Split code example into individual match cases; add code for example

blog/content/second-edition/posts/12-async-await/index.md

@@ -387,13 +387,7 @@ TODO
 
 
 ```rust
-impl Future for ExampleStateMachine {
-    type Output = String; // return type of `example`
-
-    fn poll(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Self::Output> {
-        loop {
-            match self { // TODO: handle pinning
-                ExampleStateMachine::Start(state) => {
+ExampleStateMachine::Start(state) => {
     // from body of `example`
     let foo_txt_future = async_read_file("foo.txt");
     // `.await` operation
@@ -402,8 +396,10 @@ impl Future for ExampleStateMachine {
         foo_txt_future,
     };
     *self = ExampleStateMachine::WaitingOnFooTxt(state);
-                }
-                ExampleStateMachine::WaitingOnFooTxt(state) => {
+}
+```
+```rust
+ExampleStateMachine::WaitingOnFooTxt(state) => {
     match state.foo_txt_future.poll(cx) {
         Poll::Pending => return Poll::Pending,
         Poll::Ready(content) => {
@@ -422,8 +418,10 @@ impl Future for ExampleStateMachine {
             }
         }
     }
-                }
-                ExampleStateMachine::WaitingOnFooTxt(state) => {
+}
+```
+```rust
+ExampleStateMachine::WaitingOnFooTxt(state) => {
     match state.bar_txt_future.poll(cx) {
         match state.bar_txt_future.poll(cx) {
             Poll::Pending => return Poll::Pending,
@@ -434,13 +432,20 @@ impl Future for ExampleStateMachine {
             }
         }
     }
-                }
-                ExampleStateMachine::End(_) => {
+}
+```
+```rust
+ExampleStateMachine::End(_) => {
     panic!("poll called after Poll::Ready was returned");
-                }
-            }
-        }
-    }
+}
+```
+
+
+```rust
+fn example(min_len: usize) -> ExampleStateMachine {
+    ExampleStateMachine::Start(StartState {
+        min_len,
+    })
 }
 ```