diff --git a/.vscode/settings.json b/.vscode/settings.json
new file mode 100644
index 0000000..503f68a
--- /dev/null
+++ b/.vscode/settings.json
@@ -0,0 +1,9 @@
+{
+    "rust-analyzer.showUnlinkedFileNotification": false,
+    "[rust]": {
+        "editor.rulers": [100]
+    },
+    "[markdown]": {
+        "editor.rulers": [86, 99, 100]
+    }
+}
diff --git a/Cargo.toml b/Cargo.toml
index d51cd24..61751ff 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -2,7 +2,7 @@
 name = "exercises"
 version = "0.1.0"
 edition = "2021"
-
-# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+publish = false
 
 [dependencies]
+futures = "0.3"
diff --git a/src/SUMMARY.md b/src/SUMMARY.md
index 4d621e0..e0c1e46 100644
--- a/src/SUMMARY.md
+++ b/src/SUMMARY.md
@@ -10,6 +10,7 @@
   - [A/A1/A2](./exercises/multiple_blanket.md)
   - [BoolStream](./exercises/bool_stream.md)
   - [RcChars](./exercises/rcchars.md)
+  - [Async Fn](./exercises/async_fn.md)
 - [Chapter 2](./chapter_2.md)
   - [AnyStr](./exercises/anystr.md)
   - [Mode](./exercises/mode.md)
diff --git a/src/exercises/async_fn.md b/src/exercises/async_fn.md
new file mode 100644
index 0000000..285de55
--- /dev/null
+++ b/src/exercises/async_fn.md
@@ -0,0 +1,28 @@
+# Async `to_string`
+
+Make this compile and pass tests:
+
+```rust,ignore,mdbook-runnable
+# extern crate futures;
+# use futures::{Future, executor::block_on};
+# trait AsyncToStringRef<'a> { type F: 'a + Future<Output = String>; fn to_string(&'a self, s: &'a str) -> Self::F; }
+# impl<'a, F: 'a + Future<Output = String>, T: Fn(&'a str) -> F> AsyncToStringRef<'a> for T {
+# type F = F;
+# fn to_string(&'a self, s: &'a str) -> Self::F { self(s) }
+# }
+# trait AsyncToString: for<'a> AsyncToStringRef<'a> {}
+# impl<T: for<'a> AsyncToStringRef<'a>> AsyncToString for T {}
+async fn test_generic(s: &str, f: impl AsyncToString) -> String {
+    f.to_string(s).await
+}
+
+async fn to_string_async(s: &str) -> String {
+    s.to_string()
+}
+
+async fn test_concrete(s: &str) -> String {
+    test_generic(s, to_string_async).await
+}
+
+assert_eq!(block_on(test_concrete("abc")), "abc".to_string())
+```
diff --git a/src/lib.rs b/src/lib.rs
index 7d12d9a..e69de29 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,14 +0,0 @@
-pub fn add(left: usize, right: usize) -> usize {
-    left + right
-}
-
-#[cfg(test)]
-mod tests {
-    use super::*;
-
-    #[test]
-    fn it_works() {
-        let result = add(2, 2);
-        assert_eq!(result, 4);
-    }
-}