fix: add CoroutineClosure to maybe_drop_guard type list

compiler/rustc_mir_transform/src/liveness.rs

@@ -238,6 +238,7 @@ fn maybe_drop_guard<'tcx>(
         matches!(
             ty.kind(),
             ty::Closure(..)
+                | ty::CoroutineClosure(..)
                 | ty::Coroutine(..)
                 | ty::Tuple(..)
                 | ty::Adt(..)

tests/ui/async-await/async-closures/liveness-drop-guard.rs

@@ -0,0 +1,25 @@
+// Regression test for #479: maybe_drop_guard missing CoroutineClosure in type list.
+// Before the fix, async closures holding Drop types could produce false
+// "unused variable" warnings because liveness analysis did not recognize
+// CoroutineClosure as a type that may have drop guards.
+
+//@ edition:2021
+//@ check-pass
+
+#![deny(unused_variables)]
+
+struct NeedsDrop(i32);
+
+impl Drop for NeedsDrop {
+    fn drop(&mut self) {}
+}
+
+fn main() {
+    let _ = async {
+        let d = NeedsDrop(1);
+        let c = async move || {
+            let _ = &d;
+        };
+        c().await;
+    };
+}