @@
     - [Join](async/control-flow/join_all.md)
     - [Select](async/control-flow/select.md)
   - [Pitfalls](async/pitfalls.md)
-    - [Blocking the executor](async/pitfalls/blocking-executor.md)
+    - [Blocking the Executor](async/pitfalls/blocking-executor.md)
     - [Pin](async/pitfalls/pin.md)
+    - [Async Traits](async/pitfalls/pin.md)
 - [Exercises](exercises/day-4/async.md)
 
 # Final Words

src/async/pitfalls.md

@@ -4,5 +4,6 @@ Async / await provides convenient and efficient abstraction for concurrent async
 
 ---
 
-- [Blocking the executor](pitfalls/blocking-executor.md)
+- [Blocking the Executor](pitfalls/blocking-executor.md)
 - [Pin](pitfalls/pin.md)
+- [Async Traits](pitfall/async-traits.md)

src/async/pitfalls/async-traits.md

@@ -0,0 +1,49 @@
+# Async Traits
+
+Async methods in traits are not yet supported in the stable channel ([An experimental feature exists in nightly and should be stabilized in the mid term.](https://blog.rust-lang.org/inside-rust/2022/11/17/async-fn-in-trait-nightly.html))
+
+The crate [async_trait](https://docs.rs/async-trait/latest/async_trait/) provides a workaround through a macro:
+
+```rust,editable,compile_fail
+use async_trait::async_trait;
+use tokio::time::{sleep, Duration};
+
+#[async_trait]
+trait Sleeper {
+    async fn sleep(&self);
+}
+
+struct FixedSleeper {
+    sleep_ms: u64,
+}
+
+#[async_trait]
+impl Sleeper for FixedSleeper {
+    async fn sleep(&self) {
+        sleep(Duration::from_millis(self.sleep_ms)).await;
+    }
+}
+
+async fn run_all_sleepers_multiple_times(sleepers: Vec<Box<dyn Sleeper>>, n_times: usize) {
+    for _ in 0..n_times {
+        for sleeper in &sleepers {
+            sleeper.sleep().await;
+        }
+    }
+}
+
+#[tokio::main]
+async fn main() {
+    let sleepers: Vec<Box<dyn Sleeper>> = vec![
+        Box::new(FixedSleeper { sleep_ms: 50 }),
+        Box::new(FixedSleeper { sleep_ms: 100 }),
+    ];
+    run_all_sleepers_multiple_times(sleepers, 5).await;
+}
+```
+
+<details>
+* Try creating a new sleeper struct that will sleep for a random amount of time and adding it to the Vec.
+* Try making the `sleep` call mutable.
+* Try adding an associated type for the return value that would return how much time was actually slept.
+</details>