[draft] Rework the way we treat opaques in check_opaque_meets_bounds

compiler/rustc_hir_analysis/src/check/check.rs

@@ -345,7 +345,12 @@ fn check_opaque_meets_bounds<'tcx>(
     };
     let param_env = tcx.param_env(defining_use_anchor);
 
-    let infcx = tcx.infer_ctxt().with_opaque_type_inference(defining_use_anchor).build();
+    let infcx = tcx
+        .infer_ctxt()
+        .with_opaque_type_mode(ty::OpaqueTypeMode::Reveal(
+            tcx.opaque_types_defined_by(defining_use_anchor),
+        ))
+        .build();
     let ocx = ObligationCtxt::new(&infcx);
 
     let args = match *origin {
@@ -359,42 +364,23 @@ fn check_opaque_meets_bounds<'tcx>(
         }),
     };
 
-    let opaque_ty = Ty::new_opaque(tcx, def_id.to_def_id(), args);
-
-    // `ReErased` regions appear in the "parent_args" of closures/coroutines.
-    // We're ignoring them here and replacing them with fresh region variables.
-    // See tests in ui/type-alias-impl-trait/closure_{parent_args,wf_outlives}.rs.
-    //
-    // FIXME: Consider wrapping the hidden type in an existential `Binder` and instantiating it
-    // here rather than using ReErased.
-    let hidden_ty = tcx.type_of(def_id.to_def_id()).instantiate(tcx, args);
-    let hidden_ty = tcx.fold_regions(hidden_ty, |re, _dbi| match re.kind() {
-        ty::ReErased => infcx.next_region_var(RegionVariableOrigin::MiscVariable(span)),
-        _ => re,
-    });
-
     let misc_cause = traits::ObligationCause::misc(span, def_id);
-
-    match ocx.eq(&misc_cause, param_env, opaque_ty, hidden_ty) {
-        Ok(()) => {}
-        Err(ty_err) => {
-            // Some types may be left "stranded" if they can't be reached
-            // from a lowered rustc_middle bound but they're mentioned in the HIR.
-            // This will happen, e.g., when a nested opaque is inside of a non-
-            // existent associated type, like `impl Trait<Missing = impl Trait>`.
-            // See <tests/ui/impl-trait/stranded-opaque.rs>.
-            let ty_err = ty_err.to_string(tcx);
-            let guar = tcx.dcx().span_delayed_bug(
-                span,
-                format!("could not unify `{hidden_ty}` with revealed type:\n{ty_err}"),
-            );
-            return Err(guar);
-        }
-    }
+    let predicates: Vec<_> = tcx.item_bounds(def_id).iter_instantiated(tcx, args).collect();
+    let predicates = ocx.normalize(&misc_cause, param_env, predicates);
+    ocx.register_obligations(
+        predicates
+            .into_iter()
+            .map(|clause| Obligation::new(tcx, misc_cause.clone(), param_env, clause)),
+    );
 
     // Additionally 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
     // hidden type is well formed even without those bounds.
+    let hidden_ty =
+        tcx.fold_regions(tcx.type_of(def_id).instantiate(tcx, args), |re, _| match *re {
+            ty::ReErased => infcx.next_region_var(RegionVariableOrigin::MiscVariable(span)),
+            _ => re,
+        });
     let predicate =
         ty::Binder::dummy(ty::PredicateKind::Clause(ty::ClauseKind::WellFormed(hidden_ty.into())));
     ocx.register_obligation(Obligation::new(tcx, misc_cause.clone(), param_env, predicate));
@@ -412,23 +398,7 @@ fn check_opaque_meets_bounds<'tcx>(
     let outlives_env = OutlivesEnvironment::with_bounds(param_env, implied_bounds);
     ocx.resolve_regions_and_report_errors(defining_use_anchor, &outlives_env)?;
 
-    if let hir::OpaqueTyOrigin::FnReturn(..) | hir::OpaqueTyOrigin::AsyncFn(..) = origin {
-        // HACK: this should also fall through to the hidden type check below, but the original
-        // implementation had a bug where equivalent lifetimes are not identical. This caused us
-        // to reject existing stable code that is otherwise completely fine. The real fix is to
-        // compare the hidden types via our type equivalence/relation infra instead of doing an
-        // identity check.
-        let _ = infcx.take_opaque_types();
-        Ok(())
-    } else {
-        // Check that any hidden types found during wf checking match the hidden types that `type_of` sees.
-        for (mut key, mut ty) in infcx.take_opaque_types() {
-            ty.hidden_type.ty = infcx.resolve_vars_if_possible(ty.hidden_type.ty);
-            key = infcx.resolve_vars_if_possible(key);
-            sanity_check_found_hidden_type(tcx, key, ty.hidden_type)?;
-        }
-        Ok(())
-    }
+    Ok(())
 }
 
 fn sanity_check_found_hidden_type<'tcx>(

compiler/rustc_infer/src/infer/at.rs

@@ -76,7 +76,7 @@ impl<'tcx> InferCtxt<'tcx> {
     pub fn fork_with_intercrate(&self, intercrate: bool) -> Self {
         Self {
             tcx: self.tcx,
-            defining_opaque_types: self.defining_opaque_types,
+            opaque_type_mode: self.opaque_type_mode,
             considering_regions: self.considering_regions,
             skip_leak_check: self.skip_leak_check,
             inner: self.inner.clone(),

compiler/rustc_infer/src/infer/canonical/canonicalizer.rs

@@ -60,7 +60,7 @@ impl<'tcx> InferCtxt<'tcx> {
             },
         );
 
-        param_env.defining_opaque_types = self.defining_opaque_types;
+        param_env.opaque_type_mode = self.opaque_type_mode;
 
         Canonicalizer::canonicalize_with_base(
             param_env,
@@ -544,7 +544,7 @@ impl<'cx, 'tcx> Canonicalizer<'cx, 'tcx> {
             max_universe: ty::UniverseIndex::ROOT,
             variables: List::empty(),
             value: (),
-            defining_opaque_types: infcx.map(|i| i.defining_opaque_types).unwrap_or_default(),
+            opaque_type_mode: infcx.map(|i| i.opaque_type_mode).unwrap_or_default(),
         };
         Canonicalizer::canonicalize_with_base(
             base,
@@ -614,14 +614,12 @@ impl<'cx, 'tcx> Canonicalizer<'cx, 'tcx> {
             .max()
             .unwrap_or(ty::UniverseIndex::ROOT);
 
-        assert!(
-            !infcx.is_some_and(|infcx| infcx.defining_opaque_types != base.defining_opaque_types)
-        );
+        assert!(!infcx.is_some_and(|infcx| infcx.opaque_type_mode != base.opaque_type_mode));
         Canonical {
             max_universe,
             variables: canonical_variables,
             value: (base.value, out_value),
-            defining_opaque_types: base.defining_opaque_types,
+            opaque_type_mode: base.opaque_type_mode,
         }
     }
 

compiler/rustc_infer/src/infer/mod.rs

@@ -5,6 +5,8 @@ pub use relate::combine::CombineFields;
 pub use relate::combine::ObligationEmittingRelation;
 pub use relate::StructurallyRelateAliases;
 pub use rustc_macros::{TypeFoldable, TypeVisitable};
+use rustc_middle::traits::Reveal;
+use rustc_middle::traits::TreatOpaque;
 pub use rustc_middle::ty::IntVarValue;
 pub use BoundRegionConversionTime::*;
 pub use RegionVariableOrigin::*;
@@ -246,7 +248,7 @@ pub struct InferCtxt<'tcx> {
     pub tcx: TyCtxt<'tcx>,
 
     /// The `DefIds` of the opaque types that may have their hidden types constrained.
-    defining_opaque_types: &'tcx ty::List<LocalDefId>,
+    opaque_type_mode: ty::OpaqueTypeMode<TyCtxt<'tcx>>,
 
     /// Whether this inference context should care about region obligations in
     /// the root universe. Most notably, this is used during hir typeck as region
@@ -373,8 +375,8 @@ impl<'tcx> ty::InferCtxtLike for InferCtxt<'tcx> {
         self.inner.borrow_mut().unwrap_region_constraints().opportunistic_resolve_var(self.tcx, vid)
     }
 
-    fn defining_opaque_types(&self) -> &'tcx ty::List<LocalDefId> {
-        self.defining_opaque_types
+    fn opaque_type_mode(&self) -> ty::OpaqueTypeMode<TyCtxt<'tcx>> {
+        self.opaque_type_mode
     }
 
     fn opportunistic_resolve_ty_var(&self, vid: TyVid) -> Ty<'tcx> {
@@ -621,7 +623,7 @@ impl fmt::Display for FixupError {
 /// Used to configure inference contexts before their creation.
 pub struct InferCtxtBuilder<'tcx> {
     tcx: TyCtxt<'tcx>,
-    defining_opaque_types: &'tcx ty::List<LocalDefId>,
+    opaque_type_mode: ty::OpaqueTypeMode<TyCtxt<'tcx>>,
     considering_regions: bool,
     skip_leak_check: bool,
     /// Whether we are in coherence mode.
@@ -636,7 +638,7 @@ impl<'tcx> TyCtxt<'tcx> {
     fn infer_ctxt(self) -> InferCtxtBuilder<'tcx> {
         InferCtxtBuilder {
             tcx: self,
-            defining_opaque_types: ty::List::empty(),
+            opaque_type_mode: Default::default(),
             considering_regions: true,
             skip_leak_check: false,
             intercrate: false,
@@ -646,22 +648,23 @@ impl<'tcx> TyCtxt<'tcx> {
 }
 
 impl<'tcx> InferCtxtBuilder<'tcx> {
+    pub fn with_opaque_type_mode(
+        mut self,
+        opaque_type_mode: ty::OpaqueTypeMode<TyCtxt<'tcx>>,
+    ) -> Self {
+        self.opaque_type_mode = opaque_type_mode;
+        self
+    }
+
     /// Whenever the `InferCtxt` should be able to handle defining uses of opaque types,
     /// you need to call this function. Otherwise the opaque type will be treated opaquely.
     ///
     /// It is only meant to be called in two places, for typeck
     /// (via `Inherited::build`) and for the inference context used
     /// in mir borrowck.
     pub fn with_opaque_type_inference(mut self, defining_anchor: LocalDefId) -> Self {
-        self.defining_opaque_types = self.tcx.opaque_types_defined_by(defining_anchor);
-        self
-    }
-
-    pub fn with_defining_opaque_types(
-        mut self,
-        defining_opaque_types: &'tcx ty::List<LocalDefId>,
-    ) -> Self {
-        self.defining_opaque_types = defining_opaque_types;
+        let types = self.tcx.opaque_types_defined_by(defining_anchor);
+        self.opaque_type_mode = ty::OpaqueTypeMode::Define(types);
         self
     }
 
@@ -700,23 +703,23 @@ impl<'tcx> InferCtxtBuilder<'tcx> {
     where
         T: TypeFoldable<TyCtxt<'tcx>>,
     {
-        let infcx = self.with_defining_opaque_types(canonical.defining_opaque_types).build();
+        let infcx = self.with_opaque_type_mode(canonical.opaque_type_mode).build();
         let (value, args) = infcx.instantiate_canonical(span, canonical);
         (infcx, value, args)
     }
 
     pub fn build(&mut self) -> InferCtxt<'tcx> {
         let InferCtxtBuilder {
             tcx,
-            defining_opaque_types,
+            opaque_type_mode: defining_opaque_types,
             considering_regions,
             skip_leak_check,
             intercrate,
             next_trait_solver,
         } = *self;
         InferCtxt {
             tcx,
-            defining_opaque_types,
+            opaque_type_mode: defining_opaque_types,
             considering_regions,
             skip_leak_check,
             inner: RefCell::new(InferCtxtInner::new()),
@@ -1240,10 +1243,30 @@ impl<'tcx> InferCtxt<'tcx> {
         self.inner.borrow().opaque_type_storage.opaque_types.clone()
     }
 
-    #[inline(always)]
-    pub fn can_define_opaque_ty(&self, id: impl Into<DefId>) -> bool {
-        let Some(id) = id.into().as_local() else { return false };
-        self.defining_opaque_types.contains(&id)
+    pub fn treat_opaque_ty(&self, reveal: Reveal, def_id: DefId) -> TreatOpaque {
+        if self.intercrate {
+            return TreatOpaque::Ambiguous;
+        }
+
+        match reveal {
+            Reveal::All => return TreatOpaque::Reveal,
+            Reveal::UserFacing => {}
+        }
+
+        match self.opaque_type_mode {
+            ty::OpaqueTypeMode::Define(list) => {
+                if def_id.as_local().is_some_and(|def_id| list.contains(&def_id)) {
+                    return TreatOpaque::Define;
+                }
+            }
+            ty::OpaqueTypeMode::Reveal(list) => {
+                if def_id.as_local().is_some_and(|def_id| list.contains(&def_id)) {
+                    return TreatOpaque::Reveal;
+                }
+            }
+        }
+
+        TreatOpaque::Rigid
     }
 
     pub fn ty_to_string(&self, t: Ty<'tcx>) -> String {

compiler/rustc_infer/src/infer/opaque_types/mod.rs

@@ -6,7 +6,7 @@ use hir::def_id::{DefId, LocalDefId};
 use rustc_data_structures::fx::FxIndexMap;
 use rustc_data_structures::sync::Lrc;
 use rustc_hir as hir;
-use rustc_middle::traits::ObligationCause;
+use rustc_middle::traits::{ObligationCause, TreatOpaque};
 use rustc_middle::ty::error::{ExpectedFound, TypeError};
 use rustc_middle::ty::fold::BottomUpFolder;
 use rustc_middle::ty::GenericArgKind;
@@ -58,7 +58,10 @@ impl<'tcx> InferCtxt<'tcx> {
             ct_op: |ct| ct,
             ty_op: |ty| match *ty.kind() {
                 ty::Alias(ty::Opaque, ty::AliasTy { def_id, .. })
-                    if self.can_define_opaque_ty(def_id) && !ty.has_escaping_bound_vars() =>
+                    if matches!(
+                        self.treat_opaque_ty(param_env.reveal(), def_id),
+                        TreatOpaque::Define
+                    ) && !ty.has_escaping_bound_vars() =>
                 {
                     let def_span = self.tcx.def_span(def_id);
                     let span = if span.contains(def_span) { def_span } else { span };
@@ -85,16 +88,6 @@ impl<'tcx> InferCtxt<'tcx> {
     ) -> InferResult<'tcx, ()> {
         let process = |a: Ty<'tcx>, b: Ty<'tcx>| match *a.kind() {
             ty::Alias(ty::Opaque, ty::AliasTy { def_id, args, .. }) if def_id.is_local() => {
-                let def_id = def_id.expect_local();
-                if self.intercrate {
-                    // See comment on `insert_hidden_type` for why this is sufficient in coherence
-                    return Some(self.register_hidden_type(
-                        OpaqueTypeKey { def_id, args },
-                        cause.clone(),
-                        param_env,
-                        b,
-                    ));
-                }
                 // Check that this is `impl Trait` type is
                 // declared by `parent_def_id` -- i.e., one whose
                 // value we are inferring. At present, this is
@@ -129,8 +122,18 @@ impl<'tcx> InferCtxt<'tcx> {
                 //     let x = || foo(); // returns the Opaque assoc with `foo`
                 // }
                 // ```
-                if !self.can_define_opaque_ty(def_id) {
-                    return None;
+                match self.treat_opaque_ty(param_env.reveal(), def_id) {
+                    TreatOpaque::Define => {}
+                    TreatOpaque::Reveal | TreatOpaque::Rigid => return None,
+                    TreatOpaque::Ambiguous => {
+                        // See comment on `insert_hidden_type` for why this is sufficient in coherence
+                        return Some(self.register_hidden_type(
+                            OpaqueTypeKey { def_id: def_id.expect_local(), args },
+                            cause.clone(),
+                            param_env,
+                            b,
+                        ));
+                    }
                 }
 
                 if let ty::Alias(ty::Opaque, ty::AliasTy { def_id: b_def_id, .. }) = *b.kind() {
@@ -139,8 +142,10 @@ impl<'tcx> InferCtxt<'tcx> {
                     // no one encounters it in practice.
                     // It does occur however in `fn fut() -> impl Future<Output = i32> { async { 42 } }`,
                     // where it is of no concern, so we only check for TAITs.
-                    if self.can_define_opaque_ty(b_def_id)
-                        && self.tcx.is_type_alias_impl_trait(b_def_id)
+                    if matches!(
+                        self.treat_opaque_ty(param_env.reveal(), b_def_id),
+                        TreatOpaque::Define
+                    ) && self.tcx.is_type_alias_impl_trait(b_def_id)
                     {
                         self.tcx.dcx().emit_err(OpaqueHiddenTypeDiag {
                             span: cause.span,
@@ -150,7 +155,7 @@ impl<'tcx> InferCtxt<'tcx> {
                     }
                 }
                 Some(self.register_hidden_type(
-                    OpaqueTypeKey { def_id, args },
+                    OpaqueTypeKey { def_id: def_id.expect_local(), args },
                     cause.clone(),
                     param_env,
                     b,

compiler/rustc_middle/src/infer/canonical.rs

@@ -182,7 +182,7 @@ impl<'tcx> CanonicalParamEnvCache<'tcx> {
                 max_universe: ty::UniverseIndex::ROOT,
                 variables: List::empty(),
                 value: key,
-                defining_opaque_types: ty::List::empty(),
+                opaque_type_mode: Default::default(),
             };
         }
 

compiler/rustc_middle/src/query/plumbing.rs

@@ -325,7 +325,7 @@ macro_rules! define_callbacks {
                 // Increase this limit if necessary, but do try to keep the size low if possible
                 #[cfg(target_pointer_width = "64")]
                 const _: () = {
-                    if mem::size_of::<Key<'static>>() > 72 {
+                    if mem::size_of::<Key<'static>>() > 84 {
                         panic!("{}", concat!(
                             "the query `",
                             stringify!($name),

compiler/rustc_middle/src/traits/mod.rs

@@ -81,6 +81,13 @@ pub enum Reveal {
     All,
 }
 
+pub enum TreatOpaque {
+    Reveal,
+    Define,
+    Rigid,
+    Ambiguous,
+}
+
 /// The reason why we incurred this obligation; used for error reporting.
 ///
 /// Non-misc `ObligationCauseCode`s are stored on the heap. This gives the