Silence generated unreachable code.

src/expand.rs

@@ -321,6 +321,7 @@ fn transform_block(
                 #(#stmts)*
             };
 
+            #[allow(unreachable_code)]
             __ret
         })
     );

tests/test.rs

@@ -216,7 +216,6 @@ pub async fn test_internal_items() {
 
 pub async fn test_unimplemented() {
     #[async_trait]
-    #[allow(unreachable_code)]
     pub trait Trait {
         async fn f() {
             unimplemented!()

tests/ui/unreachable.rs

@@ -0,0 +1,20 @@
+#![deny(warnings)]
+
+use async_trait::async_trait;
+
+#[async_trait]
+pub trait Trait {
+    async fn f() {
+        unimplemented!()
+    }
+}
+
+#[async_trait]
+pub trait TraitFoo {
+    async fn f() {
+        let y = unimplemented!();
+        let z = y;
+    }
+}
+
+fn main() {}