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 1d5f463015294..dbc2bf2306d0e 100644
--- a/compiler/rustc_const_eval/src/transform/check_consts/check.rs
+++ b/compiler/rustc_const_eval/src/transform/check_consts/check.rs
@@ -396,6 +396,7 @@ impl<'mir, 'tcx> Checker<'mir, 'tcx> {
                     | ty::PredicateKind::Projection(_)
                     | ty::PredicateKind::ConstEvaluatable(..)
                     | ty::PredicateKind::ConstEquate(..)
+                    | ty::PredicateKind::OpaqueType(..)
                     | ty::PredicateKind::TypeWellFormedFromEnv(..) => continue,
                     ty::PredicateKind::ObjectSafe(_) => {
                         bug!("object safe predicate on function: {:#?}", predicate)
diff --git a/compiler/rustc_infer/src/infer/opaque_types.rs b/compiler/rustc_infer/src/infer/opaque_types.rs
index c2ef0b41e27bf..6535315a5e77b 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 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};
@@ -328,6 +329,25 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             },
         });
     }
+
+    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> {
+        PredicateObligation::new(
+            cause,
+            param_env,
+            self.tcx.mk_predicate(ty::Binder::dummy(ty::PredicateKind::OpaqueType(
+                a,
+                b,
+                a_is_expected,
+            ))),
+        )
+    }
 }
 
 // Visitor that requires that (almost) all regions in the type visited outlive
diff --git a/compiler/rustc_infer/src/infer/outlives/mod.rs b/compiler/rustc_infer/src/infer/outlives/mod.rs
index 03d6c45a65345..b60ffc1878be4 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/traits/util.rs b/compiler/rustc_infer/src/traits/util.rs
index 61588147364a9..a85ea51cd29c0 100644
--- a/compiler/rustc_infer/src/traits/util.rs
+++ b/compiler/rustc_infer/src/traits/util.rs
@@ -167,6 +167,9 @@ impl 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 61695109a894a..3bd30de76b471 100644
--- a/compiler/rustc_lint/src/builtin.rs
+++ b/compiler/rustc_lint/src/builtin.rs
@@ -1661,6 +1661,7 @@ impl<'tcx> LateLintPass<'tcx> for TrivialConstraints {
                     Coerce(..) |
                     ConstEvaluatable(..) |
                     ConstEquate(..) |
+                OpaqueType(..) |
                     TypeWellFormedFromEnv(..) => continue,
                 };
                 if predicate.is_global(cx.tcx) {
diff --git a/compiler/rustc_middle/src/ty/flags.rs b/compiler/rustc_middle/src/ty/flags.rs
index a078b6fb742a7..1b6d28a3a808d 100644
--- a/compiler/rustc_middle/src/ty/flags.rs
+++ b/compiler/rustc_middle/src/ty/flags.rs
@@ -262,6 +262,10 @@ impl FlagComputation {
             ty::PredicateKind::TypeWellFormedFromEnv(ty) => {
                 self.add_ty(ty);
             }
+            ty::PredicateKind::OpaqueType(opaque, ty, _a_is_expected) => {
+                self.add_ty(opaque);
+                self.add_ty(ty);
+            }
         }
     }
 
diff --git a/compiler/rustc_middle/src/ty/mod.rs b/compiler/rustc_middle/src/ty/mod.rs
index d0d2a46fc4c1d..b1988489f2343 100644
--- a/compiler/rustc_middle/src/ty/mod.rs
+++ b/compiler/rustc_middle/src/ty/mod.rs
@@ -605,6 +605,14 @@ 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.
+    /// The arguments may be in any order, but one of them must be an opaque type.
+    /// The order and the boolean is used for resolving which opaque type defines
+    /// the other if both are opaque types and for diagnostics.
+    OpaqueType(Ty<'tcx>, Ty<'tcx>, bool),
 }
 
 /// The crate outlives map is computed during typeck and contains the
@@ -898,6 +906,7 @@ impl<'tcx> Predicate<'tcx> {
             | PredicateKind::TypeOutlives(..)
             | PredicateKind::ConstEvaluatable(..)
             | PredicateKind::ConstEquate(..)
+            | PredicateKind::OpaqueType(..)
             | PredicateKind::TypeWellFormedFromEnv(..) => None,
         }
     }
@@ -916,6 +925,7 @@ impl<'tcx> Predicate<'tcx> {
             | PredicateKind::ClosureKind(..)
             | PredicateKind::ConstEvaluatable(..)
             | PredicateKind::ConstEquate(..)
+            | PredicateKind::OpaqueType(..)
             | PredicateKind::TypeWellFormedFromEnv(..) => None,
         }
     }
diff --git a/compiler/rustc_middle/src/ty/print/pretty.rs b/compiler/rustc_middle/src/ty/print/pretty.rs
index 175295b3199e8..0b2fadacbfa5a 100644
--- a/compiler/rustc_middle/src/ty/print/pretty.rs
+++ b/compiler/rustc_middle/src/ty/print/pretty.rs
@@ -2546,6 +2546,9 @@ define_print_and_forward_display! {
             ty::PredicateKind::TypeWellFormedFromEnv(ty) => {
                 p!("the type `", print(ty), "` is found in the environment")
             }
+            ty::PredicateKind::OpaqueType(a, b, _a_is_expected) => {
+                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 28dc9767b782e..50276370a44a9 100644
--- a/compiler/rustc_middle/src/ty/structural_impls.rs
+++ b/compiler/rustc_middle/src/ty/structural_impls.rs
@@ -197,6 +197,9 @@ impl fmt::Debug for ty::PredicateKind<'tcx> {
             ty::PredicateKind::TypeWellFormedFromEnv(ty) => {
                 write!(f, "TypeWellFormedFromEnv({:?})", ty)
             }
+            ty::PredicateKind::OpaqueType(a, b, a_is_expected) => {
+                write!(f, "OpaqueType({:?}, {:?}, {:?})", a.kind(), b.kind(), a_is_expected)
+            }
         }
     }
 }
@@ -460,6 +463,9 @@ 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, a_is_expected) => {
+                Some(ty::PredicateKind::OpaqueType(tcx.lift(opaque)?, tcx.lift(ty)?, a_is_expected))
+            }
         }
     }
 }
diff --git a/compiler/rustc_middle/src/ty/sty.rs b/compiler/rustc_middle/src/ty/sty.rs
index 1c525fb55e1b5..376e19c69d537 100644
--- a/compiler/rustc_middle/src/ty/sty.rs
+++ b/compiler/rustc_middle/src/ty/sty.rs
@@ -1815,6 +1815,13 @@ impl<'tcx> TyS<'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_trait_selection/src/opaque_types.rs b/compiler/rustc_trait_selection/src/opaque_types.rs
index ea0ac6318bc9a..6b0910b475ff2 100644
--- a/compiler/rustc_trait_selection/src/opaque_types.rs
+++ b/compiler/rustc_trait_selection/src/opaque_types.rs
@@ -359,6 +359,7 @@ 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/error_reporting/mod.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
index 8833805d35c7d..8351e4113c9d6 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
@@ -713,6 +713,10 @@ 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/fulfill.rs b/compiler/rustc_trait_selection/src/traits/fulfill.rs
index 2b5dae1d751f8..0f0b6553b0670 100644
--- a/compiler/rustc_trait_selection/src/traits/fulfill.rs
+++ b/compiler/rustc_trait_selection/src/traits/fulfill.rs
@@ -394,6 +394,9 @@ 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) => {
@@ -639,6 +642,9 @@ impl<'a, 'b, 'tcx> FulfillProcessor<'a, 'b, 'tcx> {
                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
                     bug!("TypeWellFormedFromEnv is only used for Chalk")
                 }
+                ty::PredicateKind::OpaqueType(..) => {
+                    todo!("{:#?}", obligation);
+                }
             },
         }
     }
diff --git a/compiler/rustc_trait_selection/src/traits/object_safety.rs b/compiler/rustc_trait_selection/src/traits/object_safety.rs
index 4e84849bc1e23..cb063a02a336c 100644
--- a/compiler/rustc_trait_selection/src/traits/object_safety.rs
+++ b/compiler/rustc_trait_selection/src/traits/object_safety.rs
@@ -307,6 +307,7 @@ fn predicate_references_self<'tcx>(
         | ty::PredicateKind::Coerce(..)
         | ty::PredicateKind::ConstEvaluatable(..)
         | ty::PredicateKind::ConstEquate(..)
+        | ty::PredicateKind::OpaqueType(..)
         | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
     }
 }
@@ -341,6 +342,7 @@ 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/select/mod.rs b/compiler/rustc_trait_selection/src/traits/select/mod.rs
index 607deb8f90875..fd42ecaa8cc55 100644
--- a/compiler/rustc_trait_selection/src/traits/select/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/mod.rs
@@ -649,6 +649,9 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
                     bug!("TypeWellFormedFromEnv is only used for chalk")
                 }
+                ty::PredicateKind::OpaqueType(..) => {
+                    todo!("{:#?}", obligation)
+                }
             }
         });
 
diff --git a/compiler/rustc_trait_selection/src/traits/wf.rs b/compiler/rustc_trait_selection/src/traits/wf.rs
index 5875b764e9f36..6ab5a2ad57250 100644
--- a/compiler/rustc_trait_selection/src/traits/wf.rs
+++ b/compiler/rustc_trait_selection/src/traits/wf.rs
@@ -145,6 +145,10 @@ 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 e24f699adf6b3..73140790e5338 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),
             };
@@ -196,6 +197,7 @@ impl<'tcx> LowerInto<'tcx, chalk_ir::GoalData<RustInterner<'tcx>>> for ty::Predi
             | ty::PredicateKind::Subtype(..)
             | ty::PredicateKind::Coerce(..)
             | ty::PredicateKind::ConstEvaluatable(..)
+            | ty::PredicateKind::OpaqueType(..)
             | ty::PredicateKind::ConstEquate(..) => {
                 chalk_ir::GoalData::All(chalk_ir::Goals::empty(interner))
             }
@@ -597,6 +599,7 @@ 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)
             }
@@ -725,6 +728,7 @@ 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/implied_outlives_bounds.rs b/compiler/rustc_traits/src/implied_outlives_bounds.rs
index 92f2760e62c7e..dd9ee48a66584 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 87530cf9961b1..6a7f30e438c09 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(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_typeck/src/check/fn_ctxt/_impl.rs b/compiler/rustc_typeck/src/check/fn_ctxt/_impl.rs
index 738af9bfb8cc5..771bffedf2f55 100644
--- a/compiler/rustc_typeck/src/check/fn_ctxt/_impl.rs
+++ b/compiler/rustc_typeck/src/check/fn_ctxt/_impl.rs
@@ -716,6 +716,7 @@ 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))
diff --git a/compiler/rustc_typeck/src/check/method/probe.rs b/compiler/rustc_typeck/src/check/method/probe.rs
index dc7243960946b..68f84bb833c69 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,
             }
         });
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 4fb422c801b1d..48dd7e9f182e4 100644
--- a/compiler/rustc_typeck/src/impl_wf_check/min_specialization.rs
+++ b/compiler/rustc_typeck/src/impl_wf_check/min_specialization.rs
@@ -427,6 +427,7 @@ 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 bbf31de527eb3..7839ce5e4ff48 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(..) => (),
                 }
             }