Avoid query cycles in DataflowConstProp

* Avoid query cycles in DataflowConstProp
* Add -Zmir-opt-level=0 to the test

compiler/rustc_mir_transform/src/dataflow_const_prop.rs

@@ -41,6 +41,11 @@ fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
 
     #[instrument(skip_all level = "debug")]
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+        // Avoid query cycles from coroutines.
+        if body.coroutine.is_some() {
+            return;
+        }
+
         debug!(def_id = ?body.source.def_id());
         if tcx.sess.mir_opt_level() < 4 && body.basic_blocks.len() > BLOCK_LIMIT {
             debug!("aborted dataflow const prop due too many basic blocks");

tests/ui/dataflow_const_prop/recursive-async.rs

@@ -0,0 +1,11 @@
+//! Ensure DataflowConstProp doesn't cause an error with async recursion as in #155376.
+
+//@ edition:2018
+//@ check-pass
+//@ compile-flags: -Zmir-opt-level=0 -Zmir-enable-passes=+DataflowConstProp --crate-type=lib
+
+pub async fn foo(n: usize) {
+    if n > 0 {
+        Box::pin(foo(n - 1)).await;
+    }
+}