Bubble up opaque <eq> opaque operations instead of picking an order

compiler/rustc_borrowck/src/region_infer/opaque_types.rs

@@ -328,19 +328,26 @@ fn check_opaque_type_well_formed<'tcx>(
 
     // Require that the hidden type actually fulfills all the bounds of the opaque type, even without
     // the bounds that the function supplies.
-    let opaque_ty = Ty::new_opaque(tcx, def_id.to_def_id(), identity_args);
-    ocx.eq(&ObligationCause::misc(definition_span, def_id), param_env, opaque_ty, definition_ty)
-        .map_err(|err| {
-            infcx
-                .err_ctxt()
-                .report_mismatched_types(
-                    &ObligationCause::misc(definition_span, def_id),
-                    opaque_ty,
-                    definition_ty,
-                    err,
-                )
-                .emit()
-        })?;
+    let mut obligations = vec![];
+    infcx
+        .insert_hidden_type(
+            OpaqueTypeKey { def_id, args: identity_args },
+            &ObligationCause::misc(definition_span, def_id),
+            param_env,
+            definition_ty,
+            true,
+            &mut obligations,
+        )
+        .unwrap();
+    infcx.add_item_bounds_for_hidden_type(
+        def_id.to_def_id(),
+        identity_args,
+        ObligationCause::misc(definition_span, def_id),
+        param_env,
+        definition_ty,
+        &mut obligations,
+    );
+    ocx.register_obligations(obligations);
 
     // Require the hidden type to be well-formed with only the generics of the opaque type.
     // Defining use functions may have more bounds than the opaque type, which is ok, as long as the

compiler/rustc_infer/src/infer/opaque_types.rs

@@ -145,7 +145,25 @@ impl<'tcx> InferCtxt<'tcx> {
                             return None;
                         }
                     }
-                    DefiningAnchor::Bubble => {}
+                    DefiningAnchor::Bubble => {
+                        if let ty::Alias(ty::Opaque, _) = b.kind() {
+                            // In bubble mode we don't know which of the two opaque types is supposed to have the other
+                            // as a hidden type (both, none or either one of them could be in its defining scope).
+                            let predicate = ty::PredicateKind::AliasRelate(
+                                a.into(),
+                                b.into(),
+                                ty::AliasRelationDirection::Equate,
+                            );
+                            let obligation = traits::Obligation::new(
+                                self.tcx,
+                                cause.clone(),
+                                param_env,
+                                predicate,
+                            );
+                            let obligations = vec![obligation];
+                            return Some(Ok(InferOk { value: (), obligations }));
+                        }
+                    }
                     DefiningAnchor::Error => return None,
                 };
                 if let ty::Alias(ty::Opaque, ty::AliasTy { def_id: b_def_id, .. }) = *b.kind() {

compiler/rustc_trait_selection/src/traits/fulfill.rs

@@ -6,6 +6,7 @@ use rustc_infer::infer::DefineOpaqueTypes;
 use rustc_infer::traits::ProjectionCacheKey;
 use rustc_infer::traits::{PolyTraitObligation, SelectionError, TraitEngine};
 use rustc_middle::mir::interpret::ErrorHandled;
+use rustc_middle::traits::DefiningAnchor;
 use rustc_middle::ty::abstract_const::NotConstEvaluatable;
 use rustc_middle::ty::error::{ExpectedFound, TypeError};
 use rustc_middle::ty::GenericArgsRef;
@@ -623,9 +624,27 @@ impl<'a, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'tcx> {
                     }
                 }
                 ty::PredicateKind::Ambiguous => ProcessResult::Unchanged,
-                ty::PredicateKind::AliasRelate(..) => {
-                    bug!("AliasRelate is only used for new solver")
+                ty::PredicateKind::AliasRelate(..)
+                    if matches!(self.selcx.infcx.defining_use_anchor, DefiningAnchor::Bubble) =>
+                {
+                    ProcessResult::Unchanged
                 }
+                ty::PredicateKind::AliasRelate(a, b, relate) => match relate {
+                    ty::AliasRelationDirection::Equate => match self
+                        .selcx
+                        .infcx
+                        .at(&obligation.cause, obligation.param_env)
+                        .eq(DefineOpaqueTypes::Yes, a, b)
+                    {
+                        Ok(inf_ok) => ProcessResult::Changed(mk_pending(inf_ok.into_obligations())),
+                        Err(_) => ProcessResult::Error(FulfillmentErrorCode::CodeSelectionError(
+                            SelectionError::Unimplemented,
+                        )),
+                    },
+                    ty::AliasRelationDirection::Subtype => {
+                        bug!("AliasRelate with subtyping is only used for new solver")
+                    }
+                },
                 ty::PredicateKind::Clause(ty::ClauseKind::ConstArgHasType(ct, ty)) => {
                     match self.selcx.infcx.at(&obligation.cause, obligation.param_env).eq(
                         DefineOpaqueTypes::No,

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -37,6 +37,7 @@ use rustc_infer::infer::LateBoundRegionConversionTime;
 use rustc_infer::traits::TraitObligation;
 use rustc_middle::dep_graph::{DepKind, DepNodeIndex};
 use rustc_middle::mir::interpret::ErrorHandled;
+use rustc_middle::traits::DefiningAnchor;
 use rustc_middle::ty::abstract_const::NotConstEvaluatable;
 use rustc_middle::ty::fold::BottomUpFolder;
 use rustc_middle::ty::relate::TypeRelation;
@@ -960,9 +961,27 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                         }
                     }
                 }
-                ty::PredicateKind::AliasRelate(..) => {
-                    bug!("AliasRelate is only used for new solver")
+                ty::PredicateKind::AliasRelate(..)
+                    if matches!(self.infcx.defining_use_anchor, DefiningAnchor::Bubble) =>
+                {
+                    Ok(EvaluatedToAmbig)
                 }
+                ty::PredicateKind::AliasRelate(a, b, relate) => match relate {
+                    ty::AliasRelationDirection::Equate => match self
+                        .infcx
+                        .at(&obligation.cause, obligation.param_env)
+                        .eq(DefineOpaqueTypes::Yes, a, b)
+                    {
+                        Ok(inf_ok) => self.evaluate_predicates_recursively(
+                            previous_stack,
+                            inf_ok.into_obligations(),
+                        ),
+                        Err(_) => Ok(EvaluatedToErr),
+                    },
+                    ty::AliasRelationDirection::Subtype => {
+                        bug!("AliasRelate subtyping is only used for new solver")
+                    }
+                },
                 ty::PredicateKind::Ambiguous => Ok(EvaluatedToAmbig),
                 ty::PredicateKind::Clause(ty::ClauseKind::ConstArgHasType(ct, ty)) => {
                     match self.infcx.at(&obligation.cause, obligation.param_env).eq(

tests/ui/impl-trait/async_scope_creep.rs

@@ -1,6 +1,7 @@
 #![feature(type_alias_impl_trait)]
 // edition:2021
-// check-pass
+//[rpit] check-pass
+// revisions: tait rpit
 
 struct Pending {}
 
@@ -12,15 +13,23 @@ impl AsyncRead for i32 {}
 
 type PendingReader<'a> = impl AsyncRead + 'a;
 
-type OpeningReadFuture<'a> =
-    impl std::future::Future<Output = Result<PendingReader<'a>, CantOpen>>;
+#[cfg(tait)]
+type OpeningReadFuture<'a> = impl std::future::Future<Output = Result<PendingReader<'a>, CantOpen>>;
 
 impl Pending {
     async fn read(&mut self) -> Result<impl AsyncRead + '_, CantOpen> {
         Ok(42)
     }
 
+    #[cfg(tait)]
     fn read_fut(&mut self) -> OpeningReadFuture<'_> {
+        self.read() //[tait]~ ERROR: cannot satisfy `impl AsyncRead + 'a == PendingReader<'a>`
+    }
+
+    #[cfg(rpit)]
+    fn read_fut(
+        &mut self,
+    ) -> impl std::future::Future<Output = Result<PendingReader<'_>, CantOpen>> {
         self.read()
     }
 }

tests/ui/impl-trait/async_scope_creep.tait.stderr

@@ -0,0 +1,9 @@
+error[E0284]: type annotations needed: cannot satisfy `impl AsyncRead + 'a == PendingReader<'a>`
+  --> $DIR/async_scope_creep.rs:26:9
+   |
+LL |         self.read()
+   |         ^^^^^^^^^^^ cannot satisfy `impl AsyncRead + 'a == PendingReader<'a>`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0284`.

tests/ui/type-alias-impl-trait/rpit_tait_equality_in_canonical_query.rs

@@ -0,0 +1,25 @@
+//! This tries to prove the APIT's bounds in a canonical query,
+//! which doesn't know anything about the defining scope of either
+//! opaque type and thus makes a random choice as to which opaque type
+//! becomes the hidden type of the other. When we leave the canonical
+//! query, we attempt to actually check the defining anchor, but now we
+//! have a situation where the RPIT gets constrained outside its anchor.
+
+// revisions: current next
+//[next] compile-flags: -Ztrait-solver=next
+// check-pass
+
+#![feature(type_alias_impl_trait)]
+
+type Opaque = impl Sized;
+
+fn get_rpit() -> impl Clone {}
+
+fn query(_: impl FnOnce() -> Opaque) {}
+
+fn test() -> Opaque {
+    query(get_rpit);
+    get_rpit()
+}
+
+fn main() {}