From d2e10d5cc60eab699f7ae0f11fe5be4c7ccf1b53 Mon Sep 17 00:00:00 2001
From: bors <bors@rust-lang.org>
Date: Thu, 16 Sep 2021 16:46:02 +0000
Subject: [PATCH] Auto merge of #88979 - tmiasko:no-remove-zsts-in-generators,
 r=oli-obk

Disable RemoveZsts in generators to avoid query cycles

Querying layout of a generator requires its optimized MIR. Thus
computing layout during MIR optimization of a generator might create a
query cycle. Disable RemoveZsts in generators to avoid the issue
(similar approach is used in ConstProp transform already).

Fixes #88972.
---
 compiler/rustc_mir/src/transform/remove_zsts.rs |  4 ++++
 src/test/ui/mir/remove-zsts-query-cycle.rs      | 16 ++++++++++++++++
 2 files changed, 20 insertions(+)
 create mode 100644 src/test/ui/mir/remove-zsts-query-cycle.rs

diff --git a/compiler/rustc_mir/src/transform/remove_zsts.rs b/compiler/rustc_mir/src/transform/remove_zsts.rs
index 03277b1b2e0be..5876ac22c66a7 100644
--- a/compiler/rustc_mir/src/transform/remove_zsts.rs
+++ b/compiler/rustc_mir/src/transform/remove_zsts.rs
@@ -9,6 +9,10 @@ pub struct RemoveZsts;
 
 impl<'tcx> MirPass<'tcx> for RemoveZsts {
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+        // Avoid query cycles (generators require optimized MIR for layout).
+        if tcx.type_of(body.source.def_id()).is_generator() {
+            return;
+        }
         let param_env = tcx.param_env(body.source.def_id());
         let (basic_blocks, local_decls) = body.basic_blocks_and_local_decls_mut();
         for block in basic_blocks.iter_mut() {
diff --git a/src/test/ui/mir/remove-zsts-query-cycle.rs b/src/test/ui/mir/remove-zsts-query-cycle.rs
new file mode 100644
index 0000000000000..8f93c6cadff5a
--- /dev/null
+++ b/src/test/ui/mir/remove-zsts-query-cycle.rs
@@ -0,0 +1,16 @@
+// Regression test for #88972. Used to cause a query cycle:
+//   optimized mir -> remove zsts -> layout of a generator -> optimized mir.
+//
+// edition:2018
+// compile-flags: --crate-type=lib
+// build-pass
+
+pub async fn listen() -> Result<(), std::io::Error> {
+    let f = do_async();
+    std::mem::forget(f);
+    Ok(())
+}
+
+pub async fn do_async() {
+    listen().await.unwrap()
+}