Desugar for await loops

compiler/rustc_ast_lowering/src/lib.rs

@@ -130,6 +130,7 @@ struct LoweringContext<'a, 'hir> {
     allow_try_trait: Lrc<[Symbol]>,
     allow_gen_future: Lrc<[Symbol]>,
     allow_async_iterator: Lrc<[Symbol]>,
+    allow_for_await: Lrc<[Symbol]>,
 
     /// Mapping from generics `def_id`s to TAIT generics `def_id`s.
     /// For each captured lifetime (e.g., 'a), we create a new lifetime parameter that is a generic
@@ -174,6 +175,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             } else {
                 [sym::gen_future].into()
             },
+            allow_for_await: [sym::async_iterator].into(),
             // FIXME(gen_blocks): how does `closure_track_caller`/`async_fn_track_caller`
             // interact with `gen`/`async gen` blocks
             allow_async_iterator: [sym::gen_future, sym::async_iterator].into(),