fix RedundantLocals clippy caused by async and await

compiler/rustc_middle/src/lint.rs

@@ -463,3 +463,12 @@ pub fn in_external_macro(sess: &Session, span: Span) -> bool {
         ExpnKind::Macro { .. } => true, // definitely a plugin
     }
 }
+
+/// Return whether `span` is generated by `async` or `await`.
+pub fn is_from_async_await(span: Span) -> bool {
+    let expn_data = span.ctxt().outer_expn_data();
+    match expn_data.kind {
+        ExpnKind::Desugaring(DesugaringKind::Async | DesugaringKind::Await) => true,
+        _ => false,
+    }
+}