Rename await into cycle_error for the single threaded case and add some comments

src/librustc/ty/query/job.rs

@@ -91,7 +91,7 @@ pub fn new(info: QueryInfo<'tcx>, parent: Option<Lrc<QueryJob<'tcx>>>) -> Self {
     #[cfg(not(parallel_queries))]
     #[inline(never)]
     #[cold]
-    pub(super) fn await<'lcx, 'a, D: QueryDescription<'tcx>>(
+    pub(super) fn cycle_error<'lcx, 'a, D: QueryDescription<'tcx>>(
         &self,
         tcx: TyCtxt<'_, 'tcx, 'lcx>,
         span: Span,

src/librustc/ty/query/plumbing.rs

@@ -153,9 +153,13 @@ pub(super) fn try_get(
             };
             mem::drop(lock);
 
+            // If we are single-threaded we know that we have cycle error,
+            // so we just turn the errror
             #[cfg(not(parallel_queries))]
-            return job.await(tcx, span);
+            return job.cycle_error(tcx, span);
 
+            // With parallel queries we might just have to wait on some other
+            // thread
             #[cfg(parallel_queries)]
             {
                 if let Err(cycle) = job.await(tcx, span) {