Fix is_async_fn to check FnKind::Method

clippy_utils/src/lib.rs

@@ -1901,7 +1901,11 @@ pub fn if_sequence<'tcx>(mut expr: &'tcx Expr<'tcx>) -> (Vec<&'tcx Expr<'tcx>>,
 
 /// Checks if the given function kind is an async function.
 pub fn is_async_fn(kind: FnKind<'_>) -> bool {
-    matches!(kind, FnKind::ItemFn(_, _, header) if header.asyncness == IsAsync::Async)
+    match kind {
+        FnKind::ItemFn(_, _, header) => header.asyncness == IsAsync::Async,
+        FnKind::Method(_, sig) => sig.header.asyncness == IsAsync::Async,
+        FnKind::Closure => false,
+    }
 }
 
 /// Peels away all the compiler generated code surrounding the body of an async function,