diff --git a/compiler/rustc_hir_typeck/src/_match.rs b/compiler/rustc_hir_typeck/src/_match.rs
index 38319862334a8..da3d960df3132 100644
--- a/compiler/rustc_hir_typeck/src/_match.rs
+++ b/compiler/rustc_hir_typeck/src/_match.rs
@@ -601,7 +601,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             // FIXME(-Znext-solver): Remove this branch once `replace_opaque_types_with_infer` is gone.
             ty::Infer(ty::TyVar(_)) => self
                 .inner
-                .borrow()
+                .borrow_mut()
+                .opaque_types()
                 .iter_opaque_types()
                 .find(|(_, v)| v.ty == expected_ty)
                 .map(|(k, _)| (k.def_id, k.args))?,
diff --git a/compiler/rustc_hir_typeck/src/fn_ctxt/mod.rs b/compiler/rustc_hir_typeck/src/fn_ctxt/mod.rs
index de189b301092c..54089b95bb4fb 100644
--- a/compiler/rustc_hir_typeck/src/fn_ctxt/mod.rs
+++ b/compiler/rustc_hir_typeck/src/fn_ctxt/mod.rs
@@ -19,7 +19,6 @@ use rustc_middle::ty::{self, Const, Ty, TyCtxt, TypeVisitableExt};
 use rustc_session::Session;
 use rustc_span::{self, DUMMY_SP, ErrorGuaranteed, Ident, Span, sym};
 use rustc_trait_selection::error_reporting::TypeErrCtxt;
-use rustc_trait_selection::error_reporting::infer::sub_relations::SubRelations;
 use rustc_trait_selection::traits::{ObligationCause, ObligationCauseCode, ObligationCtxt};
 
 use crate::coercion::DynamicCoerceMany;
@@ -177,14 +176,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     ///
     /// [`InferCtxtErrorExt::err_ctxt`]: rustc_trait_selection::error_reporting::InferCtxtErrorExt::err_ctxt
     pub(crate) fn err_ctxt(&'a self) -> TypeErrCtxt<'a, 'tcx> {
-        let mut sub_relations = SubRelations::default();
-        sub_relations.add_constraints(
-            self,
-            self.fulfillment_cx.borrow_mut().pending_obligations().iter().map(|o| o.predicate),
-        );
         TypeErrCtxt {
             infcx: &self.infcx,
-            sub_relations: RefCell::new(sub_relations),
             typeck_results: Some(self.typeck_results.borrow()),
             fallback_has_occurred: self.fallback_has_occurred.get(),
             normalize_fn_sig: Box::new(|fn_sig| {
diff --git a/compiler/rustc_infer/src/infer/canonical/canonicalizer.rs b/compiler/rustc_infer/src/infer/canonical/canonicalizer.rs
index a1a0926cd8188..27b218220f1cc 100644
--- a/compiler/rustc_infer/src/infer/canonical/canonicalizer.rs
+++ b/compiler/rustc_infer/src/infer/canonical/canonicalizer.rs
@@ -17,8 +17,7 @@ use tracing::debug;
 
 use crate::infer::InferCtxt;
 use crate::infer::canonical::{
-    Canonical, CanonicalQueryInput, CanonicalTyVarKind, CanonicalVarInfo, CanonicalVarKind,
-    OriginalQueryValues,
+    Canonical, CanonicalQueryInput, CanonicalVarInfo, CanonicalVarKind, OriginalQueryValues,
 };
 
 impl<'tcx> InferCtxt<'tcx> {
@@ -299,6 +298,7 @@ struct Canonicalizer<'cx, 'tcx> {
     // Note that indices is only used once `var_values` is big enough to be
     // heap-allocated.
     indices: FxHashMap<GenericArg<'tcx>, BoundVar>,
+    sub_root_lookup_table: FxHashMap<ty::TyVid, usize>,
     canonicalize_mode: &'cx dyn CanonicalizeMode,
     needs_canonical_flags: TypeFlags,
 
@@ -367,9 +367,10 @@ impl<'cx, 'tcx> TypeFolder<TyCtxt<'tcx>> for Canonicalizer<'cx, 'tcx> {
                             // FIXME: perf problem described in #55921.
                             ui = ty::UniverseIndex::ROOT;
                         }
+                        let sub_root = self.get_or_insert_sub_root(vid);
                         self.canonicalize_ty_var(
                             CanonicalVarInfo {
-                                kind: CanonicalVarKind::Ty(CanonicalTyVarKind::General(ui)),
+                                kind: CanonicalVarKind::Ty { universe: ui, sub_root },
                             },
                             t,
                         )
@@ -382,10 +383,7 @@ impl<'cx, 'tcx> TypeFolder<TyCtxt<'tcx>> for Canonicalizer<'cx, 'tcx> {
                 if nt != t {
                     return self.fold_ty(nt);
                 } else {
-                    self.canonicalize_ty_var(
-                        CanonicalVarInfo { kind: CanonicalVarKind::Ty(CanonicalTyVarKind::Int) },
-                        t,
-                    )
+                    self.canonicalize_ty_var(CanonicalVarInfo { kind: CanonicalVarKind::Int }, t)
                 }
             }
             ty::Infer(ty::FloatVar(vid)) => {
@@ -393,10 +391,7 @@ impl<'cx, 'tcx> TypeFolder<TyCtxt<'tcx>> for Canonicalizer<'cx, 'tcx> {
                 if nt != t {
                     return self.fold_ty(nt);
                 } else {
-                    self.canonicalize_ty_var(
-                        CanonicalVarInfo { kind: CanonicalVarKind::Ty(CanonicalTyVarKind::Float) },
-                        t,
-                    )
+                    self.canonicalize_ty_var(CanonicalVarInfo { kind: CanonicalVarKind::Float }, t)
                 }
             }
 
@@ -576,6 +571,7 @@ impl<'cx, 'tcx> Canonicalizer<'cx, 'tcx> {
             variables: SmallVec::from_slice(base.variables),
             query_state,
             indices: FxHashMap::default(),
+            sub_root_lookup_table: Default::default(),
             binder_index: ty::INNERMOST,
         };
         if canonicalizer.query_state.var_values.spilled() {
@@ -670,6 +666,13 @@ impl<'cx, 'tcx> Canonicalizer<'cx, 'tcx> {
         }
     }
 
+    fn get_or_insert_sub_root(&mut self, vid: ty::TyVid) -> ty::BoundVar {
+        let root_vid = self.infcx.unwrap().sub_root_var(vid);
+        let idx =
+            *self.sub_root_lookup_table.entry(root_vid).or_insert_with(|| self.variables.len());
+        ty::BoundVar::from(idx)
+    }
+
     /// Replaces the universe indexes used in `var_values` with their index in
     /// `query_state.universe_map`. This minimizes the maximum universe used in
     /// the canonicalized value.
@@ -690,11 +693,11 @@ impl<'cx, 'tcx> Canonicalizer<'cx, 'tcx> {
             .iter()
             .map(|v| CanonicalVarInfo {
                 kind: match v.kind {
-                    CanonicalVarKind::Ty(CanonicalTyVarKind::Int | CanonicalTyVarKind::Float) => {
+                    CanonicalVarKind::Int | CanonicalVarKind::Float => {
                         return *v;
                     }
-                    CanonicalVarKind::Ty(CanonicalTyVarKind::General(u)) => {
-                        CanonicalVarKind::Ty(CanonicalTyVarKind::General(reverse_universe_map[&u]))
+                    CanonicalVarKind::Ty { universe, sub_root } => {
+                        CanonicalVarKind::Ty { universe: reverse_universe_map[&universe], sub_root }
                     }
                     CanonicalVarKind::Region(u) => {
                         CanonicalVarKind::Region(reverse_universe_map[&u])
diff --git a/compiler/rustc_infer/src/infer/canonical/mod.rs b/compiler/rustc_infer/src/infer/canonical/mod.rs
index 3be07dbe208f9..e8d9138e5deaa 100644
--- a/compiler/rustc_infer/src/infer/canonical/mod.rs
+++ b/compiler/rustc_infer/src/infer/canonical/mod.rs
@@ -84,13 +84,12 @@ impl<'tcx> InferCtxt<'tcx> {
         variables: &List<CanonicalVarInfo<'tcx>>,
         universe_map: impl Fn(ty::UniverseIndex) -> ty::UniverseIndex,
     ) -> CanonicalVarValues<'tcx> {
-        CanonicalVarValues {
-            var_values: self.tcx.mk_args_from_iter(
-                variables
-                    .iter()
-                    .map(|info| self.instantiate_canonical_var(span, info, &universe_map)),
-            ),
+        let mut var_values = Vec::new();
+        for info in variables.iter() {
+            let value = self.instantiate_canonical_var(span, info, &var_values, &universe_map);
+            var_values.push(value);
         }
+        CanonicalVarValues { var_values: self.tcx.mk_args(&var_values) }
     }
 
     /// Given the "info" about a canonical variable, creates a fresh
@@ -105,21 +104,22 @@ impl<'tcx> InferCtxt<'tcx> {
         &self,
         span: Span,
         cv_info: CanonicalVarInfo<'tcx>,
+        previous_var_values: &[GenericArg<'tcx>],
         universe_map: impl Fn(ty::UniverseIndex) -> ty::UniverseIndex,
     ) -> GenericArg<'tcx> {
         match cv_info.kind {
-            CanonicalVarKind::Ty(ty_kind) => {
-                let ty = match ty_kind {
-                    CanonicalTyVarKind::General(ui) => {
-                        self.next_ty_var_in_universe(span, universe_map(ui))
-                    }
-
-                    CanonicalTyVarKind::Int => self.next_int_var(),
-
-                    CanonicalTyVarKind::Float => self.next_float_var(),
-                };
-                ty.into()
+            CanonicalVarKind::Ty { universe, sub_root } => {
+                let vid = self.next_ty_var_id_in_universe(span, universe_map(universe));
+                if let Some(prev) = previous_var_values.get(sub_root.as_usize()) {
+                    let &ty::Infer(ty::TyVar(sub_root)) = prev.expect_ty().kind() else {
+                        unreachable!("expected `sub_root` to be an inference variable");
+                    };
+                    self.inner.borrow_mut().type_variables().sub(vid, sub_root);
+                }
+                Ty::new_var(self.tcx, vid).into()
             }
+            CanonicalVarKind::Int => self.next_int_var().into(),
+            CanonicalVarKind::Float => self.next_float_var().into(),
 
             CanonicalVarKind::PlaceholderTy(ty::PlaceholderType { universe, bound }) => {
                 let universe_mapped = universe_map(universe);
diff --git a/compiler/rustc_infer/src/infer/canonical/query_response.rs b/compiler/rustc_infer/src/infer/canonical/query_response.rs
index 5220071c50059..227d2817e496b 100644
--- a/compiler/rustc_infer/src/infer/canonical/query_response.rs
+++ b/compiler/rustc_infer/src/infer/canonical/query_response.rs
@@ -13,7 +13,9 @@ use std::iter;
 use rustc_index::{Idx, IndexVec};
 use rustc_middle::arena::ArenaAllocatable;
 use rustc_middle::mir::ConstraintCategory;
-use rustc_middle::ty::{self, BoundVar, GenericArg, GenericArgKind, Ty, TyCtxt, TypeFoldable};
+use rustc_middle::ty::{
+    self, BoundVar, CanonicalVarKind, GenericArg, GenericArgKind, Ty, TyCtxt, TypeFoldable,
+};
 use rustc_middle::{bug, span_bug};
 use tracing::{debug, instrument};
 
@@ -132,7 +134,13 @@ impl<'tcx> InferCtxt<'tcx> {
 
         let certainty = if errors.is_empty() { Certainty::Proven } else { Certainty::Ambiguous };
 
-        let opaque_types = self.take_opaque_types_for_query_response();
+        let opaque_types = self
+            .inner
+            .borrow_mut()
+            .opaque_type_storage
+            .take_opaque_types()
+            .map(|(k, v)| (k, v.ty))
+            .collect();
 
         Ok(QueryResponse {
             var_values: inference_vars,
@@ -143,24 +151,6 @@ impl<'tcx> InferCtxt<'tcx> {
         })
     }
 
-    /// Used by the new solver as that one takes the opaque types at the end of a probe
-    /// to deal with multiple candidates without having to recompute them.
-    pub fn clone_opaque_types_for_query_response(
-        &self,
-    ) -> Vec<(ty::OpaqueTypeKey<'tcx>, Ty<'tcx>)> {
-        self.inner
-            .borrow()
-            .opaque_type_storage
-            .opaque_types
-            .iter()
-            .map(|(k, v)| (*k, v.ty))
-            .collect()
-    }
-
-    fn take_opaque_types_for_query_response(&self) -> Vec<(ty::OpaqueTypeKey<'tcx>, Ty<'tcx>)> {
-        self.take_opaque_types().into_iter().map(|(k, v)| (k, v.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
@@ -455,32 +445,48 @@ impl<'tcx> InferCtxt<'tcx> {
         // Create result arguments: if we found a value for a
         // given variable in the loop above, use that. Otherwise, use
         // a fresh inference variable.
-        let result_args = CanonicalVarValues {
-            var_values: self.tcx.mk_args_from_iter(
-                query_response.variables.iter().enumerate().map(|(index, info)| {
-                    if info.universe() != ty::UniverseIndex::ROOT {
-                        // A variable from inside a binder of the query. While ideally these shouldn't
-                        // exist at all, we have to deal with them for now.
-                        self.instantiate_canonical_var(cause.span, info, |u| {
-                            universe_map[u.as_usize()]
-                        })
-                    } else if info.is_existential() {
-                        match opt_values[BoundVar::new(index)] {
-                            Some(k) => k,
-                            None => self.instantiate_canonical_var(cause.span, info, |u| {
-                                universe_map[u.as_usize()]
-                            }),
+        let mut var_values = Vec::new();
+        for (index, info) in query_response.variables.iter().enumerate() {
+            let value = if info.universe() != ty::UniverseIndex::ROOT {
+                // A variable from inside a binder of the query. While ideally these shouldn't
+                // exist at all, we have to deal with them for now.
+                self.instantiate_canonical_var(cause.span, info, &var_values, |u| {
+                    universe_map[u.as_usize()]
+                })
+            } else if info.is_existential() {
+                // As an optimization we sometimes avoid creating a new inference variable here.
+                // We need to still make sure to register any subtype relations returned by the
+                // query.
+                match opt_values[BoundVar::new(index)] {
+                    Some(v) => {
+                        if let CanonicalVarKind::Ty { universe: _, sub_root } = info.kind {
+                            if let Some(prev) = var_values.get(sub_root.as_usize()) {
+                                let &ty::Infer(ty::TyVar(vid)) = v.expect_ty().kind() else {
+                                    unreachable!("expected `sub_root` to be an inference variable");
+                                };
+                                let &ty::Infer(ty::TyVar(sub_root)) = prev.expect_ty().kind()
+                                else {
+                                    unreachable!("expected `sub_root` to be an inference variable");
+                                };
+                                self.inner.borrow_mut().type_variables().sub(vid, sub_root);
+                            }
                         }
-                    } else {
-                        // For placeholders which were already part of the input, we simply map this
-                        // universal bound variable back the placeholder of the input.
-                        opt_values[BoundVar::new(index)].expect(
-                            "expected placeholder to be unified with itself during response",
-                        )
+                        v
                     }
-                }),
-            ),
-        };
+                    None => self.instantiate_canonical_var(cause.span, info, &var_values, |u| {
+                        universe_map[u.as_usize()]
+                    }),
+                }
+            } else {
+                // For placeholders which were already part of the input, we simply map this
+                // universal bound variable back the placeholder of the input.
+                opt_values[BoundVar::new(index)]
+                    .expect("expected placeholder to be unified with itself during response")
+            };
+            var_values.push(value)
+        }
+
+        let result_args = CanonicalVarValues { var_values: self.tcx.mk_args(&var_values) };
 
         let mut obligations = PredicateObligations::new();
 
diff --git a/compiler/rustc_infer/src/infer/context.rs b/compiler/rustc_infer/src/infer/context.rs
index 75affa1397705..7e171cf81cb16 100644
--- a/compiler/rustc_infer/src/infer/context.rs
+++ b/compiler/rustc_infer/src/infer/context.rs
@@ -55,6 +55,9 @@ impl<'tcx> rustc_type_ir::InferCtxtLike for InferCtxt<'tcx> {
     fn root_ty_var(&self, var: ty::TyVid) -> ty::TyVid {
         self.root_var(var)
     }
+    fn sub_root_ty_var(&self, var: ty::TyVid) -> ty::TyVid {
+        self.sub_root_var(var)
+    }
 
     fn root_const_var(&self, var: ty::ConstVid) -> ty::ConstVid {
         self.root_const_var(var)
@@ -125,6 +128,10 @@ impl<'tcx> rustc_type_ir::InferCtxtLike for InferCtxt<'tcx> {
         self.inner.borrow_mut().type_variables().equate(a, b);
     }
 
+    fn sub_ty_vids_raw(&self, a: rustc_type_ir::TyVid, b: rustc_type_ir::TyVid) {
+        self.inner.borrow_mut().type_variables().sub(a, b);
+    }
+
     fn equate_int_vids_raw(&self, a: rustc_type_ir::IntVid, b: rustc_type_ir::IntVid) {
         self.inner.borrow_mut().int_unification_table().union(a, b);
     }
diff --git a/compiler/rustc_infer/src/infer/mod.rs b/compiler/rustc_infer/src/infer/mod.rs
index d25542dadd59b..3d8f3ef258e0b 100644
--- a/compiler/rustc_infer/src/infer/mod.rs
+++ b/compiler/rustc_infer/src/infer/mod.rs
@@ -31,9 +31,9 @@ use rustc_middle::traits::solve::Goal;
 use rustc_middle::ty::error::{ExpectedFound, TypeError};
 use rustc_middle::ty::{
     self, BoundVarReplacerDelegate, ConstVid, FloatVid, GenericArg, GenericArgKind, GenericArgs,
-    GenericArgsRef, GenericParamDefKind, InferConst, IntVid, PseudoCanonicalInput, Term, TermKind,
-    Ty, TyCtxt, TyVid, TypeFoldable, TypeFolder, TypeSuperFoldable, TypeVisitable,
-    TypeVisitableExt, TypingEnv, TypingMode, fold_regions,
+    GenericArgsRef, GenericParamDefKind, InferConst, IntVid, OpaqueHiddenType, OpaqueTypeKey,
+    PseudoCanonicalInput, Term, TermKind, Ty, TyCtxt, TyVid, TypeFoldable, TypeFolder,
+    TypeSuperFoldable, TypeVisitable, TypeVisitableExt, TypingEnv, TypingMode, fold_regions,
 };
 use rustc_span::{Span, Symbol};
 use snapshot::undo_log::InferCtxtUndoLogs;
@@ -198,7 +198,7 @@ impl<'tcx> InferCtxtInner<'tcx> {
     }
 
     #[inline]
-    fn opaque_types(&mut self) -> opaque_types::OpaqueTypeTable<'_, 'tcx> {
+    pub fn opaque_types(&mut self) -> opaque_types::OpaqueTypeTable<'_, 'tcx> {
         self.opaque_type_storage.with_log(&mut self.undo_log)
     }
 
@@ -224,15 +224,6 @@ impl<'tcx> InferCtxtInner<'tcx> {
             .expect("region constraints already solved")
             .with_log(&mut self.undo_log)
     }
-
-    // Iterates through the opaque type definitions without taking them; this holds the
-    // `InferCtxtInner` lock, so make sure to not do anything with `InferCtxt` side-effects
-    // while looping through this.
-    pub fn iter_opaque_types(
-        &self,
-    ) -> impl Iterator<Item = (ty::OpaqueTypeKey<'tcx>, ty::OpaqueHiddenType<'tcx>)> {
-        self.opaque_type_storage.opaque_types.iter().map(|(&k, &v)| (k, v))
-    }
 }
 
 pub struct InferCtxt<'tcx> {
@@ -733,6 +724,7 @@ impl<'tcx> InferCtxt<'tcx> {
         let r_b = self.shallow_resolve(predicate.skip_binder().b);
         match (r_a.kind(), r_b.kind()) {
             (&ty::Infer(ty::TyVar(a_vid)), &ty::Infer(ty::TyVar(b_vid))) => {
+                self.inner.borrow_mut().type_variables().sub(a_vid, b_vid);
                 return Err((a_vid, b_vid));
             }
             _ => {}
@@ -954,13 +946,13 @@ impl<'tcx> InferCtxt<'tcx> {
     }
 
     #[instrument(level = "debug", skip(self), ret)]
-    pub fn take_opaque_types(&self) -> opaque_types::OpaqueTypeMap<'tcx> {
-        std::mem::take(&mut self.inner.borrow_mut().opaque_type_storage.opaque_types)
+    pub fn take_opaque_types(&self) -> Vec<(OpaqueTypeKey<'tcx>, OpaqueHiddenType<'tcx>)> {
+        self.inner.borrow_mut().opaque_type_storage.take_opaque_types().collect()
     }
 
     #[instrument(level = "debug", skip(self), ret)]
-    pub fn clone_opaque_types(&self) -> opaque_types::OpaqueTypeMap<'tcx> {
-        self.inner.borrow().opaque_type_storage.opaque_types.clone()
+    pub fn clone_opaque_types(&self) -> Vec<(OpaqueTypeKey<'tcx>, OpaqueHiddenType<'tcx>)> {
+        self.inner.borrow_mut().opaque_type_storage.iter_opaque_types().collect()
     }
 
     #[inline(always)]
@@ -1065,6 +1057,10 @@ impl<'tcx> InferCtxt<'tcx> {
         self.inner.borrow_mut().type_variables().root_var(var)
     }
 
+    pub fn sub_root_var(&self, var: ty::TyVid) -> ty::TyVid {
+        self.inner.borrow_mut().type_variables().sub_root_var(var)
+    }
+
     pub fn root_const_var(&self, var: ty::ConstVid) -> ty::ConstVid {
         self.inner.borrow_mut().const_unification_table().find(var).vid
     }
diff --git a/compiler/rustc_infer/src/infer/opaque_types/mod.rs b/compiler/rustc_infer/src/infer/opaque_types/mod.rs
index ce5d2e6e17a96..df7144c31da5a 100644
--- a/compiler/rustc_infer/src/infer/opaque_types/mod.rs
+++ b/compiler/rustc_infer/src/infer/opaque_types/mod.rs
@@ -1,5 +1,4 @@
 use hir::def_id::{DefId, LocalDefId};
-use rustc_data_structures::fx::FxIndexMap;
 use rustc_hir as hir;
 use rustc_middle::bug;
 use rustc_middle::traits::ObligationCause;
@@ -19,7 +18,6 @@ use crate::traits::{self, Obligation, PredicateObligations};
 
 mod table;
 
-pub(crate) type OpaqueTypeMap<'tcx> = FxIndexMap<OpaqueTypeKey<'tcx>, OpaqueHiddenType<'tcx>>;
 pub(crate) use table::{OpaqueTypeStorage, OpaqueTypeTable};
 
 impl<'tcx> InferCtxt<'tcx> {
diff --git a/compiler/rustc_infer/src/infer/opaque_types/table.rs b/compiler/rustc_infer/src/infer/opaque_types/table.rs
index ba6cc0d783dd3..3c5bf9d722b97 100644
--- a/compiler/rustc_infer/src/infer/opaque_types/table.rs
+++ b/compiler/rustc_infer/src/infer/opaque_types/table.rs
@@ -1,18 +1,17 @@
+use std::ops::Deref;
+
+use rustc_data_structures::fx::FxIndexMap;
 use rustc_data_structures::undo_log::UndoLogs;
 use rustc_middle::bug;
 use rustc_middle::ty::{self, OpaqueHiddenType, OpaqueTypeKey, Ty};
 use tracing::instrument;
 
-use super::OpaqueTypeMap;
 use crate::infer::snapshot::undo_log::{InferCtxtUndoLogs, UndoLog};
 
 #[derive(Default, Debug, Clone)]
-pub(crate) 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' OpaqueHiddenType, after typeck is done with all functions.
-    pub opaque_types: OpaqueTypeMap<'tcx>,
+pub struct OpaqueTypeStorage<'tcx> {
+    opaque_types: FxIndexMap<OpaqueTypeKey<'tcx>, OpaqueHiddenType<'tcx>>,
+    duplicate_entries: Vec<(OpaqueTypeKey<'tcx>, OpaqueHiddenType<'tcx>)>,
 }
 
 impl<'tcx> OpaqueTypeStorage<'tcx> {
@@ -33,6 +32,52 @@ impl<'tcx> OpaqueTypeStorage<'tcx> {
         }
     }
 
+    pub(crate) fn pop_duplicate_entry(&mut self) {
+        let entry = self.duplicate_entries.pop();
+        assert!(entry.is_some());
+    }
+
+    pub(crate) fn is_empty(&self) -> bool {
+        let OpaqueTypeStorage { opaque_types, duplicate_entries } = self;
+        opaque_types.is_empty() && duplicate_entries.is_empty()
+    }
+
+    pub(crate) fn take_opaque_types(
+        &mut self,
+    ) -> impl Iterator<Item = (OpaqueTypeKey<'tcx>, OpaqueHiddenType<'tcx>)> {
+        let OpaqueTypeStorage { opaque_types, duplicate_entries } = self;
+        std::mem::take(opaque_types).into_iter().chain(std::mem::take(duplicate_entries))
+    }
+
+    /// Only returns the opaque types from the lookup table. These are used
+    /// when normalizing opaque types and have a unique key.
+    ///
+    /// Outside of canonicalization one should generally use `iter_opaque_types`
+    /// to also consider duplicate entries.
+    pub fn iter_lookup_table(
+        &self,
+    ) -> impl Iterator<Item = (OpaqueTypeKey<'tcx>, OpaqueHiddenType<'tcx>)> {
+        self.opaque_types.iter().map(|(k, v)| (*k, *v))
+    }
+
+    /// Only returns the opaque types which are stored in `duplicate_entries`.
+    ///
+    /// These have to considered when checking all opaque type uses but are e.g.
+    /// irrelevant for canonical inputs as nested queries never meaningfully
+    /// accesses them.
+    pub fn iter_duplicate_entries(
+        &self,
+    ) -> impl Iterator<Item = (OpaqueTypeKey<'tcx>, OpaqueHiddenType<'tcx>)> {
+        self.duplicate_entries.iter().copied()
+    }
+
+    pub fn iter_opaque_types(
+        &self,
+    ) -> impl Iterator<Item = (OpaqueTypeKey<'tcx>, OpaqueHiddenType<'tcx>)> {
+        let OpaqueTypeStorage { opaque_types, duplicate_entries } = self;
+        opaque_types.iter().map(|(k, v)| (*k, *v)).chain(duplicate_entries.iter().copied())
+    }
+
     #[inline]
     pub(crate) fn with_log<'a>(
         &'a mut self,
@@ -44,21 +89,27 @@ impl<'tcx> OpaqueTypeStorage<'tcx> {
 
 impl<'tcx> Drop for OpaqueTypeStorage<'tcx> {
     fn drop(&mut self) {
-        if !self.opaque_types.is_empty() {
+        if !self.is_empty() {
             ty::tls::with(|tcx| tcx.dcx().delayed_bug(format!("{:?}", self.opaque_types)));
         }
     }
 }
 
-pub(crate) struct OpaqueTypeTable<'a, 'tcx> {
+pub struct OpaqueTypeTable<'a, 'tcx> {
     storage: &'a mut OpaqueTypeStorage<'tcx>,
 
     undo_log: &'a mut InferCtxtUndoLogs<'tcx>,
 }
+impl<'tcx> Deref for OpaqueTypeTable<'_, 'tcx> {
+    type Target = OpaqueTypeStorage<'tcx>;
+    fn deref(&self) -> &Self::Target {
+        self.storage
+    }
+}
 
 impl<'a, 'tcx> OpaqueTypeTable<'a, 'tcx> {
     #[instrument(skip(self), level = "debug")]
-    pub(crate) fn register(
+    pub fn register(
         &mut self,
         key: OpaqueTypeKey<'tcx>,
         hidden_type: OpaqueHiddenType<'tcx>,
@@ -72,4 +123,9 @@ impl<'a, 'tcx> OpaqueTypeTable<'a, 'tcx> {
         self.undo_log.push(UndoLog::OpaqueTypes(key, None));
         None
     }
+
+    pub fn add_duplicate(&mut self, key: OpaqueTypeKey<'tcx>, hidden_type: OpaqueHiddenType<'tcx>) {
+        self.storage.duplicate_entries.push((key, hidden_type));
+        self.undo_log.push(UndoLog::DuplicateOpaqueType);
+    }
 }
diff --git a/compiler/rustc_infer/src/infer/relate/generalize.rs b/compiler/rustc_infer/src/infer/relate/generalize.rs
index 210b8f37d883d..cdcd6f6dbb6ff 100644
--- a/compiler/rustc_infer/src/infer/relate/generalize.rs
+++ b/compiler/rustc_infer/src/infer/relate/generalize.rs
@@ -503,6 +503,10 @@ impl<'tcx> TypeRelation<TyCtxt<'tcx>> for Generalizer<'_, 'tcx> {
                             let origin = inner.type_variables().var_origin(vid);
                             let new_var_id =
                                 inner.type_variables().new_var(self.for_universe, origin);
+                            // Record that `vid` and `new_var_id` have to be subtypes
+                            // of each other. This is currently only used for diagnostics.
+                            // To see why, see the docs in the `type_variables` module.
+                            inner.type_variables().sub(vid, new_var_id);
                             // If we're in the new solver and create a new inference
                             // variable inside of an alias we eagerly constrain that
                             // inference variable to prevent unexpected ambiguity errors.
diff --git a/compiler/rustc_infer/src/infer/snapshot/undo_log.rs b/compiler/rustc_infer/src/infer/snapshot/undo_log.rs
index ba7d8f588e68e..6b2f9df6bc80c 100644
--- a/compiler/rustc_infer/src/infer/snapshot/undo_log.rs
+++ b/compiler/rustc_infer/src/infer/snapshot/undo_log.rs
@@ -17,8 +17,9 @@ 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> {
+    DuplicateOpaqueType,
     OpaqueTypes(OpaqueTypeKey<'tcx>, Option<OpaqueHiddenType<'tcx>>),
-    TypeVariables(sv::UndoLog<ut::Delegate<type_variable::TyVidEqKey<'tcx>>>),
+    TypeVariables(type_variable::UndoLog<'tcx>),
     ConstUnificationTable(sv::UndoLog<ut::Delegate<ConstVidKey<'tcx>>>),
     IntUnificationTable(sv::UndoLog<ut::Delegate<ty::IntVid>>),
     FloatUnificationTable(sv::UndoLog<ut::Delegate<ty::FloatVid>>),
@@ -44,7 +45,9 @@ macro_rules! impl_from {
 impl_from! {
     RegionConstraintCollector(region_constraints::UndoLog<'tcx>),
 
+    TypeVariables(type_variable::UndoLog<'tcx>),
     TypeVariables(sv::UndoLog<ut::Delegate<type_variable::TyVidEqKey<'tcx>>>),
+    TypeVariables(sv::UndoLog<ut::Delegate<ty::TyVid>>),
     IntUnificationTable(sv::UndoLog<ut::Delegate<ty::IntVid>>),
     FloatUnificationTable(sv::UndoLog<ut::Delegate<ty::FloatVid>>),
 
@@ -58,6 +61,7 @@ impl_from! {
 impl<'tcx> Rollback<UndoLog<'tcx>> for InferCtxtInner<'tcx> {
     fn reverse(&mut self, undo: UndoLog<'tcx>) {
         match undo {
+            UndoLog::DuplicateOpaqueType => self.opaque_type_storage.pop_duplicate_entry(),
             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),
diff --git a/compiler/rustc_infer/src/infer/type_variable.rs b/compiler/rustc_infer/src/infer/type_variable.rs
index 2086483b94a71..cb64c17bdff33 100644
--- a/compiler/rustc_infer/src/infer/type_variable.rs
+++ b/compiler/rustc_infer/src/infer/type_variable.rs
@@ -13,9 +13,33 @@ use tracing::debug;
 
 use crate::infer::InferCtxtUndoLogs;
 
-impl<'tcx> Rollback<sv::UndoLog<ut::Delegate<TyVidEqKey<'tcx>>>> for TypeVariableStorage<'tcx> {
-    fn reverse(&mut self, undo: sv::UndoLog<ut::Delegate<TyVidEqKey<'tcx>>>) {
-        self.eq_relations.reverse(undo)
+/// Represents a single undo-able action that affects a type inference variable.
+#[derive(Clone)]
+pub(crate) enum UndoLog<'tcx> {
+    EqRelation(sv::UndoLog<ut::Delegate<TyVidEqKey<'tcx>>>),
+    SubRelation(sv::UndoLog<ut::Delegate<ty::TyVid>>),
+}
+
+/// Convert from a specific kind of undo to the more general UndoLog
+impl<'tcx> From<sv::UndoLog<ut::Delegate<TyVidEqKey<'tcx>>>> for UndoLog<'tcx> {
+    fn from(l: sv::UndoLog<ut::Delegate<TyVidEqKey<'tcx>>>) -> Self {
+        UndoLog::EqRelation(l)
+    }
+}
+
+/// Convert from a specific kind of undo to the more general UndoLog
+impl<'tcx> From<sv::UndoLog<ut::Delegate<ty::TyVid>>> for UndoLog<'tcx> {
+    fn from(l: sv::UndoLog<ut::Delegate<ty::TyVid>>) -> Self {
+        UndoLog::SubRelation(l)
+    }
+}
+
+impl<'tcx> Rollback<UndoLog<'tcx>> for TypeVariableStorage<'tcx> {
+    fn reverse(&mut self, undo: UndoLog<'tcx>) {
+        match undo {
+            UndoLog::EqRelation(undo) => self.eq_relations.reverse(undo),
+            UndoLog::SubRelation(undo) => self.sub_relations.reverse(undo),
+        }
     }
 }
 
@@ -27,6 +51,24 @@ pub(crate) struct TypeVariableStorage<'tcx> {
     /// constraint `?X == ?Y`. This table also stores, for each key,
     /// the known value.
     eq_relations: ut::UnificationTableStorage<TyVidEqKey<'tcx>>,
+
+    /// Only used by `-Znext-solver` andfor diagnostics.
+    ///
+    /// When reporting ambiguity errors, we sometimes want to
+    /// treat all inference vars which are subtypes of each
+    /// others as if they are equal. For this case we compute
+    /// the transitive closure of our subtype obligations here.
+    ///
+    /// E.g. when encountering ambiguity errors, we want to suggest
+    /// specifying some method argument or to add a type annotation
+    /// to a local variable. Because subtyping cannot change the
+    /// shape of a type, it's fine if the cause of the ambiguity error
+    /// is only related to the suggested variable via subtyping.
+    ///
+    /// Even for something like `let x = returns_arg(); x.method();` the
+    /// type of `x` is only a supertype of the argument of `returns_arg`. We
+    /// still want to suggest specifying the type of the argument.
+    sub_relations: ut::UnificationTableStorage<ty::TyVid>,
 }
 
 pub(crate) struct TypeVariableTable<'a, 'tcx> {
@@ -109,6 +151,16 @@ impl<'tcx> TypeVariableTable<'_, 'tcx> {
         debug_assert!(self.probe(a).is_unknown());
         debug_assert!(self.probe(b).is_unknown());
         self.eq_relations().union(a, b);
+        self.sub_relations().union(a, b);
+    }
+
+    /// Records that `a <: b`, depending on `dir`.
+    ///
+    /// Precondition: neither `a` nor `b` are known.
+    pub(crate) fn sub(&mut self, a: ty::TyVid, b: ty::TyVid) {
+        debug_assert!(self.probe(a).is_unknown());
+        debug_assert!(self.probe(b).is_unknown());
+        self.sub_relations().union(a, b);
     }
 
     /// Instantiates `vid` with the type `ty`.
@@ -142,6 +194,10 @@ impl<'tcx> TypeVariableTable<'_, 'tcx> {
         origin: TypeVariableOrigin,
     ) -> ty::TyVid {
         let eq_key = self.eq_relations().new_key(TypeVariableValue::Unknown { universe });
+
+        let sub_key = self.sub_relations().new_key(());
+        debug_assert_eq!(eq_key.vid, sub_key);
+
         let index = self.storage.values.push(TypeVariableData { origin });
         debug_assert_eq!(eq_key.vid, index);
 
@@ -164,6 +220,18 @@ impl<'tcx> TypeVariableTable<'_, 'tcx> {
         self.eq_relations().find(vid).vid
     }
 
+    /// Returns the "root" variable of `vid` in the `sub_relations`
+    /// equivalence table. All type variables that have been are
+    /// related via equality or subtyping will yield the same root
+    /// variable (per the union-find algorithm), so `sub_root_var(a)
+    /// == sub_root_var(b)` implies that:
+    /// ```text
+    /// exists X. (a <: X || X <: a) && (b <: X || X <: b)
+    /// ```
+    pub(crate) fn sub_root_var(&mut self, vid: ty::TyVid) -> ty::TyVid {
+        self.sub_relations().find(vid)
+    }
+
     /// Retrieves the type to which `vid` has been instantiated, if
     /// any.
     pub(crate) fn probe(&mut self, vid: ty::TyVid) -> TypeVariableValue<'tcx> {
@@ -181,6 +249,11 @@ impl<'tcx> TypeVariableTable<'_, 'tcx> {
         self.storage.eq_relations.with_log(self.undo_log)
     }
 
+    #[inline]
+    fn sub_relations(&mut self) -> super::UnificationTable<'_, 'tcx, ty::TyVid> {
+        self.storage.sub_relations.with_log(self.undo_log)
+    }
+
     /// Returns a range of the type variables created during the snapshot.
     pub(crate) fn vars_since_snapshot(
         &mut self,
diff --git a/compiler/rustc_middle/src/infer/canonical.rs b/compiler/rustc_middle/src/infer/canonical.rs
index 5b8603744961c..15cc01ba6a01e 100644
--- a/compiler/rustc_middle/src/infer/canonical.rs
+++ b/compiler/rustc_middle/src/infer/canonical.rs
@@ -27,7 +27,7 @@ use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::sync::Lock;
 use rustc_macros::{HashStable, TypeFoldable, TypeVisitable};
 pub use rustc_type_ir as ir;
-pub use rustc_type_ir::{CanonicalTyVarKind, CanonicalVarKind};
+pub use rustc_type_ir::CanonicalVarKind;
 use smallvec::SmallVec;
 
 use crate::mir::ConstraintCategory;
diff --git a/compiler/rustc_next_trait_solver/src/canonicalizer.rs b/compiler/rustc_next_trait_solver/src/canonicalizer.rs
index bbb4a162027dc..5097f9ec545a5 100644
--- a/compiler/rustc_next_trait_solver/src/canonicalizer.rs
+++ b/compiler/rustc_next_trait_solver/src/canonicalizer.rs
@@ -4,8 +4,8 @@ use rustc_type_ir::data_structures::{HashMap, ensure_sufficient_stack};
 use rustc_type_ir::inherent::*;
 use rustc_type_ir::solve::{Goal, QueryInput};
 use rustc_type_ir::{
-    self as ty, Canonical, CanonicalTyVarKind, CanonicalVarInfo, CanonicalVarKind, InferCtxtLike,
-    Interner, TypeFoldable, TypeFolder, TypeSuperFoldable, TypeVisitableExt,
+    self as ty, Canonical, CanonicalVarInfo, CanonicalVarKind, InferCtxtLike, Interner,
+    TypeFoldable, TypeFolder, TypeSuperFoldable, TypeVisitableExt,
 };
 
 use crate::delegate::SolverDelegate;
@@ -52,6 +52,7 @@ pub struct Canonicalizer<'a, D: SolverDelegate<Interner = I>, I: Interner> {
     variables: &'a mut Vec<I::GenericArg>,
     primitive_var_infos: Vec<CanonicalVarInfo<I>>,
     variable_lookup_table: HashMap<I::GenericArg, usize>,
+    sub_root_lookup_table: HashMap<ty::TyVid, usize>,
     binder_index: ty::DebruijnIndex,
 
     /// We only use the debruijn index during lookup. We don't need to
@@ -73,6 +74,7 @@ impl<'a, D: SolverDelegate<Interner = I>, I: Interner> Canonicalizer<'a, D, I> {
 
             variables,
             variable_lookup_table: Default::default(),
+            sub_root_lookup_table: Default::default(),
             primitive_var_infos: Vec::new(),
             binder_index: ty::INNERMOST,
 
@@ -106,6 +108,7 @@ impl<'a, D: SolverDelegate<Interner = I>, I: Interner> Canonicalizer<'a, D, I> {
 
             variables,
             variable_lookup_table: Default::default(),
+            sub_root_lookup_table: Default::default(),
             primitive_var_infos: Vec::new(),
             binder_index: ty::INNERMOST,
 
@@ -123,6 +126,7 @@ impl<'a, D: SolverDelegate<Interner = I>, I: Interner> Canonicalizer<'a, D, I> {
             // We're able to reuse the `variable_lookup_table` as whether or not
             // it already contains an entry for `'static` does not matter.
             variable_lookup_table: env_canonicalizer.variable_lookup_table,
+            sub_root_lookup_table: Default::default(),
             primitive_var_infos: env_canonicalizer.primitive_var_infos,
             binder_index: ty::INNERMOST,
 
@@ -177,6 +181,13 @@ impl<'a, D: SolverDelegate<Interner = I>, I: Interner> Canonicalizer<'a, D, I> {
         ty::BoundVar::from(idx)
     }
 
+    fn get_or_insert_sub_root(&mut self, vid: ty::TyVid) -> ty::BoundVar {
+        let root_vid = self.delegate.sub_root_ty_var(vid);
+        let idx =
+            *self.sub_root_lookup_table.entry(root_vid).or_insert_with(|| self.variables.len());
+        ty::BoundVar::from(idx)
+    }
+
     fn finalize(self) -> (ty::UniverseIndex, I::CanonicalVars) {
         let mut var_infos = self.primitive_var_infos;
         // See the rustc-dev-guide section about how we deal with universes
@@ -323,11 +334,12 @@ impl<'a, D: SolverDelegate<Interner = I>, I: Interner> Canonicalizer<'a, D, I> {
                         "ty vid should have been resolved fully before canonicalization"
                     );
 
-                    CanonicalVarKind::Ty(CanonicalTyVarKind::General(
-                        self.delegate
-                            .universe_of_ty(vid)
-                            .unwrap_or_else(|| panic!("ty var should have been resolved: {t:?}")),
-                    ))
+                    let universe = self
+                        .delegate
+                        .universe_of_ty(vid)
+                        .unwrap_or_else(|| panic!("ty var should have been resolved: {t:?}"));
+                    let sub_root = self.get_or_insert_sub_root(vid);
+                    CanonicalVarKind::Ty { universe, sub_root }
                 }
                 ty::IntVar(vid) => {
                     assert_eq!(
@@ -335,7 +347,7 @@ impl<'a, D: SolverDelegate<Interner = I>, I: Interner> Canonicalizer<'a, D, I> {
                         t,
                         "ty vid should have been resolved fully before canonicalization"
                     );
-                    CanonicalVarKind::Ty(CanonicalTyVarKind::Int)
+                    CanonicalVarKind::Int
                 }
                 ty::FloatVar(vid) => {
                     assert_eq!(
@@ -343,7 +355,7 @@ impl<'a, D: SolverDelegate<Interner = I>, I: Interner> Canonicalizer<'a, D, I> {
                         t,
                         "ty vid should have been resolved fully before canonicalization"
                     );
-                    CanonicalVarKind::Ty(CanonicalTyVarKind::Float)
+                    CanonicalVarKind::Float
                 }
                 ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_) => {
                     panic!("fresh vars not expected in canonicalization")
diff --git a/compiler/rustc_next_trait_solver/src/delegate.rs b/compiler/rustc_next_trait_solver/src/delegate.rs
index 25493970a0ceb..d6f542723a357 100644
--- a/compiler/rustc_next_trait_solver/src/delegate.rs
+++ b/compiler/rustc_next_trait_solver/src/delegate.rs
@@ -39,7 +39,10 @@ pub trait SolverDelegate: Deref<Target = Self::Infcx> + Sized {
         term: <Self::Interner as Interner>::Term,
     ) -> Option<Vec<Goal<Self::Interner, <Self::Interner as Interner>::Predicate>>>;
 
-    fn clone_opaque_types_for_query_response(
+    fn clone_opaque_types_lookup_table(
+        &self,
+    ) -> Vec<(ty::OpaqueTypeKey<Self::Interner>, <Self::Interner as Interner>::Ty)>;
+    fn clone_duplicate_opaque_types(
         &self,
     ) -> Vec<(ty::OpaqueTypeKey<Self::Interner>, <Self::Interner as Interner>::Ty)>;
 
@@ -59,6 +62,7 @@ pub trait SolverDelegate: Deref<Target = Self::Infcx> + Sized {
         &self,
         cv_info: ty::CanonicalVarInfo<Self::Interner>,
         span: <Self::Interner as Interner>::Span,
+        var_values: &[<Self::Interner as Interner>::GenericArg],
         universe_map: impl Fn(ty::UniverseIndex) -> ty::UniverseIndex,
     ) -> <Self::Interner as Interner>::GenericArg;
 
@@ -68,6 +72,12 @@ pub trait SolverDelegate: Deref<Target = Self::Infcx> + Sized {
         hidden_ty: <Self::Interner as Interner>::Ty,
         span: <Self::Interner as Interner>::Span,
     ) -> Option<<Self::Interner as Interner>::Ty>;
+    fn add_duplicate_opaque_type(
+        &self,
+        opaque_type_key: ty::OpaqueTypeKey<Self::Interner>,
+        hidden_ty: <Self::Interner as Interner>::Ty,
+        span: <Self::Interner as Interner>::Span,
+    );
 
     fn add_item_bounds_for_hidden_type(
         &self,
diff --git a/compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs b/compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs
index 101129a231e7d..364d460d6c130 100644
--- a/compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs
+++ b/compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs
@@ -6,9 +6,10 @@ use derive_where::derive_where;
 use rustc_type_ir::inherent::*;
 use rustc_type_ir::lang_items::TraitSolverLangItem;
 use rustc_type_ir::{
-    self as ty, Interner, TypeFoldable, TypeVisitableExt as _, TypingMode, Upcast as _, elaborate,
+    self as ty, Interner, TypeFoldable, TypeFolder, TypeSuperFoldable, TypeVisitableExt as _,
+    TypingMode, Upcast as _, elaborate,
 };
-use tracing::{debug, instrument};
+use tracing::instrument;
 
 use super::has_only_region_constraints;
 use super::trait_goals::TraitGoalProvenVia;
@@ -316,8 +317,7 @@ where
         };
 
         if normalized_self_ty.is_ty_var() {
-            debug!("self type has been normalized to infer");
-            return self.forced_ambiguity(MaybeCause::Ambiguity).into_iter().collect();
+            return self.try_assemble_bounds_via_registered_opaque(goal, normalized_self_ty);
         }
 
         let goal: Goal<I, G> =
@@ -828,6 +828,60 @@ where
         }
     }
 
+    fn try_assemble_bounds_via_registered_opaque<G: GoalKind<D>>(
+        &mut self,
+        goal: Goal<I, G>,
+        self_ty: I::Ty,
+    ) -> Vec<Candidate<I>> {
+        //println!("for goal {goal:#?} and {self_ty:?}, we found an alias: {:#?}", self.find_sup_as_registered_opaque(self_ty));
+
+        let Some(alias_ty) = self.find_sup_as_registered_opaque(self_ty) else {
+            return self.forced_ambiguity(MaybeCause::Ambiguity).into_iter().collect();
+        };
+
+        let mut candidates = vec![];
+        for item_bound in
+            self.cx().item_self_bounds(alias_ty.def_id).iter_instantiated(self.cx(), alias_ty.args)
+        {
+            // TODO: comment
+            let assumption =
+                item_bound.fold_with(&mut ReplaceOpaque { cx: self.cx(), alias_ty, self_ty });
+            candidates.extend(G::probe_and_match_goal_against_assumption(
+                self,
+                CandidateSource::AliasBound,
+                goal,
+                assumption,
+                |ecx| ecx.evaluate_added_goals_and_make_canonical_response(Certainty::AMBIGUOUS),
+            ));
+        }
+
+        struct ReplaceOpaque<I: Interner> {
+            cx: I,
+            alias_ty: ty::AliasTy<I>,
+            self_ty: I::Ty,
+        }
+        impl<I: Interner> TypeFolder<I> for ReplaceOpaque<I> {
+            fn cx(&self) -> I {
+                self.cx
+            }
+            fn fold_ty(&mut self, ty: I::Ty) -> I::Ty {
+                if let ty::Alias(ty::Opaque, alias_ty) = ty.kind() {
+                    if alias_ty == self.alias_ty {
+                        return self.self_ty;
+                    }
+                }
+                ty.super_fold_with(self)
+            }
+        }
+
+        // TODO:
+        if candidates.is_empty() {
+            candidates.extend(self.forced_ambiguity(MaybeCause::Ambiguity));
+        }
+
+        candidates
+    }
+
     /// Assemble and merge candidates for goals which are related to an underlying trait
     /// goal. Right now, this is normalizes-to and host effect goals.
     ///
diff --git a/compiler/rustc_next_trait_solver/src/solve/eval_ctxt/canonical.rs b/compiler/rustc_next_trait_solver/src/solve/eval_ctxt/canonical.rs
index dded84f67686b..d50e09eb829d8 100644
--- a/compiler/rustc_next_trait_solver/src/solve/eval_ctxt/canonical.rs
+++ b/compiler/rustc_next_trait_solver/src/solve/eval_ctxt/canonical.rs
@@ -15,7 +15,8 @@ use rustc_index::IndexVec;
 use rustc_type_ir::inherent::*;
 use rustc_type_ir::relate::solver_relating::RelateExt;
 use rustc_type_ir::{
-    self as ty, Canonical, CanonicalVarValues, InferCtxtLike, Interner, TypeFoldable,
+    self as ty, Canonical, CanonicalVarKind, CanonicalVarValues, InferCtxtLike, Interner,
+    TypeFoldable,
 };
 use tracing::{debug, instrument, trace};
 
@@ -56,7 +57,10 @@ where
         &self,
         goal: Goal<I, T>,
     ) -> (Vec<I::GenericArg>, CanonicalInput<I, T>) {
-        let opaque_types = self.delegate.clone_opaque_types_for_query_response();
+        // We only care about one entry per `OpaqueTypeKey` here,
+        // so we only canonicalize the lookup table and ignore
+        // duplicate entries.
+        let opaque_types = self.delegate.clone_opaque_types_lookup_table();
         let (goal, opaque_types) =
             (goal, opaque_types).fold_with(&mut EagerResolver::new(self.delegate));
 
@@ -241,19 +245,21 @@ where
             Default::default()
         };
 
-        ExternalConstraintsData {
-            region_constraints,
-            opaque_types: self
-                .delegate
-                .clone_opaque_types_for_query_response()
-                .into_iter()
-                // Only return *newly defined* opaque types.
-                .filter(|(a, _)| {
-                    self.predefined_opaques_in_body.opaque_types.iter().all(|(pa, _)| pa != a)
-                })
-                .collect(),
-            normalization_nested_goals,
-        }
+        // We only return *newly defined* opaque types from canonical queries.
+        //
+        // Constraints for any existing opaque types are already tracked by changes
+        // to the `var_values`.
+        let opaque_types = self
+            .delegate
+            .clone_opaque_types_lookup_table()
+            .into_iter()
+            .filter(|(a, _)| {
+                self.predefined_opaques_in_body.opaque_types.iter().all(|(pa, _)| pa != a)
+            })
+            .chain(self.delegate.clone_duplicate_opaque_types())
+            .collect();
+
+        ExternalConstraintsData { region_constraints, opaque_types, normalization_nested_goals }
     }
 
     /// After calling a canonical query, we apply the constraints returned
@@ -354,37 +360,52 @@ where
             }
         }
 
-        let var_values = delegate.cx().mk_args_from_iter(
-            response.variables.iter().enumerate().map(|(index, info)| {
-                if info.universe() != ty::UniverseIndex::ROOT {
-                    // A variable from inside a binder of the query. While ideally these shouldn't
-                    // exist at all (see the FIXME at the start of this method), we have to deal with
-                    // them for now.
-                    delegate.instantiate_canonical_var_with_infer(info, span, |idx| {
-                        prev_universe + idx.index()
-                    })
-                } else if info.is_existential() {
-                    // As an optimization we sometimes avoid creating a new inference variable here.
-                    //
-                    // All new inference variables we create start out in the current universe of the caller.
-                    // This is conceptually wrong as these inference variables would be able to name
-                    // more placeholders then they should be able to. However the inference variables have
-                    // to "come from somewhere", so by equating them with the original values of the caller
-                    // later on, we pull them down into their correct universe again.
-                    if let Some(v) = opt_values[ty::BoundVar::from_usize(index)] {
-                        v
-                    } else {
-                        delegate.instantiate_canonical_var_with_infer(info, span, |_| prev_universe)
+        let mut var_values = Vec::new();
+        for (index, info) in response.variables.iter().enumerate() {
+            let value = if info.universe() != ty::UniverseIndex::ROOT {
+                // A variable from inside a binder of the query. While ideally these shouldn't
+                // exist at all (see the FIXME at the start of this method), we have to deal with
+                // them for now.
+                delegate.instantiate_canonical_var_with_infer(info, span, &var_values, |idx| {
+                    prev_universe + idx.index()
+                })
+            } else if info.is_existential() {
+                // As an optimization we sometimes avoid creating a new inference variable here.
+                // We need to still make sure to register any subtype relations returned by the
+                // query.
+                if let Some(v) = opt_values[ty::BoundVar::from_usize(index)] {
+                    if let CanonicalVarKind::Ty { universe: _, sub_root } = info.kind {
+                        if let Some(prev) = var_values.get(sub_root.as_usize()) {
+                            let v = delegate.shallow_resolve(v.expect_ty());
+                            let prev = delegate.shallow_resolve(prev.expect_ty());
+                            match (v.kind(), prev.kind()) {
+                                (ty::Infer(ty::TyVar(vid)), ty::Infer(ty::TyVar(sub_root))) => {
+                                    delegate.sub_ty_vids_raw(vid, sub_root)
+                                }
+                                _ => {}
+                            }
+                        }
                     }
+                    v
                 } else {
-                    // For placeholders which were already part of the input, we simply map this
-                    // universal bound variable back the placeholder of the input.
-                    original_values[info.expect_placeholder_index()]
+                    // All new inference variables we create start out in the current universe
+                    // of the caller. This is conceptually wrong as these inference variables
+                    // would be able to name more placeholders then they should be able to.
+                    // However the inference variables have to "come from somewhere", so by
+                    // equating them with the original values of the caller later on, we pull
+                    // them down into their correct universe again.
+                    delegate.instantiate_canonical_var_with_infer(info, span, &var_values, |_| {
+                        prev_universe
+                    })
                 }
-            }),
-        );
-
-        CanonicalVarValues { var_values }
+            } else {
+                // For placeholders which were already part of the input, we simply map this
+                // universal bound variable back the placeholder of the input.
+                original_values[info.expect_placeholder_index()]
+            };
+            var_values.push(value)
+        }
+        CanonicalVarValues { var_values: delegate.cx().mk_args(&var_values) }
     }
 
     /// Unify the `original_values` with the `var_values` returned by the canonical query..
@@ -432,7 +453,16 @@ where
     fn register_new_opaque_types(&mut self, opaque_types: &[(ty::OpaqueTypeKey<I>, I::Ty)]) {
         for &(key, ty) in opaque_types {
             let prev = self.delegate.register_hidden_type_in_storage(key, ty, self.origin_span);
-            assert_eq!(prev, None);
+            // We eagerly resolve inference variables when computing the query response.
+            // This can cause previously distinct opaque type keys to now be structurally equal.
+            //
+            // To handle this, we store any duplicate entries in a separate list to check them
+            // at the end of typeck/borrowck. We could alternatively eagerly equate the hidden
+            // types here. However, doing so is difficult as it may result in nested goals and
+            // any errors may make it harder to track the control flow for diagnostics.
+            if let Some(prev) = prev {
+                self.delegate.add_duplicate_opaque_type(key, prev, self.origin_span);
+            }
         }
     }
 }
diff --git a/compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs b/compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs
index 6dd554299a697..49b2722e55e00 100644
--- a/compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs
+++ b/compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs
@@ -14,7 +14,7 @@ use rustc_type_ir::{
     TypeSuperFoldable, TypeSuperVisitable, TypeVisitable, TypeVisitableExt, TypeVisitor,
     TypingMode,
 };
-use tracing::{instrument, trace};
+use tracing::{debug, instrument, trace};
 
 use super::has_only_region_constraints;
 use crate::coherence;
@@ -361,7 +361,20 @@ where
 
         for &(key, ty) in &input.predefined_opaques_in_body.opaque_types {
             let prev = ecx.delegate.register_hidden_type_in_storage(key, ty, ecx.origin_span);
-            assert_eq!(prev, None);
+            // It may be possible that two entries in the opaque type storage end up
+            // with the same key after resolving contained inference variables.
+            //
+            // We could put them in the duplicate list but don't have to. The opaques we
+            // encounter here are already tracked in the caller, so there's no need to
+            // also store them here. We'd take them out when computing the query response
+            // and then discard them, as they're already present in the input.
+            //
+            // Ideally we'd drop duplicate opaque type definitions when computing
+            // the canonical input. This is more annoying to implement and may cause a
+            // perf regression, so we do it inside of the query for now.
+            if let Some(prev) = prev {
+                debug!(?key, ?ty, ?prev, "ignore duplicate in `opaque_type_storage`");
+            }
         }
 
         if !ecx.nested_goals.is_empty() {
@@ -844,6 +857,10 @@ where
             && goal.param_env.visit_with(&mut visitor).is_continue()
     }
 
+    pub(super) fn sub_ty_vids_raw(&self, a: ty::TyVid, b: ty::TyVid) {
+        self.delegate.sub_ty_vids_raw(a, b)
+    }
+
     #[instrument(level = "trace", skip(self, param_env), ret)]
     pub(super) fn eq<T: Relate<I>>(
         &mut self,
@@ -1065,14 +1082,13 @@ where
         &mut self,
         key: ty::OpaqueTypeKey<I>,
     ) -> Option<(ty::OpaqueTypeKey<I>, I::Ty)> {
-        let mut matching =
-            self.delegate.clone_opaque_types_for_query_response().into_iter().filter(
-                |(candidate_key, _)| {
-                    candidate_key.def_id == key.def_id
-                        && DeepRejectCtxt::relate_rigid_rigid(self.cx())
-                            .args_may_unify(candidate_key.args, key.args)
-                },
-            );
+        let mut matching = self.delegate.clone_opaque_types_lookup_table().into_iter().filter(
+            |(candidate_key, _)| {
+                candidate_key.def_id == key.def_id
+                    && DeepRejectCtxt::relate_rigid_rigid(self.cx())
+                        .args_may_unify(candidate_key.args, key.args)
+            },
+        );
         let first = matching.next();
         let second = matching.next();
         assert_eq!(second, None);
@@ -1098,6 +1114,26 @@ where
     ) -> Result<Certainty, NoSolution> {
         self.delegate.is_transmutable(dst, src, assume)
     }
+
+    pub(crate) fn find_sup_as_registered_opaque(&self, self_ty: I::Ty) -> Option<ty::AliasTy<I>> {
+        self.delegate
+            .clone_opaque_types_lookup_table()
+            .into_iter()
+            .chain(self.delegate.clone_duplicate_opaque_types())
+            .find(|(_, hidden_ty)| {
+                if let ty::Infer(ty::TyVar(self_vid)) = self_ty.kind() {
+                    if let ty::Infer(ty::TyVar(hidden_vid)) = hidden_ty.kind() {
+                        if self.delegate.sub_root_ty_var(self_vid)
+                            == self.delegate.sub_root_ty_var(hidden_vid)
+                        {
+                            return true;
+                        }
+                    }
+                }
+                false
+            })
+            .map(|(key, _)| ty::AliasTy::new_from_args(self.cx(), key.def_id.into(), key.args))
+    }
 }
 
 /// Eagerly replace aliases with inference variables, emitting `AliasRelate`
diff --git a/compiler/rustc_next_trait_solver/src/solve/mod.rs b/compiler/rustc_next_trait_solver/src/solve/mod.rs
index c9f4fc649b523..639d103f9020b 100644
--- a/compiler/rustc_next_trait_solver/src/solve/mod.rs
+++ b/compiler/rustc_next_trait_solver/src/solve/mod.rs
@@ -120,11 +120,15 @@ where
 
     #[instrument(level = "trace", skip(self))]
     fn compute_subtype_goal(&mut self, goal: Goal<I, ty::SubtypePredicate<I>>) -> QueryResult<I> {
-        if goal.predicate.a.is_ty_var() && goal.predicate.b.is_ty_var() {
-            self.evaluate_added_goals_and_make_canonical_response(Certainty::AMBIGUOUS)
-        } else {
-            self.sub(goal.param_env, goal.predicate.a, goal.predicate.b)?;
-            self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
+        match (goal.predicate.a.kind(), goal.predicate.b.kind()) {
+            (ty::Infer(ty::TyVar(a_vid)), ty::Infer(ty::TyVar(b_vid))) => {
+                self.sub_ty_vids_raw(a_vid, b_vid);
+                self.evaluate_added_goals_and_make_canonical_response(Certainty::AMBIGUOUS)
+            }
+            _ => {
+                self.sub(goal.param_env, goal.predicate.a, goal.predicate.b)?;
+                self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
+            }
         }
     }
 
diff --git a/compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs b/compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs
index fdd547448f004..00230f45a892e 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs
@@ -92,7 +92,6 @@ mod suggest;
 pub mod need_type_info;
 pub mod nice_region_error;
 pub mod region;
-pub mod sub_relations;
 
 /// Makes a valid string literal from a string by escaping special characters (" and \),
 /// unless they are already escaped.
diff --git a/compiler/rustc_trait_selection/src/error_reporting/infer/need_type_info.rs b/compiler/rustc_trait_selection/src/error_reporting/infer/need_type_info.rs
index de9a50f196234..72571a177e1e6 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/infer/need_type_info.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/infer/need_type_info.rs
@@ -945,7 +945,7 @@ impl<'a, 'tcx> FindInferSourceVisitor<'a, 'tcx> {
                 use ty::{Infer, TyVar};
                 match (inner_ty.kind(), target_ty.kind()) {
                     (&Infer(TyVar(a_vid)), &Infer(TyVar(b_vid))) => {
-                        self.tecx.sub_relations.borrow_mut().unified(self.tecx, a_vid, b_vid)
+                        self.tecx.sub_root_var(a_vid) == self.tecx.sub_root_var(b_vid)
                     }
                     _ => false,
                 }
diff --git a/compiler/rustc_trait_selection/src/error_reporting/infer/sub_relations.rs b/compiler/rustc_trait_selection/src/error_reporting/infer/sub_relations.rs
deleted file mode 100644
index ef26a8ff7b863..0000000000000
--- a/compiler/rustc_trait_selection/src/error_reporting/infer/sub_relations.rs
+++ /dev/null
@@ -1,81 +0,0 @@
-use rustc_data_structures::fx::FxHashMap;
-use rustc_data_structures::undo_log::NoUndo;
-use rustc_data_structures::unify as ut;
-use rustc_middle::ty;
-
-use crate::infer::InferCtxt;
-
-#[derive(Debug, Copy, Clone, PartialEq)]
-struct SubId(u32);
-impl ut::UnifyKey for SubId {
-    type Value = ();
-    #[inline]
-    fn index(&self) -> u32 {
-        self.0
-    }
-    #[inline]
-    fn from_index(i: u32) -> SubId {
-        SubId(i)
-    }
-    fn tag() -> &'static str {
-        "SubId"
-    }
-}
-
-/// When reporting ambiguity errors, we sometimes want to
-/// treat all inference vars which are subtypes of each
-/// others as if they are equal. For this case we compute
-/// the transitive closure of our subtype obligations here.
-///
-/// E.g. when encountering ambiguity errors, we want to suggest
-/// specifying some method argument or to add a type annotation
-/// to a local variable. Because subtyping cannot change the
-/// shape of a type, it's fine if the cause of the ambiguity error
-/// is only related to the suggested variable via subtyping.
-///
-/// Even for something like `let x = returns_arg(); x.method();` the
-/// type of `x` is only a supertype of the argument of `returns_arg`. We
-/// still want to suggest specifying the type of the argument.
-#[derive(Default)]
-pub struct SubRelations {
-    map: FxHashMap<ty::TyVid, SubId>,
-    table: ut::UnificationTableStorage<SubId>,
-}
-
-impl SubRelations {
-    fn get_id<'tcx>(&mut self, infcx: &InferCtxt<'tcx>, vid: ty::TyVid) -> SubId {
-        let root_vid = infcx.root_var(vid);
-        *self.map.entry(root_vid).or_insert_with(|| self.table.with_log(&mut NoUndo).new_key(()))
-    }
-
-    pub fn add_constraints<'tcx>(
-        &mut self,
-        infcx: &InferCtxt<'tcx>,
-        obls: impl IntoIterator<Item = ty::Predicate<'tcx>>,
-    ) {
-        for p in obls {
-            let (a, b) = match p.kind().skip_binder() {
-                ty::PredicateKind::Subtype(ty::SubtypePredicate { a_is_expected: _, a, b }) => {
-                    (a, b)
-                }
-                ty::PredicateKind::Coerce(ty::CoercePredicate { a, b }) => (a, b),
-                _ => continue,
-            };
-
-            match (a.kind(), b.kind()) {
-                (&ty::Infer(ty::TyVar(a_vid)), &ty::Infer(ty::TyVar(b_vid))) => {
-                    let a = self.get_id(infcx, a_vid);
-                    let b = self.get_id(infcx, b_vid);
-                    self.table.with_log(&mut NoUndo).unify_var_var(a, b).unwrap();
-                }
-                _ => continue,
-            }
-        }
-    }
-
-    pub fn unified<'tcx>(&mut self, infcx: &InferCtxt<'tcx>, a: ty::TyVid, b: ty::TyVid) -> bool {
-        let a = self.get_id(infcx, a);
-        let b = self.get_id(infcx, b);
-        self.table.with_log(&mut NoUndo).unioned(a, b)
-    }
-}
diff --git a/compiler/rustc_trait_selection/src/error_reporting/mod.rs b/compiler/rustc_trait_selection/src/error_reporting/mod.rs
index 82695688ae897..cce20b05c79aa 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/mod.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/mod.rs
@@ -7,8 +7,6 @@ use rustc_macros::extension;
 use rustc_middle::bug;
 use rustc_middle::ty::{self, Ty};
 
-use crate::error_reporting::infer::sub_relations;
-
 pub mod infer;
 pub mod traits;
 
@@ -21,7 +19,6 @@ pub mod traits;
 /// methods which should not be used during the happy path.
 pub struct TypeErrCtxt<'a, 'tcx> {
     pub infcx: &'a InferCtxt<'tcx>,
-    pub sub_relations: std::cell::RefCell<sub_relations::SubRelations>,
 
     pub typeck_results: Option<std::cell::Ref<'a, ty::TypeckResults<'tcx>>>,
     pub fallback_has_occurred: bool,
@@ -38,7 +35,6 @@ impl<'tcx> InferCtxt<'tcx> {
     fn err_ctxt(&self) -> TypeErrCtxt<'_, 'tcx> {
         TypeErrCtxt {
             infcx: self,
-            sub_relations: Default::default(),
             typeck_results: None,
             fallback_has_occurred: false,
             normalize_fn_sig: Box::new(|fn_sig| fn_sig),
diff --git a/compiler/rustc_trait_selection/src/error_reporting/traits/mod.rs b/compiler/rustc_trait_selection/src/error_reporting/traits/mod.rs
index 78f9287b407b3..9d2d64e4398fc 100644
--- a/compiler/rustc_trait_selection/src/error_reporting/traits/mod.rs
+++ b/compiler/rustc_trait_selection/src/error_reporting/traits/mod.rs
@@ -141,10 +141,6 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         &self,
         mut errors: Vec<FulfillmentError<'tcx>>,
     ) -> ErrorGuaranteed {
-        self.sub_relations
-            .borrow_mut()
-            .add_constraints(self, errors.iter().map(|e| e.obligation.predicate));
-
         #[derive(Debug)]
         struct ErrorDescriptor<'tcx> {
             goal: Goal<'tcx, ty::Predicate<'tcx>>,
diff --git a/compiler/rustc_trait_selection/src/solve/delegate.rs b/compiler/rustc_trait_selection/src/solve/delegate.rs
index ef64da131891f..4a184f8616116 100644
--- a/compiler/rustc_trait_selection/src/solve/delegate.rs
+++ b/compiler/rustc_trait_selection/src/solve/delegate.rs
@@ -104,8 +104,23 @@ impl<'tcx> rustc_next_trait_solver::delegate::SolverDelegate for SolverDelegate<
         .map(|obligations| obligations.into_iter().map(|obligation| obligation.as_goal()).collect())
     }
 
-    fn clone_opaque_types_for_query_response(&self) -> Vec<(ty::OpaqueTypeKey<'tcx>, Ty<'tcx>)> {
-        self.0.clone_opaque_types_for_query_response()
+    fn clone_opaque_types_lookup_table(&self) -> Vec<(ty::OpaqueTypeKey<'tcx>, Ty<'tcx>)> {
+        self.0
+            .inner
+            .borrow_mut()
+            .opaque_types()
+            .iter_lookup_table()
+            .map(|(k, h)| (k, h.ty))
+            .collect()
+    }
+    fn clone_duplicate_opaque_types(&self) -> Vec<(ty::OpaqueTypeKey<'tcx>, Ty<'tcx>)> {
+        self.0
+            .inner
+            .borrow_mut()
+            .opaque_types()
+            .iter_duplicate_entries()
+            .map(|(k, h)| (k, h.ty))
+            .collect()
     }
 
     fn make_deduplicated_outlives_constraints(
@@ -148,22 +163,35 @@ impl<'tcx> rustc_next_trait_solver::delegate::SolverDelegate for SolverDelegate<
         &self,
         cv_info: CanonicalVarInfo<'tcx>,
         span: Span,
+        var_values: &[ty::GenericArg<'tcx>],
         universe_map: impl Fn(ty::UniverseIndex) -> ty::UniverseIndex,
     ) -> ty::GenericArg<'tcx> {
-        self.0.instantiate_canonical_var(span, cv_info, universe_map)
+        self.0.instantiate_canonical_var(span, cv_info, var_values, universe_map)
     }
 
     fn register_hidden_type_in_storage(
         &self,
         opaque_type_key: ty::OpaqueTypeKey<'tcx>,
-        hidden_ty: <Self::Interner as ty::Interner>::Ty,
-        span: <Self::Interner as ty::Interner>::Span,
-    ) -> Option<<Self::Interner as ty::Interner>::Ty> {
+        hidden_ty: Ty<'tcx>,
+        span: Span,
+    ) -> Option<Ty<'tcx>> {
         self.0.register_hidden_type_in_storage(
             opaque_type_key,
             ty::OpaqueHiddenType { span, ty: hidden_ty },
         )
     }
+    fn add_duplicate_opaque_type(
+        &self,
+        opaque_type_key: ty::OpaqueTypeKey<'tcx>,
+        hidden_ty: Ty<'tcx>,
+        span: Span,
+    ) {
+        self.0
+            .inner
+            .borrow_mut()
+            .opaque_types()
+            .add_duplicate(opaque_type_key, ty::OpaqueHiddenType { span, ty: hidden_ty })
+    }
 
     fn add_item_bounds_for_hidden_type(
         &self,
diff --git a/compiler/rustc_type_ir/src/canonical.rs b/compiler/rustc_type_ir/src/canonical.rs
index 67b67df4b2817..b65547ba04b8a 100644
--- a/compiler/rustc_type_ir/src/canonical.rs
+++ b/compiler/rustc_type_ir/src/canonical.rs
@@ -110,7 +110,7 @@ impl<I: Interner> CanonicalVarInfo<I> {
 
     pub fn is_existential(&self) -> bool {
         match self.kind {
-            CanonicalVarKind::Ty(_) => true,
+            CanonicalVarKind::Ty { .. } | CanonicalVarKind::Int | CanonicalVarKind::Float => true,
             CanonicalVarKind::PlaceholderTy(_) => false,
             CanonicalVarKind::Region(_) => true,
             CanonicalVarKind::PlaceholderRegion(..) => false,
@@ -122,7 +122,9 @@ impl<I: Interner> CanonicalVarInfo<I> {
     pub fn is_region(&self) -> bool {
         match self.kind {
             CanonicalVarKind::Region(_) | CanonicalVarKind::PlaceholderRegion(_) => true,
-            CanonicalVarKind::Ty(_)
+            CanonicalVarKind::Ty { .. }
+            | CanonicalVarKind::Int
+            | CanonicalVarKind::Float
             | CanonicalVarKind::PlaceholderTy(_)
             | CanonicalVarKind::Const(_)
             | CanonicalVarKind::PlaceholderConst(_) => false,
@@ -131,7 +133,11 @@ impl<I: Interner> CanonicalVarInfo<I> {
 
     pub fn expect_placeholder_index(self) -> usize {
         match self.kind {
-            CanonicalVarKind::Ty(_) | CanonicalVarKind::Region(_) | CanonicalVarKind::Const(_) => {
+            CanonicalVarKind::Ty { .. }
+            | CanonicalVarKind::Int
+            | CanonicalVarKind::Float
+            | CanonicalVarKind::Region(_)
+            | CanonicalVarKind::Const(_) => {
                 panic!("expected placeholder: {self:?}")
             }
 
@@ -151,8 +157,18 @@ impl<I: Interner> CanonicalVarInfo<I> {
     derive(Decodable_NoContext, Encodable_NoContext, HashStable_NoContext)
 )]
 pub enum CanonicalVarKind<I: Interner> {
-    /// Some kind of type inference variable.
-    Ty(CanonicalTyVarKind),
+    /// A general type variable `?T` that can be unified with arbitrary types.
+    ///
+    /// We also store the index of the first type variable which is sub-unified
+    /// with this one. If there is no inference variable related to this one,
+    /// its `sub_root` just points to itself.
+    Ty { universe: UniverseIndex, sub_root: ty::BoundVar },
+
+    /// Integral type variable `?I` (that can only be unified with integral types).
+    Int,
+
+    /// Floating-point type variable `?F` (that can only be unified with float types).
+    Float,
 
     /// A "placeholder" that represents "any type".
     PlaceholderTy(I::PlaceholderTy),
@@ -175,15 +191,13 @@ pub enum CanonicalVarKind<I: Interner> {
 impl<I: Interner> CanonicalVarKind<I> {
     pub fn universe(self) -> UniverseIndex {
         match self {
-            CanonicalVarKind::Ty(CanonicalTyVarKind::General(ui)) => ui,
+            CanonicalVarKind::Ty { universe, sub_root: _ } => universe,
             CanonicalVarKind::Region(ui) => ui,
             CanonicalVarKind::Const(ui) => ui,
             CanonicalVarKind::PlaceholderTy(placeholder) => placeholder.universe(),
             CanonicalVarKind::PlaceholderRegion(placeholder) => placeholder.universe(),
             CanonicalVarKind::PlaceholderConst(placeholder) => placeholder.universe(),
-            CanonicalVarKind::Ty(CanonicalTyVarKind::Float | CanonicalTyVarKind::Int) => {
-                UniverseIndex::ROOT
-            }
+            CanonicalVarKind::Int | CanonicalVarKind::Float => UniverseIndex::ROOT,
         }
     }
 
@@ -193,8 +207,8 @@ impl<I: Interner> CanonicalVarKind<I> {
     /// the updated universe is not the root.
     pub fn with_updated_universe(self, ui: UniverseIndex) -> CanonicalVarKind<I> {
         match self {
-            CanonicalVarKind::Ty(CanonicalTyVarKind::General(_)) => {
-                CanonicalVarKind::Ty(CanonicalTyVarKind::General(ui))
+            CanonicalVarKind::Ty { universe: _, sub_root } => {
+                CanonicalVarKind::Ty { universe: ui, sub_root }
             }
             CanonicalVarKind::Region(_) => CanonicalVarKind::Region(ui),
             CanonicalVarKind::Const(_) => CanonicalVarKind::Const(ui),
@@ -208,7 +222,7 @@ impl<I: Interner> CanonicalVarKind<I> {
             CanonicalVarKind::PlaceholderConst(placeholder) => {
                 CanonicalVarKind::PlaceholderConst(placeholder.with_updated_universe(ui))
             }
-            CanonicalVarKind::Ty(CanonicalTyVarKind::Int | CanonicalTyVarKind::Float) => {
+            CanonicalVarKind::Int | CanonicalVarKind::Float => {
                 assert_eq!(ui, UniverseIndex::ROOT);
                 self
             }
@@ -216,28 +230,6 @@ impl<I: Interner> CanonicalVarKind<I> {
     }
 }
 
-/// Rust actually has more than one category of type variables;
-/// notably, the type variables we create for literals (e.g., 22 or
-/// 22.) can only be instantiated with integral/float types (e.g.,
-/// usize or f32). In order to faithfully reproduce a type, we need to
-/// know what set of types a given type variable can be unified with.
-#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
-#[derive(TypeVisitable_Generic, TypeFoldable_Generic)]
-#[cfg_attr(
-    feature = "nightly",
-    derive(Decodable_NoContext, Encodable_NoContext, HashStable_NoContext)
-)]
-pub enum CanonicalTyVarKind {
-    /// General type variable `?T` that can be unified with arbitrary types.
-    General(UniverseIndex),
-
-    /// Integral type variable `?I` (that can only be unified with integral types).
-    Int,
-
-    /// Floating-point type variable `?F` (that can only be unified with float types).
-    Float,
-}
-
 /// A set of values corresponding to the canonical variables from some
 /// `Canonical`. You can give these values to
 /// `canonical_value.instantiate` to instantiate them into the canonical
@@ -311,7 +303,10 @@ impl<I: Interner> CanonicalVarValues<I> {
             var_values: cx.mk_args_from_iter(infos.iter().enumerate().map(
                 |(i, info)| -> I::GenericArg {
                     match info.kind {
-                        CanonicalVarKind::Ty(_) | CanonicalVarKind::PlaceholderTy(_) => {
+                        CanonicalVarKind::Ty { .. }
+                        | CanonicalVarKind::Int
+                        | CanonicalVarKind::Float
+                        | CanonicalVarKind::PlaceholderTy(_) => {
                             Ty::new_anon_bound(cx, ty::INNERMOST, ty::BoundVar::from_usize(i))
                                 .into()
                         }
diff --git a/compiler/rustc_type_ir/src/infer_ctxt.rs b/compiler/rustc_type_ir/src/infer_ctxt.rs
index 8fa56c3599963..70467622b8ebc 100644
--- a/compiler/rustc_type_ir/src/infer_ctxt.rs
+++ b/compiler/rustc_type_ir/src/infer_ctxt.rs
@@ -149,6 +149,7 @@ pub trait InferCtxtLike: Sized {
     fn universe_of_ct(&self, ct: ty::ConstVid) -> Option<ty::UniverseIndex>;
 
     fn root_ty_var(&self, var: ty::TyVid) -> ty::TyVid;
+    fn sub_root_ty_var(&self, var: ty::TyVid) -> ty::TyVid;
     fn root_const_var(&self, var: ty::ConstVid) -> ty::ConstVid;
 
     fn opportunistic_resolve_ty_var(&self, vid: ty::TyVid) -> <Self::Interner as Interner>::Ty;
@@ -186,6 +187,7 @@ pub trait InferCtxtLike: Sized {
     ) -> U;
 
     fn equate_ty_vids_raw(&self, a: ty::TyVid, b: ty::TyVid);
+    fn sub_ty_vids_raw(&self, a: ty::TyVid, b: ty::TyVid);
     fn equate_int_vids_raw(&self, a: ty::IntVid, b: ty::IntVid);
     fn equate_float_vids_raw(&self, a: ty::FloatVid, b: ty::FloatVid);
     fn equate_const_vids_raw(&self, a: ty::ConstVid, b: ty::ConstVid);
diff --git a/tests/ui/traits/next-solver/opaques/duplicate-opaque-type-entries.rs b/tests/ui/traits/next-solver/opaques/duplicate-opaque-type-entries.rs
new file mode 100644
index 0000000000000..e0668ac3d39f4
--- /dev/null
+++ b/tests/ui/traits/next-solver/opaques/duplicate-opaque-type-entries.rs
@@ -0,0 +1,25 @@
+//@ revisions: current next
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@[next] compile-flags: -Znext-solver
+//@ check-pass
+#![crate_type = "lib"]
+trait Eq<T> {}
+impl<T> Eq<T> for T {}
+trait ConstrainAndEq<T> {}
+impl<T, U> ConstrainAndEq<T> for U
+where
+    T: FnOnce() -> u32,
+    U: FnOnce() -> u32,
+    T: Eq<U>,
+{}
+
+fn constrain_and_eq<T: ConstrainAndEq<U>, U>(_: T, _: U) {}
+fn foo<'a>() -> impl Sized + use<'a> {
+    // This proves `foo<'a>: FnOnce() -> u32` and `foo<'1>: FnOnce() -> u32`,
+    // We constrain both `opaque<'a>` and `opaque<'1>` to `u32`, resulting in
+    // two distinct opaque type uses. Proving `foo<'a>: Eq<foo<'1>>` then
+    // equates the two regions at which point the two opaque type keys are now
+    // equal. This previously caused an ICE.
+    constrain_and_eq(foo::<'a>, foo::<'_>);
+    1u32
+}