Only compute layout of opaque if coroutine is the cause of an opaque cycle

compiler/rustc_hir_analysis/src/check/check.rs

@@ -17,7 +17,7 @@ use rustc_middle::middle::stability::EvalResult;
 use rustc_middle::traits::{DefiningAnchor, ObligationCauseCode};
 use rustc_middle::ty::fold::BottomUpFolder;
 use rustc_middle::ty::layout::{LayoutError, MAX_SIMD_LANES};
-use rustc_middle::ty::util::{Discr, IntTypeExt};
+use rustc_middle::ty::util::{Discr, InspectCoroutineFields, IntTypeExt};
 use rustc_middle::ty::GenericArgKind;
 use rustc_middle::ty::{
     AdtDef, ParamEnv, RegionKind, TypeSuperVisitable, TypeVisitable, TypeVisitableExt,
@@ -232,16 +232,33 @@ pub(super) fn check_opaque_for_cycles<'tcx>(
     span: Span,
 ) -> Result<(), ErrorGuaranteed> {
     let args = GenericArgs::identity_for_item(tcx, def_id);
-    if tcx.try_expand_impl_trait_type(def_id.to_def_id(), args).is_err() {
-        let reported = opaque_type_cycle_error(tcx, def_id, span);
-        Err(reported)
-    } else if let Err(&LayoutError::Cycle(guar)) =
-        tcx.layout_of(tcx.param_env(def_id).and(Ty::new_opaque(tcx, def_id.to_def_id(), args)))
+
+    // First, try to look at any opaque expansion cycles, considering coroutine fields
+    // (even though these aren't necessarily true errors).
+    if tcx
+        .try_expand_impl_trait_type(def_id.to_def_id(), args, InspectCoroutineFields::Yes)
+        .is_err()
     {
-        Err(guar)
-    } else {
-        Ok(())
+        // Look for true opaque expansion cycles, but ignore coroutines.
+        // This will give us any true errors. Coroutines are only problematic
+        // if they cause layout computation errors.
+        if tcx
+            .try_expand_impl_trait_type(def_id.to_def_id(), args, InspectCoroutineFields::No)
+            .is_err()
+        {
+            let reported = opaque_type_cycle_error(tcx, def_id, span);
+            return Err(reported);
+        }
+
+        // And also look for cycle errors in the layout of coroutines.
+        if let Err(&LayoutError::Cycle(guar)) =
+            tcx.layout_of(tcx.param_env(def_id).and(Ty::new_opaque(tcx, def_id.to_def_id(), args)))
+        {
+            return Err(guar);
+        }
     }
+
+    Ok(())
 }
 
 /// Check that the concrete type behind `impl Trait` actually implements `Trait`.

compiler/rustc_middle/src/ty/util.rs

@@ -702,6 +702,7 @@ impl<'tcx> TyCtxt<'tcx> {
         self,
         def_id: DefId,
         args: GenericArgsRef<'tcx>,
+        inspect_coroutine_fields: InspectCoroutineFields,
     ) -> Result<Ty<'tcx>, Ty<'tcx>> {
         let mut visitor = OpaqueTypeExpander {
             seen_opaque_tys: FxHashSet::default(),
@@ -712,6 +713,7 @@ impl<'tcx> TyCtxt<'tcx> {
             check_recursion: true,
             expand_coroutines: true,
             tcx: self,
+            inspect_coroutine_fields,
         };
 
         let expanded_type = visitor.expand_opaque_ty(def_id, args).unwrap();
@@ -885,6 +887,13 @@ struct OpaqueTypeExpander<'tcx> {
     /// recursion, and 'false' otherwise to avoid unnecessary work.
     check_recursion: bool,
     tcx: TyCtxt<'tcx>,
+    inspect_coroutine_fields: InspectCoroutineFields,
+}
+
+#[derive(Copy, Clone, PartialEq, Eq, Debug)]
+pub enum InspectCoroutineFields {
+    No,
+    Yes,
 }
 
 impl<'tcx> OpaqueTypeExpander<'tcx> {
@@ -923,7 +932,20 @@ impl<'tcx> OpaqueTypeExpander<'tcx> {
         }
         let args = args.fold_with(self);
         if !self.check_recursion || self.seen_opaque_tys.insert(def_id) {
-            let expanded_ty = Ty::new_coroutine_witness(self.tcx, def_id, args);
+            let expanded_ty = match self.expanded_cache.get(&(def_id, args)) {
+                Some(expanded_ty) => *expanded_ty,
+                None => {
+                    if matches!(self.inspect_coroutine_fields, InspectCoroutineFields::Yes) {
+                        for bty in self.tcx.coroutine_hidden_types(def_id) {
+                            let hidden_ty = bty.instantiate(self.tcx, args);
+                            self.fold_ty(hidden_ty);
+                        }
+                    }
+                    let expanded_ty = Ty::new_coroutine_witness(self.tcx, def_id, args);
+                    self.expanded_cache.insert((def_id, args), expanded_ty);
+                    expanded_ty
+                }
+            };
             if self.check_recursion {
                 self.seen_opaque_tys.remove(&def_id);
             }
@@ -1495,6 +1517,7 @@ pub fn reveal_opaque_types_in_bounds<'tcx>(
         check_recursion: false,
         expand_coroutines: false,
         tcx,
+        inspect_coroutine_fields: InspectCoroutineFields::No,
     };
     val.fold_with(&mut visitor)
 }

tests/ui/type-alias-impl-trait/indirect-recursion-issue-112047.rs

@@ -1,4 +1,5 @@
 // edition: 2021
+// build-fail
 
 #![feature(impl_trait_in_assoc_type)]
 

tests/ui/type-alias-impl-trait/indirect-recursion-issue-112047.stderr

@@ -1,5 +1,5 @@
 error[E0733]: recursion in an async block requires boxing
-  --> $DIR/indirect-recursion-issue-112047.rs:21:9
+  --> $DIR/indirect-recursion-issue-112047.rs:22:9
    |
 LL |         async move { recur(self).await; }
    |         ^^^^^^^^^^^^^-----------------^^^
@@ -7,7 +7,7 @@ LL |         async move { recur(self).await; }
    |                      recursive call here
    |
 note: which leads to this async fn
-  --> $DIR/indirect-recursion-issue-112047.rs:13:1
+  --> $DIR/indirect-recursion-issue-112047.rs:14:1
    |
 LL | async fn recur(t: impl Recur) {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^