Revert "Auto merge of #93893 - oli-obk:sad_revert, r=oli-obk"

This reverts commit 6499c5e7fc173a3f55b7a3bd1e6a50e9edef782d, reversing changes made to 78450d2d602b06d9b94349aaf8cece1a4acaf3a8.
author: Oli Scherer <git-spam-no-reply9815368754983@oli-obk.de> 2022-02-14 16:10:22 +0000
committer: Oli Scherer <git-spam-no-reply9815368754983@oli-obk.de> 2022-03-28 16:27:14 +0000
commit: 264cd05b16198c2838ec6cd585bd122491e3f7a9 (patch)
tree: 133ca794d093e515a783cd92db831ff51e197820 /compiler
parent: 0e4524e5b4a9c5656fef69f532f96eb9959a1803 (diff)
download: rust-264cd05b16198c2838ec6cd585bd122491e3f7a9.tar.gz
rust-264cd05b16198c2838ec6cd585bd122491e3f7a9.zip
77 files changed, 1327 insertions, 875 deletions
diff --git a/compiler/rustc_borrowck/src/diagnostics/bound_region_errors.rs b/compiler/rustc_borrowck/src/diagnostics/bound_region_errors.rs
index 7507ea76ed0..8601fbe27f3 100644
--- a/compiler/rustc_borrowck/src/diagnostics/bound_region_errors.rs
+++ b/compiler/rustc_borrowck/src/diagnostics/bound_region_errors.rs
@@ -2,9 +2,13 @@ use rustc_errors::{DiagnosticBuilder, ErrorGuaranteed};
 use rustc_infer::infer::canonical::Canonical;
 use rustc_infer::infer::error_reporting::nice_region_error::NiceRegionError;
 use rustc_infer::infer::region_constraints::Constraint;
+use rustc_infer::infer::region_constraints::RegionConstraintData;
+use rustc_infer::infer::RegionVariableOrigin;
 use rustc_infer::infer::{InferCtxt, RegionResolutionError, SubregionOrigin, TyCtxtInferExt as _};
 use rustc_infer::traits::{Normalized, ObligationCause, TraitEngine, TraitEngineExt};
 use rustc_middle::ty::error::TypeError;
+use rustc_middle::ty::RegionVid;
+use rustc_middle::ty::UniverseIndex;
 use rustc_middle::ty::{self, Ty, TyCtxt, TypeFoldable};
 use rustc_span::Span;
 use rustc_trait_selection::traits::query::type_op;
@@ -76,6 +80,15 @@ crate trait ToUniverseInfo<'tcx> {
     fn to_universe_info(self, base_universe: ty::UniverseIndex) -> UniverseInfo<'tcx>;
 }
 
+impl<'tcx> ToUniverseInfo<'tcx> for crate::type_check::InstantiateOpaqueType<'tcx> {
+    fn to_universe_info(self, base_universe: ty::UniverseIndex) -> UniverseInfo<'tcx> {
+        UniverseInfo(UniverseInfoInner::TypeOp(Rc::new(crate::type_check::InstantiateOpaqueType {
+            base_universe: Some(base_universe),
+            ..self
+        })))
+    }
+}
+
 impl<'tcx> ToUniverseInfo<'tcx>
     for Canonical<'tcx, ty::ParamEnvAnd<'tcx, type_op::prove_predicate::ProvePredicate<'tcx>>>
 {
@@ -116,6 +129,12 @@ impl<'tcx, F, G> ToUniverseInfo<'tcx> for Canonical<'tcx, type_op::custom::Custo
     }
 }
 
+impl<'tcx> ToUniverseInfo<'tcx> for ! {
+    fn to_universe_info(self, _base_universe: ty::UniverseIndex) -> UniverseInfo<'tcx> {
+        self
+    }
+}
+
 #[allow(unused_lifetimes)]
 trait TypeOpInfo<'tcx> {
     /// Returns an error to be reported if rerunning the type op fails to
@@ -130,7 +149,7 @@ trait TypeOpInfo<'tcx> {
 
     fn nice_error(
         &self,
-        tcx: TyCtxt<'tcx>,
+        mbcx: &mut MirBorrowckCtxt<'_, 'tcx>,
         cause: ObligationCause<'tcx>,
         placeholder_region: ty::Region<'tcx>,
         error_region: Option<ty::Region<'tcx>>,
@@ -175,7 +194,7 @@ trait TypeOpInfo<'tcx> {
         debug!(?placeholder_region);
 
         let span = cause.span;
-        let nice_error = self.nice_error(tcx, cause, placeholder_region, error_region);
+        let nice_error = self.nice_error(mbcx, cause, placeholder_region, error_region);
 
         if let Some(nice_error) = nice_error {
             mbcx.buffer_error(nice_error);
@@ -208,16 +227,16 @@ impl<'tcx> TypeOpInfo<'tcx> for PredicateQuery<'tcx> {
 
     fn nice_error(
         &self,
-        tcx: TyCtxt<'tcx>,
+        mbcx: &mut MirBorrowckCtxt<'_, 'tcx>,
         cause: ObligationCause<'tcx>,
         placeholder_region: ty::Region<'tcx>,
         error_region: Option<ty::Region<'tcx>>,
     ) -> Option<DiagnosticBuilder<'tcx, ErrorGuaranteed>> {
-        tcx.infer_ctxt().enter_with_canonical(
+        mbcx.infcx.tcx.infer_ctxt().enter_with_canonical(
             cause.span,
             &self.canonical_query,
             |ref infcx, key, _| {
-                let mut fulfill_cx = <dyn TraitEngine<'_>>::new(tcx);
+                let mut fulfill_cx = <dyn TraitEngine<'_>>::new(infcx.tcx);
                 type_op_prove_predicate_with_cause(infcx, &mut *fulfill_cx, key, cause);
                 try_extract_error_from_fulfill_cx(
                     fulfill_cx,
@@ -255,16 +274,16 @@ where
 
     fn nice_error(
         &self,
-        tcx: TyCtxt<'tcx>,
+        mbcx: &mut MirBorrowckCtxt<'_, 'tcx>,
         cause: ObligationCause<'tcx>,
         placeholder_region: ty::Region<'tcx>,
         error_region: Option<ty::Region<'tcx>>,
     ) -> Option<DiagnosticBuilder<'tcx, ErrorGuaranteed>> {
-        tcx.infer_ctxt().enter_with_canonical(
+        mbcx.infcx.tcx.infer_ctxt().enter_with_canonical(
             cause.span,
             &self.canonical_query,
             |ref infcx, key, _| {
-                let mut fulfill_cx = <dyn TraitEngine<'_>>::new(tcx);
+                let mut fulfill_cx = <dyn TraitEngine<'_>>::new(infcx.tcx);
 
                 let mut selcx = SelectionContext::new(infcx);
 
@@ -316,16 +335,16 @@ impl<'tcx> TypeOpInfo<'tcx> for AscribeUserTypeQuery<'tcx> {
 
     fn nice_error(
         &self,
-        tcx: TyCtxt<'tcx>,
+        mbcx: &mut MirBorrowckCtxt<'_, 'tcx>,
         cause: ObligationCause<'tcx>,
         placeholder_region: ty::Region<'tcx>,
         error_region: Option<ty::Region<'tcx>>,
     ) -> Option<DiagnosticBuilder<'tcx, ErrorGuaranteed>> {
-        tcx.infer_ctxt().enter_with_canonical(
+        mbcx.infcx.tcx.infer_ctxt().enter_with_canonical(
             cause.span,
             &self.canonical_query,
             |ref infcx, key, _| {
-                let mut fulfill_cx = <dyn TraitEngine<'_>>::new(tcx);
+                let mut fulfill_cx = <dyn TraitEngine<'_>>::new(infcx.tcx);
                 type_op_ascribe_user_type_with_span(infcx, &mut *fulfill_cx, key, Some(cause.span))
                     .ok()?;
                 try_extract_error_from_fulfill_cx(
@@ -339,6 +358,43 @@ impl<'tcx> TypeOpInfo<'tcx> for AscribeUserTypeQuery<'tcx> {
     }
 }
 
+impl<'tcx> TypeOpInfo<'tcx> for crate::type_check::InstantiateOpaqueType<'tcx> {
+    fn fallback_error(
+        &self,
+        tcx: TyCtxt<'tcx>,
+        span: Span,
+    ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
+        // FIXME: This error message isn't great, but it doesn't show up in the existing UI tests,
+        // and is only the fallback when the nice error fails. Consider improving this some more.
+        tcx.sess.struct_span_err(span, "higher-ranked lifetime error for opaque type!")
+    }
+
+    fn base_universe(&self) -> ty::UniverseIndex {
+        self.base_universe.unwrap()
+    }
+
+    fn nice_error(
+        &self,
+        mbcx: &mut MirBorrowckCtxt<'_, 'tcx>,
+        _cause: ObligationCause<'tcx>,
+        placeholder_region: ty::Region<'tcx>,
+        error_region: Option<ty::Region<'tcx>>,
+    ) -> Option<DiagnosticBuilder<'tcx, ErrorGuaranteed>> {
+        try_extract_error_from_region_constraints(
+            mbcx.infcx,
+            placeholder_region,
+            error_region,
+            self.region_constraints.as_ref().unwrap(),
+            // We're using the original `InferCtxt` that we
+            // started MIR borrowchecking with, so the region
+            // constraints have already been taken. Use the data from
+            // our `mbcx` instead.
+            |vid| mbcx.regioncx.var_infos[vid].origin,
+            |vid| mbcx.regioncx.var_infos[vid].universe,
+        )
+    }
+}
+
 #[instrument(skip(fulfill_cx, infcx), level = "debug")]
 fn try_extract_error_from_fulfill_cx<'tcx>(
     mut fulfill_cx: Box<dyn TraitEngine<'tcx> + 'tcx>,
@@ -346,15 +402,30 @@ fn try_extract_error_from_fulfill_cx<'tcx>(
     placeholder_region: ty::Region<'tcx>,
     error_region: Option<ty::Region<'tcx>>,
 ) -> Option<DiagnosticBuilder<'tcx, ErrorGuaranteed>> {
-    let tcx = infcx.tcx;
-
     // We generally shouldn't have errors here because the query was
     // already run, but there's no point using `delay_span_bug`
     // when we're going to emit an error here anyway.
     let _errors = fulfill_cx.select_all_or_error(infcx);
+    let region_constraints = infcx.with_region_constraints(|r| r.clone());
+    try_extract_error_from_region_constraints(
+        infcx,
+        placeholder_region,
+        error_region,
+        &region_constraints,
+        |vid| infcx.region_var_origin(vid),
+        |vid| infcx.universe_of_region(infcx.tcx.mk_region(ty::ReVar(vid))),
+    )
+}
 
-    let (sub_region, cause) = infcx.with_region_constraints(|region_constraints| {
-        debug!("{:#?}", region_constraints);
+fn try_extract_error_from_region_constraints<'tcx>(
+    infcx: &InferCtxt<'_, 'tcx>,
+    placeholder_region: ty::Region<'tcx>,
+    error_region: Option<ty::Region<'tcx>>,
+    region_constraints: &RegionConstraintData<'tcx>,
+    mut region_var_origin: impl FnMut(RegionVid) -> RegionVariableOrigin,
+    mut universe_of_region: impl FnMut(RegionVid) -> UniverseIndex,
+) -> Option<DiagnosticBuilder<'tcx, ErrorGuaranteed>> {
+    let (sub_region, cause) =
         region_constraints.constraints.iter().find_map(|(constraint, cause)| {
             match *constraint {
                 Constraint::RegSubReg(sub, sup) if sup == placeholder_region && sup != sub => {
@@ -362,12 +433,11 @@ fn try_extract_error_from_fulfill_cx<'tcx>(
                 }
                 // FIXME: Should this check the universe of the var?
                 Constraint::VarSubReg(vid, sup) if sup == placeholder_region => {
-                    Some((tcx.mk_region(ty::ReVar(vid)), cause.clone()))
+                    Some((infcx.tcx.mk_region(ty::ReVar(vid)), cause.clone()))
                 }
                 _ => None,
             }
-        })
-    })?;
+        })?;
 
     debug!(?sub_region, "cause = {:#?}", cause);
     let nice_error = match (error_region, *sub_region) {
@@ -375,7 +445,7 @@ fn try_extract_error_from_fulfill_cx<'tcx>(
             infcx,
             RegionResolutionError::SubSupConflict(
                 vid,
-                infcx.region_var_origin(vid),
+                region_var_origin(vid),
                 cause.clone(),
                 error_region,
                 cause.clone(),
@@ -392,8 +462,8 @@ fn try_extract_error_from_fulfill_cx<'tcx>(
             infcx,
             RegionResolutionError::UpperBoundUniverseConflict(
                 vid,
-                infcx.region_var_origin(vid),
-                infcx.universe_of_region(sub_region),
+                region_var_origin(vid),
+                universe_of_region(vid),
                 cause.clone(),
                 placeholder_region,
             ),
diff --git a/compiler/rustc_borrowck/src/lib.rs b/compiler/rustc_borrowck/src/lib.rs
index 6f78c816093..58b246ec873 100644
--- a/compiler/rustc_borrowck/src/lib.rs
+++ b/compiler/rustc_borrowck/src/lib.rs
@@ -7,6 +7,7 @@
 #![feature(let_chains)]
 #![feature(let_else)]
 #![feature(min_specialization)]
+#![feature(never_type)]
 #![feature(stmt_expr_attributes)]
 #![feature(trusted_step)]
 #![feature(try_blocks)]
@@ -125,8 +126,9 @@ fn mir_borrowck<'tcx>(
 ) -> &'tcx BorrowCheckResult<'tcx> {
     let (input_body, promoted) = tcx.mir_promoted(def);
     debug!("run query mir_borrowck: {}", tcx.def_path_str(def.did.to_def_id()));
+    let hir_owner = tcx.hir().local_def_id_to_hir_id(def.did).owner;
 
-    let opt_closure_req = tcx.infer_ctxt().with_opaque_type_inference(def.did).enter(|infcx| {
+    let opt_closure_req = tcx.infer_ctxt().with_opaque_type_inference(hir_owner).enter(|infcx| {
         let input_body: &Body<'_> = &input_body.borrow();
         let promoted: &IndexVec<_, _> = &promoted.borrow();
         do_mir_borrowck(&infcx, input_body, promoted, false).0
@@ -141,7 +143,7 @@ fn mir_borrowck<'tcx>(
 /// If `return_body_with_facts` is true, then return the body with non-erased
 /// region ids on which the borrow checking was performed together with Polonius
 /// facts.
-#[instrument(skip(infcx, input_body, input_promoted), level = "debug")]
+#[instrument(skip(infcx, input_body, input_promoted), fields(id=?input_body.source.with_opt_param().as_local().unwrap()), level = "debug")]
 fn do_mir_borrowck<'a, 'tcx>(
     infcx: &InferCtxt<'a, 'tcx>,
     input_body: &Body<'tcx>,
diff --git a/compiler/rustc_borrowck/src/region_infer/mod.rs b/compiler/rustc_borrowck/src/region_infer/mod.rs
index 20f54d04777..1f81f7dda13 100644
--- a/compiler/rustc_borrowck/src/region_infer/mod.rs
+++ b/compiler/rustc_borrowck/src/region_infer/mod.rs
@@ -45,6 +45,7 @@ mod reverse_sccs;
 pub mod values;
 
 pub struct RegionInferenceContext<'tcx> {
+    pub var_infos: VarInfos,
     /// Contains the definition for every region variable. Region
     /// variables are identified by their index (`RegionVid`). The
     /// definition contains information about where the region came
@@ -267,7 +268,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
     ) -> Self {
         // Create a RegionDefinition for each inference variable.
         let definitions: IndexVec<_, _> = var_infos
-            .into_iter()
+            .iter()
             .map(|info| RegionDefinition::new(info.universe, info.origin))
             .collect();
 
@@ -292,6 +293,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
             Rc::new(member_constraints_in.into_mapped(|r| constraint_sccs.scc(r)));
 
         let mut result = Self {
+            var_infos,
             definitions,
             liveness_constraints,
             constraints,
diff --git a/compiler/rustc_borrowck/src/region_infer/opaque_types.rs b/compiler/rustc_borrowck/src/region_infer/opaque_types.rs
index 60c48d8a764..a2378802802 100644
--- a/compiler/rustc_borrowck/src/region_infer/opaque_types.rs
+++ b/compiler/rustc_borrowck/src/region_infer/opaque_types.rs
@@ -1,7 +1,6 @@
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::vec_map::VecMap;
 use rustc_hir::OpaqueTyOrigin;
-use rustc_infer::infer::opaque_types::OpaqueTypeDecl;
 use rustc_infer::infer::InferCtxt;
 use rustc_middle::ty::subst::GenericArgKind;
 use rustc_middle::ty::{self, OpaqueTypeKey, Ty, TyCtxt, TypeFoldable};
@@ -54,27 +53,44 @@ impl<'tcx> RegionInferenceContext<'tcx> {
     pub(crate) fn infer_opaque_types(
         &self,
         infcx: &InferCtxt<'_, 'tcx>,
-        opaque_ty_decls: VecMap<OpaqueTypeKey<'tcx>, OpaqueTypeDecl<'tcx>>,
+        opaque_ty_decls: VecMap<OpaqueTypeKey<'tcx>, (Ty<'tcx>, Span, OpaqueTyOrigin)>,
         span: Span,
     ) -> VecMap<OpaqueTypeKey<'tcx>, Ty<'tcx>> {
         opaque_ty_decls
             .into_iter()
-            .filter_map(|(opaque_type_key, decl)| {
+            .map(|(opaque_type_key, (concrete_type, decl_span, origin))| {
                 let substs = opaque_type_key.substs;
-                let concrete_type = decl.concrete_ty;
+                // FIXME: why are the spans in decl_span often DUMMY_SP?
+                let span = decl_span.substitute_dummy(span);
                 debug!(?concrete_type, ?substs);
 
                 let mut subst_regions = vec![self.universal_regions.fr_static];
                 let universal_substs = infcx.tcx.fold_regions(substs, &mut false, |region, _| {
-                    let vid = self.universal_regions.to_region_vid(region);
-                    subst_regions.push(vid);
-                    self.definitions[vid].external_name.unwrap_or_else(|| {
-                        infcx
-                            .tcx
-                            .sess
-                            .delay_span_bug(span, "opaque type with non-universal region substs");
-                        infcx.tcx.lifetimes.re_static
-                    })
+                    if let ty::RePlaceholder(..) = region.kind() {
+                        // Higher kinded regions don't need remapping, they don't refer to anything outside of this the substs.
+                        return region;
+                    }
+                    let vid = self.to_region_vid(region);
+                    trace!(?vid);
+                    let scc = self.constraint_sccs.scc(vid);
+                    trace!(?scc);
+                    match self.scc_values.universal_regions_outlived_by(scc).find_map(|lb| {
+                        self.eval_equal(vid, lb).then_some(self.definitions[lb].external_name?)
+                    }) {
+                        Some(region) => {
+                            let vid = self.universal_regions.to_region_vid(region);
+                            subst_regions.push(vid);
+                            region
+                        }
+                        None => {
+                            subst_regions.push(vid);
+                            infcx.tcx.sess.delay_span_bug(
+                                span,
+                                "opaque type with non-universal region substs",
+                            );
+                            infcx.tcx.lifetimes.re_static
+                        }
+                    }
                 });
 
                 subst_regions.sort();
@@ -100,12 +116,14 @@ impl<'tcx> RegionInferenceContext<'tcx> {
                     span,
                 );
 
-                check_opaque_type_parameter_valid(
-                    infcx.tcx,
+                (
                     opaque_type_key,
-                    OpaqueTypeDecl { concrete_ty: remapped_type, ..decl },
+                    if check_opaque_type_parameter_valid(infcx.tcx, opaque_type_key, origin, span) {
+                        remapped_type
+                    } else {
+                        infcx.tcx.ty_error()
+                    },
                 )
-                .then_some((opaque_type_key, remapped_type))
             })
             .collect()
     }
@@ -149,9 +167,10 @@ impl<'tcx> RegionInferenceContext<'tcx> {
 fn check_opaque_type_parameter_valid(
     tcx: TyCtxt<'_>,
     opaque_type_key: OpaqueTypeKey<'_>,
-    decl: OpaqueTypeDecl<'_>,
+    origin: OpaqueTyOrigin,
+    span: Span,
 ) -> bool {
-    match decl.origin {
+    match origin {
         // No need to check return position impl trait (RPIT)
         // because for type and const parameters they are correct
         // by construction: we convert
@@ -177,7 +196,6 @@ fn check_opaque_type_parameter_valid(
         // Check these
         OpaqueTyOrigin::TyAlias => {}
     }
-    let span = decl.definition_span;
     let opaque_generics = tcx.generics_of(opaque_type_key.def_id);
     let mut seen_params: FxHashMap<_, Vec<_>> = FxHashMap::default();
     for (i, arg) in opaque_type_key.substs.iter().enumerate() {
diff --git a/compiler/rustc_borrowck/src/type_check/canonical.rs b/compiler/rustc_borrowck/src/type_check/canonical.rs
index 0fa72ed8241..3856b7f4a4b 100644
--- a/compiler/rustc_borrowck/src/type_check/canonical.rs
+++ b/compiler/rustc_borrowck/src/type_check/canonical.rs
@@ -33,12 +33,11 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
     ) -> Fallible<R>
     where
         Op: type_op::TypeOp<'tcx, Output = R>,
-        Canonical<'tcx, Op>: ToUniverseInfo<'tcx>,
+        Op::ErrorInfo: ToUniverseInfo<'tcx>,
     {
         let old_universe = self.infcx.universe();
 
-        let TypeOpOutput { output, constraints, canonicalized_query } =
-            op.fully_perform(self.infcx)?;
+        let TypeOpOutput { output, constraints, error_info } = op.fully_perform(self.infcx)?;
 
         if let Some(data) = &constraints {
             self.push_region_constraints(locations, category, data);
@@ -47,8 +46,8 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
         let universe = self.infcx.universe();
 
         if old_universe != universe {
-            let universe_info = match canonicalized_query {
-                Some(canonicalized_query) => canonicalized_query.to_universe_info(old_universe),
+            let universe_info = match error_info {
+                Some(error_info) => error_info.to_universe_info(old_universe),
                 None => UniverseInfo::other(),
             };
             for u in old_universe..universe {
diff --git a/compiler/rustc_borrowck/src/type_check/free_region_relations.rs b/compiler/rustc_borrowck/src/type_check/free_region_relations.rs
index 2e7798beb64..0b9d0a0e922 100644
--- a/compiler/rustc_borrowck/src/type_check/free_region_relations.rs
+++ b/compiler/rustc_borrowck/src/type_check/free_region_relations.rs
@@ -267,7 +267,7 @@ impl<'tcx> UniversalRegionRelationsBuilder<'_, 'tcx> {
                         TypeOpOutput {
                             output: self.infcx.tcx.ty_error(),
                             constraints: None,
-                            canonicalized_query: None,
+                            error_info: None,
                         }
                     });
                 // Note: we need this in examples like
diff --git a/compiler/rustc_borrowck/src/type_check/input_output.rs b/compiler/rustc_borrowck/src/type_check/input_output.rs
index bc740de5150..83c8ecba1f1 100644
--- a/compiler/rustc_borrowck/src/type_check/input_output.rs
+++ b/compiler/rustc_borrowck/src/type_check/input_output.rs
@@ -147,9 +147,9 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
         // Return types are a bit more complex. They may contain opaque `impl Trait` types.
         let mir_output_ty = body.local_decls[RETURN_PLACE].ty;
         let output_span = body.local_decls[RETURN_PLACE].source_info.span;
-        if let Err(terr) = self.eq_opaque_type_and_type(
-            mir_output_ty,
+        if let Err(terr) = self.eq_types(
             normalized_output_ty,
+            mir_output_ty,
             Locations::All(output_span),
             ConstraintCategory::BoringNoLocation,
         ) {
@@ -169,9 +169,9 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
             let user_provided_output_ty = user_provided_sig.output();
             let user_provided_output_ty =
                 self.normalize(user_provided_output_ty, Locations::All(output_span));
-            if let Err(err) = self.eq_opaque_type_and_type(
-                mir_output_ty,
+            if let Err(err) = self.eq_types(
                 user_provided_output_ty,
+                mir_output_ty,
                 Locations::All(output_span),
                 ConstraintCategory::BoringNoLocation,
             ) {
diff --git a/compiler/rustc_borrowck/src/type_check/mod.rs b/compiler/rustc_borrowck/src/type_check/mod.rs
index a7cf25d43d2..2d4bb272df5 100644
--- a/compiler/rustc_borrowck/src/type_check/mod.rs
+++ b/compiler/rustc_borrowck/src/type_check/mod.rs
@@ -5,6 +5,7 @@ use std::{fmt, iter, mem};
 
 use either::Either;
 
+use hir::OpaqueTyOrigin;
 use rustc_data_structures::frozen::Frozen;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_data_structures::vec_map::VecMap;
@@ -15,8 +16,8 @@ use rustc_hir::def_id::LocalDefId;
 use rustc_hir::lang_items::LangItem;
 use rustc_index::vec::{Idx, IndexVec};
 use rustc_infer::infer::canonical::QueryRegionConstraints;
-use rustc_infer::infer::opaque_types::OpaqueTypeDecl;
 use rustc_infer::infer::outlives::env::RegionBoundPairs;
+use rustc_infer::infer::region_constraints::RegionConstraintData;
 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use rustc_infer::infer::{
     InferCtxt, InferOk, LateBoundRegionConversionTime, NllRegionVariableOrigin,
@@ -39,9 +40,11 @@ use rustc_target::abi::VariantIdx;
 use rustc_trait_selection::infer::InferCtxtExt as _;
 use rustc_trait_selection::traits::error_reporting::InferCtxtExt as _;
 use rustc_trait_selection::traits::query::type_op;
+use rustc_trait_selection::traits::query::type_op::custom::scrape_region_constraints;
 use rustc_trait_selection::traits::query::type_op::custom::CustomTypeOp;
+use rustc_trait_selection::traits::query::type_op::{TypeOp, TypeOpOutput};
 use rustc_trait_selection::traits::query::Fallible;
-use rustc_trait_selection::traits::{self, ObligationCause, PredicateObligations};
+use rustc_trait_selection::traits::{self, ObligationCause, PredicateObligation};
 
 use rustc_const_eval::transform::{
     check_consts::ConstCx, promote_consts::is_const_fn_in_array_repeat_expression,
@@ -75,7 +78,7 @@ macro_rules! span_mirbug {
             $context.last_span,
             &format!(
                 "broken MIR in {:?} ({:?}): {}",
-                $context.body.source.def_id(),
+                $context.body().source.def_id(),
                 $elem,
                 format_args!($($message)*),
             ),
@@ -199,59 +202,44 @@ pub(crate) fn type_check<'mir, 'tcx>(
             );
 
             translate_outlives_facts(&mut cx);
-            let opaque_type_values = mem::take(&mut infcx.inner.borrow_mut().opaque_types);
+            let opaque_type_values =
+                infcx.inner.borrow_mut().opaque_type_storage.take_opaque_types();
 
             opaque_type_values
                 .into_iter()
-                .filter_map(|(opaque_type_key, mut decl)| {
-                    decl.concrete_ty = infcx.resolve_vars_if_possible(decl.concrete_ty);
+                .map(|(opaque_type_key, decl)| {
+                    cx.fully_perform_op(
+                        Locations::All(body.span),
+                        ConstraintCategory::OpaqueType,
+                        CustomTypeOp::new(
+                            |infcx| {
+                                infcx.register_member_constraints(
+                                    param_env,
+                                    opaque_type_key,
+                                    decl.hidden_type.ty,
+                                    decl.hidden_type.span,
+                                );
+                                Ok(InferOk { value: (), obligations: vec![] })
+                            },
+                            || "opaque_type_map".to_string(),
+                        ),
+                    )
+                    .unwrap();
+                    let mut hidden_type = infcx.resolve_vars_if_possible(decl.hidden_type.ty);
                     trace!(
                         "finalized opaque type {:?} to {:#?}",
                         opaque_type_key,
-                        decl.concrete_ty.kind()
+                        hidden_type.kind()
                     );
-                    if decl.concrete_ty.has_infer_types_or_consts() {
+                    if hidden_type.has_infer_types_or_consts() {
                         infcx.tcx.sess.delay_span_bug(
-                            body.span,
-                            &format!("could not resolve {:#?}", decl.concrete_ty.kind()),
+                            decl.hidden_type.span,
+                            &format!("could not resolve {:#?}", hidden_type.kind()),
                         );
-                        decl.concrete_ty = infcx.tcx.ty_error();
+                        hidden_type = infcx.tcx.ty_error();
                     }
-                    let concrete_is_opaque = if let ty::Opaque(def_id, _) = decl.concrete_ty.kind()
-                    {
-                        *def_id == opaque_type_key.def_id
-                    } else {
-                        false
-                    };
 
-                    if concrete_is_opaque {
-                        // We're using an opaque `impl Trait` type without
-                        // 'revealing' it. For example, code like this:
-                        //
-                        // type Foo = impl Debug;
-                        // fn foo1() -> Foo { ... }
-                        // fn foo2() -> Foo { foo1() }
-                        //
-                        // In `foo2`, we're not revealing the type of `Foo` - we're
-                        // just treating it as the opaque type.
-                        //
-                        // When this occurs, we do *not* want to try to equate
-                        // the concrete type with the underlying defining type
-                        // of the opaque type - this will always fail, since
-                        // the defining type of an opaque type is always
-                        // some other type (e.g. not itself)
-                        // Essentially, none of the normal obligations apply here -
-                        // we're just passing around some unknown opaque type,
-                        // without actually looking at the underlying type it
-                        // gets 'revealed' into
-                        debug!(
-                            "eq_opaque_type_and_type: non-defining use of {:?}",
-                            opaque_type_key.def_id,
-                        );
-                        None
-                    } else {
-                        Some((opaque_type_key, decl))
-                    }
+                    (opaque_type_key, (hidden_type, decl.hidden_type.span, decl.origin))
                 })
                 .collect()
         },
@@ -283,7 +271,7 @@ fn type_check_internal<'a, 'tcx, R>(
         borrowck_context,
     );
     let errors_reported = {
-        let mut verifier = TypeVerifier::new(&mut checker, body, promoted);
+        let mut verifier = TypeVerifier::new(&mut checker, promoted);
         verifier.visit_body(&body);
         verifier.errors_reported
     };
@@ -340,7 +328,6 @@ enum FieldAccessError {
 /// is a problem.
 struct TypeVerifier<'a, 'b, 'tcx> {
     cx: &'a mut TypeChecker<'b, 'tcx>,
-    body: &'b Body<'tcx>,
     promoted: &'b IndexVec<Promoted, Body<'tcx>>,
     last_span: Span,
     errors_reported: bool,
@@ -476,7 +463,7 @@ impl<'a, 'b, 'tcx> Visitor<'tcx> for TypeVerifier<'a, 'b, 'tcx> {
 
     fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {
         self.super_rvalue(rvalue, location);
-        let rval_ty = rvalue.ty(self.body, self.tcx());
+        let rval_ty = rvalue.ty(self.body(), self.tcx());
         self.sanitize_type(rvalue, rval_ty);
     }
 
@@ -535,10 +522,13 @@ impl<'a, 'b, 'tcx> Visitor<'tcx> for TypeVerifier<'a, 'b, 'tcx> {
 impl<'a, 'b, 'tcx> TypeVerifier<'a, 'b, 'tcx> {
     fn new(
         cx: &'a mut TypeChecker<'b, 'tcx>,
-        body: &'b Body<'tcx>,
         promoted: &'b IndexVec<Promoted, Body<'tcx>>,
     ) -> Self {
-        TypeVerifier { body, promoted, cx, last_span: body.span, errors_reported: false }
+        TypeVerifier { promoted, last_span: cx.body.span, cx, errors_reported: false }
+    }
+
+    fn body(&self) -> &Body<'tcx> {
+        self.cx.body
     }
 
     fn tcx(&self) -> TyCtxt<'tcx> {
@@ -563,7 +553,7 @@ impl<'a, 'b, 'tcx> TypeVerifier<'a, 'b, 'tcx> {
     ) -> PlaceTy<'tcx> {
         debug!("sanitize_place: {:?}", place);
 
-        let mut place_ty = PlaceTy::from_ty(self.body.local_decls[place.local].ty);
+        let mut place_ty = PlaceTy::from_ty(self.body().local_decls[place.local].ty);
 
         for elem in place.projection.iter() {
             if place_ty.variant_index.is_none() {
@@ -608,7 +598,7 @@ impl<'a, 'b, 'tcx> TypeVerifier<'a, 'b, 'tcx> {
         // checker on the promoted MIR, then transfer the constraints back to
         // the main MIR, changing the locations to the provided location.
 
-        let parent_body = mem::replace(&mut self.body, promoted_body);
+        let parent_body = mem::replace(&mut self.cx.body, promoted_body);
 
         // Use new sets of constraints and closure bounds so that we can
         // modify their locations.
@@ -644,7 +634,7 @@ impl<'a, 'b, 'tcx> TypeVerifier<'a, 'b, 'tcx> {
             self.cx.typeck_mir(promoted_body);
         }
 
-        self.body = parent_body;
+        self.cx.body = parent_body;
         // Merge the outlives constraints back in, at the given location.
         swap_constraints(self);
 
@@ -706,7 +696,7 @@ impl<'a, 'b, 'tcx> TypeVerifier<'a, 'b, 'tcx> {
                 }))
             }
             ProjectionElem::Index(i) => {
-                let index_ty = Place::from(i).ty(self.body, tcx).ty;
+                let index_ty = Place::from(i).ty(self.body(), tcx).ty;
                 if index_ty != tcx.types.usize {
                     PlaceTy::from_ty(span_mirbug_and_err!(self, i, "index by non-usize {:?}", i))
                 } else {
@@ -915,7 +905,7 @@ struct BorrowCheckContext<'a, 'tcx> {
 crate struct MirTypeckResults<'tcx> {
     crate constraints: MirTypeckRegionConstraints<'tcx>,
     crate universal_region_relations: Frozen<UniversalRegionRelations<'tcx>>,
-    crate opaque_type_values: VecMap<OpaqueTypeKey<'tcx>, OpaqueTypeDecl<'tcx>>,
+    crate opaque_type_values: VecMap<OpaqueTypeKey<'tcx>, (Ty<'tcx>, Span, OpaqueTyOrigin)>,
 }
 
 /// A collection of region constraints that must be satisfied for the
@@ -1065,17 +1055,19 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
         checker
     }
 
+    fn body(&self) -> &Body<'tcx> {
+        self.body
+    }
+
     fn unsized_feature_enabled(&self) -> bool {
         let features = self.tcx().features();
         features.unsized_locals || features.unsized_fn_params
     }
 
     /// Equate the inferred type and the annotated type for user type annotations
+    #[instrument(skip(self), level = "debug")]
     fn check_user_type_annotations(&mut self) {
-        debug!(
-            "check_user_type_annotations: user_type_annotations={:?}",
-            self.user_type_annotations
-        );
+        debug!(?self.user_type_annotations);
         for user_annotation in self.user_type_annotations {
             let CanonicalUserTypeAnnotation { span, ref user_ty, inferred_ty } = *user_annotation;
             let inferred_ty = self.normalize(inferred_ty, Locations::All(span));
@@ -1216,131 +1208,6 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
         Ok(())
     }
 
-    /// Equates a type `anon_ty` that may contain opaque types whose
-    /// values are to be inferred by the MIR.
-    ///
-    /// The type `revealed_ty` contains the same type as `anon_ty`, but with the
-    /// hidden types for impl traits revealed.
-    ///
-    /// # Example
-    ///
-    /// Consider a piece of code like
-    ///
-    /// ```rust
-    /// type Foo<U> = impl Debug;
-    ///
-    /// fn foo<T: Debug>(t: T) -> Box<Foo<T>> {
-    ///      Box::new((t, 22_u32))
-    /// }
-    /// ```
-    ///
-    /// Here, the function signature would be something like
-    /// `fn(T) -> Box<impl Debug>`. The MIR return slot would have
-    /// the type with the opaque type revealed, so `Box<(T, u32)>`.
-    ///
-    /// In terms of our function parameters:
-    ///
-    /// * `anon_ty` would be `Box<Foo<T>>` where `Foo<T>` is an opaque type
-    ///   scoped to this function (note that it is parameterized by the
-    ///   generics of `foo`). Note that `anon_ty` is not just the opaque type,
-    ///   but the entire return type (which may contain opaque types within it).
-    /// * `revealed_ty` would be `Box<(T, u32)>`
-    #[instrument(skip(self), level = "debug")]
-    fn eq_opaque_type_and_type(
-        &mut self,
-        revealed_ty: Ty<'tcx>,
-        anon_ty: Ty<'tcx>,
-        locations: Locations,
-        category: ConstraintCategory,
-    ) -> Fallible<()> {
-        // Fast path for the common case.
-        if !anon_ty.has_opaque_types() {
-            if let Err(terr) = self.eq_types(anon_ty, revealed_ty, locations, category) {
-                span_mirbug!(
-                    self,
-                    locations,
-                    "eq_opaque_type_and_type: `{:?}=={:?}` failed with `{:?}`",
-                    revealed_ty,
-                    anon_ty,
-                    terr
-                );
-            }
-            return Ok(());
-        }
-
-        let param_env = self.param_env;
-        let body = self.body;
-        let mir_def_id = body.source.def_id().expect_local();
-
-        debug!(?mir_def_id);
-        self.fully_perform_op(
-            locations,
-            category,
-            CustomTypeOp::new(
-                |infcx| {
-                    let mut obligations = ObligationAccumulator::default();
-
-                    let dummy_body_id = hir::CRATE_HIR_ID;
-
-                    // Replace the opaque types defined by this function with
-                    // inference variables, creating a map. In our example above,
-                    // this would transform the type `Box<Foo<T>>` (where `Foo` is an opaque type)
-                    // to `Box<?T>`, returning an `opaque_type_map` mapping `{Foo<T> -> ?T}`.
-                    // (Note that the key of the map is both the def-id of `Foo` along with
-                    // any generic parameters.)
-                    let output_ty = obligations.add(infcx.instantiate_opaque_types(
-                        dummy_body_id,
-                        param_env,
-                        anon_ty,
-                        locations.span(body),
-                    ));
-                    debug!(?output_ty, ?revealed_ty);
-
-                    // Make sure that the inferred types are well-formed. I'm
-                    // not entirely sure this is needed (the HIR type check
-                    // didn't do this) but it seems sensible to prevent opaque
-                    // types hiding ill-formed types.
-                    obligations.obligations.push(traits::Obligation::new(
-                        ObligationCause::dummy(),
-                        param_env,
-                        ty::Binder::dummy(ty::PredicateKind::WellFormed(revealed_ty.into()))
-                            .to_predicate(infcx.tcx),
-                    ));
-                    obligations.add(
-                        infcx
-                            .at(&ObligationCause::dummy(), param_env)
-                            .eq(output_ty, revealed_ty)?,
-                    );
-
-                    debug!("equated");
-
-                    Ok(InferOk { value: (), obligations: obligations.into_vec() })
-                },
-                || "input_output".to_string(),
-            ),
-        )?;
-
-        // Finally, if we instantiated the anon types successfully, we
-        // have to solve any bounds (e.g., `-> impl Iterator` needs to
-        // prove that `T: Iterator` where `T` is the type we
-        // instantiated it with).
-        let opaque_type_map = self.infcx.inner.borrow().opaque_types.clone();
-        for (opaque_type_key, opaque_decl) in opaque_type_map {
-            self.fully_perform_op(
-                locations,
-                ConstraintCategory::OpaqueType,
-                CustomTypeOp::new(
-                    |infcx| {
-                        infcx.constrain_opaque_type(opaque_type_key, &opaque_decl);
-                        Ok(InferOk { value: (), obligations: vec![] })
-                    },
-                    || "opaque_type_map".to_string(),
-                ),
-            )?;
-        }
-        Ok(())
-    }
-
     fn tcx(&self) -> TyCtxt<'tcx> {
         self.infcx.tcx
     }
@@ -2772,19 +2639,31 @@ impl NormalizeLocation for Location {
     }
 }
 
-#[derive(Debug, Default)]
-struct ObligationAccumulator<'tcx> {
-    obligations: PredicateObligations<'tcx>,
+/// Runs `infcx.instantiate_opaque_types`. Unlike other `TypeOp`s,
+/// this is not canonicalized - it directly affects the main `InferCtxt`
+/// that we use during MIR borrowchecking.
+#[derive(Debug)]
+pub(super) struct InstantiateOpaqueType<'tcx> {
+    pub base_universe: Option<ty::UniverseIndex>,
+    pub region_constraints: Option<RegionConstraintData<'tcx>>,
+    pub obligation: PredicateObligation<'tcx>,
 }
 
-impl<'tcx> ObligationAccumulator<'tcx> {
-    fn add<T>(&mut self, value: InferOk<'tcx, T>) -> T {
-        let InferOk { value, obligations } = value;
-        self.obligations.extend(obligations);
-        value
-    }
-
-    fn into_vec(self) -> PredicateObligations<'tcx> {
-        self.obligations
+impl<'tcx> TypeOp<'tcx> for InstantiateOpaqueType<'tcx> {
+    type Output = ();
+    /// We use this type itself to store the information used
+    /// when reporting errors. Since this is not a query, we don't
+    /// re-run anything during error reporting - we just use the information
+    /// we saved to help extract an error from the already-existing region
+    /// constraints in our `InferCtxt`
+    type ErrorInfo = InstantiateOpaqueType<'tcx>;
+
+    fn fully_perform(mut self, infcx: &InferCtxt<'_, 'tcx>) -> Fallible<TypeOpOutput<'tcx, Self>> {
+        let (mut output, region_constraints) = scrape_region_constraints(infcx, || {
+            Ok(InferOk { value: (), obligations: vec![self.obligation.clone()] })
+        })?;
+        self.region_constraints = Some(region_constraints);
+        output.error_info = Some(self);
+        Ok(output)
     }
 }
diff --git a/compiler/rustc_borrowck/src/type_check/relate_tys.rs b/compiler/rustc_borrowck/src/type_check/relate_tys.rs
index 590a7a91641..a032095bf98 100644
--- a/compiler/rustc_borrowck/src/type_check/relate_tys.rs
+++ b/compiler/rustc_borrowck/src/type_check/relate_tys.rs
@@ -1,13 +1,15 @@
 use rustc_infer::infer::nll_relate::{NormalizationStrategy, TypeRelating, TypeRelatingDelegate};
 use rustc_infer::infer::NllRegionVariableOrigin;
+use rustc_infer::traits::ObligationCause;
 use rustc_middle::mir::ConstraintCategory;
 use rustc_middle::ty::relate::TypeRelation;
 use rustc_middle::ty::{self, Const, Ty};
+use rustc_span::Span;
 use rustc_trait_selection::traits::query::Fallible;
 
 use crate::constraints::OutlivesConstraint;
 use crate::diagnostics::UniverseInfo;
-use crate::type_check::{Locations, TypeChecker};
+use crate::type_check::{InstantiateOpaqueType, Locations, TypeChecker};
 
 impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
     /// Adds sufficient constraints to ensure that `a R b` where `R` depends on `v`:
@@ -63,6 +65,10 @@ impl<'me, 'bccx, 'tcx> NllTypeRelatingDelegate<'me, 'bccx, 'tcx> {
 }
 
 impl<'tcx> TypeRelatingDelegate<'tcx> for NllTypeRelatingDelegate<'_, '_, 'tcx> {
+    fn span(&self) -> Span {
+        self.locations.span(self.type_checker.body)
+    }
+
     fn param_env(&self) -> ty::ParamEnv<'tcx> {
         self.type_checker.param_env
     }
@@ -117,6 +123,9 @@ impl<'tcx> TypeRelatingDelegate<'tcx> for NllTypeRelatingDelegate<'_, '_, 'tcx>
 
     // We don't have to worry about the equality of consts during borrow checking
     // as consts always have a static lifetime.
+    // FIXME(oli-obk): is this really true? We can at least have HKL and with
+    // inline consts we may have further lifetimes that may be unsound to treat as
+    // 'static.
     fn const_equate(&mut self, _a: Const<'tcx>, _b: Const<'tcx>) {}
 
     fn normalization() -> NormalizationStrategy {
@@ -126,4 +135,30 @@ impl<'tcx> TypeRelatingDelegate<'tcx> for NllTypeRelatingDelegate<'_, '_, 'tcx>
     fn forbid_inference_vars() -> bool {
         true
     }
+
+    fn register_opaque_type(&mut self, a: Ty<'tcx>, b: Ty<'tcx>, a_is_expected: bool) {
+        let param_env = self.param_env();
+        let span = self.span();
+        let def_id = self.type_checker.body.source.def_id().expect_local();
+        let body_id = self.type_checker.tcx().hir().local_def_id_to_hir_id(def_id);
+        let cause = ObligationCause::misc(span, body_id);
+        self.type_checker
+            .fully_perform_op(
+                self.locations,
+                self.category,
+                InstantiateOpaqueType {
+                    obligation: self.type_checker.infcx.opaque_ty_obligation(
+                        a,
+                        b,
+                        a_is_expected,
+                        param_env,
+                        cause,
+                    ),
+                    // These fields are filled in during exectuion of the operation
+                    base_universe: None,
+                    region_constraints: None,
+                },
+            )
+            .unwrap();
+    }
 }
diff --git a/compiler/rustc_borrowck/src/universal_regions.rs b/compiler/rustc_borrowck/src/universal_regions.rs
index 478dbe31fba..e9c64049817 100644
--- a/compiler/rustc_borrowck/src/universal_regions.rs
+++ b/compiler/rustc_borrowck/src/universal_regions.rs
@@ -725,6 +725,7 @@ impl<'cx, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'cx, 'tcx> {
         self.tcx.fold_regions(value, &mut false, |_region, _depth| self.next_nll_region_var(origin))
     }
 
+    #[instrument(level = "debug", skip(self, indices))]
     fn replace_bound_regions_with_nll_infer_vars<T>(
         &self,
         origin: NllRegionVariableOrigin,
@@ -735,22 +736,15 @@ impl<'cx, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'cx, 'tcx> {
     where
         T: TypeFoldable<'tcx>,
     {
-        debug!(
-            "replace_bound_regions_with_nll_infer_vars(value={:?}, all_outlive_scope={:?})",
-            value, all_outlive_scope,
-        );
         let (value, _map) = self.tcx.replace_late_bound_regions(value, |br| {
-            debug!("replace_bound_regions_with_nll_infer_vars: br={:?}", br);
+            debug!(?br);
             let liberated_region = self.tcx.mk_region(ty::ReFree(ty::FreeRegion {
                 scope: all_outlive_scope.to_def_id(),
                 bound_region: br.kind,
             }));
             let region_vid = self.next_nll_region_var(origin);
             indices.insert_late_bound_region(liberated_region, region_vid.to_region_vid());
-            debug!(
-                "replace_bound_regions_with_nll_infer_vars: liberated_region={:?} => {:?}",
-                liberated_region, region_vid
-            );
+            debug!(?liberated_region, ?region_vid);
             region_vid
         });
         value
@@ -765,6 +759,7 @@ impl<'cx, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'cx, 'tcx> {
     /// entries for them and store them in the indices map. This code iterates over the complete
     /// set of late-bound regions and checks for any that we have not yet seen, adding them to the
     /// inputs vector.
+    #[instrument(skip(self, indices))]
     fn replace_late_bound_regions_with_nll_infer_vars(
         &self,
         mir_def_id: LocalDefId,
@@ -776,6 +771,7 @@ impl<'cx, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'cx, 'tcx> {
             debug!("replace_late_bound_regions_with_nll_infer_vars: r={:?}", r);
             if !indices.indices.contains_key(&r) {
                 let region_vid = self.next_nll_region_var(FR);
+                debug!(?region_vid);
                 indices.insert_late_bound_region(r, region_vid.to_region_vid());
             }
         });
diff --git a/compiler/rustc_const_eval/src/transform/check_consts/check.rs b/compiler/rustc_const_eval/src/transform/check_consts/check.rs
index 223b3ad0cf9..8790baa010c 100644
--- a/compiler/rustc_const_eval/src/transform/check_consts/check.rs
+++ b/compiler/rustc_const_eval/src/transform/check_consts/check.rs
@@ -11,7 +11,7 @@ use rustc_middle::mir::*;
 use rustc_middle::ty::cast::CastTy;
 use rustc_middle::ty::subst::{GenericArgKind, InternalSubsts};
 use rustc_middle::ty::{self, adjustment::PointerCast, Instance, InstanceDef, Ty, TyCtxt};
-use rustc_middle::ty::{Binder, TraitPredicate, TraitRef};
+use rustc_middle::ty::{Binder, TraitPredicate, TraitRef, TypeFoldable};
 use rustc_mir_dataflow::{self, Analysis};
 use rustc_span::{sym, Span, Symbol};
 use rustc_trait_selection::traits::error_reporting::InferCtxtExt;
@@ -47,7 +47,10 @@ impl<'mir, 'tcx> Qualifs<'mir, 'tcx> {
         location: Location,
     ) -> bool {
         let ty = ccx.body.local_decls[local].ty;
-        if !NeedsDrop::in_any_value_of_ty(ccx, ty) {
+        // Peeking into opaque types causes cycles if the current function declares said opaque
+        // type. Thus we avoid short circuiting on the type and instead run the more expensive
+        // analysis that looks at the actual usage within this function
+        if !ty.has_opaque_types() && !NeedsDrop::in_any_value_of_ty(ccx, ty) {
             return false;
         }
 
@@ -101,7 +104,10 @@ impl<'mir, 'tcx> Qualifs<'mir, 'tcx> {
         location: Location,
     ) -> bool {
         let ty = ccx.body.local_decls[local].ty;
-        if !HasMutInterior::in_any_value_of_ty(ccx, ty) {
+        // Peeking into opaque types causes cycles if the current function declares said opaque
+        // type. Thus we avoid short circuiting on the type and instead run the more expensive
+        // analysis that looks at the actual usage within this function
+        if !ty.has_opaque_types() && !HasMutInterior::in_any_value_of_ty(ccx, ty) {
             return false;
         }
 
@@ -148,7 +154,12 @@ impl<'mir, 'tcx> Qualifs<'mir, 'tcx> {
 
             // If we know that all values of the return type are structurally matchable, there's no
             // need to run dataflow.
-            _ if !CustomEq::in_any_value_of_ty(ccx, ccx.body.return_ty()) => false,
+            // Opaque types do not participate in const generics or pattern matching, so we can safely count them out.
+            _ if ccx.body.return_ty().has_opaque_types()
+                || !CustomEq::in_any_value_of_ty(ccx, ccx.body.return_ty()) =>
+            {
+                false
+            }
 
             hir::ConstContext::Const | hir::ConstContext::Static(_) => {
                 let mut cursor = FlowSensitiveAnalysis::new(CustomEq, ccx)
diff --git a/compiler/rustc_const_eval/src/transform/validate.rs b/compiler/rustc_const_eval/src/transform/validate.rs
index deeca78b75d..e68e1a055d9 100644
--- a/compiler/rustc_const_eval/src/transform/validate.rs
+++ b/compiler/rustc_const_eval/src/transform/validate.rs
@@ -79,7 +79,6 @@ pub fn equal_up_to_regions<'tcx>(
     }
 
     // Normalize lifetimes away on both sides, then compare.
-    let param_env = param_env.with_reveal_all_normalized(tcx);
     let normalize = |ty: Ty<'tcx>| {
         tcx.normalize_erasing_regions(
             param_env,
@@ -171,9 +170,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
             return true;
         }
         // Normalize projections and things like that.
-        // FIXME: We need to reveal_all, as some optimizations change types in ways
-        // that require unfolding opaque types.
-        let param_env = self.param_env.with_reveal_all_normalized(self.tcx);
+        let param_env = self.param_env;
         let src = self.tcx.normalize_erasing_regions(param_env, src);
         let dest = self.tcx.normalize_erasing_regions(param_env, dest);
 
diff --git a/compiler/rustc_data_structures/src/vec_map.rs b/compiler/rustc_data_structures/src/vec_map.rs
index cc7ec9432fa..2f4b3844430 100644
--- a/compiler/rustc_data_structures/src/vec_map.rs
+++ b/compiler/rustc_data_structures/src/vec_map.rs
@@ -30,6 +30,11 @@ where
         }
     }
 
+    /// Removes the entry from the map and returns the removed value
+    pub fn remove(&mut self, k: &K) -> Option<V> {
+        self.0.iter().position(|(k2, _)| k2 == k).map(|pos| self.0.remove(pos).1)
+    }
+
     /// Gets a reference to the value in the entry.
     pub fn get<Q: ?Sized>(&self, k: &Q) -> Option<&V>
     where
@@ -39,6 +44,15 @@ where
         self.0.iter().find(|(key, _)| k == key.borrow()).map(|elem| &elem.1)
     }
 
+    /// Gets a mutable reference to the value in the entry.
+    pub fn get_mut<Q: ?Sized>(&mut self, k: &Q) -> Option<&mut V>
+    where
+        K: Borrow<Q>,
+        Q: Eq,
+    {
+        self.0.iter_mut().find(|(key, _)| k == key.borrow()).map(|elem| &mut elem.1)
+    }
+
     /// Returns the any value corresponding to the supplied predicate filter.
     ///
     /// The supplied predicate will be applied to each (key, value) pair and it will return a
@@ -58,7 +72,7 @@ where
         // This should return just one element, otherwise it's a bug
         assert!(
             filter.next().is_none(),
-            "Collection {:?} should have just one matching element",
+            "Collection {:#?} should have just one matching element",
             self
         );
         Some(value)
diff --git a/compiler/rustc_infer/src/infer/at.rs b/compiler/rustc_infer/src/infer/at.rs
index ff8082840e1..71ee4f1e76d 100644
--- a/compiler/rustc_infer/src/infer/at.rs
+++ b/compiler/rustc_infer/src/infer/at.rs
@@ -34,6 +34,12 @@ pub struct At<'a, 'tcx> {
     pub infcx: &'a InferCtxt<'a, 'tcx>,
     pub cause: &'a ObligationCause<'tcx>,
     pub param_env: ty::ParamEnv<'tcx>,
+    /// Whether we should define opaque types
+    /// or just treat them opaquely.
+    /// Currently only used to prevent predicate
+    /// matching from matching anything against opaque
+    /// types.
+    pub define_opaque_types: bool,
 }
 
 pub struct Trace<'a, 'tcx> {
@@ -49,7 +55,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         cause: &'a ObligationCause<'tcx>,
         param_env: ty::ParamEnv<'tcx>,
     ) -> At<'a, 'tcx> {
-        At { infcx: self, cause, param_env }
+        At { infcx: self, cause, param_env, define_opaque_types: true }
     }
 
     /// Forks the inference context, creating a new inference context with the same inference
@@ -59,6 +65,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         Self {
             tcx: self.tcx.clone(),
             defining_use_anchor: self.defining_use_anchor.clone(),
+            reveal_defining_opaque_types: self.reveal_defining_opaque_types.clone(),
             in_progress_typeck_results: self.in_progress_typeck_results.clone(),
             inner: self.inner.clone(),
             skip_leak_check: self.skip_leak_check.clone(),
@@ -86,6 +93,10 @@ pub trait ToTrace<'tcx>: Relate<'tcx> + Copy {
 }
 
 impl<'a, 'tcx> At<'a, 'tcx> {
+    pub fn define_opaque_types(self, define_opaque_types: bool) -> Self {
+        Self { define_opaque_types, ..self }
+    }
+
     /// Hacky routine for equating two impl headers in coherence.
     pub fn eq_impl_headers(
         self,
@@ -216,7 +227,7 @@ impl<'a, 'tcx> Trace<'a, 'tcx> {
     {
         let Trace { at, trace, a_is_expected } = self;
         at.infcx.commit_if_ok(|_| {
-            let mut fields = at.infcx.combine_fields(trace, at.param_env);
+            let mut fields = at.infcx.combine_fields(trace, at.param_env, at.define_opaque_types);
             fields
                 .sub(a_is_expected)
                 .relate(a, b)
@@ -233,7 +244,7 @@ impl<'a, 'tcx> Trace<'a, 'tcx> {
     {
         let Trace { at, trace, a_is_expected } = self;
         at.infcx.commit_if_ok(|_| {
-            let mut fields = at.infcx.combine_fields(trace, at.param_env);
+            let mut fields = at.infcx.combine_fields(trace, at.param_env, at.define_opaque_types);
             fields
                 .equate(a_is_expected)
                 .relate(a, b)
@@ -248,7 +259,7 @@ impl<'a, 'tcx> Trace<'a, 'tcx> {
     {
         let Trace { at, trace, a_is_expected } = self;
         at.infcx.commit_if_ok(|_| {
-            let mut fields = at.infcx.combine_fields(trace, at.param_env);
+            let mut fields = at.infcx.combine_fields(trace, at.param_env, at.define_opaque_types);
             fields
                 .lub(a_is_expected)
                 .relate(a, b)
@@ -263,7 +274,7 @@ impl<'a, 'tcx> Trace<'a, 'tcx> {
     {
         let Trace { at, trace, a_is_expected } = self;
         at.infcx.commit_if_ok(|_| {
-            let mut fields = at.infcx.combine_fields(trace, at.param_env);
+            let mut fields = at.infcx.combine_fields(trace, at.param_env, at.define_opaque_types);
             fields
                 .glb(a_is_expected)
                 .relate(a, b)
diff --git a/compiler/rustc_infer/src/infer/canonical/query_response.rs b/compiler/rustc_infer/src/infer/canonical/query_response.rs
index 48d5c21f9eb..20f65962db4 100644
--- a/compiler/rustc_infer/src/infer/canonical/query_response.rs
+++ b/compiler/rustc_infer/src/infer/canonical/query_response.rs
@@ -26,6 +26,7 @@ use rustc_middle::ty::fold::TypeFoldable;
 use rustc_middle::ty::relate::TypeRelation;
 use rustc_middle::ty::subst::{GenericArg, GenericArgKind};
 use rustc_middle::ty::{self, BoundVar, Const, ToPredicate, Ty, TyCtxt};
+use rustc_span::Span;
 use std::fmt::Debug;
 use std::iter;
 
@@ -89,6 +90,7 @@ impl<'cx, 'tcx> InferCtxt<'cx, 'tcx> {
             var_values: inference_vars,
             region_constraints: QueryRegionConstraints::default(),
             certainty: Certainty::Proven, // Ambiguities are OK!
+            opaque_types: vec![],
             value: answer,
         })
     }
@@ -133,14 +135,27 @@ impl<'cx, 'tcx> InferCtxt<'cx, 'tcx> {
         let certainty =
             if ambig_errors.is_empty() { Certainty::Proven } else { Certainty::Ambiguous };
 
+        let opaque_types = self.take_opaque_types_for_query_response();
+
         Ok(QueryResponse {
             var_values: inference_vars,
             region_constraints,
             certainty,
             value: answer,
+            opaque_types,
         })
     }
 
+    fn take_opaque_types_for_query_response(&self) -> Vec<(Ty<'tcx>, Ty<'tcx>)> {
+        self.inner
+            .borrow_mut()
+            .opaque_type_storage
+            .take_opaque_types()
+            .into_iter()
+            .map(|(k, v)| (self.tcx.mk_opaque(k.def_id, k.substs), v.hidden_type.ty))
+            .collect()
+    }
+
     /// Given the (canonicalized) result to a canonical query,
     /// instantiates the result so it can be used, plugging in the
     /// values from the canonical query. (Note that the result may
@@ -223,13 +238,12 @@ impl<'cx, 'tcx> InferCtxt<'cx, 'tcx> {
     where
         R: Debug + TypeFoldable<'tcx>,
     {
-        let result_subst =
-            self.query_response_substitution_guess(cause, original_values, query_response);
+        let InferOk { value: result_subst, mut obligations } = self
+            .query_response_substitution_guess(cause, param_env, original_values, query_response)?;
 
         // Compute `QueryOutlivesConstraint` values that unify each of
         // the original values `v_o` that was canonicalized into a
         // variable...
-        let mut obligations = vec![];
 
         for (index, original_value) in original_values.var_values.iter().enumerate() {
             // ...with the value `v_r` of that variable from the query.
@@ -343,20 +357,25 @@ impl<'cx, 'tcx> InferCtxt<'cx, 'tcx> {
             original_values, query_response,
         );
 
-        let result_subst =
-            self.query_response_substitution_guess(cause, original_values, query_response);
+        let mut value = self.query_response_substitution_guess(
+            cause,
+            param_env,
+            original_values,
+            query_response,
+        )?;
 
-        let obligations = self
-            .unify_query_response_substitution_guess(
+        value.obligations.extend(
+            self.unify_query_response_substitution_guess(
                 cause,
                 param_env,
                 original_values,
-                &result_subst,
+                &value.value,
                 query_response,
             )?
-            .into_obligations();
+            .into_obligations(),
+        );
 
-        Ok(InferOk { value: result_subst, obligations })
+        Ok(value)
     }
 
     /// Given the original values and the (canonicalized) result from
@@ -371,9 +390,10 @@ impl<'cx, 'tcx> InferCtxt<'cx, 'tcx> {
     fn query_response_substitution_guess<R>(
         &self,
         cause: &ObligationCause<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
         original_values: &OriginalQueryValues<'tcx>,
         query_response: &Canonical<'tcx, QueryResponse<'tcx, R>>,
-    ) -> CanonicalVarValues<'tcx>
+    ) -> InferResult<'tcx, CanonicalVarValues<'tcx>>
     where
         R: Debug + TypeFoldable<'tcx>,
     {
@@ -473,7 +493,16 @@ impl<'cx, 'tcx> InferCtxt<'cx, 'tcx> {
                 .collect(),
         };
 
-        result_subst
+        let mut obligations = vec![];
+
+        // Carry all newly resolved opaque types to the caller's scope
+        for &(a, b) in &query_response.value.opaque_types {
+            let a = substitute_value(self.tcx, &result_subst, a);
+            let b = substitute_value(self.tcx, &result_subst, b);
+            obligations.extend(self.handle_opaque_type(a, b, cause, param_env)?.obligations);
+        }
+
+        Ok(InferOk { value: result_subst, obligations })
     }
 
     /// Given a "guess" at the values for the canonical variables in
@@ -629,6 +658,10 @@ struct QueryTypeRelatingDelegate<'a, 'tcx> {
 }
 
 impl<'tcx> TypeRelatingDelegate<'tcx> for QueryTypeRelatingDelegate<'_, 'tcx> {
+    fn span(&self) -> Span {
+        self.cause.span
+    }
+
     fn param_env(&self) -> ty::ParamEnv<'tcx> {
         self.param_env
     }
@@ -684,4 +717,14 @@ impl<'tcx> TypeRelatingDelegate<'tcx> for QueryTypeRelatingDelegate<'_, 'tcx> {
     fn forbid_inference_vars() -> bool {
         true
     }
+
+    fn register_opaque_type(&mut self, a: Ty<'tcx>, b: Ty<'tcx>, a_is_expected: bool) {
+        self.obligations.push(self.infcx.opaque_ty_obligation(
+            a,
+            b,
+            a_is_expected,
+            self.param_env,
+            self.cause.clone(),
+        ));
+    }
 }
diff --git a/compiler/rustc_infer/src/infer/combine.rs b/compiler/rustc_infer/src/infer/combine.rs
index e1b5d04ccfb..41995ca509e 100644
--- a/compiler/rustc_infer/src/infer/combine.rs
+++ b/compiler/rustc_infer/src/infer/combine.rs
@@ -51,6 +51,12 @@ pub struct CombineFields<'infcx, 'tcx> {
     pub cause: Option<ty::relate::Cause>,
     pub param_env: ty::ParamEnv<'tcx>,
     pub obligations: PredicateObligations<'tcx>,
+    /// Whether we should define opaque types
+    /// or just treat them opaquely.
+    /// Currently only used to prevent predicate
+    /// matching from matching anything against opaque
+    /// types.
+    pub define_opaque_types: bool,
 }
 
 #[derive(Copy, Clone, Debug)]
@@ -322,6 +328,7 @@ impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
     /// will first instantiate `b_vid` with a *generalized* version
     /// of `a_ty`. Generalization introduces other inference
     /// variables wherever subtyping could occur.
+    #[instrument(skip(self), level = "debug")]
     pub fn instantiate(
         &mut self,
         a_ty: Ty<'tcx>,
@@ -334,8 +341,6 @@ impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
         // Get the actual variable that b_vid has been inferred to
         debug_assert!(self.infcx.inner.borrow_mut().type_variables().probe(b_vid).is_unknown());
 
-        debug!("instantiate(a_ty={:?} dir={:?} b_vid={:?})", a_ty, dir, b_vid);
-
         // Generalize type of `a_ty` appropriately depending on the
         // direction.  As an example, assume:
         //
@@ -348,10 +353,7 @@ impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
         // variables. (Down below, we will relate `a_ty <: b_ty`,
         // adding constraints like `'x: '?2` and `?1 <: ?3`.)
         let Generalization { ty: b_ty, needs_wf } = self.generalize(a_ty, b_vid, dir)?;
-        debug!(
-            "instantiate(a_ty={:?}, dir={:?}, b_vid={:?}, generalized b_ty={:?})",
-            a_ty, dir, b_vid, b_ty
-        );
+        debug!(?b_ty);
         self.infcx.inner.borrow_mut().type_variables().instantiate(b_vid, b_ty);
 
         if needs_wf {
@@ -392,13 +394,13 @@ impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
     /// Preconditions:
     ///
     /// - `for_vid` is a "root vid"
+    #[instrument(skip(self), level = "trace")]
     fn generalize(
         &self,
         ty: Ty<'tcx>,
         for_vid: ty::TyVid,
         dir: RelationDir,
     ) -> RelateResult<'tcx, Generalization<'tcx>> {
-        debug!("generalize(ty={:?}, for_vid={:?}, dir={:?}", ty, for_vid, dir);
         // Determine the ambient variance within which `ty` appears.
         // The surrounding equation is:
         //
@@ -412,7 +414,7 @@ impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
             RelationDir::SupertypeOf => ty::Contravariant,
         };
 
-        debug!("generalize: ambient_variance = {:?}", ambient_variance);
+        trace!(?ambient_variance);
 
         let for_universe = match self.infcx.inner.borrow_mut().type_variables().probe(for_vid) {
             v @ TypeVariableValue::Known { .. } => {
@@ -421,8 +423,8 @@ impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
             TypeVariableValue::Unknown { universe } => universe,
         };
 
-        debug!("generalize: for_universe = {:?}", for_universe);
-        debug!("generalize: trace = {:?}", self.trace);
+        trace!(?for_universe);
+        trace!(?self.trace);
 
         let mut generalize = Generalizer {
             infcx: self.infcx,
@@ -439,12 +441,12 @@ impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
         let ty = match generalize.relate(ty, ty) {
             Ok(ty) => ty,
             Err(e) => {
-                debug!("generalize: failure {:?}", e);
+                debug!(?e, "failure");
                 return Err(e);
             }
         };
         let needs_wf = generalize.needs_wf;
-        debug!("generalize: success {{ {:?}, {:?} }}", ty, needs_wf);
+        trace!(?ty, ?needs_wf, "success");
         Ok(Generalization { ty, needs_wf })
     }
 
diff --git a/compiler/rustc_infer/src/infer/equate.rs b/compiler/rustc_infer/src/infer/equate.rs
index 5ac9ad6850c..c1c075e9a4c 100644
--- a/compiler/rustc_infer/src/infer/equate.rs
+++ b/compiler/rustc_infer/src/infer/equate.rs
@@ -66,18 +66,19 @@ impl<'tcx> TypeRelation<'tcx> for Equate<'_, '_, 'tcx> {
         self.relate(a, b)
     }
 
+    #[instrument(skip(self), level = "debug")]
     fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
-        debug!("{}.tys({:?}, {:?})", self.tag(), a, b);
         if a == b {
             return Ok(a);
         }
 
+        trace!(a = ?a.kind(), b = ?b.kind());
+
         let infcx = self.fields.infcx;
+
         let a = infcx.inner.borrow_mut().type_variables().replace_if_possible(a);
         let b = infcx.inner.borrow_mut().type_variables().replace_if_possible(b);
 
-        debug!("{}.tys: replacements ({:?}, {:?})", self.tag(), a, b);
-
         match (a.kind(), b.kind()) {
             (&ty::Infer(TyVar(a_id)), &ty::Infer(TyVar(b_id))) => {
                 infcx.inner.borrow_mut().type_variables().equate(a_id, b_id);
@@ -91,6 +92,21 @@ impl<'tcx> TypeRelation<'tcx> for Equate<'_, '_, 'tcx> {
                 self.fields.instantiate(a, RelationDir::EqTo, b_id, self.a_is_expected)?;
             }
 
+            (&ty::Opaque(a_def_id, _), &ty::Opaque(b_def_id, _)) if a_def_id == b_def_id => {
+                self.fields.infcx.super_combine_tys(self, a, b)?;
+            }
+            (&ty::Opaque(did, ..), _) | (_, &ty::Opaque(did, ..))
+                if self.fields.define_opaque_types && did.is_local() =>
+            {
+                self.fields.obligations.push(infcx.opaque_ty_obligation(
+                    a,
+                    b,
+                    self.a_is_expected(),
+                    self.param_env(),
+                    self.fields.trace.cause.clone(),
+                ));
+            }
+
             _ => {
                 self.fields.infcx.super_combine_tys(self, a, b)?;
             }
diff --git a/compiler/rustc_infer/src/infer/glb.rs b/compiler/rustc_infer/src/infer/glb.rs
index e98f33ea86e..0d38b94965a 100644
--- a/compiler/rustc_infer/src/infer/glb.rs
+++ b/compiler/rustc_infer/src/infer/glb.rs
@@ -6,7 +6,7 @@ use super::InferCtxt;
 use super::Subtype;
 
 use crate::infer::combine::ConstEquateRelation;
-use crate::traits::ObligationCause;
+use crate::traits::{ObligationCause, PredicateObligation};
 use rustc_middle::ty::relate::{Relate, RelateResult, TypeRelation};
 use rustc_middle::ty::{self, Ty, TyCtxt};
 
@@ -113,12 +113,20 @@ impl<'combine, 'infcx, 'tcx> LatticeDir<'infcx, 'tcx> for Glb<'combine, 'infcx,
         &self.fields.trace.cause
     }
 
+    fn add_obligations(&mut self, obligations: Vec<PredicateObligation<'tcx>>) {
+        self.fields.obligations.extend(obligations)
+    }
+
     fn relate_bound(&mut self, v: Ty<'tcx>, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, ()> {
         let mut sub = self.fields.sub(self.a_is_expected);
         sub.relate(v, a)?;
         sub.relate(v, b)?;
         Ok(())
     }
+
+    fn define_opaque_types(&self) -> bool {
+        self.fields.define_opaque_types
+    }
 }
 
 impl<'tcx> ConstEquateRelation<'tcx> for Glb<'_, '_, 'tcx> {
diff --git a/compiler/rustc_infer/src/infer/lattice.rs b/compiler/rustc_infer/src/infer/lattice.rs
index 32affd6a14e..f4c749492d3 100644
--- a/compiler/rustc_infer/src/infer/lattice.rs
+++ b/compiler/rustc_infer/src/infer/lattice.rs
@@ -20,7 +20,7 @@
 use super::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use super::InferCtxt;
 
-use crate::traits::ObligationCause;
+use crate::traits::{ObligationCause, PredicateObligation};
 use rustc_middle::ty::relate::{RelateResult, TypeRelation};
 use rustc_middle::ty::TyVar;
 use rustc_middle::ty::{self, Ty};
@@ -35,6 +35,10 @@ pub trait LatticeDir<'f, 'tcx>: TypeRelation<'tcx> {
 
     fn cause(&self) -> &ObligationCause<'tcx>;
 
+    fn add_obligations(&mut self, obligations: Vec<PredicateObligation<'tcx>>);
+
+    fn define_opaque_types(&self) -> bool;
+
     // Relates the type `v` to `a` and `b` such that `v` represents
     // the LUB/GLB of `a` and `b` as appropriate.
     //
@@ -45,6 +49,7 @@ pub trait LatticeDir<'f, 'tcx>: TypeRelation<'tcx> {
 }
 
 /// Relates two types using a given lattice.
+#[instrument(skip(this), level = "debug")]
 pub fn super_lattice_tys<'a, 'tcx: 'a, L>(
     this: &mut L,
     a: Ty<'tcx>,
@@ -53,15 +58,17 @@ pub fn super_lattice_tys<'a, 'tcx: 'a, L>(
 where
     L: LatticeDir<'a, 'tcx>,
 {
-    debug!("{}.lattice_tys({:?}, {:?})", this.tag(), a, b);
+    debug!("{}", this.tag());
 
     if a == b {
         return Ok(a);
     }
 
     let infcx = this.infcx();
+
     let a = infcx.inner.borrow_mut().type_variables().replace_if_possible(a);
     let b = infcx.inner.borrow_mut().type_variables().replace_if_possible(b);
+
     match (a.kind(), b.kind()) {
         // If one side is known to be a variable and one is not,
         // create a variable (`v`) to represent the LUB. Make sure to
@@ -98,6 +105,22 @@ where
             Ok(v)
         }
 
+        (&ty::Opaque(a_def_id, _), &ty::Opaque(b_def_id, _)) if a_def_id == b_def_id => {
+            infcx.super_combine_tys(this, a, b)
+        }
+        (&ty::Opaque(did, ..), _) | (_, &ty::Opaque(did, ..))
+            if this.define_opaque_types() && did.is_local() =>
+        {
+            this.add_obligations(vec![infcx.opaque_ty_obligation(
+                a,
+                b,
+                this.a_is_expected(),
+                this.param_env(),
+                this.cause().clone(),
+            )]);
+            Ok(a)
+        }
+
         _ => infcx.super_combine_tys(this, a, b),
     }
 }
diff --git a/compiler/rustc_infer/src/infer/lub.rs b/compiler/rustc_infer/src/infer/lub.rs
index bc85a4ac609..498c1e907c4 100644
--- a/compiler/rustc_infer/src/infer/lub.rs
+++ b/compiler/rustc_infer/src/infer/lub.rs
@@ -6,7 +6,7 @@ use super::InferCtxt;
 use super::Subtype;
 
 use crate::infer::combine::ConstEquateRelation;
-use crate::traits::ObligationCause;
+use crate::traits::{ObligationCause, PredicateObligation};
 use rustc_middle::ty::relate::{Relate, RelateResult, TypeRelation};
 use rustc_middle::ty::{self, Ty, TyCtxt};
 
@@ -119,10 +119,18 @@ impl<'combine, 'infcx, 'tcx> LatticeDir<'infcx, 'tcx> for Lub<'combine, 'infcx,
         &self.fields.trace.cause
     }
 
+    fn add_obligations(&mut self, obligations: Vec<PredicateObligation<'tcx>>) {
+        self.fields.obligations.extend(obligations)
+    }
+
     fn relate_bound(&mut self, v: Ty<'tcx>, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, ()> {
         let mut sub = self.fields.sub(self.a_is_expected);
         sub.relate(a, v)?;
         sub.relate(b, v)?;
         Ok(())
     }
+
+    fn define_opaque_types(&self) -> bool {
+        self.fields.define_opaque_types
+    }
 }
diff --git a/compiler/rustc_infer/src/infer/mod.rs b/compiler/rustc_infer/src/infer/mod.rs
index 83ba9c96978..63ffb5bde38 100644
--- a/compiler/rustc_infer/src/infer/mod.rs
+++ b/compiler/rustc_infer/src/infer/mod.rs
@@ -5,7 +5,7 @@ pub use self::RegionVariableOrigin::*;
 pub use self::SubregionOrigin::*;
 pub use self::ValuePairs::*;
 
-use self::opaque_types::OpaqueTypeMap;
+use self::opaque_types::OpaqueTypeStorage;
 pub(crate) use self::undo_log::{InferCtxtUndoLogs, Snapshot, UndoLog};
 
 use crate::traits::{self, ObligationCause, PredicateObligations, TraitEngine};
@@ -191,18 +191,8 @@ pub struct InferCtxtInner<'tcx> {
 
     undo_log: InferCtxtUndoLogs<'tcx>,
 
-    // Opaque types found in explicit return types and their
-    // associated fresh inference variable. Writeback resolves these
-    // variables to get the concrete type, which can be used to
-    // 'de-opaque' OpaqueTypeDecl outside of type inference.
-    pub opaque_types: OpaqueTypeMap<'tcx>,
-
-    /// A map from inference variables created from opaque
-    /// type instantiations (`ty::Infer`) to the actual opaque
-    /// type (`ty::Opaque`). Used during fallback to map unconstrained
-    /// opaque type inference variables to their corresponding
-    /// opaque type.
-    pub opaque_types_vars: FxHashMap<Ty<'tcx>, Ty<'tcx>>,
+    /// Caches for opaque type inference.
+    pub opaque_type_storage: OpaqueTypeStorage<'tcx>,
 }
 
 impl<'tcx> InferCtxtInner<'tcx> {
@@ -216,8 +206,7 @@ impl<'tcx> InferCtxtInner<'tcx> {
             float_unification_storage: ut::UnificationTableStorage::new(),
             region_constraint_storage: Some(RegionConstraintStorage::new()),
             region_obligations: vec![],
-            opaque_types: Default::default(),
-            opaque_types_vars: Default::default(),
+            opaque_type_storage: Default::default(),
         }
     }
 
@@ -237,6 +226,11 @@ impl<'tcx> InferCtxtInner<'tcx> {
     }
 
     #[inline]
+    pub fn opaque_types(&mut self) -> opaque_types::OpaqueTypeTable<'_, 'tcx> {
+        self.opaque_type_storage.with_log(&mut self.undo_log)
+    }
+
+    #[inline]
     fn int_unification_table(
         &mut self,
     ) -> ut::UnificationTable<
@@ -296,6 +290,10 @@ pub struct InferCtxt<'a, 'tcx> {
     /// to the outside until the end up in an `InferCtxt` for typeck or borrowck.
     pub defining_use_anchor: Option<LocalDefId>,
 
+    /// Used by WF-checking to not have to figure out hidden types itself, but
+    /// to just invoke type_of to get the already computed hidden type from typeck.
+    pub reveal_defining_opaque_types: bool,
+
     /// During type-checking/inference of a body, `in_progress_typeck_results`
     /// contains a reference to the typeck results being built up, which are
     /// used for reading closure kinds/signatures as they are inferred,
@@ -571,6 +569,7 @@ pub struct InferCtxtBuilder<'tcx> {
     tcx: TyCtxt<'tcx>,
     fresh_typeck_results: Option<RefCell<ty::TypeckResults<'tcx>>>,
     defining_use_anchor: Option<LocalDefId>,
+    reveal_defining_opaque_types: bool,
 }
 
 pub trait TyCtxtInferExt<'tcx> {
@@ -579,7 +578,12 @@ pub trait TyCtxtInferExt<'tcx> {
 
 impl<'tcx> TyCtxtInferExt<'tcx> for TyCtxt<'tcx> {
     fn infer_ctxt(self) -> InferCtxtBuilder<'tcx> {
-        InferCtxtBuilder { tcx: self, defining_use_anchor: None, fresh_typeck_results: None }
+        InferCtxtBuilder {
+            tcx: self,
+            defining_use_anchor: None,
+            fresh_typeck_results: None,
+            reveal_defining_opaque_types: false,
+        }
     }
 }
 
@@ -603,6 +607,13 @@ impl<'tcx> InferCtxtBuilder<'tcx> {
         self
     }
 
+    /// WF-checking doesn't need to recompute opaque types and can instead use
+    /// the type_of query to get them from typeck.
+    pub fn reveal_defining_opaque_types(mut self) -> Self {
+        self.reveal_defining_opaque_types = true;
+        self
+    }
+
     /// Given a canonical value `C` as a starting point, create an
     /// inference context that contains each of the bound values
     /// within instantiated as a fresh variable. The `f` closure is
@@ -627,11 +638,17 @@ impl<'tcx> InferCtxtBuilder<'tcx> {
     }
 
     pub fn enter<R>(&mut self, f: impl for<'a> FnOnce(InferCtxt<'a, 'tcx>) -> R) -> R {
-        let InferCtxtBuilder { tcx, defining_use_anchor, ref fresh_typeck_results } = *self;
+        let InferCtxtBuilder {
+            tcx,
+            defining_use_anchor,
+            reveal_defining_opaque_types,
+            ref fresh_typeck_results,
+        } = *self;
         let in_progress_typeck_results = fresh_typeck_results.as_ref();
         f(InferCtxt {
             tcx,
             defining_use_anchor,
+            reveal_defining_opaque_types,
             in_progress_typeck_results,
             inner: RefCell::new(InferCtxtInner::new()),
             lexical_region_resolutions: RefCell::new(None),
@@ -766,6 +783,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         &'a self,
         trace: TypeTrace<'tcx>,
         param_env: ty::ParamEnv<'tcx>,
+        define_opaque_types: bool,
     ) -> CombineFields<'a, 'tcx> {
         CombineFields {
             infcx: self,
@@ -773,6 +791,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             cause: None,
             param_env,
             obligations: PredicateObligations::new(),
+            define_opaque_types,
         }
     }
 
@@ -1088,12 +1107,20 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         self.tcx.mk_ty_var(self.next_ty_var_id(origin))
     }
 
+    pub fn next_ty_var_id_in_universe(
+        &self,
+        origin: TypeVariableOrigin,
+        universe: ty::UniverseIndex,
+    ) -> TyVid {
+        self.inner.borrow_mut().type_variables().new_var(universe, origin)
+    }
+
     pub fn next_ty_var_in_universe(
         &self,
         origin: TypeVariableOrigin,
         universe: ty::UniverseIndex,
     ) -> Ty<'tcx> {
-        let vid = self.inner.borrow_mut().type_variables().new_var(universe, origin);
+        let vid = self.next_ty_var_id_in_universe(origin, universe);
         self.tcx.mk_ty_var(vid)
     }
 
diff --git a/compiler/rustc_infer/src/infer/nll_relate/mod.rs b/compiler/rustc_infer/src/infer/nll_relate/mod.rs
index 69db6509b79..2c7f3adafdc 100644
--- a/compiler/rustc_infer/src/infer/nll_relate/mod.rs
+++ b/compiler/rustc_infer/src/infer/nll_relate/mod.rs
@@ -24,11 +24,13 @@
 use crate::infer::combine::ConstEquateRelation;
 use crate::infer::InferCtxt;
 use crate::infer::{ConstVarValue, ConstVariableValue};
+use crate::infer::{TypeVariableOrigin, TypeVariableOriginKind};
 use rustc_data_structures::fx::FxHashMap;
 use rustc_middle::ty::error::TypeError;
 use rustc_middle::ty::fold::{TypeFoldable, TypeVisitor};
 use rustc_middle::ty::relate::{self, Relate, RelateResult, TypeRelation};
 use rustc_middle::ty::{self, InferConst, Ty, TyCtxt};
+use rustc_span::Span;
 use std::fmt::Debug;
 use std::ops::ControlFlow;
 
@@ -75,6 +77,7 @@ where
 
 pub trait TypeRelatingDelegate<'tcx> {
     fn param_env(&self) -> ty::ParamEnv<'tcx>;
+    fn span(&self) -> Span;
 
     /// Push a constraint `sup: sub` -- this constraint must be
     /// satisfied for the two types to be related. `sub` and `sup` may
@@ -88,6 +91,7 @@ pub trait TypeRelatingDelegate<'tcx> {
     );
 
     fn const_equate(&mut self, a: ty::Const<'tcx>, b: ty::Const<'tcx>);
+    fn register_opaque_type(&mut self, a: Ty<'tcx>, b: Ty<'tcx>, a_is_expected: bool);
 
     /// Creates a new universe index. Used when instantiating placeholders.
     fn create_next_universe(&mut self) -> ty::UniverseIndex;
@@ -277,7 +281,6 @@ where
         projection_ty: ty::ProjectionTy<'tcx>,
         value_ty: Ty<'tcx>,
     ) -> Ty<'tcx> {
-        use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
         use rustc_span::DUMMY_SP;
 
         match *value_ty.kind() {
@@ -286,6 +289,8 @@ where
                     kind: TypeVariableOriginKind::MiscVariable,
                     span: DUMMY_SP,
                 });
+                // FIXME(lazy-normalization): This will always ICE, because the recursive
+                // call will end up in the _ arm below.
                 self.relate_projection_ty(projection_ty, var);
                 self.relate_projection_ty(other_projection_ty, var);
                 var
@@ -531,6 +536,8 @@ where
 
     #[instrument(skip(self), level = "debug")]
     fn tys(&mut self, a: Ty<'tcx>, mut b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
+        let infcx = self.infcx;
+
         let a = self.infcx.shallow_resolve(a);
 
         if !D::forbid_inference_vars() {
@@ -559,6 +566,35 @@ where
 
             (&ty::Infer(ty::TyVar(vid)), _) => self.relate_ty_var((vid, b)),
 
+            (&ty::Opaque(a_def_id, _), &ty::Opaque(b_def_id, _)) if a_def_id == b_def_id => {
+                self.infcx.super_combine_tys(self, a, b)
+            }
+            (&ty::Opaque(did, ..), _) | (_, &ty::Opaque(did, ..)) if did.is_local() => {
+                let (a, b) = if self.a_is_expected() { (a, b) } else { (b, a) };
+                let mut generalize = |ty, ty_is_expected| {
+                    let var = infcx.next_ty_var_id_in_universe(
+                        TypeVariableOrigin {
+                            kind: TypeVariableOriginKind::MiscVariable,
+                            span: self.delegate.span(),
+                        },
+                        ty::UniverseIndex::ROOT,
+                    );
+                    if ty_is_expected {
+                        self.relate_ty_var((ty, var))
+                    } else {
+                        self.relate_ty_var((var, ty))
+                    }
+                };
+                let (a, b) = match (a.kind(), b.kind()) {
+                    (&ty::Opaque(..), _) => (a, generalize(b, false)?),
+                    (_, &ty::Opaque(..)) => (generalize(a, true)?, b),
+                    _ => unreachable!(),
+                };
+                self.delegate.register_opaque_type(a, b, true);
+                trace!(a = ?a.kind(), b = ?b.kind(), "opaque type instantiated");
+                Ok(a)
+            }
+
             (&ty::Projection(projection_ty), _)
                 if D::normalization() == NormalizationStrategy::Lazy =>
             {
diff --git a/compiler/rustc_infer/src/infer/opaque_types.rs b/compiler/rustc_infer/src/infer/opaque_types.rs
index e7dca94806c..46420fbe0c3 100644
--- a/compiler/rustc_infer/src/infer/opaque_types.rs
+++ b/compiler/rustc_infer/src/infer/opaque_types.rs
@@ -1,10 +1,11 @@
-use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use crate::infer::{InferCtxt, InferOk};
-use crate::traits;
+use crate::traits::{self, PredicateObligation};
+use hir::def_id::{DefId, LocalDefId};
+use hir::OpaqueTyOrigin;
 use rustc_data_structures::sync::Lrc;
 use rustc_data_structures::vec_map::VecMap;
 use rustc_hir as hir;
-use rustc_hir::def_id::LocalDefId;
+use rustc_middle::traits::ObligationCause;
 use rustc_middle::ty::fold::BottomUpFolder;
 use rustc_middle::ty::subst::{GenericArgKind, Subst};
 use rustc_middle::ty::{self, OpaqueTypeKey, Ty, TyCtxt, TypeFoldable, TypeVisitor};
@@ -14,14 +15,28 @@ use std::ops::ControlFlow;
 
 pub type OpaqueTypeMap<'tcx> = VecMap<OpaqueTypeKey<'tcx>, OpaqueTypeDecl<'tcx>>;
 
+mod table;
+
+pub use table::{OpaqueTypeStorage, OpaqueTypeTable};
+
+use super::InferResult;
+
 /// Information about the opaque types whose values we
 /// are inferring in this function (these are the `impl Trait` that
 /// appear in the return type).
-#[derive(Copy, Clone, Debug)]
+#[derive(Clone, Debug)]
 pub struct OpaqueTypeDecl<'tcx> {
-    /// The opaque type (`ty::Opaque`) for this declaration.
-    pub opaque_type: Ty<'tcx>,
+    /// The hidden types that have been inferred for this opaque type.
+    /// There can be multiple, but they are all `lub`ed together at the end
+    /// to obtain the canonical hidden type.
+    pub hidden_type: OpaqueHiddenType<'tcx>,
+
+    /// The origin of the opaque type.
+    pub origin: hir::OpaqueTyOrigin,
+}
 
+#[derive(Copy, Clone, Debug, TypeFoldable)]
+pub struct OpaqueHiddenType<'tcx> {
     /// The span of this particular definition of the opaque type. So
     /// for example:
     ///
@@ -35,7 +50,7 @@ pub struct OpaqueTypeDecl<'tcx> {
     /// In cases where the fn returns `(impl Trait, impl Trait)` or
     /// other such combinations, the result is currently
     /// over-approximated, but better than nothing.
-    pub definition_span: Span,
+    pub span: Span,
 
     /// The type variable that represents the value of the opaque type
     /// that we require. In other words, after we compile this function,
@@ -49,54 +64,132 @@ pub struct OpaqueTypeDecl<'tcx> {
     /// those that are arguments to `Foo` in the constraint above. (In
     /// other words, `?C` should not include `'b`, even though it's a
     /// lifetime parameter on `foo`.)
-    pub concrete_ty: Ty<'tcx>,
-
-    /// The origin of the opaque type.
-    pub origin: hir::OpaqueTyOrigin,
+    pub ty: Ty<'tcx>,
 }
 
 impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
-    /// Replaces all opaque types in `value` with fresh inference variables
-    /// and creates appropriate obligations. For example, given the input:
-    ///
-    ///     impl Iterator<Item = impl Debug>
-    ///
-    /// this method would create two type variables, `?0` and `?1`. It would
-    /// return the type `?0` but also the obligations:
-    ///
-    ///     ?0: Iterator<Item = ?1>
-    ///     ?1: Debug
-    ///
-    /// Moreover, it returns an `OpaqueTypeMap` that would map `?0` to
-    /// info about the `impl Iterator<..>` type and `?1` to info about
-    /// the `impl Debug` type.
-    ///
-    /// # Parameters
-    ///
-    /// - `parent_def_id` -- the `DefId` of the function in which the opaque type
-    ///   is defined
-    /// - `body_id` -- the body-id with which the resulting obligations should
-    ///   be associated
-    /// - `param_env` -- the in-scope parameter environment to be used for
-    ///   obligations
-    /// - `value` -- the value within which we are instantiating opaque types
-    /// - `value_span` -- the span where the value came from, used in error reporting
-    pub fn instantiate_opaque_types<T: TypeFoldable<'tcx>>(
+    pub fn handle_opaque_type(
         &self,
-        body_id: hir::HirId,
+        a: Ty<'tcx>,
+        b: Ty<'tcx>,
+        cause: &ObligationCause<'tcx>,
         param_env: ty::ParamEnv<'tcx>,
-        value: T,
-        value_span: Span,
-    ) -> InferOk<'tcx, T> {
-        debug!(
-            "instantiate_opaque_types(value={:?}, body_id={:?}, \
-             param_env={:?}, value_span={:?})",
-            value, body_id, param_env, value_span,
-        );
-        let mut instantiator =
-            Instantiator { infcx: self, body_id, param_env, value_span, obligations: vec![] };
-        let value = instantiator.instantiate_opaque_types_in_map(value);
-        InferOk { value, obligations: instantiator.obligations }
+    ) -> InferResult<'tcx, ()> {
+        if a.references_error() || b.references_error() {
+            return Ok(InferOk { value: (), obligations: vec![] });
+        }
+        if self.defining_use_anchor.is_some() {
+            let process = |a: Ty<'tcx>, b: Ty<'tcx>| match *a.kind() {
+                ty::Opaque(def_id, substs) => {
+                    if let ty::Opaque(did2, _) = *b.kind() {
+                        // We could accept this, but there are various ways to handle this situation, and we don't
+                        // want to make a decision on it right now. Likely this case is so super rare anyway, that
+                        // 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 let Some(OpaqueTyOrigin::TyAlias) =
+                            self.opaque_type_origin(did2, cause.span)
+                        {
+                            self.tcx
+                                .sess
+                                .struct_span_err(
+                                    cause.span,
+                                    "opaque type's hidden type cannot be another opaque type from the same scope",
+                                )
+                                .span_label(cause.span, "one of the two opaque types used here has to be outside its defining scope")
+                                .span_note(
+                                    self.tcx.def_span(def_id),
+                                    "opaque type whose hidden type is being assigned",
+                                )
+                                .span_note(
+                                    self.tcx.def_span(did2),
+                                    "opaque type being used as hidden type",
+                                )
+                                .emit();
+                        }
+                    }
+                    Some(self.register_hidden_type(
+                        OpaqueTypeKey { def_id, substs },
+                        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
+                        // always true during the first phase of
+                        // type-check, but not always true later on during
+                        // NLL. Once we support named opaque types more fully,
+                        // this same scenario will be able to arise during all phases.
+                        //
+                        // Here is an example using type alias `impl Trait`
+                        // that indicates the distinction we are checking for:
+                        //
+                        // ```rust
+                        // mod a {
+                        //   pub type Foo = impl Iterator;
+                        //   pub fn make_foo() -> Foo { .. }
+                        // }
+                        //
+                        // mod b {
+                        //   fn foo() -> a::Foo { a::make_foo() }
+                        // }
+                        // ```
+                        //
+                        // Here, the return type of `foo` references an
+                        // `Opaque` indeed, but not one whose value is
+                        // presently being inferred. You can get into a
+                        // similar situation with closure return types
+                        // today:
+                        //
+                        // ```rust
+                        // fn foo() -> impl Iterator { .. }
+                        // fn bar() {
+                        //     let x = || foo(); // returns the Opaque assoc with `foo`
+                        // }
+                        // ```
+                        self.opaque_type_origin(def_id, cause.span)?,
+                    ))
+                }
+                _ => None,
+            };
+            if let Some(res) = process(a, b) {
+                res
+            } else if let Some(res) = process(b, a) {
+                res
+            } else {
+                // Rerun equality check, but this time error out due to
+                // different types.
+                match self.at(cause, param_env).define_opaque_types(false).eq(a, b) {
+                    Ok(_) => span_bug!(
+                        cause.span,
+                        "opaque types are never equal to anything but themselves: {:#?}",
+                        (a, b)
+                    ),
+                    Err(e) => Err(e),
+                }
+            }
+        } else {
+            let (opaque_type, hidden_ty) = match (a.kind(), b.kind()) {
+                (ty::Opaque(..), _) => (a, b),
+                (_, ty::Opaque(..)) => (b, a),
+                types => span_bug!(
+                    cause.span,
+                    "opaque type obligations only work for opaque types: {:#?}",
+                    types
+                ),
+            };
+            let key = opaque_type.expect_opaque_type();
+            let origin = self.opaque_ty_origin_unchecked(key.def_id, cause.span);
+            let prev = self.inner.borrow_mut().opaque_types().register(
+                key,
+                OpaqueHiddenType { ty: hidden_ty, span: cause.span },
+                origin,
+            );
+            match prev {
+                Some(prev) => self.at(cause, param_env).eq(prev, hidden_ty),
+                None => Ok(InferOk { value: (), obligations: vec![] }),
+            }
+        }
     }
 
     /// Given the map `opaque_types` containing the opaque
@@ -231,51 +324,23 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     /// but this is not necessary, because the opaque type we
     /// create will be allowed to reference `T`. So we only generate a
     /// constraint that `'0: 'a`.
-    ///
-    /// # The `free_region_relations` parameter
-    ///
-    /// The `free_region_relations` argument is used to find the
-    /// "minimum" of the regions supplied to a given opaque type.
-    /// It must be a relation that can answer whether `'a <= 'b`,
-    /// where `'a` and `'b` are regions that appear in the "substs"
-    /// for the opaque type references (the `<'a>` in `Foo1<'a>`).
-    ///
-    /// Note that we do not impose the constraints based on the
-    /// generic regions from the `Foo1` definition (e.g., `'x`). This
-    /// is because the constraints we are imposing here is basically
-    /// the concern of the one generating the constraining type C1,
-    /// which is the current function. It also means that we can
-    /// take "implied bounds" into account in some cases:
-    ///
-    /// ```text
-    /// trait SomeTrait<'a, 'b> { }
-    /// fn foo<'a, 'b>(_: &'a &'b u32) -> impl SomeTrait<'a, 'b> { .. }
-    /// ```
-    ///
-    /// Here, the fact that `'b: 'a` is known only because of the
-    /// implied bounds from the `&'a &'b u32` parameter, and is not
-    /// "inherent" to the opaque type definition.
-    ///
-    /// # Parameters
-    ///
-    /// - `opaque_types` -- the map produced by `instantiate_opaque_types`
-    /// - `free_region_relations` -- something that can be used to relate
-    ///   the free regions (`'a`) that appear in the impl trait.
     #[instrument(level = "debug", skip(self))]
-    pub fn constrain_opaque_type(
+    pub fn register_member_constraints(
         &self,
+        param_env: ty::ParamEnv<'tcx>,
         opaque_type_key: OpaqueTypeKey<'tcx>,
-        opaque_defn: &OpaqueTypeDecl<'tcx>,
+        concrete_ty: Ty<'tcx>,
+        span: Span,
     ) {
         let def_id = opaque_type_key.def_id;
 
         let tcx = self.tcx;
 
-        let concrete_ty = self.resolve_vars_if_possible(opaque_defn.concrete_ty);
+        let concrete_ty = self.resolve_vars_if_possible(concrete_ty);
 
         debug!(?concrete_ty);
 
-        let first_own_region = match opaque_defn.origin {
+        let first_own_region = match self.opaque_ty_origin_unchecked(def_id, span) {
             hir::OpaqueTyOrigin::FnReturn(..) | hir::OpaqueTyOrigin::AsyncFn(..) => {
                 // We lower
                 //
@@ -319,7 +384,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             op: |r| {
                 self.member_constraint(
                     opaque_type_key.def_id,
-                    opaque_defn.definition_span,
+                    span,
                     concrete_ty,
                     r,
                     &choice_regions,
@@ -328,15 +393,34 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         });
     }
 
-    fn opaque_type_origin(&self, def_id: LocalDefId) -> Option<hir::OpaqueTyOrigin> {
-        let tcx = self.tcx;
-        let opaque_hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
+    pub fn opaque_ty_obligation(
+        &self,
+        a: Ty<'tcx>,
+        b: Ty<'tcx>,
+        a_is_expected: bool,
+        param_env: ty::ParamEnv<'tcx>,
+        cause: ObligationCause<'tcx>,
+    ) -> PredicateObligation<'tcx> {
+        let (a, b) = if a_is_expected { (a, b) } else { (b, a) };
+        PredicateObligation::new(
+            cause,
+            param_env,
+            self.tcx.mk_predicate(ty::Binder::dummy(ty::PredicateKind::OpaqueType(a, b))),
+        )
+    }
+
+    #[instrument(skip(self), level = "trace")]
+    pub fn opaque_type_origin(&self, opaque_def_id: DefId, span: Span) -> Option<OpaqueTyOrigin> {
+        let def_id = opaque_def_id.as_local()?;
+        let opaque_hir_id = self.tcx.hir().local_def_id_to_hir_id(def_id);
         let parent_def_id = self.defining_use_anchor?;
-        let item_kind = &tcx.hir().expect_item(def_id).kind;
+        let item_kind = &self.tcx.hir().expect_item(def_id).kind;
+
         let hir::ItemKind::OpaqueTy(hir::OpaqueTy { origin, ..  }) = item_kind else {
             span_bug!(
-                tcx.def_span(def_id),
-                "weird opaque type: {:#?}",
+                span,
+                "weird opaque type: {:#?}, {:#?}",
+                opaque_def_id,
                 item_kind
             )
         };
@@ -347,11 +431,29 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             hir::OpaqueTyOrigin::FnReturn(parent) => parent == parent_def_id,
             // Named `type Foo = impl Bar;`
             hir::OpaqueTyOrigin::TyAlias => {
-                may_define_opaque_type(tcx, parent_def_id, opaque_hir_id)
+                may_define_opaque_type(self.tcx, parent_def_id, opaque_hir_id)
             }
         };
+        trace!(?origin);
         in_definition_scope.then_some(*origin)
     }
+
+    #[instrument(skip(self), level = "trace")]
+    fn opaque_ty_origin_unchecked(&self, opaque_def_id: DefId, span: Span) -> OpaqueTyOrigin {
+        let def_id = opaque_def_id.as_local().unwrap();
+        let origin = match self.tcx.hir().expect_item(def_id).kind {
+            hir::ItemKind::OpaqueTy(hir::OpaqueTy { origin, .. }) => origin,
+            ref itemkind => {
+                span_bug!(span, "weird opaque type: {:?}, {:#?}", opaque_def_id, itemkind)
+            }
+        };
+        trace!(?origin);
+        origin
+    }
+
+    pub fn opaque_types(&self) -> OpaqueTypeMap<'tcx> {
+        self.inner.borrow().opaque_type_storage.opaque_types()
+    }
 }
 
 // Visitor that requires that (almost) all regions in the type visited outlive
@@ -426,180 +528,93 @@ where
     }
 }
 
-struct Instantiator<'a, 'tcx> {
-    infcx: &'a InferCtxt<'a, 'tcx>,
-    body_id: hir::HirId,
-    param_env: ty::ParamEnv<'tcx>,
-    value_span: Span,
-    obligations: Vec<traits::PredicateObligation<'tcx>>,
+pub enum UseKind {
+    DefiningUse,
+    OpaqueUse,
 }
 
-impl<'a, 'tcx> Instantiator<'a, 'tcx> {
-    fn instantiate_opaque_types_in_map<T: TypeFoldable<'tcx>>(&mut self, value: T) -> T {
-        let tcx = self.infcx.tcx;
-        value.fold_with(&mut BottomUpFolder {
-            tcx,
-            ty_op: |ty| {
-                if ty.references_error() {
-                    return tcx.ty_error();
-                } else if let ty::Opaque(def_id, substs) = ty.kind() {
-                    // 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
-                    // always true during the first phase of
-                    // type-check, but not always true later on during
-                    // NLL. Once we support named opaque types more fully,
-                    // this same scenario will be able to arise during all phases.
-                    //
-                    // Here is an example using type alias `impl Trait`
-                    // that indicates the distinction we are checking for:
-                    //
-                    // ```rust
-                    // mod a {
-                    //   pub type Foo = impl Iterator;
-                    //   pub fn make_foo() -> Foo { .. }
-                    // }
-                    //
-                    // mod b {
-                    //   fn foo() -> a::Foo { a::make_foo() }
-                    // }
-                    // ```
-                    //
-                    // Here, the return type of `foo` references an
-                    // `Opaque` indeed, but not one whose value is
-                    // presently being inferred. You can get into a
-                    // similar situation with closure return types
-                    // today:
-                    //
-                    // ```rust
-                    // fn foo() -> impl Iterator { .. }
-                    // fn bar() {
-                    //     let x = || foo(); // returns the Opaque assoc with `foo`
-                    // }
-                    // ```
-                    if let Some(def_id) = def_id.as_local() {
-                        if let Some(origin) = self.infcx.opaque_type_origin(def_id) {
-                            let opaque_type_key =
-                                OpaqueTypeKey { def_id: def_id.to_def_id(), substs };
-                            return self.fold_opaque_ty(ty, opaque_type_key, origin);
-                        }
-
-                        debug!(
-                            "instantiate_opaque_types_in_map: \
-                             encountered opaque outside its definition scope \
-                             def_id={:?}",
-                            def_id,
-                        );
-                    }
-                }
-
-                ty
-            },
-            lt_op: |lt| lt,
-            ct_op: |ct| ct,
-        })
+impl UseKind {
+    pub fn is_defining(self) -> bool {
+        match self {
+            UseKind::DefiningUse => true,
+            UseKind::OpaqueUse => false,
+        }
     }
+}
 
+impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
     #[instrument(skip(self), level = "debug")]
-    fn fold_opaque_ty(
-        &mut self,
-        ty: Ty<'tcx>,
+    fn register_hidden_type(
+        &self,
         opaque_type_key: OpaqueTypeKey<'tcx>,
+        cause: ObligationCause<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+        hidden_ty: Ty<'tcx>,
         origin: hir::OpaqueTyOrigin,
-    ) -> Ty<'tcx> {
-        let infcx = self.infcx;
-        let tcx = infcx.tcx;
+    ) -> InferResult<'tcx, ()> {
+        let tcx = self.tcx;
         let OpaqueTypeKey { def_id, substs } = opaque_type_key;
 
-        // Use the same type variable if the exact same opaque type appears more
-        // than once in the return type (e.g., if it's passed to a type alias).
-        if let Some(opaque_defn) = infcx.inner.borrow().opaque_types.get(&opaque_type_key) {
-            debug!("re-using cached concrete type {:?}", opaque_defn.concrete_ty.kind());
-            return opaque_defn.concrete_ty;
-        }
-
-        let ty_var = infcx.next_ty_var(TypeVariableOrigin {
-            kind: TypeVariableOriginKind::TypeInference,
-            span: self.value_span,
-        });
-
         // Ideally, we'd get the span where *this specific `ty` came
         // from*, but right now we just use the span from the overall
         // value being folded. In simple cases like `-> impl Foo`,
         // these are the same span, but not in cases like `-> (impl
         // Foo, impl Bar)`.
-        let definition_span = self.value_span;
+        let span = cause.span;
 
-        {
-            let mut infcx = self.infcx.inner.borrow_mut();
-            infcx.opaque_types.insert(
-                OpaqueTypeKey { def_id, substs },
-                OpaqueTypeDecl { opaque_type: ty, definition_span, concrete_ty: ty_var, origin },
-            );
-            infcx.opaque_types_vars.insert(ty_var, ty);
+        let mut obligations = vec![];
+        let prev = self.inner.borrow_mut().opaque_types().register(
+            OpaqueTypeKey { def_id, substs },
+            OpaqueHiddenType { ty: hidden_ty, span },
+            origin,
+        );
+        if let Some(prev) = prev {
+            obligations = self.at(&cause, param_env).eq(prev, hidden_ty)?.obligations;
         }
 
-        debug!("generated new type inference var {:?}", ty_var.kind());
-
         let item_bounds = tcx.explicit_item_bounds(def_id);
 
-        self.obligations.reserve(item_bounds.len());
         for (predicate, _) in item_bounds {
             debug!(?predicate);
             let predicate = predicate.subst(tcx, substs);
-            debug!(?predicate);
 
             let predicate = predicate.fold_with(&mut BottomUpFolder {
                 tcx,
                 ty_op: |ty| match *ty.kind() {
-                    // Replace all other mentions of the same opaque type with the hidden type,
-                    // as the bounds must hold on the hidden type after all.
-                    ty::Opaque(def_id2, substs2) if def_id == def_id2 && substs == substs2 => {
-                        ty_var
-                    }
-                    // Instantiate nested instances of `impl Trait`.
-                    ty::Opaque(..) => self.instantiate_opaque_types_in_map(ty),
-                    _ => ty,
-                },
-                lt_op: |lt| lt,
-                ct_op: |ct| ct,
-            });
-
-            // We can't normalize associated types from `rustc_infer`, but we can eagerly register inference variables for them.
-            let predicate = predicate.fold_with(&mut BottomUpFolder {
-                tcx,
-                ty_op: |ty| match ty.kind() {
+                    // We can't normalize associated types from `rustc_infer`,
+                    // but we can eagerly register inference variables for them.
                     ty::Projection(projection_ty) if !projection_ty.has_escaping_bound_vars() => {
-                        infcx.infer_projection(
-                            self.param_env,
-                            *projection_ty,
-                            traits::ObligationCause::misc(self.value_span, self.body_id),
+                        self.infer_projection(
+                            param_env,
+                            projection_ty,
+                            cause.clone(),
                             0,
-                            &mut self.obligations,
+                            &mut obligations,
                         )
                     }
+                    // Replace all other mentions of the same opaque type with the hidden type,
+                    // as the bounds must hold on the hidden type after all.
+                    ty::Opaque(def_id2, substs2) if def_id == def_id2 && substs == substs2 => {
+                        hidden_ty
+                    }
                     _ => ty,
                 },
                 lt_op: |lt| lt,
                 ct_op: |ct| ct,
             });
-            debug!(?predicate);
 
             if let ty::PredicateKind::Projection(projection) = predicate.kind().skip_binder() {
                 if projection.term.references_error() {
-                    return tcx.ty_error();
+                    // No point on adding these obligations since there's a type error involved.
+                    return Ok(InferOk { value: (), obligations: vec![] });
                 }
+                trace!("{:#?}", projection.term);
             }
-
-            let cause =
-                traits::ObligationCause::new(self.value_span, self.body_id, traits::OpaqueType);
-
             // Require that the predicate holds for the concrete type.
             debug!(?predicate);
-            self.obligations.push(traits::Obligation::new(cause, self.param_env, predicate));
+            obligations.push(traits::Obligation::new(cause.clone(), param_env, predicate));
         }
-
-        ty_var
+        Ok(InferOk { value: (), obligations })
     }
 }
 
diff --git a/compiler/rustc_infer/src/infer/opaque_types/table.rs b/compiler/rustc_infer/src/infer/opaque_types/table.rs
new file mode 100644
index 00000000000..202afea559c
--- /dev/null
+++ b/compiler/rustc_infer/src/infer/opaque_types/table.rs
@@ -0,0 +1,88 @@
+use rustc_data_structures::undo_log::UndoLogs;
+use rustc_hir::OpaqueTyOrigin;
+use rustc_middle::ty::{self, OpaqueTypeKey, Ty};
+use rustc_span::DUMMY_SP;
+
+use crate::infer::{InferCtxtUndoLogs, UndoLog};
+
+use super::{OpaqueHiddenType, OpaqueTypeDecl, OpaqueTypeMap};
+
+#[derive(Default, Debug, Clone)]
+pub struct OpaqueTypeStorage<'tcx> {
+    // Opaque types found in explicit return types and their
+    // associated fresh inference variable. Writeback resolves these
+    // variables to get the concrete type, which can be used to
+    // 'de-opaque' OpaqueTypeDecl, after typeck is done with all functions.
+    pub opaque_types: OpaqueTypeMap<'tcx>,
+}
+
+impl<'tcx> OpaqueTypeStorage<'tcx> {
+    #[instrument(level = "debug")]
+    pub(crate) fn remove(&mut self, key: OpaqueTypeKey<'tcx>, idx: Option<OpaqueHiddenType<'tcx>>) {
+        if let Some(idx) = idx {
+            self.opaque_types.get_mut(&key).unwrap().hidden_type = idx;
+        } else {
+            match self.opaque_types.remove(&key) {
+                None => bug!("reverted opaque type inference that was never registered: {:?}", key),
+                Some(_) => {}
+            }
+        }
+    }
+
+    pub fn get_decl(&self, key: &OpaqueTypeKey<'tcx>) -> Option<&OpaqueTypeDecl<'tcx>> {
+        self.opaque_types.get(key)
+    }
+
+    pub fn opaque_types(&self) -> OpaqueTypeMap<'tcx> {
+        self.opaque_types.clone()
+    }
+
+    #[instrument(level = "debug")]
+    pub fn take_opaque_types(&mut self) -> OpaqueTypeMap<'tcx> {
+        std::mem::take(&mut self.opaque_types)
+    }
+
+    #[inline]
+    pub(crate) fn with_log<'a>(
+        &'a mut self,
+        undo_log: &'a mut InferCtxtUndoLogs<'tcx>,
+    ) -> OpaqueTypeTable<'a, 'tcx> {
+        OpaqueTypeTable { storage: self, undo_log }
+    }
+}
+
+impl<'tcx> Drop for OpaqueTypeStorage<'tcx> {
+    fn drop(&mut self) {
+        if !self.opaque_types.is_empty() {
+            ty::tls::with(|tcx| {
+                tcx.sess.delay_span_bug(DUMMY_SP, &format!("{:?}", self.opaque_types))
+            });
+        }
+    }
+}
+
+pub struct OpaqueTypeTable<'a, 'tcx> {
+    storage: &'a mut OpaqueTypeStorage<'tcx>,
+
+    undo_log: &'a mut InferCtxtUndoLogs<'tcx>,
+}
+
+impl<'a, 'tcx> OpaqueTypeTable<'a, 'tcx> {
+    #[instrument(skip(self), level = "debug")]
+    pub fn register(
+        &mut self,
+        key: OpaqueTypeKey<'tcx>,
+        hidden_type: OpaqueHiddenType<'tcx>,
+        origin: OpaqueTyOrigin,
+    ) -> Option<Ty<'tcx>> {
+        if let Some(decl) = self.storage.opaque_types.get_mut(&key) {
+            let prev = std::mem::replace(&mut decl.hidden_type, hidden_type);
+            self.undo_log.push(UndoLog::OpaqueTypes(key, Some(prev)));
+            return Some(prev.ty);
+        }
+        let decl = OpaqueTypeDecl { hidden_type, origin };
+        self.storage.opaque_types.insert(key, decl);
+        self.undo_log.push(UndoLog::OpaqueTypes(key, None));
+        None
+    }
+}
diff --git a/compiler/rustc_infer/src/infer/outlives/mod.rs b/compiler/rustc_infer/src/infer/outlives/mod.rs
index 03d6c45a653..b60ffc1878b 100644
--- a/compiler/rustc_infer/src/infer/outlives/mod.rs
+++ b/compiler/rustc_infer/src/infer/outlives/mod.rs
@@ -28,6 +28,7 @@ pub fn explicit_outlives_bounds<'tcx>(
             | ty::PredicateKind::TypeOutlives(..)
             | ty::PredicateKind::ConstEvaluatable(..)
             | ty::PredicateKind::ConstEquate(..)
+            | ty::PredicateKind::OpaqueType(..)
             | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
             ty::PredicateKind::RegionOutlives(ty::OutlivesPredicate(r_a, r_b)) => {
                 Some(OutlivesBound::RegionSubRegion(r_b, r_a))
diff --git a/compiler/rustc_infer/src/infer/outlives/obligations.rs b/compiler/rustc_infer/src/infer/outlives/obligations.rs
index 5d8cc94e05c..45aa2bcba86 100644
--- a/compiler/rustc_infer/src/infer/outlives/obligations.rs
+++ b/compiler/rustc_infer/src/infer/outlives/obligations.rs
@@ -153,6 +153,7 @@ impl<'cx, 'tcx> InferCtxt<'cx, 'tcx> {
     /// This function may have to perform normalizations, and hence it
     /// returns an `InferOk` with subobligations that must be
     /// processed.
+    #[instrument(level = "debug", skip(self, region_bound_pairs_map))]
     pub fn process_registered_region_obligations(
         &self,
         region_bound_pairs_map: &FxHashMap<hir::HirId, RegionBoundPairs<'tcx>>,
@@ -164,8 +165,6 @@ impl<'cx, 'tcx> InferCtxt<'cx, 'tcx> {
             "cannot process registered region obligations in a snapshot"
         );
 
-        debug!(?param_env, "process_registered_region_obligations()");
-
         let my_region_obligations = self.take_registered_region_obligations();
 
         for (body_id, RegionObligation { sup_type, sub_region, origin }) in my_region_obligations {
diff --git a/compiler/rustc_infer/src/infer/sub.rs b/compiler/rustc_infer/src/infer/sub.rs
index 9ec1b3390d0..85bae8b823b 100644
--- a/compiler/rustc_infer/src/infer/sub.rs
+++ b/compiler/rustc_infer/src/infer/sub.rs
@@ -2,6 +2,7 @@ use super::combine::{CombineFields, RelationDir};
 use super::SubregionOrigin;
 
 use crate::infer::combine::ConstEquateRelation;
+use crate::infer::{TypeVariableOrigin, TypeVariableOriginKind};
 use crate::traits::Obligation;
 use rustc_middle::ty::fold::TypeFoldable;
 use rustc_middle::ty::relate::{Cause, Relate, RelateResult, TypeRelation};
@@ -74,9 +75,8 @@ impl<'tcx> TypeRelation<'tcx> for Sub<'_, '_, 'tcx> {
         }
     }
 
+    #[instrument(skip(self), level = "debug")]
     fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
-        debug!("{}.tys({:?}, {:?})", self.tag(), a, b);
-
         if a == b {
             return Ok(a);
         }
@@ -84,6 +84,7 @@ impl<'tcx> TypeRelation<'tcx> for Sub<'_, '_, 'tcx> {
         let infcx = self.fields.infcx;
         let a = infcx.inner.borrow_mut().type_variables().replace_if_possible(a);
         let b = infcx.inner.borrow_mut().type_variables().replace_if_possible(b);
+
         match (a.kind(), b.kind()) {
             (&ty::Infer(TyVar(_)), &ty::Infer(TyVar(_))) => {
                 // Shouldn't have any LBR here, so we can safely put
@@ -121,6 +122,40 @@ impl<'tcx> TypeRelation<'tcx> for Sub<'_, '_, 'tcx> {
                 Ok(self.tcx().ty_error())
             }
 
+            (&ty::Opaque(a_def_id, _), &ty::Opaque(b_def_id, _)) if a_def_id == b_def_id => {
+                self.fields.infcx.super_combine_tys(self, a, b)?;
+                Ok(a)
+            }
+            (&ty::Opaque(did, ..), _) | (_, &ty::Opaque(did, ..))
+                if self.fields.define_opaque_types && did.is_local() =>
+            {
+                let mut generalize = |ty, ty_is_expected| {
+                    let var = infcx.next_ty_var_id_in_universe(
+                        TypeVariableOrigin {
+                            kind: TypeVariableOriginKind::MiscVariable,
+                            span: self.fields.trace.cause.span,
+                        },
+                        ty::UniverseIndex::ROOT,
+                    );
+                    self.fields.instantiate(ty, RelationDir::SubtypeOf, var, ty_is_expected)?;
+                    Ok(infcx.tcx.mk_ty_var(var))
+                };
+                let (a, b) = if self.a_is_expected { (a, b) } else { (b, a) };
+                let (a, b) = match (a.kind(), b.kind()) {
+                    (&ty::Opaque(..), _) => (a, generalize(b, true)?),
+                    (_, &ty::Opaque(..)) => (generalize(a, false)?, b),
+                    _ => unreachable!(),
+                };
+                self.fields.obligations.push(infcx.opaque_ty_obligation(
+                    a,
+                    b,
+                    true,
+                    self.param_env(),
+                    self.fields.trace.cause.clone(),
+                ));
+                Ok(a)
+            }
+
             _ => {
                 self.fields.infcx.super_combine_tys(self, a, b)?;
                 Ok(a)
diff --git a/compiler/rustc_infer/src/infer/undo_log.rs b/compiler/rustc_infer/src/infer/undo_log.rs
index ecd886b5478..49bef4face3 100644
--- a/compiler/rustc_infer/src/infer/undo_log.rs
+++ b/compiler/rustc_infer/src/infer/undo_log.rs
@@ -4,13 +4,15 @@ use rustc_data_structures::snapshot_vec as sv;
 use rustc_data_structures::undo_log::{Rollback, UndoLogs};
 use rustc_data_structures::unify as ut;
 use rustc_middle::infer::unify_key::RegionVidKey;
-use rustc_middle::ty;
+use rustc_middle::ty::{self, OpaqueTypeKey};
 
 use crate::{
     infer::{region_constraints, type_variable, InferCtxtInner},
     traits,
 };
 
+use super::opaque_types::OpaqueHiddenType;
+
 pub struct Snapshot<'tcx> {
     pub(crate) undo_len: usize,
     _marker: PhantomData<&'tcx ()>,
@@ -19,6 +21,7 @@ pub struct Snapshot<'tcx> {
 /// Records the "undo" data for a single operation that affects some form of inference variable.
 #[derive(Clone)]
 pub(crate) enum UndoLog<'tcx> {
+    OpaqueTypes(OpaqueTypeKey<'tcx>, Option<OpaqueHiddenType<'tcx>>),
     TypeVariables(type_variable::UndoLog<'tcx>),
     ConstUnificationTable(sv::UndoLog<ut::Delegate<ty::ConstVid<'tcx>>>),
     IntUnificationTable(sv::UndoLog<ut::Delegate<ty::IntVid>>),
@@ -65,6 +68,7 @@ impl_from! {
 impl<'tcx> Rollback<UndoLog<'tcx>> for InferCtxtInner<'tcx> {
     fn reverse(&mut self, undo: UndoLog<'tcx>) {
         match undo {
+            UndoLog::OpaqueTypes(key, idx) => self.opaque_type_storage.remove(key, idx),
             UndoLog::TypeVariables(undo) => self.type_variable_storage.reverse(undo),
             UndoLog::ConstUnificationTable(undo) => self.const_unification_storage.reverse(undo),
             UndoLog::IntUnificationTable(undo) => self.int_unification_storage.reverse(undo),
diff --git a/compiler/rustc_infer/src/traits/util.rs b/compiler/rustc_infer/src/traits/util.rs
index 674c75fdee5..6600d5e4d02 100644
--- a/compiler/rustc_infer/src/traits/util.rs
+++ b/compiler/rustc_infer/src/traits/util.rs
@@ -167,6 +167,9 @@ impl<'tcx> Elaborator<'tcx> {
                 // Currently, we do not elaborate WF predicates,
                 // although we easily could.
             }
+            ty::PredicateKind::OpaqueType(..) => {
+                todo!("{:#?}", obligation)
+            }
             ty::PredicateKind::ObjectSafe(..) => {
                 // Currently, we do not elaborate object-safe
                 // predicates.
diff --git a/compiler/rustc_lint/src/builtin.rs b/compiler/rustc_lint/src/builtin.rs
index 50a3df21a3b..cec32823ee2 100644
--- a/compiler/rustc_lint/src/builtin.rs
+++ b/compiler/rustc_lint/src/builtin.rs
@@ -1675,6 +1675,7 @@ impl<'tcx> LateLintPass<'tcx> for TrivialConstraints {
                     Coerce(..) |
                     ConstEvaluatable(..) |
                     ConstEquate(..) |
+                    OpaqueType(..) |
                     TypeWellFormedFromEnv(..) => continue,
                 };
                 if predicate.is_global() {
diff --git a/compiler/rustc_middle/src/infer/canonical.rs b/compiler/rustc_middle/src/infer/canonical.rs
index 419ed429246..f3ea83a32b5 100644
--- a/compiler/rustc_middle/src/infer/canonical.rs
+++ b/compiler/rustc_middle/src/infer/canonical.rs
@@ -178,6 +178,12 @@ pub struct QueryResponse<'tcx, R> {
     pub var_values: CanonicalVarValues<'tcx>,
     pub region_constraints: QueryRegionConstraints<'tcx>,
     pub certainty: Certainty,
+    /// List of opaque types which we tried to compare to another type.
+    /// Inside the query we don't know yet whether the opaque type actually
+    /// should get its hidden type inferred. So we bubble the opaque type
+    /// and the type it was compared against upwards and let the query caller
+    /// handle it.
+    pub opaque_types: Vec<(Ty<'tcx>, Ty<'tcx>)>,
     pub value: R,
 }
 
diff --git a/compiler/rustc_middle/src/ty/_match.rs b/compiler/rustc_middle/src/ty/_match.rs
index 738c48dbb5c..3243ef28ff0 100644
--- a/compiler/rustc_middle/src/ty/_match.rs
+++ b/compiler/rustc_middle/src/ty/_match.rs
@@ -53,17 +53,17 @@ impl<'tcx> TypeRelation<'tcx> for Match<'tcx> {
         self.relate(a, b)
     }
 
+    #[instrument(skip(self), level = "debug")]
     fn regions(
         &mut self,
         a: ty::Region<'tcx>,
         b: ty::Region<'tcx>,
     ) -> RelateResult<'tcx, ty::Region<'tcx>> {
-        debug!("{}.regions({:?}, {:?})", self.tag(), a, b);
         Ok(a)
     }
 
+    #[instrument(skip(self), level = "debug")]
     fn tys(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
-        debug!("{}.tys({:?}, {:?})", self.tag(), a, b);
         if a == b {
             return Ok(a);
         }
diff --git a/compiler/rustc_middle/src/ty/context.rs b/compiler/rustc_middle/src/ty/context.rs
index f51e6c2bc1f..ac9f04ee055 100644
--- a/compiler/rustc_middle/src/ty/context.rs
+++ b/compiler/rustc_middle/src/ty/context.rs
@@ -33,6 +33,7 @@ use rustc_data_structures::sharded::{IntoPointer, ShardedHashMap};
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
 use rustc_data_structures::steal::Steal;
 use rustc_data_structures::sync::{self, Lock, Lrc, WorkerLocal};
+use rustc_data_structures::vec_map::VecMap;
 use rustc_errors::ErrorGuaranteed;
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
@@ -485,9 +486,13 @@ pub struct TypeckResults<'tcx> {
     /// this field will be set to `Some(ErrorGuaranteed)`.
     pub tainted_by_errors: Option<ErrorGuaranteed>,
 
-    /// All the opaque types that are restricted to concrete types
-    /// by this function.
-    pub concrete_opaque_types: FxHashSet<DefId>,
+    /// All the opaque types that have hidden types set
+    /// by this function. For return-position-impl-trait we also store the
+    /// type here, so that mir-borrowck can figure out hidden types,
+    /// even if they are only set in dead code (which doesn't show up in MIR).
+    /// For type-alias-impl-trait, this map is only used to prevent query cycles,
+    /// so the hidden types are all `None`.
+    pub concrete_opaque_types: VecMap<DefId, Option<Ty<'tcx>>>,
 
     /// Tracks the minimum captures required for a closure;
     /// see `MinCaptureInformationMap` for more details.
diff --git a/compiler/rustc_middle/src/ty/flags.rs b/compiler/rustc_middle/src/ty/flags.rs
index 7a3d615862c..06ac21e5d57 100644
--- a/compiler/rustc_middle/src/ty/flags.rs
+++ b/compiler/rustc_middle/src/ty/flags.rs
@@ -265,6 +265,10 @@ impl FlagComputation {
             ty::PredicateKind::TypeWellFormedFromEnv(ty) => {
                 self.add_ty(ty);
             }
+            ty::PredicateKind::OpaqueType(opaque, ty) => {
+                self.add_ty(opaque);
+                self.add_ty(ty);
+            }
         }
     }
 
diff --git a/compiler/rustc_middle/src/ty/fold.rs b/compiler/rustc_middle/src/ty/fold.rs
index 780d380da36..388109e6512 100644
--- a/compiler/rustc_middle/src/ty/fold.rs
+++ b/compiler/rustc_middle/src/ty/fold.rs
@@ -1243,15 +1243,11 @@ impl<'tcx> TypeVisitor<'tcx> for HasTypeFlagsVisitor {
     type BreakTy = FoundFlags;
 
     #[inline]
-    #[instrument(level = "trace")]
-    fn visit_ty(&mut self, t: Ty<'_>) -> ControlFlow<Self::BreakTy> {
-        debug!(
-            "HasTypeFlagsVisitor: t={:?} t.flags={:?} self.flags={:?}",
-            t,
-            t.flags(),
-            self.flags
-        );
-        if t.flags().intersects(self.flags) {
+    #[instrument(skip(self), level = "trace")]
+    fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
+        let flags = t.flags();
+        trace!(t.flags=?t.flags());
+        if flags.intersects(self.flags) {
             ControlFlow::Break(FoundFlags)
         } else {
             ControlFlow::CONTINUE
diff --git a/compiler/rustc_middle/src/ty/mod.rs b/compiler/rustc_middle/src/ty/mod.rs
index 31db66dc242..4208ef1a7f4 100644
--- a/compiler/rustc_middle/src/ty/mod.rs
+++ b/compiler/rustc_middle/src/ty/mod.rs
@@ -632,6 +632,11 @@ pub enum PredicateKind<'tcx> {
     ///
     /// Only used for Chalk.
     TypeWellFormedFromEnv(Ty<'tcx>),
+
+    /// Represents a hidden type assignment for an opaque type.
+    /// Such obligations get processed by checking whether the item currently being
+    /// type-checked may acually define it.
+    OpaqueType(Ty<'tcx>, Ty<'tcx>),
 }
 
 /// The crate outlives map is computed during typeck and contains the
@@ -999,6 +1004,7 @@ impl<'tcx> Predicate<'tcx> {
             | PredicateKind::TypeOutlives(..)
             | PredicateKind::ConstEvaluatable(..)
             | PredicateKind::ConstEquate(..)
+            | PredicateKind::OpaqueType(..)
             | PredicateKind::TypeWellFormedFromEnv(..) => None,
         }
     }
@@ -1017,6 +1023,7 @@ impl<'tcx> Predicate<'tcx> {
             | PredicateKind::ClosureKind(..)
             | PredicateKind::ConstEvaluatable(..)
             | PredicateKind::ConstEquate(..)
+            | PredicateKind::OpaqueType(..)
             | PredicateKind::TypeWellFormedFromEnv(..) => None,
         }
     }
@@ -1057,7 +1064,18 @@ impl<'tcx> InstantiatedPredicates<'tcx> {
     }
 }
 
-#[derive(Copy, Clone, Debug, PartialEq, Eq, HashStable, TyEncodable, TyDecodable, TypeFoldable)]
+#[derive(
+    Copy,
+    Clone,
+    Debug,
+    PartialEq,
+    Eq,
+    HashStable,
+    TyEncodable,
+    TyDecodable,
+    TypeFoldable,
+    Lift
+)]
 pub struct OpaqueTypeKey<'tcx> {
     pub def_id: DefId,
     pub substs: SubstsRef<'tcx>,
diff --git a/compiler/rustc_middle/src/ty/print/pretty.rs b/compiler/rustc_middle/src/ty/print/pretty.rs
index 5cfd9a5edfb..d5f5a1572b8 100644
--- a/compiler/rustc_middle/src/ty/print/pretty.rs
+++ b/compiler/rustc_middle/src/ty/print/pretty.rs
@@ -647,20 +647,23 @@ pub trait PrettyPrinter<'tcx>:
                     return Ok(self);
                 }
 
-                return with_no_queries!({
-                    let def_key = self.tcx().def_key(def_id);
-                    if let Some(name) = def_key.disambiguated_data.data.get_opt_name() {
-                        p!(write("{}", name));
-                        // FIXME(eddyb) print this with `print_def_path`.
-                        if !substs.is_empty() {
-                            p!("::");
-                            p!(generic_delimiters(|cx| cx.comma_sep(substs.iter())));
+                let parent = self.tcx().parent(def_id).expect("opaque types always have a parent");
+                match self.tcx().def_kind(parent) {
+                    DefKind::TyAlias | DefKind::AssocTy => {
+                        if let ty::Opaque(d, _) = *self.tcx().type_of(parent).kind() {
+                            if d == def_id {
+                                // If the type alias directly starts with the `impl` of the
+                                // opaque type we're printing, then skip the `::{opaque#1}`.
+                                p!(print_def_path(parent, substs));
+                                return Ok(self);
+                            }
                         }
+                        // Complex opaque type, e.g. `type Foo = (i32, impl Debug);`
+                        p!(print_def_path(def_id, substs));
                         return Ok(self);
                     }
-
-                    self.pretty_print_opaque_impl_type(def_id, substs)
-                });
+                    _ => return self.pretty_print_opaque_impl_type(def_id, substs),
+                }
             }
             ty::Str => p!("str"),
             ty::Generator(did, substs, movability) => {
@@ -2651,6 +2654,9 @@ define_print_and_forward_display! {
             ty::PredicateKind::TypeWellFormedFromEnv(ty) => {
                 p!("the type `", print(ty), "` is found in the environment")
             }
+            ty::PredicateKind::OpaqueType(a, b) => {
+                p!("opaque type assigment with `", print(a), "` == `", print(b) ,"`")
+            }
         }
     }
 
diff --git a/compiler/rustc_middle/src/ty/structural_impls.rs b/compiler/rustc_middle/src/ty/structural_impls.rs
index 5c7910db362..576f4121c42 100644
--- a/compiler/rustc_middle/src/ty/structural_impls.rs
+++ b/compiler/rustc_middle/src/ty/structural_impls.rs
@@ -198,6 +198,9 @@ impl<'tcx> fmt::Debug for ty::PredicateKind<'tcx> {
             ty::PredicateKind::TypeWellFormedFromEnv(ty) => {
                 write!(f, "TypeWellFormedFromEnv({:?})", ty)
             }
+            ty::PredicateKind::OpaqueType(a, b) => {
+                write!(f, "OpaqueType({:?}, {:?})", a.kind(), b.kind())
+            }
         }
     }
 }
@@ -471,6 +474,9 @@ impl<'a, 'tcx> Lift<'tcx> for ty::PredicateKind<'a> {
             ty::PredicateKind::TypeWellFormedFromEnv(ty) => {
                 tcx.lift(ty).map(ty::PredicateKind::TypeWellFormedFromEnv)
             }
+            ty::PredicateKind::OpaqueType(opaque, ty) => {
+                Some(ty::PredicateKind::OpaqueType(tcx.lift(opaque)?, tcx.lift(ty)?))
+            }
         }
     }
 }
diff --git a/compiler/rustc_middle/src/ty/sty.rs b/compiler/rustc_middle/src/ty/sty.rs
index dc16460ca43..335ddeb66db 100644
--- a/compiler/rustc_middle/src/ty/sty.rs
+++ b/compiler/rustc_middle/src/ty/sty.rs
@@ -1925,6 +1925,13 @@ impl<'tcx> Ty<'tcx> {
         }
     }
 
+    pub fn expect_opaque_type(self) -> ty::OpaqueTypeKey<'tcx> {
+        match *self.kind() {
+            Opaque(def_id, substs) => ty::OpaqueTypeKey { def_id, substs },
+            _ => bug!("`expect_opaque_type` called on non-opaque type: {}", self),
+        }
+    }
+
     pub fn simd_size_and_type(self, tcx: TyCtxt<'tcx>) -> (u64, Ty<'tcx>) {
         match self.kind() {
             Adt(def, substs) => {
diff --git a/compiler/rustc_mir_build/src/build/block.rs b/compiler/rustc_mir_build/src/build/block.rs
index f98025f7953..679043bdc30 100644
--- a/compiler/rustc_mir_build/src/build/block.rs
+++ b/compiler/rustc_mir_build/src/build/block.rs
@@ -1,8 +1,8 @@
 use crate::build::matches::ArmHasGuard;
 use crate::build::ForGuard::OutsideGuard;
 use crate::build::{BlockAnd, BlockAndExtension, BlockFrame, Builder};
-use rustc_middle::mir::*;
 use rustc_middle::thir::*;
+use rustc_middle::{mir::*, ty};
 use rustc_span::Span;
 
 impl<'a, 'tcx> Builder<'a, 'tcx> {
@@ -190,7 +190,9 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             // This return type is usually `()`, unless the block is diverging, in which case the
             // return type is `!`. For the unit type, we need to actually return the unit, but in
             // the case of `!`, no return value is required, as the block will never return.
-            if destination_ty.is_unit() {
+            // Opaque types of empty bodies also need this unit assignment, in order to infer that their
+            // type is actually unit. Otherwise there will be no defining use found in the MIR.
+            if destination_ty.is_unit() || matches!(destination_ty.kind(), ty::Opaque(..)) {
                 // We only want to assign an implicit `()` as the return value of the block if the
                 // block does not diverge. (Otherwise, we may try to assign a unit to a `!`-type.)
                 this.cfg.push_assign_unit(block, source_info, destination, this.tcx);
diff --git a/compiler/rustc_query_system/src/dep_graph/serialized.rs b/compiler/rustc_query_system/src/dep_graph/serialized.rs
index 9778f77384c..7fd20b45e77 100644
--- a/compiler/rustc_query_system/src/dep_graph/serialized.rs
+++ b/compiler/rustc_query_system/src/dep_graph/serialized.rs
@@ -181,7 +181,6 @@ impl<K: DepKind> EncoderState<K> {
         }
     }
 
-    #[instrument(level = "debug", skip(self, record_graph))]
     fn encode_node(
         &mut self,
         node: &NodeInfo<K>,
@@ -208,7 +207,6 @@ impl<K: DepKind> EncoderState<K> {
             stat.edge_counter += edge_count as u64;
         }
 
-        debug!(?index, ?node);
         let encoder = &mut self.encoder;
         if self.result.is_ok() {
             self.result = node.encode(encoder);
diff --git a/compiler/rustc_trait_selection/src/opaque_types.rs b/compiler/rustc_trait_selection/src/opaque_types.rs
index c93ff0aa6e2..c5531222fef 100644
--- a/compiler/rustc_trait_selection/src/opaque_types.rs
+++ b/compiler/rustc_trait_selection/src/opaque_types.rs
@@ -33,7 +33,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
     /// purpose of this function is to do that translation.
     ///
     /// (*) C1 and C2 were introduced in the comments on
-    /// `constrain_opaque_type`. Read that comment for more context.
+    /// `register_member_constraints`. Read that comment for more context.
     ///
     /// # Parameters
     ///
@@ -48,6 +48,10 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         instantiated_ty: Ty<'tcx>,
         span: Span,
     ) -> Ty<'tcx> {
+        if self.is_tainted_by_errors() {
+            return self.tcx.ty_error();
+        }
+
         let OpaqueTypeKey { def_id, substs } = opaque_type_key;
 
         // Use substs to build up a reverse map from regions to their
@@ -67,7 +71,6 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
         // after producing an error for each of them.
         let definition_ty = instantiated_ty.fold_with(&mut ReverseMapper::new(
             self.tcx,
-            self.is_tainted_by_errors(),
             def_id,
             map,
             instantiated_ty,
@@ -82,10 +85,6 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
 struct ReverseMapper<'tcx> {
     tcx: TyCtxt<'tcx>,
 
-    /// If errors have already been reported in this fn, we suppress
-    /// our own errors because they are sometimes derivative.
-    tainted_by_errors: bool,
-
     opaque_type_def_id: DefId,
     map: FxHashMap<GenericArg<'tcx>, GenericArg<'tcx>>,
     map_missing_regions_to_empty: bool,
@@ -100,7 +99,6 @@ struct ReverseMapper<'tcx> {
 impl<'tcx> ReverseMapper<'tcx> {
     fn new(
         tcx: TyCtxt<'tcx>,
-        tainted_by_errors: bool,
         opaque_type_def_id: DefId,
         map: FxHashMap<GenericArg<'tcx>, GenericArg<'tcx>>,
         hidden_ty: Ty<'tcx>,
@@ -108,7 +106,6 @@ impl<'tcx> ReverseMapper<'tcx> {
     ) -> Self {
         Self {
             tcx,
-            tainted_by_errors,
             opaque_type_def_id,
             map,
             map_missing_regions_to_empty: false,
@@ -167,9 +164,7 @@ impl<'tcx> TypeFolder<'tcx> for ReverseMapper<'tcx> {
         match self.map.get(&r.into()).map(|k| k.unpack()) {
             Some(GenericArgKind::Lifetime(r1)) => r1,
             Some(u) => panic!("region mapped to unexpected kind: {:?}", u),
-            None if self.map_missing_regions_to_empty || self.tainted_by_errors => {
-                self.tcx.lifetimes.re_root_empty
-            }
+            None if self.map_missing_regions_to_empty => self.tcx.lifetimes.re_root_empty,
             None if generics.parent.is_some() => {
                 if let Some(hidden_ty) = self.hidden_ty.take() {
                     unexpected_hidden_region_diagnostic(
@@ -359,6 +354,7 @@ crate fn required_region_bounds<'tcx>(
                 | ty::PredicateKind::RegionOutlives(..)
                 | ty::PredicateKind::ConstEvaluatable(..)
                 | ty::PredicateKind::ConstEquate(..)
+                | ty::PredicateKind::OpaqueType(..)
                 | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
                 ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(ref t, ref r)) => {
                     // Search for a bound of the form `erased_self_ty
diff --git a/compiler/rustc_trait_selection/src/traits/auto_trait.rs b/compiler/rustc_trait_selection/src/traits/auto_trait.rs
index ee9983ee8b8..ee8b3a62453 100644
--- a/compiler/rustc_trait_selection/src/traits/auto_trait.rs
+++ b/compiler/rustc_trait_selection/src/traits/auto_trait.rs
@@ -865,6 +865,7 @@ impl<'tcx> AutoTraitFinder<'tcx> {
                 | ty::PredicateKind::Subtype(..)
                 | ty::PredicateKind::ConstEvaluatable(..)
                 | ty::PredicateKind::Coerce(..)
+                | ty::PredicateKind::OpaqueType(..)
                 | ty::PredicateKind::TypeWellFormedFromEnv(..) => {}
             };
         }
diff --git a/compiler/rustc_trait_selection/src/traits/codegen.rs b/compiler/rustc_trait_selection/src/traits/codegen.rs
index a607fb6c1b8..22182faa0c6 100644
--- a/compiler/rustc_trait_selection/src/traits/codegen.rs
+++ b/compiler/rustc_trait_selection/src/traits/codegen.rs
@@ -92,6 +92,11 @@ pub fn codegen_fulfill_obligation<'tcx>(
         });
         let impl_source = drain_fulfillment_cx_or_panic(&infcx, &mut fulfill_cx, impl_source);
 
+        // We may constrain the hidden types of opaque types in this query, but this is
+        // not information our callers need, as all that information is handled by borrowck
+        // and typeck.
+        drop(infcx.inner.borrow_mut().opaque_type_storage.take_opaque_types());
+
         debug!("Cache miss: {:?} => {:?}", trait_ref, impl_source);
         Ok(&*tcx.arena.alloc(impl_source))
     })
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
index 468c7a3c55b..218abdd573d 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
@@ -797,6 +797,10 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
                         span,
                         "TypeWellFormedFromEnv predicate should only exist in the environment"
                     ),
+
+                    ty::PredicateKind::OpaqueType(..) => {
+                        todo!("{:#?}", obligation);
+                    }
                 }
             }
 
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
index d65b390af34..39da99ac252 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
@@ -1294,6 +1294,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
             ty::Generator(..) => "generator",
             _ => "function",
         };
+        let span = self.tcx.sess.source_map().guess_head_span(span);
         let mut err = struct_span_err!(
             self.tcx.sess,
             span,
@@ -1673,6 +1674,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
             ));
 
             let original_span = err.span.primary_span().unwrap();
+            let original_span = self.tcx.sess.source_map().guess_head_span(original_span);
             let mut span = MultiSpan::from_span(original_span);
 
             let message = outer_generator
diff --git a/compiler/rustc_trait_selection/src/traits/fulfill.rs b/compiler/rustc_trait_selection/src/traits/fulfill.rs
index 1b862834467..d55d06ebbf4 100644
--- a/compiler/rustc_trait_selection/src/traits/fulfill.rs
+++ b/compiler/rustc_trait_selection/src/traits/fulfill.rs
@@ -413,6 +413,9 @@ impl<'a, 'b, 'tcx> FulfillProcessor<'a, 'b, 'tcx> {
                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
                     bug!("TypeWellFormedFromEnv is only used for Chalk")
                 }
+                ty::PredicateKind::OpaqueType(..) => {
+                    todo!("{:#?}", obligation);
+                }
             },
             Some(pred) => match pred {
                 ty::PredicateKind::Trait(data) => {
@@ -662,6 +665,20 @@ impl<'a, 'b, 'tcx> FulfillProcessor<'a, 'b, 'tcx> {
                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
                     bug!("TypeWellFormedFromEnv is only used for Chalk")
                 }
+                ty::PredicateKind::OpaqueType(a, b) => {
+                    match self.selcx.infcx().handle_opaque_type(
+                        a,
+                        b,
+                        &obligation.cause,
+                        obligation.param_env,
+                    ) {
+                        Ok(value) => ProcessResult::Changed(mk_pending(value.obligations)),
+                        Err(err) => ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
+                            ExpectedFound::new(true, a, b),
+                            err,
+                        )),
+                    }
+                }
             },
         }
     }
diff --git a/compiler/rustc_trait_selection/src/traits/object_safety.rs b/compiler/rustc_trait_selection/src/traits/object_safety.rs
index 6cceec86213..eab207600ab 100644
--- a/compiler/rustc_trait_selection/src/traits/object_safety.rs
+++ b/compiler/rustc_trait_selection/src/traits/object_safety.rs
@@ -313,6 +313,7 @@ fn predicate_references_self<'tcx>(
         | ty::PredicateKind::Coerce(..)
         | ty::PredicateKind::ConstEvaluatable(..)
         | ty::PredicateKind::ConstEquate(..)
+        | ty::PredicateKind::OpaqueType(..)
         | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
     }
 }
@@ -344,6 +345,7 @@ fn generics_require_sized_self(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
             | ty::PredicateKind::TypeOutlives(..)
             | ty::PredicateKind::ConstEvaluatable(..)
             | ty::PredicateKind::ConstEquate(..)
+            | ty::PredicateKind::OpaqueType(..)
             | ty::PredicateKind::TypeWellFormedFromEnv(..) => false,
         }
     })
diff --git a/compiler/rustc_trait_selection/src/traits/query/type_op/custom.rs b/compiler/rustc_trait_selection/src/traits/query/type_op/custom.rs
index 0a85676f431..605c9ace5ed 100644
--- a/compiler/rustc_trait_selection/src/traits/query/type_op/custom.rs
+++ b/compiler/rustc_trait_selection/src/traits/query/type_op/custom.rs
@@ -3,7 +3,8 @@ use crate::infer::{InferCtxt, InferOk};
 use crate::traits::engine::TraitEngineExt as _;
 use crate::traits::query::type_op::TypeOpOutput;
 use crate::traits::query::Fallible;
-use crate::traits::{ObligationCause, TraitEngine};
+use crate::traits::TraitEngine;
+use rustc_infer::infer::region_constraints::RegionConstraintData;
 use rustc_infer::traits::TraitEngineExt as _;
 use rustc_span::source_map::DUMMY_SP;
 
@@ -31,6 +32,9 @@ where
     G: Fn() -> String,
 {
     type Output = R;
+    /// We can't do any custom error reporting for `CustomTypeOp`, so
+    /// we can use `!` to enforce that the implementation never provides it.
+    type ErrorInfo = !;
 
     /// Processes the operation and all resulting obligations,
     /// returning the final result along with any region constraints
@@ -40,7 +44,7 @@ where
             info!("fully_perform({:?})", self);
         }
 
-        scrape_region_constraints(infcx, || (self.closure)(infcx))
+        Ok(scrape_region_constraints(infcx, || (self.closure)(infcx))?.0)
     }
 }
 
@@ -55,12 +59,11 @@ where
 
 /// Executes `op` and then scrapes out all the "old style" region
 /// constraints that result, creating query-region-constraints.
-fn scrape_region_constraints<'tcx, Op: super::TypeOp<'tcx, Output = R>, R>(
+pub fn scrape_region_constraints<'tcx, Op: super::TypeOp<'tcx, Output = R>, R>(
     infcx: &InferCtxt<'_, 'tcx>,
     op: impl FnOnce() -> Fallible<InferOk<'tcx, R>>,
-) -> Fallible<TypeOpOutput<'tcx, Op>> {
+) -> Fallible<(TypeOpOutput<'tcx, Op>, RegionConstraintData<'tcx>)> {
     let mut fulfill_cx = <dyn TraitEngine<'_>>::new(infcx.tcx);
-    let dummy_body_id = ObligationCause::dummy().body_id;
 
     // During NLL, we expect that nobody will register region
     // obligations **except** as part of a custom type op (and, at the
@@ -75,7 +78,6 @@ fn scrape_region_constraints<'tcx, Op: super::TypeOp<'tcx, Output = R>, R>(
     );
 
     let InferOk { value, obligations } = infcx.commit_if_ok(|_| op())?;
-    debug_assert!(obligations.iter().all(|o| o.cause.body_id == dummy_body_id));
     fulfill_cx.register_predicate_obligations(infcx, obligations);
     let errors = fulfill_cx.select_all_or_error(infcx);
     if !errors.is_empty() {
@@ -99,12 +101,18 @@ fn scrape_region_constraints<'tcx, Op: super::TypeOp<'tcx, Output = R>, R>(
     );
 
     if region_constraints.is_empty() {
-        Ok(TypeOpOutput { output: value, constraints: None, canonicalized_query: None })
+        Ok((
+            TypeOpOutput { output: value, constraints: None, error_info: None },
+            region_constraint_data,
+        ))
     } else {
-        Ok(TypeOpOutput {
-            output: value,
-            constraints: Some(Rc::new(region_constraints)),
-            canonicalized_query: None,
-        })
+        Ok((
+            TypeOpOutput {
+                output: value,
+                constraints: Some(Rc::new(region_constraints)),
+                error_info: None,
+            },
+            region_constraint_data,
+        ))
     }
 }
diff --git a/compiler/rustc_trait_selection/src/traits/query/type_op/mod.rs b/compiler/rustc_trait_selection/src/traits/query/type_op/mod.rs
index d662f61e2cf..1e72dd69339 100644
--- a/compiler/rustc_trait_selection/src/traits/query/type_op/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/query/type_op/mod.rs
@@ -28,6 +28,7 @@ pub use rustc_middle::traits::query::type_op::*;
 /// cannot be completed).
 pub trait TypeOp<'tcx>: Sized + fmt::Debug {
     type Output;
+    type ErrorInfo;
 
     /// Processes the operation and all resulting obligations,
     /// returning the final result along with any region constraints
@@ -41,9 +42,8 @@ pub struct TypeOpOutput<'tcx, Op: TypeOp<'tcx>> {
     pub output: Op::Output,
     /// Any region constraints from performing the type op.
     pub constraints: Option<Rc<QueryRegionConstraints<'tcx>>>,
-    /// The canonicalized form of the query.
-    /// This for error reporting to be able to rerun the query.
-    pub canonicalized_query: Option<Canonical<'tcx, Op>>,
+    /// Used for error reporting to be able to rerun the query
+    pub error_info: Option<Op::ErrorInfo>,
 }
 
 /// "Query type ops" are type ops that are implemented using a
@@ -119,10 +119,11 @@ where
     Q: QueryTypeOp<'tcx>,
 {
     type Output = Q::QueryResponse;
+    type ErrorInfo = Canonical<'tcx, ParamEnvAnd<'tcx, Q>>;
 
     fn fully_perform(self, infcx: &InferCtxt<'_, 'tcx>) -> Fallible<TypeOpOutput<'tcx, Self>> {
         let mut region_constraints = QueryRegionConstraints::default();
-        let (output, canonicalized_query, mut obligations, _) =
+        let (output, error_info, mut obligations, _) =
             Q::fully_perform_into(self, infcx, &mut region_constraints)?;
 
         // Typically, instantiating NLL query results does not
@@ -160,6 +161,6 @@ where
         let region_constraints =
             if region_constraints.is_empty() { None } else { Some(Rc::new(region_constraints)) };
 
-        Ok(TypeOpOutput { output, constraints: region_constraints, canonicalized_query })
+        Ok(TypeOpOutput { output, constraints: region_constraints, error_info })
     }
 }
diff --git a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
index 3e7a2252318..cf472813e9e 100644
--- a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
@@ -256,6 +256,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         };
 
         if obligation.predicate.skip_binder().self_ty().is_ty_var() {
+            debug!(ty = ?obligation.predicate.skip_binder().self_ty(), "ambiguous inference var or opaque type");
             // Self is a type variable (e.g., `_: AsRef<str>`).
             //
             // This is somewhat problematic, as the current scheme can't really
diff --git a/compiler/rustc_trait_selection/src/traits/select/mod.rs b/compiler/rustc_trait_selection/src/traits/select/mod.rs
index 72d156067a1..9b8a49e512b 100644
--- a/compiler/rustc_trait_selection/src/traits/select/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/mod.rs
@@ -33,6 +33,7 @@ use rustc_middle::dep_graph::{DepKind, DepNodeIndex};
 use rustc_middle::mir::interpret::ErrorHandled;
 use rustc_middle::thir::abstract_const::NotConstEvaluatable;
 use rustc_middle::ty::fast_reject::{self, TreatParams};
+use rustc_middle::ty::fold::BottomUpFolder;
 use rustc_middle::ty::print::with_no_trimmed_paths;
 use rustc_middle::ty::relate::TypeRelation;
 use rustc_middle::ty::subst::{GenericArgKind, Subst, SubstsRef};
@@ -672,6 +673,19 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
                     bug!("TypeWellFormedFromEnv is only used for chalk")
                 }
+                ty::PredicateKind::OpaqueType(a, b) => {
+                    match self.infcx().handle_opaque_type(
+                        a,
+                        b,
+                        &obligation.cause,
+                        obligation.param_env,
+                    ) {
+                        Ok(res) => {
+                            self.evaluate_predicates_recursively(previous_stack, res.obligations)
+                        }
+                        Err(_) => Ok(EvaluatedToErr),
+                    }
+                }
             }
         });
 
@@ -1313,6 +1327,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         }
     }
 
+    #[instrument(skip(self, param_env, cache_fresh_trait_pred, dep_node), level = "debug")]
     fn insert_candidate_cache(
         &mut self,
         mut param_env: ty::ParamEnv<'tcx>,
@@ -1353,6 +1368,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     /// a projection, look at the bounds of `T::Bar`, see if we can find a
     /// `Baz` bound. We return indexes into the list returned by
     /// `tcx.item_bounds` for any applicable bounds.
+    #[instrument(level = "debug", skip(self))]
     fn match_projection_obligation_against_definition_bounds(
         &mut self,
         obligation: &TraitObligation<'tcx>,
@@ -1360,10 +1376,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         let poly_trait_predicate = self.infcx().resolve_vars_if_possible(obligation.predicate);
         let placeholder_trait_predicate =
             self.infcx().replace_bound_vars_with_placeholders(poly_trait_predicate);
-        debug!(
-            ?placeholder_trait_predicate,
-            "match_projection_obligation_against_definition_bounds"
-        );
+        debug!(?placeholder_trait_predicate);
 
         let tcx = self.infcx.tcx;
         let (def_id, substs) = match *placeholder_trait_predicate.trait_ref.self_ty().kind() {
@@ -1414,7 +1427,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             })
             .collect();
 
-        debug!(?matching_bounds, "match_projection_obligation_against_definition_bounds");
+        debug!(?matching_bounds);
         matching_bounds
     }
 
@@ -1444,6 +1457,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         });
         self.infcx
             .at(&obligation.cause, obligation.param_env)
+            .define_opaque_types(false)
             .sup(ty::Binder::dummy(placeholder_trait_ref), trait_bound)
             .map(|InferOk { obligations: _, value: () }| {
                 // This method is called within a probe, so we can't have
@@ -1506,6 +1520,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         let is_match = self
             .infcx
             .at(&obligation.cause, obligation.param_env)
+            .define_opaque_types(false)
             .sup(obligation.predicate, infer_projection)
             .map_or(false, |InferOk { obligations, value: () }| {
                 self.evaluate_predicates_recursively(
@@ -2081,11 +2096,22 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         match self.match_impl(impl_def_id, obligation) {
             Ok(substs) => substs,
             Err(()) => {
-                bug!(
-                    "Impl {:?} was matchable against {:?} but now is not",
-                    impl_def_id,
-                    obligation
+                self.infcx.tcx.sess.delay_span_bug(
+                    obligation.cause.span,
+                    &format!(
+                        "Impl {:?} was matchable against {:?} but now is not",
+                        impl_def_id, obligation
+                    ),
                 );
+                let value = self.infcx.fresh_substs_for_item(obligation.cause.span, impl_def_id);
+                let err = self.tcx().ty_error();
+                let value = value.fold_with(&mut BottomUpFolder {
+                    tcx: self.tcx(),
+                    ty_op: |_| err,
+                    lt_op: |l| l,
+                    ct_op: |c| c,
+                });
+                Normalized { value, obligations: vec![] }
             }
         }
     }
@@ -2220,6 +2246,11 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     ) -> Result<Vec<PredicateObligation<'tcx>>, ()> {
         self.infcx
             .at(&obligation.cause, obligation.param_env)
+            // We don't want predicates for opaque types to just match all other types,
+            // if there is an obligation on the opaque type, then that obligation must be met
+            // opaquely. Otherwise we'd match any obligation to the opaque type and then error
+            // out later.
+            .define_opaque_types(false)
             .sup(obligation.predicate.to_poly_trait_ref(), poly_trait_ref)
             .map(|InferOk { obligations, .. }| obligations)
             .map_err(|_| ())
diff --git a/compiler/rustc_trait_selection/src/traits/wf.rs b/compiler/rustc_trait_selection/src/traits/wf.rs
index b4ed5b95b10..2ac68cea544 100644
--- a/compiler/rustc_trait_selection/src/traits/wf.rs
+++ b/compiler/rustc_trait_selection/src/traits/wf.rs
@@ -148,6 +148,10 @@ pub fn predicate_obligations<'a, 'tcx>(
             wf.compute(c1.into());
             wf.compute(c2.into());
         }
+        ty::PredicateKind::OpaqueType(opaque, ty) => {
+            wf.compute(opaque.into());
+            wf.compute(ty.into());
+        }
         ty::PredicateKind::TypeWellFormedFromEnv(..) => {
             bug!("TypeWellFormedFromEnv is only used for Chalk")
         }
diff --git a/compiler/rustc_traits/src/chalk/lowering.rs b/compiler/rustc_traits/src/chalk/lowering.rs
index e3c865ce9e6..1d5b5c6c3cd 100644
--- a/compiler/rustc_traits/src/chalk/lowering.rs
+++ b/compiler/rustc_traits/src/chalk/lowering.rs
@@ -110,6 +110,7 @@ impl<'tcx> LowerInto<'tcx, chalk_ir::InEnvironment<chalk_ir::Goal<RustInterner<'
                 | ty::PredicateKind::ClosureKind(..)
                 | ty::PredicateKind::Subtype(..)
                 | ty::PredicateKind::Coerce(..)
+                | ty::PredicateKind::OpaqueType(..)
                 | ty::PredicateKind::ConstEvaluatable(..)
                 | ty::PredicateKind::ConstEquate(..) => bug!("unexpected predicate {}", predicate),
             };
@@ -202,6 +203,7 @@ impl<'tcx> LowerInto<'tcx, chalk_ir::GoalData<RustInterner<'tcx>>> for ty::Predi
             ty::PredicateKind::ClosureKind(..)
             | ty::PredicateKind::Coerce(..)
             | ty::PredicateKind::ConstEvaluatable(..)
+            | ty::PredicateKind::OpaqueType(..)
             | ty::PredicateKind::ConstEquate(..) => {
                 chalk_ir::GoalData::All(chalk_ir::Goals::empty(interner))
             }
@@ -621,6 +623,7 @@ impl<'tcx> LowerInto<'tcx, Option<chalk_ir::QuantifiedWhereClause<RustInterner<'
             | ty::PredicateKind::Coerce(..)
             | ty::PredicateKind::ConstEvaluatable(..)
             | ty::PredicateKind::ConstEquate(..)
+            | ty::PredicateKind::OpaqueType(..)
             | ty::PredicateKind::TypeWellFormedFromEnv(..) => {
                 bug!("unexpected predicate {}", &self)
             }
@@ -750,6 +753,7 @@ impl<'tcx> LowerInto<'tcx, Option<chalk_solve::rust_ir::QuantifiedInlineBound<Ru
             | ty::PredicateKind::Coerce(..)
             | ty::PredicateKind::ConstEvaluatable(..)
             | ty::PredicateKind::ConstEquate(..)
+            | ty::PredicateKind::OpaqueType(..)
             | ty::PredicateKind::TypeWellFormedFromEnv(..) => {
                 bug!("unexpected predicate {}", &self)
             }
diff --git a/compiler/rustc_traits/src/chalk/mod.rs b/compiler/rustc_traits/src/chalk/mod.rs
index 3c2a266dab9..4e19479da87 100644
--- a/compiler/rustc_traits/src/chalk/mod.rs
+++ b/compiler/rustc_traits/src/chalk/mod.rs
@@ -134,6 +134,7 @@ crate fn evaluate_goal<'tcx>(
                 var_values: CanonicalVarValues { var_values },
                 region_constraints: QueryRegionConstraints::default(),
                 certainty: Certainty::Proven,
+                opaque_types: vec![],
                 value: (),
             },
         };
@@ -162,6 +163,7 @@ crate fn evaluate_goal<'tcx>(
                                     .make_identity(tcx),
                                 region_constraints: QueryRegionConstraints::default(),
                                 certainty: Certainty::Ambiguous,
+                                opaque_types: vec![],
                                 value: (),
                             },
                         };
diff --git a/compiler/rustc_traits/src/implied_outlives_bounds.rs b/compiler/rustc_traits/src/implied_outlives_bounds.rs
index 90c698db8fb..1535a46a01b 100644
--- a/compiler/rustc_traits/src/implied_outlives_bounds.rs
+++ b/compiler/rustc_traits/src/implied_outlives_bounds.rs
@@ -105,6 +105,7 @@ fn compute_implied_outlives_bounds<'tcx>(
                     | ty::PredicateKind::ObjectSafe(..)
                     | ty::PredicateKind::ConstEvaluatable(..)
                     | ty::PredicateKind::ConstEquate(..)
+                    | ty::PredicateKind::OpaqueType(..)
                     | ty::PredicateKind::TypeWellFormedFromEnv(..) => vec![],
                     ty::PredicateKind::WellFormed(arg) => {
                         wf_args.push(arg);
diff --git a/compiler/rustc_traits/src/normalize_erasing_regions.rs b/compiler/rustc_traits/src/normalize_erasing_regions.rs
index a4aa965ec95..1d26f6a12a3 100644
--- a/compiler/rustc_traits/src/normalize_erasing_regions.rs
+++ b/compiler/rustc_traits/src/normalize_erasing_regions.rs
@@ -69,6 +69,7 @@ fn not_outlives_predicate<'tcx>(p: ty::Predicate<'tcx>) -> bool {
         | ty::PredicateKind::Coerce(..)
         | ty::PredicateKind::ConstEvaluatable(..)
         | ty::PredicateKind::ConstEquate(..)
+        | ty::PredicateKind::OpaqueType(..)
         | ty::PredicateKind::TypeWellFormedFromEnv(..) => true,
     }
 }
diff --git a/compiler/rustc_type_ir/src/lib.rs b/compiler/rustc_type_ir/src/lib.rs
index e26f0033156..b015e40b683 100644
--- a/compiler/rustc_type_ir/src/lib.rs
+++ b/compiler/rustc_type_ir/src/lib.rs
@@ -61,6 +61,14 @@ bitflags! {
                                           | TypeFlags::HAS_CT_INFER.bits
                                           | TypeFlags::HAS_TY_PLACEHOLDER.bits
                                           | TypeFlags::HAS_CT_PLACEHOLDER.bits
+                                          // The `evaluate_obligation` query does not return further
+                                          // obligations. If it evaluates an obligation with an opaque
+                                          // type, that opaque type may get compared to another type,
+                                          // constraining it. We would lose this information.
+                                          // FIXME: differentiate between crate-local opaque types
+                                          // and opaque types from other crates, as only opaque types
+                                          // from the local crate can possibly be a local name
+                                          | TypeFlags::HAS_TY_OPAQUE.bits
                                           // We consider 'freshened' types and constants
                                           // to depend on a particular fn.
                                           // The freshening process throws away information,
diff --git a/compiler/rustc_typeck/src/check/_match.rs b/compiler/rustc_typeck/src/check/_match.rs
index 9529e1528a8..79be8bde55a 100644
--- a/compiler/rustc_typeck/src/check/_match.rs
+++ b/compiler/rustc_typeck/src/check/_match.rs
@@ -4,7 +4,7 @@ use rustc_errors::{Applicability, Diagnostic};
 use rustc_hir::{self as hir, ExprKind};
 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use rustc_infer::traits::Obligation;
-use rustc_middle::ty::{self, ToPredicate, Ty};
+use rustc_middle::ty::{self, ToPredicate, Ty, TypeFoldable};
 use rustc_span::{MultiSpan, Span};
 use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
 use rustc_trait_selection::traits::{
@@ -98,8 +98,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             let arm_ty = self.check_expr_with_expectation(&arm.body, expected);
             all_arms_diverge &= self.diverges.get();
 
-            let opt_suggest_box_span =
-                self.opt_suggest_box_span(arm.body.span, arm_ty, orig_expected);
+            let opt_suggest_box_span = self.opt_suggest_box_span(arm_ty, orig_expected);
 
             let (arm_span, semi_span) =
                 self.get_appropriate_arm_semicolon_removal_span(&arms, i, prior_arm_ty, arm_ty);
@@ -504,20 +503,15 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     // provide a structured suggestion in that case.
     pub(crate) fn opt_suggest_box_span(
         &self,
-        span: Span,
         outer_ty: Ty<'tcx>,
         orig_expected: Expectation<'tcx>,
     ) -> Option<Span> {
-        match (orig_expected, self.ret_coercion_impl_trait.map(|ty| (self.body_id.owner, ty))) {
-            (Expectation::ExpectHasType(expected), Some((_id, ty)))
-                if self.in_tail_expr && self.can_coerce(outer_ty, expected) =>
+        match orig_expected {
+            Expectation::ExpectHasType(expected)
+                if self.in_tail_expr
+                    && self.ret_coercion.as_ref()?.borrow().merged_ty().has_opaque_types()
+                    && self.can_coerce(outer_ty, expected) =>
             {
-                let impl_trait_ret_ty =
-                    self.infcx.instantiate_opaque_types(self.body_id, self.param_env, ty, span);
-                assert!(
-                    impl_trait_ret_ty.obligations.is_empty(),
-                    "we should never get new obligations here"
-                );
                 let obligations = self.fulfillment_cx.borrow().pending_obligations();
                 let mut suggest_box = !obligations.is_empty();
                 for o in obligations {
diff --git a/compiler/rustc_typeck/src/check/callee.rs b/compiler/rustc_typeck/src/check/callee.rs
index d5187c109e3..2a5cf03e9d0 100644
--- a/compiler/rustc_typeck/src/check/callee.rs
+++ b/compiler/rustc_typeck/src/check/callee.rs
@@ -538,7 +538,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         expected: Expectation<'tcx>,
         fn_sig: ty::FnSig<'tcx>,
     ) -> Ty<'tcx> {
-        // `fn_sig` is the *signature* of the cosure being called. We
+        // `fn_sig` is the *signature* of the closure being called. We
         // don't know the full details yet (`Fn` vs `FnMut` etc), but we
         // do know the types expected for each argument and the return
         // type.
diff --git a/compiler/rustc_typeck/src/check/check.rs b/compiler/rustc_typeck/src/check/check.rs
index e57d55fdc23..1a5f118ea50 100644
--- a/compiler/rustc_typeck/src/check/check.rs
+++ b/compiler/rustc_typeck/src/check/check.rs
@@ -17,7 +17,7 @@ use rustc_middle::ty::fold::TypeFoldable;
 use rustc_middle::ty::layout::{LayoutError, MAX_SIMD_LANES};
 use rustc_middle::ty::subst::GenericArgKind;
 use rustc_middle::ty::util::{Discr, IntTypeExt};
-use rustc_middle::ty::{self, OpaqueTypeKey, ParamEnv, Ty, TyCtxt};
+use rustc_middle::ty::{self, ParamEnv, Ty, TyCtxt};
 use rustc_session::lint::builtin::{UNINHABITED_STATIC, UNSUPPORTED_CALLING_CONVENTIONS};
 use rustc_span::symbol::sym;
 use rustc_span::{self, MultiSpan, Span};
@@ -83,8 +83,6 @@ pub(super) fn check_fn<'a, 'tcx>(
     can_be_generator: Option<hir::Movability>,
     return_type_pre_known: bool,
 ) -> (FnCtxt<'a, 'tcx>, Option<GeneratorTypes<'tcx>>) {
-    let mut fn_sig = fn_sig;
-
     // Create the function context. This is either derived from scratch or,
     // in the case of closures, based on the outer context.
     let mut fcx = FnCtxt::new(inherited, param_env, body.value.hir_id);
@@ -97,21 +95,8 @@ pub(super) fn check_fn<'a, 'tcx>(
 
     let declared_ret_ty = fn_sig.output();
 
-    let revealed_ret_ty =
-        fcx.instantiate_opaque_types_from_value(declared_ret_ty, decl.output.span());
-    debug!("check_fn: declared_ret_ty: {}, revealed_ret_ty: {}", declared_ret_ty, revealed_ret_ty);
-    fcx.ret_coercion = Some(RefCell::new(CoerceMany::new(revealed_ret_ty)));
+    fcx.ret_coercion = Some(RefCell::new(CoerceMany::new(declared_ret_ty)));
     fcx.ret_type_span = Some(decl.output.span());
-    if let ty::Opaque(..) = declared_ret_ty.kind() {
-        fcx.ret_coercion_impl_trait = Some(declared_ret_ty);
-    }
-    fn_sig = tcx.mk_fn_sig(
-        fn_sig.inputs().iter().cloned(),
-        revealed_ret_ty,
-        fn_sig.c_variadic,
-        fn_sig.unsafety,
-        fn_sig.abi,
-    );
 
     let span = body.value.span;
 
@@ -253,7 +238,7 @@ pub(super) fn check_fn<'a, 'tcx>(
             fcx.next_ty_var(TypeVariableOrigin { kind: TypeVariableOriginKind::DynReturnFn, span });
         debug!("actual_return_ty replaced with {:?}", actual_return_ty);
     }
-    fcx.demand_suptype(span, revealed_ret_ty, actual_return_ty);
+    fcx.demand_suptype(span, declared_ret_ty, actual_return_ty);
 
     // Check that a function marked as `#[panic_handler]` has signature `fn(&PanicInfo) -> !`
     if let Some(panic_impl_did) = tcx.lang_items().panic_impl() {
@@ -656,6 +641,8 @@ fn check_opaque_meets_bounds<'tcx>(
     span: Span,
     origin: &hir::OpaqueTyOrigin,
 ) {
+    let hidden_type = tcx.type_of(def_id).subst(tcx, substs);
+
     let hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
     let defining_use_anchor = match *origin {
         hir::OpaqueTyOrigin::FnReturn(did) | hir::OpaqueTyOrigin::AsyncFn(did) => did,
@@ -670,25 +657,13 @@ fn check_opaque_meets_bounds<'tcx>(
 
         let misc_cause = traits::ObligationCause::misc(span, hir_id);
 
-        let _ = inh.register_infer_ok_obligations(
-            infcx.instantiate_opaque_types(hir_id, param_env, opaque_ty, span),
-        );
-
-        let opaque_type_map = infcx.inner.borrow().opaque_types.clone();
-        for (OpaqueTypeKey { def_id, substs }, opaque_defn) in opaque_type_map {
-            let hidden_type = tcx.type_of(def_id).subst(tcx, substs);
-            trace!(?hidden_type);
-            match infcx.at(&misc_cause, param_env).eq(opaque_defn.concrete_ty, hidden_type) {
-                Ok(infer_ok) => inh.register_infer_ok_obligations(infer_ok),
-                Err(ty_err) => {
-                    tcx.sess.delay_span_bug(
-                        span,
-                        &format!(
-                            "could not check bounds on revealed type `{}`:\n{}",
-                            hidden_type, ty_err,
-                        ),
-                    );
-                }
+        match infcx.at(&misc_cause, param_env).eq(opaque_ty, hidden_type) {
+            Ok(infer_ok) => inh.register_infer_ok_obligations(infer_ok),
+            Err(ty_err) => {
+                tcx.sess.delay_span_bug(
+                    span,
+                    &format!("could not unify `{}` with revealed type:\n{}", hidden_type, ty_err,),
+                );
             }
         }
 
@@ -701,7 +676,7 @@ fn check_opaque_meets_bounds<'tcx>(
 
         match origin {
             // Checked when type checking the function containing them.
-            hir::OpaqueTyOrigin::FnReturn(..) | hir::OpaqueTyOrigin::AsyncFn(..) => return,
+            hir::OpaqueTyOrigin::FnReturn(..) | hir::OpaqueTyOrigin::AsyncFn(..) => {}
             // Can have different predicates to their defining use
             hir::OpaqueTyOrigin::TyAlias => {
                 // Finally, resolve all regions. This catches wily misuses of
@@ -710,6 +685,9 @@ fn check_opaque_meets_bounds<'tcx>(
                 fcx.regionck_item(hir_id, span, FxHashSet::default());
             }
         }
+
+        // Clean up after ourselves
+        let _ = infcx.inner.borrow_mut().opaque_type_storage.take_opaque_types();
     });
 }
 
diff --git a/compiler/rustc_typeck/src/check/closure.rs b/compiler/rustc_typeck/src/check/closure.rs
index 59c8febdc30..83ef76a48cd 100644
--- a/compiler/rustc_typeck/src/check/closure.rs
+++ b/compiler/rustc_typeck/src/check/closure.rs
@@ -3,16 +3,20 @@
 use super::{check_fn, Expectation, FnCtxt, GeneratorTypes};
 
 use crate::astconv::AstConv;
+use crate::rustc_middle::ty::subst::Subst;
+use hir::OpaqueTyOrigin;
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_hir::lang_items::LangItem;
 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use rustc_infer::infer::LateBoundRegionConversionTime;
 use rustc_infer::infer::{InferOk, InferResult};
+use rustc_infer::traits::ObligationCause;
 use rustc_middle::ty::fold::TypeFoldable;
 use rustc_middle::ty::subst::InternalSubsts;
 use rustc_middle::ty::{self, Ty};
 use rustc_span::source_map::Span;
+use rustc_span::DUMMY_SP;
 use rustc_target::spec::abi::Abi;
 use rustc_trait_selection::traits::error_reporting::ArgKind;
 use rustc_trait_selection::traits::error_reporting::InferCtxtExt as _;
@@ -172,6 +176,29 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         expected_ty: Ty<'tcx>,
     ) -> (Option<ExpectedSig<'tcx>>, Option<ty::ClosureKind>) {
         match *expected_ty.kind() {
+            ty::Opaque(def_id, substs) => {
+                let bounds = self.tcx.explicit_item_bounds(def_id);
+                let sig = bounds.iter().find_map(|(pred, span)| match pred.kind().skip_binder() {
+                    ty::PredicateKind::Projection(proj_predicate) => self
+                        .deduce_sig_from_projection(
+                            Some(*span),
+                            pred.kind().rebind(proj_predicate.subst(self.tcx, substs)),
+                        ),
+                    _ => None,
+                });
+
+                let kind = bounds
+                    .iter()
+                    .filter_map(|(pred, _)| match pred.kind().skip_binder() {
+                        ty::PredicateKind::Trait(tp) => {
+                            self.tcx.fn_trait_kind_from_lang_item(tp.def_id())
+                        }
+                        _ => None,
+                    })
+                    .fold(None, |best, cur| Some(best.map_or(cur, |best| cmp::min(best, cur))));
+                trace!(?sig, ?kind);
+                (sig, kind)
+            }
             ty::Dynamic(ref object_type, ..) => {
                 let sig = object_type.projection_bounds().find_map(|pb| {
                     let pb = pb.with_self_ty(self.tcx, self.tcx.types.trait_object_dummy_self);
@@ -197,10 +224,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     ) -> (Option<ExpectedSig<'tcx>>, Option<ty::ClosureKind>) {
         let expected_sig =
             self.obligations_for_self_ty(expected_vid).find_map(|(_, obligation)| {
-                debug!(
-                    "deduce_expectations_from_obligations: obligation.predicate={:?}",
-                    obligation.predicate
-                );
+                debug!(?obligation.predicate);
 
                 let bound_predicate = obligation.predicate.kind();
                 if let ty::PredicateKind::Projection(proj_predicate) =
@@ -235,6 +259,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// The `cause_span` should be the span that caused us to
     /// have this expected signature, or `None` if we can't readily
     /// know that.
+    #[instrument(level = "debug", skip(self, cause_span))]
     fn deduce_sig_from_projection(
         &self,
         cause_span: Option<Span>,
@@ -242,15 +267,13 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     ) -> Option<ExpectedSig<'tcx>> {
         let tcx = self.tcx;
 
-        debug!("deduce_sig_from_projection({:?})", projection);
-
         let trait_def_id = projection.trait_def_id(tcx);
 
         let is_fn = tcx.fn_trait_kind_from_lang_item(trait_def_id).is_some();
         let gen_trait = tcx.require_lang_item(LangItem::Generator, cause_span);
         let is_gen = gen_trait == trait_def_id;
         if !is_fn && !is_gen {
-            debug!("deduce_sig_from_projection: not fn or generator");
+            debug!("not fn or generator");
             return None;
         }
 
@@ -259,7 +282,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             // associated item and not yield.
             let return_assoc_item = self.tcx.associated_item_def_ids(gen_trait)[1];
             if return_assoc_item != projection.projection_def_id() {
-                debug!("deduce_sig_from_projection: not return assoc item of generator");
+                debug!("not return assoc item of generator");
                 return None;
             }
         }
@@ -267,7 +290,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let input_tys = if is_fn {
             let arg_param_ty = projection.skip_binder().projection_ty.substs.type_at(1);
             let arg_param_ty = self.resolve_vars_if_possible(arg_param_ty);
-            debug!("deduce_sig_from_projection: arg_param_ty={:?}", arg_param_ty);
+            debug!(?arg_param_ty);
 
             match arg_param_ty.kind() {
                 &ty::Tuple(tys) => tys,
@@ -282,7 +305,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         // Since this is a return parameter type it is safe to unwrap.
         let ret_param_ty = projection.skip_binder().term.ty().unwrap();
         let ret_param_ty = self.resolve_vars_if_possible(ret_param_ty);
-        debug!("deduce_sig_from_projection: ret_param_ty={:?}", ret_param_ty);
+        debug!(?ret_param_ty);
 
         let sig = projection.rebind(self.tcx.mk_fn_sig(
             input_tys.iter(),
@@ -291,7 +314,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             hir::Unsafety::Normal,
             Abi::Rust,
         ));
-        debug!("deduce_sig_from_projection: sig={:?}", sig);
+        debug!(?sig);
 
         Some(ExpectedSig { cause_span, sig })
     }
@@ -401,9 +424,14 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         // in this binder we are creating.
         assert!(!expected_sig.sig.skip_binder().has_vars_bound_above(ty::INNERMOST));
         let bound_sig = expected_sig.sig.map_bound(|sig| {
+            let output = self.hide_parent_opaque_types(
+                sig.output(),
+                expected_sig.cause_span.unwrap_or(DUMMY_SP),
+                body.id().hir_id,
+            );
             self.tcx.mk_fn_sig(
                 sig.inputs().iter().cloned(),
-                sig.output(),
+                output,
                 sig.c_variadic,
                 hir::Unsafety::Normal,
                 Abi::RustCall,
@@ -590,6 +618,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 _ => astconv.ty_infer(None, decl.output.span()),
             },
         };
+        let supplied_return =
+            self.hide_parent_opaque_types(supplied_return, decl.output.span(), body.id().hir_id);
 
         let result = ty::Binder::bind_with_vars(
             self.tcx.mk_fn_sig(
@@ -610,27 +640,57 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         result
     }
 
+    fn hide_parent_opaque_types(&self, ty: Ty<'tcx>, span: Span, body_id: hir::HirId) -> Ty<'tcx> {
+        ty.fold_with(&mut ty::fold::BottomUpFolder {
+            tcx: self.infcx.tcx,
+            lt_op: |lt| lt,
+            ct_op: |ct| ct,
+            ty_op: |ty| match *ty.kind() {
+                // Closures can't create hidden types for opaque types of their parent, as they
+                // do not have all the outlives information available. Also `type_of` looks for
+                // hidden types in the owner (so the closure's parent), so it would not find these
+                // definitions.
+                ty::Opaque(def_id, _substs)
+                    if matches!(
+                        self.infcx.opaque_type_origin(def_id, DUMMY_SP),
+                        Some(OpaqueTyOrigin::FnReturn(..))
+                    ) =>
+                {
+                    let ty_var = self.next_ty_var(TypeVariableOrigin {
+                        kind: TypeVariableOriginKind::TypeInference,
+                        span,
+                    });
+                    let cause = ObligationCause::misc(span, body_id);
+                    self.register_predicates(vec![self.infcx.opaque_ty_obligation(
+                        ty,
+                        ty_var,
+                        true,
+                        self.param_env,
+                        cause,
+                    )]);
+                    ty_var
+                }
+                _ => ty,
+            },
+        })
+    }
+
     /// Invoked when we are translating the generator that results
     /// from desugaring an `async fn`. Returns the "sugared" return
     /// type of the `async fn` -- that is, the return type that the
     /// user specified. The "desugared" return type is an `impl
     /// Future<Output = T>`, so we do this by searching through the
     /// obligations to extract the `T`.
+    #[instrument(skip(self), level = "debug")]
     fn deduce_future_output_from_obligations(&self, expr_def_id: DefId) -> Option<Ty<'tcx>> {
-        debug!("deduce_future_output_from_obligations(expr_def_id={:?})", expr_def_id);
-
         let ret_coercion = self.ret_coercion.as_ref().unwrap_or_else(|| {
             span_bug!(self.tcx.def_span(expr_def_id), "async fn generator outside of a fn")
         });
 
-        // In practice, the return type of the surrounding function is
-        // always a (not yet resolved) inference variable, because it
-        // is the hidden type for an `impl Trait` that we are going to
-        // be inferring.
         let ret_ty = ret_coercion.borrow().expected_ty();
         let ret_ty = self.inh.infcx.shallow_resolve(ret_ty);
-        let ret_vid = match *ret_ty.kind() {
-            ty::Infer(ty::TyVar(ret_vid)) => ret_vid,
+        let (def_id, substs) = match *ret_ty.kind() {
+            ty::Opaque(def_id, substs) => (def_id, substs),
             ty::Error(_) => return None,
             _ => span_bug!(
                 self.tcx.def_span(expr_def_id),
@@ -638,17 +698,19 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             ),
         };
 
+        let item_bounds = self.tcx.explicit_item_bounds(def_id);
+
         // Search for a pending obligation like
         //
         // `<R as Future>::Output = T`
         //
         // where R is the return type we are expecting. This type `T`
         // will be our output.
-        let output_ty = self.obligations_for_self_ty(ret_vid).find_map(|(_, obligation)| {
-            let bound_predicate = obligation.predicate.kind();
+        let output_ty = item_bounds.iter().find_map(|&(predicate, span)| {
+            let bound_predicate = predicate.subst(self.tcx, substs).kind();
             if let ty::PredicateKind::Projection(proj_predicate) = bound_predicate.skip_binder() {
                 self.deduce_future_output_from_projection(
-                    obligation.cause.span,
+                    span,
                     bound_predicate.rebind(proj_predicate),
                 )
             } else {
diff --git a/compiler/rustc_typeck/src/check/coercion.rs b/compiler/rustc_typeck/src/check/coercion.rs
index c0eda02a999..2f973a54f88 100644
--- a/compiler/rustc_typeck/src/check/coercion.rs
+++ b/compiler/rustc_typeck/src/check/coercion.rs
@@ -1272,7 +1272,7 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
 
     /// Returns the current "merged type", representing our best-guess
     /// at the LUB of the expressions we've seen so far (if any). This
-    /// isn't *final* until you call `self.final()`, which will return
+    /// isn't *final* until you call `self.complete()`, which will return
     /// the merged type.
     pub fn merged_ty(&self) -> Ty<'tcx> {
         self.final_ty.unwrap_or(self.expected_ty)
diff --git a/compiler/rustc_typeck/src/check/expectation.rs b/compiler/rustc_typeck/src/check/expectation.rs
index e9e81034477..9e1a70b7dfb 100644
--- a/compiler/rustc_typeck/src/check/expectation.rs
+++ b/compiler/rustc_typeck/src/check/expectation.rs
@@ -1,5 +1,6 @@
 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use rustc_middle::ty::{self, Ty};
+use rustc_span::DUMMY_SP;
 use rustc_span::{self, Span};
 
 use super::Expectation::*;
@@ -43,7 +44,7 @@ impl<'a, 'tcx> Expectation<'tcx> {
     // when checking the 'then' block which are incompatible with the
     // 'else' branch.
     pub(super) fn adjust_for_branches(&self, fcx: &FnCtxt<'a, 'tcx>) -> Expectation<'tcx> {
-        match *self {
+        match self.strip_opaque(fcx) {
             ExpectHasType(ety) => {
                 let ety = fcx.shallow_resolve(ety);
                 if !ety.is_ty_var() { ExpectHasType(ety) } else { NoExpectation }
@@ -104,14 +105,35 @@ impl<'a, 'tcx> Expectation<'tcx> {
     /// for the program to type-check). `only_has_type` will return
     /// such a constraint, if it exists.
     pub(super) fn only_has_type(self, fcx: &FnCtxt<'a, 'tcx>) -> Option<Ty<'tcx>> {
-        match self {
-            ExpectHasType(ty) => Some(fcx.resolve_vars_if_possible(ty)),
+        match self.strip_opaque(fcx) {
+            ExpectHasType(ty) => Some(ty),
             NoExpectation | ExpectCastableToType(_) | ExpectRvalueLikeUnsized(_) | IsLast(_) => {
                 None
             }
         }
     }
 
+    /// We must not treat opaque types as expected types in their defining scope, as that
+    /// will break `fn foo() -> impl Trait { if cond { a } else { b } }` if `a` and `b` are
+    /// only "equal" if they coerce to a common target, like two different function items
+    /// coercing to a function pointer if they have the same signature.
+    fn strip_opaque(self, fcx: &FnCtxt<'a, 'tcx>) -> Self {
+        match self {
+            ExpectHasType(ty) => {
+                let ty = fcx.resolve_vars_if_possible(ty);
+                match *ty.kind() {
+                    ty::Opaque(def_id, _)
+                        if fcx.infcx.opaque_type_origin(def_id, DUMMY_SP).is_some() =>
+                    {
+                        NoExpectation
+                    }
+                    _ => self,
+                }
+            }
+            _ => self,
+        }
+    }
+
     /// Like `only_has_type`, but instead of returning `None` if no
     /// hard constraint exists, creates a fresh type variable.
     pub(super) fn coercion_target_type(self, fcx: &FnCtxt<'a, 'tcx>, span: Span) -> Ty<'tcx> {
diff --git a/compiler/rustc_typeck/src/check/expr.rs b/compiler/rustc_typeck/src/check/expr.rs
index e5784259ce8..81cebaef3aa 100644
--- a/compiler/rustc_typeck/src/check/expr.rs
+++ b/compiler/rustc_typeck/src/check/expr.rs
@@ -965,8 +965,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             };
             let else_diverges = self.diverges.get();
 
-            let opt_suggest_box_span =
-                self.opt_suggest_box_span(else_expr.span, else_ty, orig_expected);
+            let opt_suggest_box_span = self.opt_suggest_box_span(else_ty, orig_expected);
             let if_cause =
                 self.if_cause(sp, then_expr, else_expr, then_ty, else_ty, opt_suggest_box_span);
 
diff --git a/compiler/rustc_typeck/src/check/fallback.rs b/compiler/rustc_typeck/src/check/fallback.rs
index 7214cdf3312..85132317824 100644
--- a/compiler/rustc_typeck/src/check/fallback.rs
+++ b/compiler/rustc_typeck/src/check/fallback.rs
@@ -24,7 +24,7 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
             self.fulfillment_cx.borrow_mut().pending_obligations()
         );
 
-        // Check if we have any unsolved varibales. If not, no need for fallback.
+        // Check if we have any unsolved variables. If not, no need for fallback.
         let unsolved_variables = self.unsolved_variables();
         if unsolved_variables.is_empty() {
             return false;
@@ -66,16 +66,6 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
         // refer to opaque types.
         self.select_obligations_where_possible(fallback_has_occurred, |_| {});
 
-        // We now run fallback again, but this time we allow it to replace
-        // unconstrained opaque type variables, in addition to performing
-        // other kinds of fallback.
-        for ty in &self.unsolved_variables() {
-            fallback_has_occurred |= self.fallback_opaque_type_vars(*ty);
-        }
-
-        // See if we can make any more progress.
-        self.select_obligations_where_possible(fallback_has_occurred, |_| {});
-
         fallback_has_occurred
     }
 
@@ -136,59 +126,6 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
         true
     }
 
-    /// Second round of fallback: Unconstrained type variables created
-    /// from the instantiation of an opaque type fall back to the
-    /// opaque type itself. This is a somewhat incomplete attempt to
-    /// manage "identity passthrough" for `impl Trait` types.
-    ///
-    /// For example, in this code:
-    ///
-    ///```
-    /// type MyType = impl Copy;
-    /// fn defining_use() -> MyType { true }
-    /// fn other_use() -> MyType { defining_use() }
-    /// ```
-    ///
-    /// `defining_use` will constrain the instantiated inference
-    /// variable to `bool`, while `other_use` will constrain
-    /// the instantiated inference variable to `MyType`.
-    ///
-    /// When we process opaque types during writeback, we
-    /// will handle cases like `other_use`, and not count
-    /// them as defining usages
-    ///
-    /// However, we also need to handle cases like this:
-    ///
-    /// ```rust
-    /// pub type Foo = impl Copy;
-    /// fn produce() -> Option<Foo> {
-    ///     None
-    ///  }
-    ///  ```
-    ///
-    /// In the above snippet, the inference variable created by
-    /// instantiating `Option<Foo>` will be completely unconstrained.
-    /// We treat this as a non-defining use by making the inference
-    /// variable fall back to the opaque type itself.
-    fn fallback_opaque_type_vars(&self, ty: Ty<'tcx>) -> bool {
-        let span = self
-            .infcx
-            .type_var_origin(ty)
-            .map(|origin| origin.span)
-            .unwrap_or(rustc_span::DUMMY_SP);
-        let oty = self.inner.borrow().opaque_types_vars.get(&ty).copied();
-        if let Some(opaque_ty) = oty {
-            debug!(
-                "fallback_opaque_type_vars(ty={:?}): falling back to opaque type {:?}",
-                ty, opaque_ty
-            );
-            self.demand_eqtype(span, ty, opaque_ty);
-            true
-        } else {
-            return false;
-        }
-    }
-
     /// The "diverging fallback" system is rather complicated. This is
     /// a result of our need to balance 'do the right thing' with
     /// backwards compatibility.
diff --git a/compiler/rustc_typeck/src/check/fn_ctxt/_impl.rs b/compiler/rustc_typeck/src/check/fn_ctxt/_impl.rs
index e3439a6f1d9..83587951f25 100644
--- a/compiler/rustc_typeck/src/check/fn_ctxt/_impl.rs
+++ b/compiler/rustc_typeck/src/check/fn_ctxt/_impl.rs
@@ -372,23 +372,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         (result, spans)
     }
 
-    /// Replaces the opaque types from the given value with type variables,
-    /// and records the `OpaqueTypeMap` for later use during writeback. See
-    /// `InferCtxt::instantiate_opaque_types` for more details.
-    #[instrument(skip(self, value_span), level = "debug")]
-    pub(in super::super) fn instantiate_opaque_types_from_value<T: TypeFoldable<'tcx>>(
-        &self,
-        value: T,
-        value_span: Span,
-    ) -> T {
-        self.register_infer_ok_obligations(self.instantiate_opaque_types(
-            self.body_id,
-            self.param_env,
-            value,
-            value_span,
-        ))
-    }
-
     /// Convenience method which tracks extra diagnostic information for normalization
     /// that occurs as a result of WF checking. The `hir_id` is the `HirId` of the hir item
     /// whose type is being wf-checked - this is used to construct a more precise span if
@@ -749,6 +732,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     // inference variable.
                     ty::PredicateKind::ClosureKind(..) => None,
                     ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
+                    ty::PredicateKind::OpaqueType(..) => None,
                 }
             })
             .filter(move |(tr, _)| self.self_type_matches_expected_vid(*tr, ty_var_root))
@@ -775,6 +759,31 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     ) -> Vec<Ty<'tcx>> {
         let formal_ret = self.resolve_vars_with_obligations(formal_ret);
         let Some(ret_ty) = expected_ret.only_has_type(self) else { return Vec::new() };
+
+        // HACK(oli-obk): This is a backwards compatibility hack. Without it, the inference
+        // variable will get instantiated with the opaque type. The inference variable often
+        // has various helpful obligations registered for it that help closures figure out their
+        // signature. If we infer the inference var to the opaque type, the closure won't be able
+        // to find those obligations anymore, and it can't necessarily find them from the opaque
+        // type itself. We could be more powerful with inference if we *combined* the obligations
+        // so that we got both the obligations from the opaque type and the ones from the inference
+        // variable. That will accept more code than we do right now, so we need to carefully consider
+        // the implications.
+        // Note: this check is pessimistic, as the inference type could be matched with something other
+        // than the opaque type, but then we need a new `TypeRelation` just for this specific case and
+        // can't re-use `sup` below.
+        if formal_ret.has_infer_types() {
+            for ty in ret_ty.walk() {
+                if let ty::subst::GenericArgKind::Type(ty) = ty.unpack() {
+                    if let ty::Opaque(def_id, _) = *ty.kind() {
+                        if self.infcx.opaque_type_origin(def_id, DUMMY_SP).is_some() {
+                            return Vec::new();
+                        }
+                    }
+                }
+            }
+        }
+
         let expect_args = self
             .fudge_inference_if_ok(|| {
                 // Attempt to apply a subtyping relationship between the formal
diff --git a/compiler/rustc_typeck/src/check/fn_ctxt/mod.rs b/compiler/rustc_typeck/src/check/fn_ctxt/mod.rs
index 222c14d0d47..6c55f821294 100644
--- a/compiler/rustc_typeck/src/check/fn_ctxt/mod.rs
+++ b/compiler/rustc_typeck/src/check/fn_ctxt/mod.rs
@@ -57,8 +57,6 @@ pub struct FnCtxt<'a, 'tcx> {
     /// any).
     pub(super) ret_coercion: Option<RefCell<DynamicCoerceMany<'tcx>>>,
 
-    pub(super) ret_coercion_impl_trait: Option<Ty<'tcx>>,
-
     pub(super) ret_type_span: Option<Span>,
 
     /// Used exclusively to reduce cost of advanced evaluation used for
@@ -130,7 +128,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             param_env,
             err_count_on_creation: inh.tcx.sess.err_count(),
             ret_coercion: None,
-            ret_coercion_impl_trait: None,
             ret_type_span: None,
             in_tail_expr: false,
             ret_coercion_span: Cell::new(None),
diff --git a/compiler/rustc_typeck/src/check/inherited.rs b/compiler/rustc_typeck/src/check/inherited.rs
index beb6b371b2b..b775f24f8ef 100644
--- a/compiler/rustc_typeck/src/check/inherited.rs
+++ b/compiler/rustc_typeck/src/check/inherited.rs
@@ -95,6 +95,13 @@ impl<'tcx> InheritedBuilder<'tcx> {
         let def_id = self.def_id;
         self.infcx.enter(|infcx| f(Inherited::new(infcx, def_id)))
     }
+
+    /// WF-checking doesn't need to recompute opaque types and can instead use
+    /// the type_of query to get them from typeck.
+    pub fn reveal_defining_opaque_types(mut self) -> Self {
+        self.infcx = self.infcx.reveal_defining_opaque_types();
+        self
+    }
 }
 
 impl<'a, 'tcx> Inherited<'a, 'tcx> {
@@ -119,8 +126,8 @@ impl<'a, 'tcx> Inherited<'a, 'tcx> {
         }
     }
 
+    #[instrument(level = "debug", skip(self))]
     pub(super) fn register_predicate(&self, obligation: traits::PredicateObligation<'tcx>) {
-        debug!("register_predicate({:?})", obligation);
         if obligation.has_escaping_bound_vars() {
             span_bug!(obligation.cause.span, "escaping bound vars in predicate {:?}", obligation);
         }
diff --git a/compiler/rustc_typeck/src/check/method/probe.rs b/compiler/rustc_typeck/src/check/method/probe.rs
index 704866c2cc2..26119e5a064 100644
--- a/compiler/rustc_typeck/src/check/method/probe.rs
+++ b/compiler/rustc_typeck/src/check/method/probe.rs
@@ -858,6 +858,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
                 | ty::PredicateKind::TypeOutlives(..)
                 | ty::PredicateKind::ConstEvaluatable(..)
                 | ty::PredicateKind::ConstEquate(..)
+                | ty::PredicateKind::OpaqueType(..)
                 | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
             }
         });
@@ -1476,6 +1477,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
             TraitCandidate(trait_ref) => self.probe(|_| {
                 let _ = self
                     .at(&ObligationCause::dummy(), self.param_env)
+                    .define_opaque_types(false)
                     .sup(candidate.xform_self_ty, self_ty);
                 match self.select_trait_candidate(trait_ref) {
                     Ok(Some(traits::ImplSource::UserDefined(ref impl_data))) => {
@@ -1505,6 +1507,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
             // First check that the self type can be related.
             let sub_obligations = match self
                 .at(&ObligationCause::dummy(), self.param_env)
+                .define_opaque_types(false)
                 .sup(probe.xform_self_ty, self_ty)
             {
                 Ok(InferOk { obligations, value: () }) => obligations,
@@ -1655,6 +1658,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
                     );
                     if self
                         .at(&ObligationCause::dummy(), self.param_env)
+                        .define_opaque_types(false)
                         .sup(return_ty, xform_ret_ty)
                         .is_err()
                     {
diff --git a/compiler/rustc_typeck/src/check/mod.rs b/compiler/rustc_typeck/src/check/mod.rs
index 31cc3fa2ac6..b6111793678 100644
--- a/compiler/rustc_typeck/src/check/mod.rs
+++ b/compiler/rustc_typeck/src/check/mod.rs
@@ -343,6 +343,7 @@ fn diagnostic_only_typeck<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> &ty::T
     typeck_with_fallback(tcx, def_id, fallback)
 }
 
+#[instrument(skip(tcx, fallback))]
 fn typeck_with_fallback<'tcx>(
     tcx: TyCtxt<'tcx>,
     def_id: LocalDefId,
diff --git a/compiler/rustc_typeck/src/check/upvar.rs b/compiler/rustc_typeck/src/check/upvar.rs
index 7106f8f9d7a..70bc46c5123 100644
--- a/compiler/rustc_typeck/src/check/upvar.rs
+++ b/compiler/rustc_typeck/src/check/upvar.rs
@@ -335,11 +335,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     // Returns a list of `Ty`s for each upvar.
     fn final_upvar_tys(&self, closure_id: DefId) -> Vec<Ty<'tcx>> {
-        // Presently an unboxed closure type cannot "escape" out of a
-        // function, so we will only encounter ones that originated in the
-        // local crate or were inlined into it along with some function.
-        // This may change if abstract return types of some sort are
-        // implemented.
         self.typeck_results
             .borrow()
             .closure_min_captures_flattened(closure_id)
diff --git a/compiler/rustc_typeck/src/check/wfcheck.rs b/compiler/rustc_typeck/src/check/wfcheck.rs
index d6eeef3323c..250576bff04 100644
--- a/compiler/rustc_typeck/src/check/wfcheck.rs
+++ b/compiler/rustc_typeck/src/check/wfcheck.rs
@@ -968,7 +968,7 @@ fn for_item<'tcx>(tcx: TyCtxt<'tcx>, item: &hir::Item<'_>) -> CheckWfFcxBuilder<
 
 fn for_id(tcx: TyCtxt<'_>, def_id: LocalDefId, span: Span) -> CheckWfFcxBuilder<'_> {
     CheckWfFcxBuilder {
-        inherited: Inherited::build(tcx, def_id),
+        inherited: Inherited::build(tcx, def_id).reveal_defining_opaque_types(),
         id: hir::HirId::make_owner(def_id),
         span,
         param_env: tcx.param_env(def_id),
diff --git a/compiler/rustc_typeck/src/check/writeback.rs b/compiler/rustc_typeck/src/check/writeback.rs
index c1cb9a1a8d1..35f1ddb65aa 100644
--- a/compiler/rustc_typeck/src/check/writeback.rs
+++ b/compiler/rustc_typeck/src/check/writeback.rs
@@ -18,7 +18,6 @@ use rustc_middle::ty::fold::{TypeFoldable, TypeFolder};
 use rustc_middle::ty::{self, ClosureSizeProfileData, Ty, TyCtxt};
 use rustc_span::symbol::sym;
 use rustc_span::Span;
-use rustc_trait_selection::opaque_types::InferCtxtExt;
 
 use std::mem;
 
@@ -65,7 +64,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         wbcx.visit_closures();
         wbcx.visit_liberated_fn_sigs();
         wbcx.visit_fru_field_types();
-        wbcx.visit_opaque_types(body.value.span);
+        wbcx.visit_opaque_types();
         wbcx.visit_coercion_casts();
         wbcx.visit_user_provided_tys();
         wbcx.visit_user_provided_sigs();
@@ -497,64 +496,18 @@ impl<'cx, 'tcx> WritebackCx<'cx, 'tcx> {
             fcx_typeck_results.generator_interior_types.clone();
     }
 
-    #[instrument(skip(self, span), level = "debug")]
-    fn visit_opaque_types(&mut self, span: Span) {
-        let opaque_types = self.fcx.infcx.inner.borrow().opaque_types.clone();
-        for (opaque_type_key, opaque_defn) in opaque_types {
-            let hir_id =
-                self.tcx().hir().local_def_id_to_hir_id(opaque_type_key.def_id.expect_local());
-            let instantiated_ty = self.resolve(opaque_defn.concrete_ty, &hir_id);
-
-            debug_assert!(!instantiated_ty.has_escaping_bound_vars());
-
-            let opaque_type_key = self.fcx.fully_resolve(opaque_type_key).unwrap();
-
-            // Prevent:
-            // * `fn foo<T>() -> Foo<T>`
-            // * `fn foo<T: Bound + Other>() -> Foo<T>`
-            // from being defining.
-
-            // Also replace all generic params with the ones from the opaque type
-            // definition so that
-            // ```rust
-            // type Foo<T> = impl Baz + 'static;
-            // fn foo<U>() -> Foo<U> { .. }
-            // ```
-            // figures out the concrete type with `U`, but the stored type is with `T`.
-
-            // FIXME: why are we calling this here? This seems too early, and duplicated.
-            let definition_ty = self.fcx.infer_opaque_definition_from_instantiation(
-                opaque_type_key,
-                instantiated_ty,
-                span,
-            );
-
-            let mut skip_add = false;
-
-            if let ty::Opaque(definition_ty_def_id, _substs) = *definition_ty.kind() {
-                if opaque_defn.origin == hir::OpaqueTyOrigin::TyAlias {
-                    if opaque_type_key.def_id == definition_ty_def_id {
-                        debug!(
-                            "skipping adding concrete definition for opaque type {:?} {:?}",
-                            opaque_defn, opaque_type_key.def_id
-                        );
-                        skip_add = true;
-                    }
+    #[instrument(skip(self), level = "debug")]
+    fn visit_opaque_types(&mut self) {
+        let opaque_types =
+            self.fcx.infcx.inner.borrow_mut().opaque_type_storage.take_opaque_types();
+        for (opaque_type_key, decl) in opaque_types {
+            let hidden_type = match decl.origin {
+                hir::OpaqueTyOrigin::FnReturn(_) | hir::OpaqueTyOrigin::AsyncFn(_) => {
+                    Some(self.resolve(decl.hidden_type.ty, &decl.hidden_type.span))
                 }
-            }
-
-            if opaque_type_key.substs.needs_infer() {
-                span_bug!(span, "{:#?} has inference variables", opaque_type_key.substs)
-            }
-
-            // We only want to add an entry into `concrete_opaque_types`
-            // if we actually found a defining usage of this opaque type.
-            // Otherwise, we do nothing - we'll either find a defining usage
-            // in some other location, or we'll end up emitting an error due
-            // to the lack of defining usage
-            if !skip_add {
-                self.typeck_results.concrete_opaque_types.insert(opaque_type_key.def_id);
-            }
+                hir::OpaqueTyOrigin::TyAlias => None,
+            };
+            self.typeck_results.concrete_opaque_types.insert(opaque_type_key.def_id, hidden_type);
         }
     }
 
diff --git a/compiler/rustc_typeck/src/collect/type_of.rs b/compiler/rustc_typeck/src/collect/type_of.rs
index d422f355ad3..885713cd2f3 100644
--- a/compiler/rustc_typeck/src/collect/type_of.rs
+++ b/compiler/rustc_typeck/src/collect/type_of.rs
@@ -8,7 +8,7 @@ use rustc_hir::{HirId, Node};
 use rustc_middle::hir::nested_filter;
 use rustc_middle::ty::subst::InternalSubsts;
 use rustc_middle::ty::util::IntTypeExt;
-use rustc_middle::ty::{self, Ty, TyCtxt, TypeFoldable, TypeFolder};
+use rustc_middle::ty::{self, DefIdTree, Ty, TyCtxt, TypeFoldable, TypeFolder};
 use rustc_span::symbol::Ident;
 use rustc_span::{Span, DUMMY_SP};
 
@@ -359,28 +359,22 @@ pub(super) fn type_of(tcx: TyCtxt<'_>, def_id: DefId) -> Ty<'_> {
                         .get_value_matching(|(key, _)| key.def_id == def_id.to_def_id())
                         .copied()
                         .unwrap_or_else(|| {
-                            tcx.sess.delay_span_bug(
-                                DUMMY_SP,
-                                &format!(
-                                    "owner {:?} has no opaque type for {:?} in its typeck results",
-                                    owner, def_id,
-                                ),
-                            );
-                            if let Some(_) =
-                                tcx.typeck(owner).tainted_by_errors
-                            {
+                            let table = tcx.typeck(owner);
+                            if let Some(_) = table.tainted_by_errors {
                                 // Some error in the
                                 // owner fn prevented us from populating
                                 // the `concrete_opaque_types` table.
                                 tcx.ty_error()
                             } else {
-                                // We failed to resolve the opaque type or it
-                                // resolves to itself. Return the non-revealed
-                                // type, which should result in E0720.
-                                tcx.mk_opaque(
-                                    def_id.to_def_id(),
-                                    InternalSubsts::identity_for_item(tcx, def_id.to_def_id()),
-                                )
+                                table.concrete_opaque_types.get(&def_id.to_def_id()).copied().unwrap_or_else(|| {
+                                    // We failed to resolve the opaque type or it
+                                    // resolves to itself. We interpret this as the
+                                    // no values of the hidden type ever being constructed,
+                                    // so we can just make the hidden type be `!`.
+                                    // For backwards compatibility reasons, we fall back to
+                                    // `()` until we the diverging default is changed.
+                                    Some(tcx.mk_diverging_default())
+                                }).expect("RPIT always have a hidden type from typeck")
                             }
                         });
                     debug!("concrete_ty = {:?}", concrete_ty);
@@ -575,7 +569,21 @@ fn find_opaque_ty_constraints(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Ty<'_> {
             }
             // Calling `mir_borrowck` can lead to cycle errors through
             // const-checking, avoid calling it if we don't have to.
-            if !self.tcx.typeck(def_id).concrete_opaque_types.contains(&self.def_id) {
+            // ```rust
+            // type Foo = impl Fn() -> usize; // when computing type for this
+            // const fn bar() -> Foo {
+            //     || 0usize
+            // }
+            // const BAZR: Foo = bar(); // we would mir-borrowck this, causing cycles
+            // // because we again need to reveal `Foo` so we can check whether the
+            // // constant does not contain interior mutability.
+            // ```
+            let tables = self.tcx.typeck(def_id);
+            if let Some(_) = tables.tainted_by_errors {
+                self.found = Some((DUMMY_SP, self.tcx.ty_error()));
+                return;
+            }
+            if tables.concrete_opaque_types.get(&self.def_id).is_none() {
                 debug!("no constraints in typeck results");
                 return;
             }
@@ -629,7 +637,7 @@ fn find_opaque_ty_constraints(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Ty<'_> {
             intravisit::walk_expr(self, ex);
         }
         fn visit_item(&mut self, it: &'tcx Item<'tcx>) {
-            debug!("find_existential_constraints: visiting {:?}", it);
+            trace!(?it.def_id);
             // The opaque type itself or its children are not within its reveal scope.
             if it.def_id.to_def_id() != self.def_id {
                 self.check(it.def_id);
@@ -637,7 +645,7 @@ fn find_opaque_ty_constraints(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Ty<'_> {
             }
         }
         fn visit_impl_item(&mut self, it: &'tcx ImplItem<'tcx>) {
-            debug!("find_existential_constraints: visiting {:?}", it);
+            trace!(?it.def_id);
             // The opaque type itself or its children are not within its reveal scope.
             if it.def_id.to_def_id() != self.def_id {
                 self.check(it.def_id);
@@ -645,7 +653,7 @@ fn find_opaque_ty_constraints(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Ty<'_> {
             }
         }
         fn visit_trait_item(&mut self, it: &'tcx TraitItem<'tcx>) {
-            debug!("find_existential_constraints: visiting {:?}", it);
+            trace!(?it.def_id);
             self.check(it.def_id);
             intravisit::walk_trait_item(self, it);
         }
@@ -655,12 +663,12 @@ fn find_opaque_ty_constraints(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Ty<'_> {
     let scope = tcx.hir().get_defining_scope(hir_id);
     let mut locator = ConstraintLocator { def_id: def_id.to_def_id(), tcx, found: None };
 
-    debug!("find_opaque_ty_constraints: scope={:?}", scope);
+    debug!(?scope);
 
     if scope == hir::CRATE_HIR_ID {
         tcx.hir().walk_toplevel_module(&mut locator);
     } else {
-        debug!("find_opaque_ty_constraints: scope={:?}", tcx.hir().get(scope));
+        trace!("scope={:#?}", tcx.hir().get(scope));
         match tcx.hir().get(scope) {
             // We explicitly call `visit_*` methods, instead of using `intravisit::walk_*` methods
             // This allows our visitor to process the defining item itself, causing
@@ -687,7 +695,12 @@ fn find_opaque_ty_constraints(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Ty<'_> {
         Some((_, ty)) => ty,
         None => {
             let span = tcx.def_span(def_id);
-            tcx.sess.span_err(span, "could not find defining uses");
+            let name = tcx.item_name(tcx.parent(def_id.to_def_id()).unwrap());
+            let label = format!(
+                "`{}` must be used in combination with a concrete type within the same module",
+                name
+            );
+            tcx.sess.struct_span_err(span, "unconstrained opaque type").note(&label).emit();
             tcx.ty_error()
         }
     }
diff --git a/compiler/rustc_typeck/src/impl_wf_check/min_specialization.rs b/compiler/rustc_typeck/src/impl_wf_check/min_specialization.rs
index 6cef3e9d940..737e6002005 100644
--- a/compiler/rustc_typeck/src/impl_wf_check/min_specialization.rs
+++ b/compiler/rustc_typeck/src/impl_wf_check/min_specialization.rs
@@ -425,6 +425,7 @@ fn trait_predicate_kind<'tcx>(
         | ty::PredicateKind::ClosureKind(..)
         | ty::PredicateKind::ConstEvaluatable(..)
         | ty::PredicateKind::ConstEquate(..)
+        | ty::PredicateKind::OpaqueType(..)
         | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
     }
 }
diff --git a/compiler/rustc_typeck/src/outlives/explicit.rs b/compiler/rustc_typeck/src/outlives/explicit.rs
index bbf31de527e..7839ce5e4ff 100644
--- a/compiler/rustc_typeck/src/outlives/explicit.rs
+++ b/compiler/rustc_typeck/src/outlives/explicit.rs
@@ -59,6 +59,7 @@ impl<'tcx> ExplicitPredicatesMap<'tcx> {
                     | ty::PredicateKind::Coerce(..)
                     | ty::PredicateKind::ConstEvaluatable(..)
                     | ty::PredicateKind::ConstEquate(..)
+                    | ty::PredicateKind::OpaqueType(..)
                     | ty::PredicateKind::TypeWellFormedFromEnv(..) => (),
                 }
             }
author	Oli Scherer <git-spam-no-reply9815368754983@oli-obk.de>	2022-02-14 16:10:22 +0000
committer	Oli Scherer <git-spam-no-reply9815368754983@oli-obk.de>	2022-03-28 16:27:14 +0000
commit	264cd05b16198c2838ec6cd585bd122491e3f7a9 (patch)
tree	133ca794d093e515a783cd92db831ff51e197820 /compiler
parent	0e4524e5b4a9c5656fef69f532f96eb9959a1803 (diff)
download	rust-264cd05b16198c2838ec6cd585bd122491e3f7a9.tar.gz rust-264cd05b16198c2838ec6cd585bd122491e3f7a9.zip