Add must_use to async trait methods

src/expand.rs

@@ -75,6 +75,7 @@ pub fn expand(input: &mut Item, is_local: bool) {
                         }
                         let has_default = method.default.is_some();
                         transform_sig(context, sig, has_self, has_default, is_local);
+                        method.attrs.push(parse_quote!(#[must_use]));
                     }
                 }
             }

tests/ui/must-use.rs

@@ -0,0 +1,21 @@
+#![deny(unused_must_use)]
+
+use async_trait::async_trait;
+
+#[async_trait]
+trait Interface {
+    async fn f(&self);
+}
+
+struct Thing;
+
+#[async_trait]
+impl Interface for Thing {
+    async fn f(&self) {}
+}
+
+pub async fn f() {
+    Thing.f();
+}
+
+fn main() {}

tests/ui/must-use.stderr

@@ -0,0 +1,11 @@
+error: unused return value of `Interface::f` that must be used
+  --> $DIR/must-use.rs:18:5
+   |
+18 |     Thing.f();
+   |     ^^^^^^^^^^
+   |
+note: the lint level is defined here
+  --> $DIR/must-use.rs:1:9
+   |
+1  | #![deny(unused_must_use)]
+   |         ^^^^^^^^^^^^^^^