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).

compiler/rustc_mir_transform/src/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() {

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()
+}