Merging upstream version 1.71.1+dfsg1.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

33 files changed, 958 insertions, 1117 deletions

diff --git a/compiler/rustc_infer/src/infer/at.rs b/compiler/rustc_infer/src/infer/at.rs
index d240d8e49..0c8854e96 100644
--- a/compiler/rustc_infer/src/infer/at.rs
+++ b/compiler/rustc_infer/src/infer/at.rs
@@ -30,8 +30,6 @@ use super::*;
 use rustc_middle::ty::relate::{Relate, TypeRelation};
 use rustc_middle::ty::{Const, ImplSubject};
 
-use std::cell::Cell;
-
 /// Whether we should define opaque types or just treat them opaquely.
 ///
 /// Currently only used to prevent predicate matching from matching anything
@@ -84,7 +82,6 @@ impl<'tcx> InferCtxt<'tcx> {
             in_snapshot: self.in_snapshot.clone(),
             universe: self.universe.clone(),
             intercrate: self.intercrate,
-            inside_canonicalization_ctxt: Cell::new(self.inside_canonicalization_ctxt()),
         }
     }
 }
diff --git a/compiler/rustc_infer/src/infer/canonical/canonicalizer.rs b/compiler/rustc_infer/src/infer/canonical/canonicalizer.rs
index e808911a3..427d05c8b 100644
--- a/compiler/rustc_infer/src/infer/canonical/canonicalizer.rs
+++ b/compiler/rustc_infer/src/infer/canonical/canonicalizer.rs
@@ -16,7 +16,7 @@ use rustc_middle::ty::{self, BoundVar, InferConst, List, Ty, TyCtxt, TypeFlags,
 use std::sync::atomic::Ordering;
 
 use rustc_data_structures::fx::FxHashMap;
-use rustc_index::vec::Idx;
+use rustc_index::Idx;
 use smallvec::SmallVec;
 
 impl<'tcx> InferCtxt<'tcx> {
@@ -205,7 +205,7 @@ impl CanonicalizeMode for CanonicalizeQueryResponse {
                 // `delay_span_bug` to allow type error over an ICE.
                 canonicalizer.tcx.sess.delay_span_bug(
                     rustc_span::DUMMY_SP,
-                    &format!("unexpected region in query response: `{:?}`", r),
+                    format!("unexpected region in query response: `{:?}`", r),
                 );
                 r
             }
@@ -561,15 +561,13 @@ impl<'cx, 'tcx> Canonicalizer<'cx, 'tcx> {
     where
         V: TypeFoldable<TyCtxt<'tcx>>,
     {
-        let _inside_canonical_ctxt_guard = infcx.set_canonicalization_ctxt();
-
         let needs_canonical_flags = if canonicalize_region_mode.any() {
-            TypeFlags::NEEDS_INFER |
+            TypeFlags::HAS_INFER |
             TypeFlags::HAS_FREE_REGIONS | // `HAS_RE_PLACEHOLDER` implies `HAS_FREE_REGIONS`
             TypeFlags::HAS_TY_PLACEHOLDER |
             TypeFlags::HAS_CT_PLACEHOLDER
         } else {
-            TypeFlags::NEEDS_INFER
+            TypeFlags::HAS_INFER
                 | TypeFlags::HAS_RE_PLACEHOLDER
                 | TypeFlags::HAS_TY_PLACEHOLDER
                 | TypeFlags::HAS_CT_PLACEHOLDER
@@ -600,7 +598,7 @@ impl<'cx, 'tcx> Canonicalizer<'cx, 'tcx> {
         // Once we have canonicalized `out_value`, it should not
         // contain anything that ties it to this inference context
         // anymore.
-        debug_assert!(!out_value.needs_infer() && !out_value.has_placeholders());
+        debug_assert!(!out_value.has_infer() && !out_value.has_placeholders());
 
         let canonical_variables =
             tcx.mk_canonical_var_infos(&canonicalizer.universe_canonicalized_variables());
diff --git a/compiler/rustc_infer/src/infer/canonical/mod.rs b/compiler/rustc_infer/src/infer/canonical/mod.rs
index fbb2257bf..2abdd5b0a 100644
--- a/compiler/rustc_infer/src/infer/canonical/mod.rs
+++ b/compiler/rustc_infer/src/infer/canonical/mod.rs
@@ -23,7 +23,7 @@
 
 use crate::infer::{ConstVariableOrigin, ConstVariableOriginKind};
 use crate::infer::{InferCtxt, RegionVariableOrigin, TypeVariableOrigin, TypeVariableOriginKind};
-use rustc_index::vec::IndexVec;
+use rustc_index::IndexVec;
 use rustc_middle::ty::fold::TypeFoldable;
 use rustc_middle::ty::subst::GenericArg;
 use rustc_middle::ty::{self, List, TyCtxt};
diff --git a/compiler/rustc_infer/src/infer/canonical/query_response.rs b/compiler/rustc_infer/src/infer/canonical/query_response.rs
index e98f68ae5..88256c819 100644
--- a/compiler/rustc_infer/src/infer/canonical/query_response.rs
+++ b/compiler/rustc_infer/src/infer/canonical/query_response.rs
@@ -15,12 +15,12 @@ use crate::infer::canonical::{
 use crate::infer::nll_relate::{TypeRelating, TypeRelatingDelegate};
 use crate::infer::region_constraints::{Constraint, RegionConstraintData};
 use crate::infer::{DefineOpaqueTypes, InferCtxt, InferOk, InferResult, NllRegionVariableOrigin};
-use crate::traits::query::{Fallible, NoSolution};
+use crate::traits::query::NoSolution;
 use crate::traits::{Obligation, ObligationCause, PredicateObligation};
 use crate::traits::{PredicateObligations, TraitEngine, TraitEngineExt};
 use rustc_data_structures::captures::Captures;
-use rustc_index::vec::Idx;
-use rustc_index::vec::IndexVec;
+use rustc_index::Idx;
+use rustc_index::IndexVec;
 use rustc_middle::arena::ArenaAllocatable;
 use rustc_middle::mir::ConstraintCategory;
 use rustc_middle::ty::fold::TypeFoldable;
@@ -57,7 +57,7 @@ impl<'tcx> InferCtxt<'tcx> {
         inference_vars: CanonicalVarValues<'tcx>,
         answer: T,
         fulfill_cx: &mut dyn TraitEngine<'tcx>,
-    ) -> Fallible<CanonicalQueryResponse<'tcx, T>>
+    ) -> Result<CanonicalQueryResponse<'tcx, T>, NoSolution>
     where
         T: Debug + TypeFoldable<TyCtxt<'tcx>>,
         Canonical<'tcx, QueryResponse<'tcx, T>>: ArenaAllocatable<'tcx>,
@@ -153,20 +153,22 @@ 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<'tcx>, Ty<'tcx>)> {
+    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)| (self.tcx.mk_opaque(k.def_id.to_def_id(), k.substs), v.hidden_type.ty))
+            .map(|(k, v)| (*k, v.hidden_type.ty))
             .collect()
     }
 
-    fn take_opaque_types_for_query_response(&self) -> Vec<(Ty<'tcx>, Ty<'tcx>)> {
+    fn take_opaque_types_for_query_response(&self) -> Vec<(ty::OpaqueTypeKey<'tcx>, Ty<'tcx>)> {
         std::mem::take(&mut self.inner.borrow_mut().opaque_type_storage.opaque_types)
             .into_iter()
-            .map(|(k, v)| (self.tcx.mk_opaque(k.def_id.to_def_id(), k.substs), v.hidden_type.ty))
+            .map(|(k, v)| (k, v.hidden_type.ty))
             .collect()
     }
 
@@ -467,11 +469,11 @@ impl<'tcx> InferCtxt<'tcx> {
                     }
                 }
                 GenericArgKind::Const(result_value) => {
-                    if let ty::ConstKind::Bound(debrujin, b) = result_value.kind() {
+                    if let ty::ConstKind::Bound(debruijn, b) = result_value.kind() {
                         // ...in which case we would set `canonical_vars[0]` to `Some(const X)`.
 
                         // We only allow a `ty::INNERMOST` index in substitutions.
-                        assert_eq!(debrujin, ty::INNERMOST);
+                        assert_eq!(debruijn, ty::INNERMOST);
                         opt_values[b] = Some(*original_value);
                     }
                 }
@@ -507,8 +509,22 @@ impl<'tcx> InferCtxt<'tcx> {
             let a = substitute_value(self.tcx, &result_subst, a);
             let b = substitute_value(self.tcx, &result_subst, b);
             debug!(?a, ?b, "constrain opaque type");
-            obligations
-                .extend(self.at(cause, param_env).eq(DefineOpaqueTypes::Yes, a, b)?.obligations);
+            // We use equate here instead of, for example, just registering the
+            // opaque type's hidden value directly, because we may be instantiating
+            // a query response that was canonicalized in an InferCtxt that had
+            // a different defining anchor. In that case, we may have inferred
+            // `NonLocalOpaque := LocalOpaque` but can only instantiate it in
+            // the other direction as `LocalOpaque := NonLocalOpaque`. Using eq
+            // here allows us to try both directions (in `InferCtxt::handle_opaque_type`).
+            obligations.extend(
+                self.at(cause, param_env)
+                    .eq(
+                        DefineOpaqueTypes::Yes,
+                        self.tcx.mk_opaque(a.def_id.to_def_id(), a.substs),
+                        b,
+                    )?
+                    .obligations,
+            );
         }
 
         Ok(InferOk { value: result_subst, obligations })
diff --git a/compiler/rustc_infer/src/infer/combine.rs b/compiler/rustc_infer/src/infer/combine.rs
index fe45b5ebe..b6b935de6 100644
--- a/compiler/rustc_infer/src/infer/combine.rs
+++ b/compiler/rustc_infer/src/infer/combine.rs
@@ -26,24 +26,17 @@ use super::equate::Equate;
 use super::glb::Glb;
 use super::lub::Lub;
 use super::sub::Sub;
-use super::type_variable::TypeVariableValue;
-use super::{DefineOpaqueTypes, InferCtxt, MiscVariable, TypeTrace};
+use super::{DefineOpaqueTypes, InferCtxt, TypeTrace};
+use crate::infer::generalize::{self, CombineDelegate, Generalization};
 use crate::traits::{Obligation, PredicateObligations};
-use rustc_data_structures::sso::SsoHashMap;
-use rustc_hir::def_id::DefId;
 use rustc_middle::infer::canonical::OriginalQueryValues;
 use rustc_middle::infer::unify_key::{ConstVarValue, ConstVariableValue};
 use rustc_middle::infer::unify_key::{ConstVariableOrigin, ConstVariableOriginKind};
-use rustc_middle::traits::ObligationCause;
 use rustc_middle::ty::error::{ExpectedFound, TypeError};
-use rustc_middle::ty::relate::{self, Relate, RelateResult, TypeRelation};
-use rustc_middle::ty::subst::SubstsRef;
-use rustc_middle::ty::{
-    self, AliasKind, FallibleTypeFolder, InferConst, ToPredicate, Ty, TyCtxt, TypeFoldable,
-    TypeSuperFoldable, TypeVisitableExt,
-};
+use rustc_middle::ty::relate::{RelateResult, TypeRelation};
+use rustc_middle::ty::{self, AliasKind, InferConst, ToPredicate, Ty, TyCtxt, TypeVisitableExt};
 use rustc_middle::ty::{IntType, UintType};
-use rustc_span::{Span, DUMMY_SP};
+use rustc_span::DUMMY_SP;
 
 #[derive(Clone)]
 pub struct CombineFields<'infcx, 'tcx> {
@@ -55,13 +48,6 @@ pub struct CombineFields<'infcx, 'tcx> {
     pub define_opaque_types: DefineOpaqueTypes,
 }
 
-#[derive(Copy, Clone, Debug)]
-pub enum RelationDir {
-    SubtypeOf,
-    SupertypeOf,
-    EqTo,
-}
-
 impl<'tcx> InferCtxt<'tcx> {
     pub fn super_combine_tys<R>(
         &self,
@@ -73,6 +59,8 @@ impl<'tcx> InferCtxt<'tcx> {
         R: ObligationEmittingRelation<'tcx>,
     {
         let a_is_expected = relation.a_is_expected();
+        debug_assert!(!a.has_escaping_bound_vars());
+        debug_assert!(!b.has_escaping_bound_vars());
 
         match (a.kind(), b.kind()) {
             // Relate integral variables to other types
@@ -125,9 +113,7 @@ impl<'tcx> InferCtxt<'tcx> {
                 bug!()
             }
 
-            (_, ty::Alias(AliasKind::Projection, _)) | (ty::Alias(AliasKind::Projection, _), _)
-                if self.tcx.trait_solver_next() =>
-            {
+            (_, ty::Alias(..)) | (ty::Alias(..), _) if self.tcx.trait_solver_next() => {
                 relation.register_type_relate_obligation(a, b);
                 Ok(a)
             }
@@ -149,7 +135,7 @@ impl<'tcx> InferCtxt<'tcx> {
                 Ok(a)
             }
 
-            _ => ty::relate::super_relate_tys(relation, a, b),
+            _ => ty::relate::structurally_relate_tys(relation, a, b),
         }
     }
 
@@ -163,6 +149,8 @@ impl<'tcx> InferCtxt<'tcx> {
         R: ObligationEmittingRelation<'tcx>,
     {
         debug!("{}.consts({:?}, {:?})", relation.tag(), a, b);
+        debug_assert!(!a.has_escaping_bound_vars());
+        debug_assert!(!b.has_escaping_bound_vars());
         if a == b {
             return Ok(a);
         }
@@ -192,7 +180,7 @@ impl<'tcx> InferCtxt<'tcx> {
             self.tcx.check_tys_might_be_eq(canonical).map_err(|_| {
                 self.tcx.sess.delay_span_bug(
                     DUMMY_SP,
-                    &format!("cannot relate consts of different types (a={:?}, b={:?})", a, b,),
+                    format!("cannot relate consts of different types (a={:?}, b={:?})", a, b,),
                 )
             })
         });
@@ -204,13 +192,13 @@ impl<'tcx> InferCtxt<'tcx> {
             // HACK: equating both sides with `[const error]` eagerly prevents us
             // from leaving unconstrained inference vars during things like impl
             // matching in the solver.
-            let a_error = self.tcx.const_error_with_guaranteed(a.ty(), guar);
+            let a_error = self.tcx.const_error(a.ty(), guar);
             if let ty::ConstKind::Infer(InferConst::Var(vid)) = a.kind() {
-                return self.unify_const_variable(vid, a_error);
+                return self.unify_const_variable(vid, a_error, relation.param_env());
             }
-            let b_error = self.tcx.const_error_with_guaranteed(b.ty(), guar);
+            let b_error = self.tcx.const_error(b.ty(), guar);
             if let ty::ConstKind::Infer(InferConst::Var(vid)) = b.kind() {
-                return self.unify_const_variable(vid, b_error);
+                return self.unify_const_variable(vid, b_error, relation.param_env());
             }
 
             return Ok(if relation.a_is_expected() { a_error } else { b_error });
@@ -232,32 +220,22 @@ impl<'tcx> InferCtxt<'tcx> {
             }
 
             (ty::ConstKind::Infer(InferConst::Var(vid)), _) => {
-                return self.unify_const_variable(vid, b);
+                return self.unify_const_variable(vid, b, relation.param_env());
             }
 
             (_, ty::ConstKind::Infer(InferConst::Var(vid))) => {
-                return self.unify_const_variable(vid, a);
+                return self.unify_const_variable(vid, a, relation.param_env());
             }
-            (ty::ConstKind::Unevaluated(..), _) if self.tcx.lazy_normalization() => {
-                // FIXME(#59490): Need to remove the leak check to accommodate
-                // escaping bound variables here.
-                if !a.has_escaping_bound_vars() && !b.has_escaping_bound_vars() {
-                    relation.register_const_equate_obligation(a, b);
-                }
+            (ty::ConstKind::Unevaluated(..), _) | (_, ty::ConstKind::Unevaluated(..))
+                if self.tcx.lazy_normalization() =>
+            {
+                relation.register_const_equate_obligation(a, b);
                 return Ok(b);
             }
-            (_, ty::ConstKind::Unevaluated(..)) if self.tcx.lazy_normalization() => {
-                // FIXME(#59490): Need to remove the leak check to accommodate
-                // escaping bound variables here.
-                if !a.has_escaping_bound_vars() && !b.has_escaping_bound_vars() {
-                    relation.register_const_equate_obligation(a, b);
-                }
-                return Ok(a);
-            }
             _ => {}
         }
 
-        ty::relate::super_relate_consts(relation, a, b)
+        ty::relate::structurally_relate_consts(relation, a, b)
     }
 
     /// Unifies the const variable `target_vid` with the given constant.
@@ -299,24 +277,17 @@ impl<'tcx> InferCtxt<'tcx> {
         &self,
         target_vid: ty::ConstVid<'tcx>,
         ct: ty::Const<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
     ) -> RelateResult<'tcx, ty::Const<'tcx>> {
-        let (for_universe, span) = {
-            let mut inner = self.inner.borrow_mut();
-            let variable_table = &mut inner.const_unification_table();
-            let var_value = variable_table.probe_value(target_vid);
-            match var_value.val {
-                ConstVariableValue::Known { value } => {
-                    bug!("instantiating {:?} which has a known value {:?}", target_vid, value)
-                }
-                ConstVariableValue::Unknown { universe } => (universe, var_value.origin.span),
-            }
-        };
-        let value = ct.try_fold_with(&mut ConstInferUnifier {
-            infcx: self,
-            span,
-            for_universe,
+        let span =
+            self.inner.borrow_mut().const_unification_table().probe_value(target_vid).origin.span;
+        let Generalization { value, needs_wf: _ } = generalize::generalize(
+            self,
+            &mut CombineDelegate { infcx: self, span, param_env },
+            ct,
             target_vid,
-        })?;
+            ty::Variance::Invariant,
+        )?;
 
         self.inner.borrow_mut().const_unification_table().union_value(
             target_vid,
@@ -397,12 +368,10 @@ impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
     pub fn instantiate(
         &mut self,
         a_ty: Ty<'tcx>,
-        dir: RelationDir,
+        ambient_variance: ty::Variance,
         b_vid: ty::TyVid,
         a_is_expected: bool,
     ) -> RelateResult<'tcx, ()> {
-        use self::RelationDir::*;
-
         // Get the actual variable that b_vid has been inferred to
         debug_assert!(self.infcx.inner.borrow_mut().type_variables().probe(b_vid).is_unknown());
 
@@ -417,7 +386,18 @@ impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
         // `'?2` and `?3` are fresh region/type inference
         // variables. (Down below, we will relate `a_ty <: b_ty`,
         // adding constraints like `'x: '?2` and `?1 <: ?3`.)
-        let Generalization { ty: b_ty, needs_wf } = self.generalize(a_ty, b_vid, dir)?;
+        let Generalization { value: b_ty, needs_wf } = generalize::generalize(
+            self.infcx,
+            &mut CombineDelegate {
+                infcx: self.infcx,
+                param_env: self.param_env,
+                span: self.trace.span(),
+            },
+            a_ty,
+            b_vid,
+            ambient_variance,
+        )?;
+
         debug!(?b_ty);
         self.infcx.inner.borrow_mut().type_variables().instantiate(b_vid, b_ty);
 
@@ -436,78 +416,23 @@ impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
         // relations wind up attributed to the same spans. We need
         // to associate causes/spans with each of the relations in
         // the stack to get this right.
-        match dir {
-            EqTo => self.equate(a_is_expected).relate(a_ty, b_ty),
-            SubtypeOf => self.sub(a_is_expected).relate(a_ty, b_ty),
-            SupertypeOf => self.sub(a_is_expected).relate_with_variance(
+        match ambient_variance {
+            ty::Variance::Invariant => self.equate(a_is_expected).relate(a_ty, b_ty),
+            ty::Variance::Covariant => self.sub(a_is_expected).relate(a_ty, b_ty),
+            ty::Variance::Contravariant => self.sub(a_is_expected).relate_with_variance(
                 ty::Contravariant,
                 ty::VarianceDiagInfo::default(),
                 a_ty,
                 b_ty,
             ),
+            ty::Variance::Bivariant => {
+                unreachable!("no code should be generalizing bivariantly (currently)")
+            }
         }?;
 
         Ok(())
     }
 
-    /// Attempts to generalize `ty` for the type variable `for_vid`.
-    /// This checks for cycle -- that is, whether the type `ty`
-    /// references `for_vid`. The `dir` is the "direction" for which we
-    /// a performing the generalization (i.e., are we producing a type
-    /// that can be used as a supertype etc).
-    ///
-    /// Preconditions:
-    ///
-    /// - `for_vid` is a "root vid"
-    #[instrument(skip(self), level = "trace", ret)]
-    fn generalize(
-        &self,
-        ty: Ty<'tcx>,
-        for_vid: ty::TyVid,
-        dir: RelationDir,
-    ) -> RelateResult<'tcx, Generalization<'tcx>> {
-        // Determine the ambient variance within which `ty` appears.
-        // The surrounding equation is:
-        //
-        //     ty [op] ty2
-        //
-        // where `op` is either `==`, `<:`, or `:>`. This maps quite
-        // naturally.
-        let ambient_variance = match dir {
-            RelationDir::EqTo => ty::Invariant,
-            RelationDir::SubtypeOf => ty::Covariant,
-            RelationDir::SupertypeOf => ty::Contravariant,
-        };
-
-        trace!(?ambient_variance);
-
-        let for_universe = match self.infcx.inner.borrow_mut().type_variables().probe(for_vid) {
-            v @ TypeVariableValue::Known { .. } => {
-                bug!("instantiating {:?} which has a known value {:?}", for_vid, v,)
-            }
-            TypeVariableValue::Unknown { universe } => universe,
-        };
-
-        trace!(?for_universe);
-        trace!(?self.trace);
-
-        let mut generalize = Generalizer {
-            infcx: self.infcx,
-            cause: &self.trace.cause,
-            for_vid_sub_root: self.infcx.inner.borrow_mut().type_variables().sub_root_var(for_vid),
-            for_universe,
-            ambient_variance,
-            needs_wf: false,
-            root_ty: ty,
-            param_env: self.param_env,
-            cache: SsoHashMap::new(),
-        };
-
-        let ty = generalize.relate(ty, ty)?;
-        let needs_wf = generalize.needs_wf;
-        Ok(Generalization { ty, needs_wf })
-    }
-
     pub fn register_obligations(&mut self, obligations: PredicateObligations<'tcx>) {
         self.obligations.extend(obligations.into_iter());
     }
@@ -519,320 +444,13 @@ impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
     }
 }
 
-struct Generalizer<'cx, 'tcx> {
-    infcx: &'cx InferCtxt<'tcx>,
-
-    /// The span, used when creating new type variables and things.
-    cause: &'cx ObligationCause<'tcx>,
-
-    /// The vid of the type variable that is in the process of being
-    /// instantiated; if we find this within the type we are folding,
-    /// that means we would have created a cyclic type.
-    for_vid_sub_root: ty::TyVid,
-
-    /// The universe of the type variable that is in the process of
-    /// being instantiated. Any fresh variables that we create in this
-    /// process should be in that same universe.
-    for_universe: ty::UniverseIndex,
-
-    /// Track the variance as we descend into the type.
-    ambient_variance: ty::Variance,
-
-    /// See the field `needs_wf` in `Generalization`.
-    needs_wf: bool,
-
-    /// The root type that we are generalizing. Used when reporting cycles.
-    root_ty: Ty<'tcx>,
-
-    param_env: ty::ParamEnv<'tcx>,
-
-    cache: SsoHashMap<Ty<'tcx>, Ty<'tcx>>,
-}
-
-/// Result from a generalization operation. This includes
-/// not only the generalized type, but also a bool flag
-/// indicating whether further WF checks are needed.
-#[derive(Debug)]
-struct Generalization<'tcx> {
-    ty: Ty<'tcx>,
-
-    /// If true, then the generalized type may not be well-formed,
-    /// even if the source type is well-formed, so we should add an
-    /// additional check to enforce that it is. This arises in
-    /// particular around 'bivariant' type parameters that are only
-    /// constrained by a where-clause. As an example, imagine a type:
-    ///
-    ///     struct Foo<A, B> where A: Iterator<Item = B> {
-    ///         data: A
-    ///     }
-    ///
-    /// here, `A` will be covariant, but `B` is
-    /// unconstrained. However, whatever it is, for `Foo` to be WF, it
-    /// must be equal to `A::Item`. If we have an input `Foo<?A, ?B>`,
-    /// then after generalization we will wind up with a type like
-    /// `Foo<?C, ?D>`. When we enforce that `Foo<?A, ?B> <: Foo<?C,
-    /// ?D>` (or `>:`), we will wind up with the requirement that `?A
-    /// <: ?C`, but no particular relationship between `?B` and `?D`
-    /// (after all, we do not know the variance of the normalized form
-    /// of `A::Item` with respect to `A`). If we do nothing else, this
-    /// may mean that `?D` goes unconstrained (as in #41677). So, in
-    /// this scenario where we create a new type variable in a
-    /// bivariant context, we set the `needs_wf` flag to true. This
-    /// will force the calling code to check that `WF(Foo<?C, ?D>)`
-    /// holds, which in turn implies that `?C::Item == ?D`. So once
-    /// `?C` is constrained, that should suffice to restrict `?D`.
-    needs_wf: bool,
-}
-
-impl<'tcx> TypeRelation<'tcx> for Generalizer<'_, 'tcx> {
-    fn tcx(&self) -> TyCtxt<'tcx> {
-        self.infcx.tcx
-    }
-
-    fn param_env(&self) -> ty::ParamEnv<'tcx> {
-        self.param_env
-    }
-
-    fn tag(&self) -> &'static str {
-        "Generalizer"
-    }
-
-    fn a_is_expected(&self) -> bool {
-        true
-    }
-
-    fn binders<T>(
-        &mut self,
-        a: ty::Binder<'tcx, T>,
-        b: ty::Binder<'tcx, T>,
-    ) -> RelateResult<'tcx, ty::Binder<'tcx, T>>
-    where
-        T: Relate<'tcx>,
-    {
-        Ok(a.rebind(self.relate(a.skip_binder(), b.skip_binder())?))
-    }
-
-    fn relate_item_substs(
-        &mut self,
-        item_def_id: DefId,
-        a_subst: SubstsRef<'tcx>,
-        b_subst: SubstsRef<'tcx>,
-    ) -> RelateResult<'tcx, SubstsRef<'tcx>> {
-        if self.ambient_variance == ty::Variance::Invariant {
-            // Avoid fetching the variance if we are in an invariant
-            // context; no need, and it can induce dependency cycles
-            // (e.g., #41849).
-            relate::relate_substs(self, a_subst, b_subst)
-        } else {
-            let tcx = self.tcx();
-            let opt_variances = tcx.variances_of(item_def_id);
-            relate::relate_substs_with_variances(
-                self,
-                item_def_id,
-                &opt_variances,
-                a_subst,
-                b_subst,
-                true,
-            )
-        }
-    }
-
-    fn relate_with_variance<T: Relate<'tcx>>(
-        &mut self,
-        variance: ty::Variance,
-        _info: ty::VarianceDiagInfo<'tcx>,
-        a: T,
-        b: T,
-    ) -> RelateResult<'tcx, T> {
-        let old_ambient_variance = self.ambient_variance;
-        self.ambient_variance = self.ambient_variance.xform(variance);
-
-        let result = self.relate(a, b);
-        self.ambient_variance = old_ambient_variance;
-        result
-    }
-
-    fn tys(&mut self, t: Ty<'tcx>, t2: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
-        assert_eq!(t, t2); // we are abusing TypeRelation here; both LHS and RHS ought to be ==
-
-        if let Some(&result) = self.cache.get(&t) {
-            return Ok(result);
-        }
-        debug!("generalize: t={:?}", t);
-
-        // Check to see whether the type we are generalizing references
-        // any other type variable related to `vid` via
-        // subtyping. This is basically our "occurs check", preventing
-        // us from creating infinitely sized types.
-        let result = match *t.kind() {
-            ty::Infer(ty::TyVar(vid)) => {
-                let vid = self.infcx.inner.borrow_mut().type_variables().root_var(vid);
-                let sub_vid = self.infcx.inner.borrow_mut().type_variables().sub_root_var(vid);
-                if sub_vid == self.for_vid_sub_root {
-                    // If sub-roots are equal, then `for_vid` and
-                    // `vid` are related via subtyping.
-                    Err(TypeError::CyclicTy(self.root_ty))
-                } else {
-                    let probe = self.infcx.inner.borrow_mut().type_variables().probe(vid);
-                    match probe {
-                        TypeVariableValue::Known { value: u } => {
-                            debug!("generalize: known value {:?}", u);
-                            self.relate(u, u)
-                        }
-                        TypeVariableValue::Unknown { universe } => {
-                            match self.ambient_variance {
-                                // Invariant: no need to make a fresh type variable.
-                                ty::Invariant => {
-                                    if self.for_universe.can_name(universe) {
-                                        return Ok(t);
-                                    }
-                                }
-
-                                // Bivariant: make a fresh var, but we
-                                // may need a WF predicate. See
-                                // comment on `needs_wf` field for
-                                // more info.
-                                ty::Bivariant => self.needs_wf = true,
-
-                                // Co/contravariant: this will be
-                                // sufficiently constrained later on.
-                                ty::Covariant | ty::Contravariant => (),
-                            }
-
-                            let origin =
-                                *self.infcx.inner.borrow_mut().type_variables().var_origin(vid);
-                            let new_var_id = self
-                                .infcx
-                                .inner
-                                .borrow_mut()
-                                .type_variables()
-                                .new_var(self.for_universe, origin);
-                            let u = self.tcx().mk_ty_var(new_var_id);
-
-                            // Record that we replaced `vid` with `new_var_id` as part of a generalization
-                            // operation. This is needed to detect cyclic types. To see why, see the
-                            // docs in the `type_variables` module.
-                            self.infcx.inner.borrow_mut().type_variables().sub(vid, new_var_id);
-                            debug!("generalize: replacing original vid={:?} with new={:?}", vid, u);
-                            Ok(u)
-                        }
-                    }
-                }
-            }
-            ty::Infer(ty::IntVar(_) | ty::FloatVar(_)) => {
-                // No matter what mode we are in,
-                // integer/floating-point types must be equal to be
-                // relatable.
-                Ok(t)
-            }
-            ty::Alias(ty::Opaque, ty::AliasTy { def_id, substs, .. }) => {
-                let s = self.relate(substs, substs)?;
-                Ok(if s == substs { t } else { self.infcx.tcx.mk_opaque(def_id, s) })
-            }
-            _ => relate::super_relate_tys(self, t, t),
-        }?;
-
-        self.cache.insert(t, result);
-        Ok(result)
-    }
-
-    fn regions(
-        &mut self,
-        r: ty::Region<'tcx>,
-        r2: ty::Region<'tcx>,
-    ) -> RelateResult<'tcx, ty::Region<'tcx>> {
-        assert_eq!(r, r2); // we are abusing TypeRelation here; both LHS and RHS ought to be ==
-
-        debug!("generalize: regions r={:?}", r);
-
-        match *r {
-            // Never make variables for regions bound within the type itself,
-            // nor for erased regions.
-            ty::ReLateBound(..) | ty::ReErased => {
-                return Ok(r);
-            }
-
-            ty::ReError(_) => {
-                return Ok(r);
-            }
-
-            ty::RePlaceholder(..)
-            | ty::ReVar(..)
-            | ty::ReStatic
-            | ty::ReEarlyBound(..)
-            | ty::ReFree(..) => {
-                // see common code below
-            }
-        }
-
-        // If we are in an invariant context, we can re-use the region
-        // as is, unless it happens to be in some universe that we
-        // can't name. (In the case of a region *variable*, we could
-        // use it if we promoted it into our universe, but we don't
-        // bother.)
-        if let ty::Invariant = self.ambient_variance {
-            let r_universe = self.infcx.universe_of_region(r);
-            if self.for_universe.can_name(r_universe) {
-                return Ok(r);
-            }
-        }
-
-        // FIXME: This is non-ideal because we don't give a
-        // very descriptive origin for this region variable.
-        Ok(self.infcx.next_region_var_in_universe(MiscVariable(self.cause.span), self.for_universe))
-    }
-
-    fn consts(
-        &mut self,
-        c: ty::Const<'tcx>,
-        c2: ty::Const<'tcx>,
-    ) -> RelateResult<'tcx, ty::Const<'tcx>> {
-        assert_eq!(c, c2); // we are abusing TypeRelation here; both LHS and RHS ought to be ==
-
-        match c.kind() {
-            ty::ConstKind::Infer(InferConst::Var(vid)) => {
-                let mut inner = self.infcx.inner.borrow_mut();
-                let variable_table = &mut inner.const_unification_table();
-                let var_value = variable_table.probe_value(vid);
-                match var_value.val {
-                    ConstVariableValue::Known { value: u } => {
-                        drop(inner);
-                        self.relate(u, u)
-                    }
-                    ConstVariableValue::Unknown { universe } => {
-                        if self.for_universe.can_name(universe) {
-                            Ok(c)
-                        } else {
-                            let new_var_id = variable_table.new_key(ConstVarValue {
-                                origin: var_value.origin,
-                                val: ConstVariableValue::Unknown { universe: self.for_universe },
-                            });
-                            Ok(self.tcx().mk_const(new_var_id, c.ty()))
-                        }
-                    }
-                }
-            }
-            ty::ConstKind::Unevaluated(ty::UnevaluatedConst { def, substs }) => {
-                let substs = self.relate_with_variance(
-                    ty::Variance::Invariant,
-                    ty::VarianceDiagInfo::default(),
-                    substs,
-                    substs,
-                )?;
-                Ok(self.tcx().mk_const(ty::UnevaluatedConst { def, substs }, c.ty()))
-            }
-            _ => relate::super_relate_consts(self, c, c),
-        }
-    }
-}
-
 pub trait ObligationEmittingRelation<'tcx>: TypeRelation<'tcx> {
     /// Register obligations that must hold in order for this relation to hold
     fn register_obligations(&mut self, obligations: PredicateObligations<'tcx>);
 
     /// Register predicates that must hold in order for this relation to hold. Uses
     /// a default obligation cause, [`ObligationEmittingRelation::register_obligations`] should
-    /// be used if control over the obligaton causes is required.
+    /// be used if control over the obligation causes is required.
     fn register_predicates(&mut self, obligations: impl IntoIterator<Item: ToPredicate<'tcx>>);
 
     /// Register an obligation that both constants must be equal to each other.
@@ -878,135 +496,3 @@ fn float_unification_error<'tcx>(
     let (ty::FloatVarValue(a), ty::FloatVarValue(b)) = v;
     TypeError::FloatMismatch(ExpectedFound::new(a_is_expected, a, b))
 }
-
-struct ConstInferUnifier<'cx, 'tcx> {
-    infcx: &'cx InferCtxt<'tcx>,
-
-    span: Span,
-
-    for_universe: ty::UniverseIndex,
-
-    /// The vid of the const variable that is in the process of being
-    /// instantiated; if we find this within the const we are folding,
-    /// that means we would have created a cyclic const.
-    target_vid: ty::ConstVid<'tcx>,
-}
-
-impl<'tcx> FallibleTypeFolder<TyCtxt<'tcx>> for ConstInferUnifier<'_, 'tcx> {
-    type Error = TypeError<'tcx>;
-
-    fn interner(&self) -> TyCtxt<'tcx> {
-        self.infcx.tcx
-    }
-
-    #[instrument(level = "debug", skip(self), ret)]
-    fn try_fold_ty(&mut self, t: Ty<'tcx>) -> Result<Ty<'tcx>, TypeError<'tcx>> {
-        match t.kind() {
-            &ty::Infer(ty::TyVar(vid)) => {
-                let vid = self.infcx.inner.borrow_mut().type_variables().root_var(vid);
-                let probe = self.infcx.inner.borrow_mut().type_variables().probe(vid);
-                match probe {
-                    TypeVariableValue::Known { value: u } => {
-                        debug!("ConstOccursChecker: known value {:?}", u);
-                        u.try_fold_with(self)
-                    }
-                    TypeVariableValue::Unknown { universe } => {
-                        if self.for_universe.can_name(universe) {
-                            return Ok(t);
-                        }
-
-                        let origin =
-                            *self.infcx.inner.borrow_mut().type_variables().var_origin(vid);
-                        let new_var_id = self
-                            .infcx
-                            .inner
-                            .borrow_mut()
-                            .type_variables()
-                            .new_var(self.for_universe, origin);
-                        Ok(self.interner().mk_ty_var(new_var_id))
-                    }
-                }
-            }
-            ty::Infer(ty::IntVar(_) | ty::FloatVar(_)) => Ok(t),
-            _ => t.try_super_fold_with(self),
-        }
-    }
-
-    #[instrument(level = "debug", skip(self), ret)]
-    fn try_fold_region(
-        &mut self,
-        r: ty::Region<'tcx>,
-    ) -> Result<ty::Region<'tcx>, TypeError<'tcx>> {
-        debug!("ConstInferUnifier: r={:?}", r);
-
-        match *r {
-            // Never make variables for regions bound within the type itself,
-            // nor for erased regions.
-            ty::ReLateBound(..) | ty::ReErased | ty::ReError(_) => {
-                return Ok(r);
-            }
-
-            ty::RePlaceholder(..)
-            | ty::ReVar(..)
-            | ty::ReStatic
-            | ty::ReEarlyBound(..)
-            | ty::ReFree(..) => {
-                // see common code below
-            }
-        }
-
-        let r_universe = self.infcx.universe_of_region(r);
-        if self.for_universe.can_name(r_universe) {
-            return Ok(r);
-        } else {
-            // FIXME: This is non-ideal because we don't give a
-            // very descriptive origin for this region variable.
-            Ok(self.infcx.next_region_var_in_universe(MiscVariable(self.span), self.for_universe))
-        }
-    }
-
-    #[instrument(level = "debug", skip(self), ret)]
-    fn try_fold_const(&mut self, c: ty::Const<'tcx>) -> Result<ty::Const<'tcx>, TypeError<'tcx>> {
-        match c.kind() {
-            ty::ConstKind::Infer(InferConst::Var(vid)) => {
-                // Check if the current unification would end up
-                // unifying `target_vid` with a const which contains
-                // an inference variable which is unioned with `target_vid`.
-                //
-                // Not doing so can easily result in stack overflows.
-                if self
-                    .infcx
-                    .inner
-                    .borrow_mut()
-                    .const_unification_table()
-                    .unioned(self.target_vid, vid)
-                {
-                    return Err(TypeError::CyclicConst(c));
-                }
-
-                let var_value =
-                    self.infcx.inner.borrow_mut().const_unification_table().probe_value(vid);
-                match var_value.val {
-                    ConstVariableValue::Known { value: u } => u.try_fold_with(self),
-                    ConstVariableValue::Unknown { universe } => {
-                        if self.for_universe.can_name(universe) {
-                            Ok(c)
-                        } else {
-                            let new_var_id =
-                                self.infcx.inner.borrow_mut().const_unification_table().new_key(
-                                    ConstVarValue {
-                                        origin: var_value.origin,
-                                        val: ConstVariableValue::Unknown {
-                                            universe: self.for_universe,
-                                        },
-                                    },
-                                );
-                            Ok(self.interner().mk_const(new_var_id, c.ty()))
-                        }
-                    }
-                }
-            }
-            _ => c.try_super_fold_with(self),
-        }
-    }
-}
diff --git a/compiler/rustc_infer/src/infer/equate.rs b/compiler/rustc_infer/src/infer/equate.rs
index fe4a2dd38..42dfe4f6b 100644
--- a/compiler/rustc_infer/src/infer/equate.rs
+++ b/compiler/rustc_infer/src/infer/equate.rs
@@ -1,7 +1,7 @@
 use crate::infer::DefineOpaqueTypes;
 use crate::traits::PredicateObligations;
 
-use super::combine::{CombineFields, ObligationEmittingRelation, RelationDir};
+use super::combine::{CombineFields, ObligationEmittingRelation};
 use super::Subtype;
 
 use rustc_middle::ty::relate::{self, Relate, RelateResult, TypeRelation};
@@ -88,11 +88,11 @@ impl<'tcx> TypeRelation<'tcx> for Equate<'_, '_, 'tcx> {
             }
 
             (&ty::Infer(TyVar(a_id)), _) => {
-                self.fields.instantiate(b, RelationDir::EqTo, a_id, self.a_is_expected)?;
+                self.fields.instantiate(b, ty::Invariant, a_id, self.a_is_expected)?;
             }
 
             (_, &ty::Infer(TyVar(b_id))) => {
-                self.fields.instantiate(a, RelationDir::EqTo, b_id, self.a_is_expected)?;
+                self.fields.instantiate(a, ty::Invariant, b_id, self.a_is_expected)?;
             }
 
             (
@@ -104,7 +104,8 @@ impl<'tcx> TypeRelation<'tcx> for Equate<'_, '_, 'tcx> {
             (&ty::Alias(ty::Opaque, ty::AliasTy { def_id, .. }), _)
             | (_, &ty::Alias(ty::Opaque, ty::AliasTy { def_id, .. }))
                 if self.fields.define_opaque_types == DefineOpaqueTypes::Yes
-                    && def_id.is_local() =>
+                    && def_id.is_local()
+                    && !self.tcx().trait_solver_next() =>
             {
                 self.fields.obligations.extend(
                     infcx
@@ -178,7 +179,7 @@ impl<'tcx> TypeRelation<'tcx> for Equate<'_, '_, 'tcx> {
     where
         T: Relate<'tcx>,
     {
-        // A binder is equal to itself if it's structually equal to itself
+        // A binder is equal to itself if it's structurally equal to itself
         if a == b {
             return Ok(a);
         }
diff --git a/compiler/rustc_infer/src/infer/error_reporting/mod.rs b/compiler/rustc_infer/src/infer/error_reporting/mod.rs
index 9e5f6d107..35c05e80b 100644
--- a/compiler/rustc_infer/src/infer/error_reporting/mod.rs
+++ b/compiler/rustc_infer/src/infer/error_reporting/mod.rs
@@ -74,7 +74,6 @@ use rustc_middle::ty::{
     self, error::TypeError, List, Region, Ty, TyCtxt, TypeFoldable, TypeSuperVisitable,
     TypeVisitable, TypeVisitableExt,
 };
-use rustc_span::DUMMY_SP;
 use rustc_span::{sym, symbol::kw, BytePos, DesugaringKind, Pos, Span};
 use rustc_target::spec::abi;
 use std::ops::{ControlFlow, Deref};
@@ -138,7 +137,7 @@ impl Drop for TypeErrCtxt<'_, '_> {
             self.infcx
                 .tcx
                 .sess
-                .delay_span_bug(DUMMY_SP, "used a `TypeErrCtxt` without failing compilation");
+                .delay_good_path_bug("used a `TypeErrCtxt` without raising an error or lint");
         }
     }
 }
@@ -283,9 +282,9 @@ fn emit_msg_span(
     let message = format!("{}{}{}", prefix, description, suffix);
 
     if let Some(span) = span {
-        err.span_note(span, &message);
+        err.span_note(span, message);
     } else {
-        err.note(&message);
+        err.note(message);
     }
 }
 
@@ -299,9 +298,9 @@ fn label_msg_span(
     let message = format!("{}{}{}", prefix, description, suffix);
 
     if let Some(span) = span {
-        err.span_label(span, &message);
+        err.span_label(span, message);
     } else {
-        err.note(&message);
+        err.note(message);
     }
 }
 
@@ -403,7 +402,7 @@ impl<'tcx> InferCtxt<'tcx> {
         let future_trait = self.tcx.require_lang_item(LangItem::Future, None);
         let item_def_id = self.tcx.associated_item_def_ids(future_trait)[0];
 
-        self.tcx.bound_explicit_item_bounds(def_id).subst_iter_copied(self.tcx, substs).find_map(
+        self.tcx.explicit_item_bounds(def_id).subst_iter_copied(self.tcx, substs).find_map(
             |(predicate, _)| {
                 predicate
                     .kind()
@@ -1826,7 +1825,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                         s
                     };
                     if !(values.expected.is_simple_text() && values.found.is_simple_text())
-                        || (exp_found.map_or(false, |ef| {
+                        || (exp_found.is_some_and(|ef| {
                             // This happens when the type error is a subset of the expectation,
                             // like when you have two references but one is `usize` and the other
                             // is `f32`. In those cases we still want to show the `note`. If the
@@ -1878,7 +1877,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         let exp_found = match terr {
             // `terr` has more accurate type information than `exp_found` in match expressions.
             ty::error::TypeError::Sorts(terr)
-                if exp_found.map_or(false, |ef| terr.found == ef.found) =>
+                if exp_found.is_some_and(|ef| terr.found == ef.found) =>
             {
                 Some(terr)
             }
@@ -1927,6 +1926,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         {
             let span = self.tcx.def_span(def_id);
             diag.span_note(span, "this closure does not fulfill the lifetime requirements");
+            self.suggest_for_all_lifetime_closure(span, self.tcx.hir().get_by_def_id(def_id), &exp_found, diag);
         }
 
         // It reads better to have the error origin as the final
@@ -1961,7 +1961,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                     if let Ok(code) = self.tcx.sess().source_map().span_to_snippet(span)
                         && let Some(code) = code.strip_prefix('\'').and_then(|s| s.strip_suffix('\''))
                         && !code.starts_with("\\u") // forbid all Unicode escapes
-                        && code.chars().next().map_or(false, |c| c.is_ascii()) // forbids literal Unicode characters beyond ASCII
+                        && code.chars().next().is_some_and(|c| c.is_ascii()) // forbids literal Unicode characters beyond ASCII
                     {
                         suggestions.push(TypeErrorAdditionalDiags::MeantByteLiteral { span, code: escape_literal(code) })
                     }
@@ -2329,7 +2329,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                         .source_map()
                         .span_to_prev_source(p.span.shrink_to_hi())
                         .ok()
-                        .map_or(false, |s| *s.as_bytes().last().unwrap() == b'&')
+                        .is_some_and(|s| *s.as_bytes().last().unwrap() == b'&')
                     {
                         add_lt_suggs
                             .push(Some(
@@ -2354,7 +2354,9 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         let labeled_user_string = match bound_kind {
             GenericKind::Param(ref p) => format!("the parameter type `{}`", p),
             GenericKind::Alias(ref p) => match p.kind(self.tcx) {
-                ty::AliasKind::Projection => format!("the associated type `{}`", p),
+                ty::AliasKind::Projection | ty::AliasKind::Inherent => {
+                    format!("the associated type `{}`", p)
+                }
                 ty::AliasKind::Opaque => format!("the opaque type `{}`", p),
             },
         };
@@ -2395,7 +2397,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                 );
             } else {
                 let consider = format!("{} `{}: {}`...", msg, bound_kind, sub);
-                err.help(&consider);
+                err.help(consider);
             }
         }
 
@@ -2625,7 +2627,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
             );
             err.span_note(
                 sup_trace.cause.span,
-                &format!("...so that the {}", sup_trace.cause.as_requirement_str()),
+                format!("...so that the {}", sup_trace.cause.as_requirement_str()),
             );
 
             err.note_expected_found(&"", sup_expected, &"", sup_found);
@@ -2721,7 +2723,7 @@ impl<'tcx> TypeRelation<'tcx> for SameTypeModuloInfer<'_, 'tcx> {
             | (ty::Infer(ty::InferTy::TyVar(_)), _)
             | (_, ty::Infer(ty::InferTy::TyVar(_))) => Ok(a),
             (ty::Infer(_), _) | (_, ty::Infer(_)) => Err(TypeError::Mismatch),
-            _ => relate::super_relate_tys(self, a, b),
+            _ => relate::structurally_relate_tys(self, a, b),
         }
     }
 
@@ -2885,7 +2887,7 @@ impl<'tcx> ObligationCauseExt<'tcx> for ObligationCause<'tcx> {
             LetElse => ObligationCauseFailureCode::NoDiverge { span, subdiags },
             MainFunctionType => ObligationCauseFailureCode::FnMainCorrectType { span },
             StartFunctionType => ObligationCauseFailureCode::FnStartCorrectType { span, subdiags },
-            IntrinsicType => ObligationCauseFailureCode::IntristicCorrectType { span, subdiags },
+            IntrinsicType => ObligationCauseFailureCode::IntrinsicCorrectType { span, subdiags },
             MethodReceiver => ObligationCauseFailureCode::MethodCorrectType { span, subdiags },
 
             // In the case where we have no more specific thing to
@@ -2942,7 +2944,7 @@ impl IntoDiagnosticArg for ObligationCauseAsDiagArg<'_> {
             IfExpressionWithNoElse => "no_else",
             MainFunctionType => "fn_main_correct_type",
             StartFunctionType => "fn_start_correct_type",
-            IntrinsicType => "intristic_correct_type",
+            IntrinsicType => "intrinsic_correct_type",
             MethodReceiver => "method_correct_type",
             _ => "other",
         }
diff --git a/compiler/rustc_infer/src/infer/error_reporting/need_type_info.rs b/compiler/rustc_infer/src/infer/error_reporting/need_type_info.rs
index 75cc4e257..f3b2ec4c5 100644
--- a/compiler/rustc_infer/src/infer/error_reporting/need_type_info.rs
+++ b/compiler/rustc_infer/src/infer/error_reporting/need_type_info.rs
@@ -1,5 +1,5 @@
 use crate::errors::{
-    AmbigousReturn, AmbiguousImpl, AnnotationRequired, InferenceBadError, NeedTypeInfoInGenerator,
+    AmbiguousImpl, AmbiguousReturn, AnnotationRequired, InferenceBadError, NeedTypeInfoInGenerator,
     SourceKindMultiSuggestion, SourceKindSubdiag,
 };
 use crate::infer::error_reporting::TypeErrCtxt;
@@ -31,7 +31,7 @@ pub enum TypeAnnotationNeeded {
     /// ```
     E0282,
     /// An implementation cannot be chosen unambiguously because of lack of information.
-    /// ```compile_fail,E0283
+    /// ```compile_fail,E0790
     /// let _ = Default::default();
     /// ```
     E0283,
@@ -368,7 +368,7 @@ impl<'tcx> InferCtxt<'tcx> {
                 bad_label,
             }
             .into_diagnostic(&self.tcx.sess.parse_sess.span_diagnostic),
-            TypeAnnotationNeeded::E0284 => AmbigousReturn {
+            TypeAnnotationNeeded::E0284 => AmbiguousReturn {
                 span,
                 source_kind,
                 source_name,
@@ -573,7 +573,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                 bad_label: None,
             }
             .into_diagnostic(&self.tcx.sess.parse_sess.span_diagnostic),
-            TypeAnnotationNeeded::E0284 => AmbigousReturn {
+            TypeAnnotationNeeded::E0284 => AmbiguousReturn {
                 span,
                 source_kind,
                 source_name: &name,
@@ -671,7 +671,7 @@ impl<'tcx> InferSource<'tcx> {
                 receiver.span.from_expansion()
             }
             InferSourceKind::ClosureReturn { data, should_wrap_expr, .. } => {
-                data.span().from_expansion() || should_wrap_expr.map_or(false, Span::from_expansion)
+                data.span().from_expansion() || should_wrap_expr.is_some_and(Span::from_expansion)
             }
         };
         source_from_expansion || self.span.from_expansion()
@@ -984,7 +984,7 @@ impl<'a, 'tcx> FindInferSourceVisitor<'a, 'tcx> {
     ) -> impl Iterator<Item = InsertableGenericArgs<'tcx>> + 'a {
         let tcx = self.infcx.tcx;
         let have_turbofish = path.segments.iter().any(|segment| {
-            segment.args.map_or(false, |args| args.args.iter().any(|arg| arg.is_ty_or_const()))
+            segment.args.is_some_and(|args| args.args.iter().any(|arg| arg.is_ty_or_const()))
         });
         // The last segment of a path often has `Res::Err` and the
         // correct `Res` is the one of the whole path.
diff --git a/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/different_lifetimes.rs b/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/different_lifetimes.rs
index da0271a34..1a60bab18 100644
--- a/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/different_lifetimes.rs
+++ b/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/different_lifetimes.rs
@@ -21,7 +21,7 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
     ///
     /// Consider a case where we have
     ///
-    /// ```compile_fail,E0623
+    /// ```compile_fail
     /// fn foo(x: &mut Vec<&u8>, y: &u8) {
     ///     x.push(y);
     /// }
diff --git a/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/find_anon_type.rs b/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/find_anon_type.rs
index fec04af23..0df417d09 100644
--- a/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/find_anon_type.rs
+++ b/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/find_anon_type.rs
@@ -14,7 +14,7 @@ use rustc_middle::ty::{self, Region, TyCtxt};
 /// br - the bound region corresponding to the above region which is of type `BrAnon(_)`
 ///
 /// # Example
-/// ```compile_fail,E0623
+/// ```compile_fail
 /// fn foo(x: &mut Vec<&u8>, y: &u8)
 ///    { x.push(y); }
 /// ```
diff --git a/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/placeholder_error.rs b/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/placeholder_error.rs
index c1ea0a0d9..c9c1f0aea 100644
--- a/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/placeholder_error.rs
+++ b/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/placeholder_error.rs
@@ -261,11 +261,16 @@ impl<'tcx> NiceRegionError<'_, 'tcx> {
                 (false, None, None, Some(span), String::new())
             };
 
-        let expected_trait_ref = self
-            .cx
-            .resolve_vars_if_possible(self.cx.tcx.mk_trait_ref(trait_def_id, expected_substs));
-        let actual_trait_ref =
-            self.cx.resolve_vars_if_possible(self.cx.tcx.mk_trait_ref(trait_def_id, actual_substs));
+        let expected_trait_ref = self.cx.resolve_vars_if_possible(ty::TraitRef::new(
+            self.cx.tcx,
+            trait_def_id,
+            expected_substs,
+        ));
+        let actual_trait_ref = self.cx.resolve_vars_if_possible(ty::TraitRef::new(
+            self.cx.tcx,
+            trait_def_id,
+            actual_substs,
+        ));
 
         // Search the expected and actual trait references to see (a)
         // whether the sub/sup placeholders appear in them (sometimes
diff --git a/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/static_impl_trait.rs b/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/static_impl_trait.rs
index 22c1e3871..aad988582 100644
--- a/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/static_impl_trait.rs
+++ b/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/static_impl_trait.rs
@@ -299,7 +299,7 @@ pub fn suggest_new_region_bound(
                     if let Some(explicit_static) = &explicit_static {
                         err.span_suggestion_verbose(
                             span,
-                            &format!("{consider} `{ty}`'s {explicit_static}"),
+                            format!("{consider} `{ty}`'s {explicit_static}"),
                             &lifetime_name,
                             Applicability::MaybeIncorrect,
                         );
@@ -312,13 +312,10 @@ pub fn suggest_new_region_bound(
                             Applicability::MaybeIncorrect,
                         );
                     }
-                } else if opaque.bounds.iter().any(|arg| match arg {
-                    GenericBound::Outlives(Lifetime { ident, .. })
-                        if ident.name.to_string() == lifetime_name =>
-                    {
-                        true
-                    }
-                    _ => false,
+                } else if opaque.bounds.iter().any(|arg| {
+                    matches!(arg,
+                        GenericBound::Outlives(Lifetime { ident, .. })
+                        if ident.name.to_string() == lifetime_name )
                 }) {
                 } else {
                     // get a lifetime name of existing named lifetimes if any
@@ -370,7 +367,7 @@ pub fn suggest_new_region_bound(
                         spans_suggs
                             .push((fn_return.span.shrink_to_hi(), format!(" + {name} ")));
                         err.multipart_suggestion_verbose(
-                            &format!(
+                            format!(
                                 "{declare} `{ty}` {captures}, {use_lt}",
                             ),
                             spans_suggs,
@@ -379,7 +376,7 @@ pub fn suggest_new_region_bound(
                     } else {
                         err.span_suggestion_verbose(
                             fn_return.span.shrink_to_hi(),
-                            &format!("{declare} `{ty}` {captures}, {explicit}",),
+                            format!("{declare} `{ty}` {captures}, {explicit}",),
                             &plus_lt,
                             Applicability::MaybeIncorrect,
                         );
@@ -390,7 +387,7 @@ pub fn suggest_new_region_bound(
                 if let LifetimeName::ImplicitObjectLifetimeDefault = lt.res {
                     err.span_suggestion_verbose(
                         fn_return.span.shrink_to_hi(),
-                        &format!(
+                        format!(
                             "{declare} the trait object {captures}, {explicit}",
                             declare = declare,
                             captures = captures,
@@ -407,7 +404,7 @@ pub fn suggest_new_region_bound(
                     if let Some(explicit_static) = &explicit_static {
                         err.span_suggestion_verbose(
                             lt.ident.span,
-                            &format!("{} the trait object's {}", consider, explicit_static),
+                            format!("{} the trait object's {}", consider, explicit_static),
                             &lifetime_name,
                             Applicability::MaybeIncorrect,
                         );
diff --git a/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/trait_impl_difference.rs b/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/trait_impl_difference.rs
index 2875448ee..ce70bcc5c 100644
--- a/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/trait_impl_difference.rs
+++ b/compiler/rustc_infer/src/infer/error_reporting/nice_region_error/trait_impl_difference.rs
@@ -13,7 +13,7 @@ use rustc_hir::intravisit::Visitor;
 use rustc_middle::hir::nested_filter;
 use rustc_middle::ty::error::ExpectedFound;
 use rustc_middle::ty::print::RegionHighlightMode;
-use rustc_middle::ty::{self, Ty, TyCtxt, TypeSuperVisitable, TypeVisitor};
+use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitor};
 use rustc_span::Span;
 
 use std::ops::ControlFlow;
@@ -81,7 +81,7 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
                     self.highlight.highlighting_region(r, self.counter);
                     self.counter += 1;
                 }
-                r.super_visit_with(self)
+                ControlFlow::Continue(())
             }
         }
 
diff --git a/compiler/rustc_infer/src/infer/error_reporting/note_and_explain.rs b/compiler/rustc_infer/src/infer/error_reporting/note_and_explain.rs
index b38bbdfe7..421eb807a 100644
--- a/compiler/rustc_infer/src/infer/error_reporting/note_and_explain.rs
+++ b/compiler/rustc_infer/src/infer/error_reporting/note_and_explain.rs
@@ -2,6 +2,7 @@ use super::TypeErrCtxt;
 use rustc_errors::Applicability::{MachineApplicable, MaybeIncorrect};
 use rustc_errors::{pluralize, Diagnostic, MultiSpan};
 use rustc_hir as hir;
+use rustc_hir::def::DefKind;
 use rustc_middle::traits::ObligationCauseCode;
 use rustc_middle::ty::error::ExpectedFound;
 use rustc_middle::ty::print::Printer;
@@ -71,9 +72,10 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                              #traits-as-parameters",
                         );
                     }
-                    (ty::Alias(ty::Projection, _), ty::Alias(ty::Projection, _)) => {
+                    (ty::Alias(ty::Projection | ty::Inherent, _), ty::Alias(ty::Projection | ty::Inherent, _)) => {
                         diag.note("an associated type was expected, but a different one was found");
                     }
+                    // FIXME(inherent_associated_types): Extend this to support `ty::Inherent`, too.
                     (ty::Param(p), ty::Alias(ty::Projection, proj)) | (ty::Alias(ty::Projection, proj), ty::Param(p))
                         if !tcx.is_impl_trait_in_trait(proj.def_id) =>
                     {
@@ -209,7 +211,7 @@ impl<T> Trait<T> for X {
                         if !sp.contains(p_span) {
                             diag.span_label(p_span, "this type parameter");
                         }
-                        diag.help(&format!(
+                        diag.help(format!(
                             "every closure has a distinct type and so could not always match the \
                              caller-chosen type of parameter `{}`",
                             p
@@ -222,7 +224,7 @@ impl<T> Trait<T> for X {
                             diag.span_label(p_span, "this type parameter");
                         }
                     }
-                    (ty::Alias(ty::Projection, proj_ty), _) if !tcx.is_impl_trait_in_trait(proj_ty.def_id) => {
+                    (ty::Alias(ty::Projection | ty::Inherent, proj_ty), _) if !tcx.is_impl_trait_in_trait(proj_ty.def_id) => {
                         self.expected_projection(
                             diag,
                             proj_ty,
@@ -231,7 +233,7 @@ impl<T> Trait<T> for X {
                             cause.code(),
                         );
                     }
-                    (_, ty::Alias(ty::Projection, proj_ty)) if !tcx.is_impl_trait_in_trait(proj_ty.def_id) => {
+                    (_, ty::Alias(ty::Projection | ty::Inherent, proj_ty)) if !tcx.is_impl_trait_in_trait(proj_ty.def_id) => {
                         let msg = format!(
                             "consider constraining the associated type `{}` to `{}`",
                             values.found, values.expected,
@@ -248,13 +250,22 @@ impl<T> Trait<T> for X {
                             proj_ty,
                             values.expected,
                         )) {
-                            diag.help(&msg);
+                            diag.help(msg);
                             diag.note(
                                 "for more information, visit \
                                 https://doc.rust-lang.org/book/ch19-03-advanced-traits.html",
                             );
                         }
                     }
+                    (ty::Alias(ty::Opaque, alias), _) | (_, ty::Alias(ty::Opaque, alias)) if alias.def_id.is_local() && matches!(tcx.def_kind(body_owner_def_id), DefKind::AssocFn | DefKind::AssocConst) => {
+                        if tcx.is_type_alias_impl_trait(alias.def_id) {
+                            if !tcx.opaque_types_defined_by(body_owner_def_id.expect_local()).contains(&alias.def_id.expect_local()) {
+                                diag.span_note(tcx.def_span(body_owner_def_id), "\
+                                    this item must have the opaque type in its signature \
+                                    in order to be able to register hidden types");
+                            }
+                        }
+                    }
                     (ty::FnPtr(_), ty::FnDef(def, _))
                     if let hir::def::DefKind::Fn = tcx.def_kind(def) => {
                         diag.note(
@@ -415,12 +426,12 @@ impl<T> Trait<T> for X {
         if !impl_comparison {
             // Generic suggestion when we can't be more specific.
             if callable_scope {
-                diag.help(&format!(
+                diag.help(format!(
                     "{} or calling a method that returns `{}`",
                     msg, values.expected
                 ));
             } else {
-                diag.help(&msg);
+                diag.help(msg);
             }
             diag.note(
                 "for more information, visit \
@@ -462,10 +473,7 @@ fn foo(&self) -> Self::T { String::new() }
         if let ty::Alias(ty::Opaque, ty::AliasTy { def_id, .. }) = *proj_ty.self_ty().kind() {
             let opaque_local_def_id = def_id.as_local();
             let opaque_hir_ty = if let Some(opaque_local_def_id) = opaque_local_def_id {
-                match &tcx.hir().expect_item(opaque_local_def_id).kind {
-                    hir::ItemKind::OpaqueTy(opaque_hir_ty) => opaque_hir_ty,
-                    _ => bug!("The HirId comes from a `ty::Opaque`"),
-                }
+                tcx.hir().expect_item(opaque_local_def_id).expect_opaque_ty()
             } else {
                 return false;
             };
@@ -539,7 +547,7 @@ fn foo(&self) -> Self::T { String::new() }
             for (sp, label) in methods.into_iter() {
                 span.push_span_label(sp, label);
             }
-            diag.span_help(span, &msg);
+            diag.span_help(span, msg);
             return true;
         }
         false
diff --git a/compiler/rustc_infer/src/infer/error_reporting/suggest.rs b/compiler/rustc_infer/src/infer/error_reporting/suggest.rs
index b5aeca12a..d885d0407 100644
--- a/compiler/rustc_infer/src/infer/error_reporting/suggest.rs
+++ b/compiler/rustc_infer/src/infer/error_reporting/suggest.rs
@@ -1,16 +1,15 @@
 use hir::def::CtorKind;
 use hir::intravisit::{walk_expr, walk_stmt, Visitor};
 use rustc_data_structures::fx::FxIndexSet;
-use rustc_errors::Diagnostic;
+use rustc_errors::{Applicability, Diagnostic};
 use rustc_hir as hir;
 use rustc_middle::traits::{
     IfExpressionCause, MatchExpressionArmCause, ObligationCause, ObligationCauseCode,
     StatementAsExpression,
 };
 use rustc_middle::ty::print::with_no_trimmed_paths;
-use rustc_middle::ty::{self as ty, IsSuggestable, Ty, TypeVisitableExt};
+use rustc_middle::ty::{self as ty, GenericArgKind, IsSuggestable, Ty, TypeVisitableExt};
 use rustc_span::{sym, BytePos, Span};
-use rustc_target::abi::FieldIdx;
 
 use crate::errors::{
     ConsiderAddingAwait, FnConsiderCasting, FnItemsAreDistinct, FnUniqTypes,
@@ -114,7 +113,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                         variant.fields.len() == 1 && variant.ctor_kind() == Some(CtorKind::Fn)
                     })
                     .filter_map(|variant| {
-                        let sole_field = &variant.fields[FieldIdx::from_u32(0)];
+                        let sole_field = &variant.single_field();
                         let sole_field_ty = sole_field.ty(self.tcx, substs);
                         if self.same_type_modulo_infer(sole_field_ty, exp_found.found) {
                             let variant_path =
@@ -536,6 +535,82 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         }
         None
     }
+
+    /// For "one type is more general than the other" errors on closures, suggest changing the lifetime
+    /// of the parameters to accept all lifetimes.
+    pub(super) fn suggest_for_all_lifetime_closure(
+        &self,
+        span: Span,
+        hir: hir::Node<'_>,
+        exp_found: &ty::error::ExpectedFound<ty::PolyTraitRef<'tcx>>,
+        diag: &mut Diagnostic,
+    ) {
+        // 0. Extract fn_decl from hir
+        let hir::Node::Expr(hir::Expr { kind: hir::ExprKind::Closure(hir::Closure { body, fn_decl, .. }), .. }) = hir else { return; };
+        let hir::Body { params, .. } = self.tcx.hir().body(*body);
+
+        // 1. Get the substs of the closure.
+        // 2. Assume exp_found is FnOnce / FnMut / Fn, we can extract function parameters from [1].
+        let Some(expected) = exp_found.expected.skip_binder().substs.get(1) else { return; };
+        let Some(found) = exp_found.found.skip_binder().substs.get(1) else { return; };
+        let expected = expected.unpack();
+        let found = found.unpack();
+        // 3. Extract the tuple type from Fn trait and suggest the change.
+        if let GenericArgKind::Type(expected) = expected &&
+            let GenericArgKind::Type(found) = found &&
+            let ty::Tuple(expected) = expected.kind() &&
+            let ty::Tuple(found)= found.kind() &&
+            expected.len() == found.len() {
+            let mut suggestion = "|".to_string();
+            let mut is_first = true;
+            let mut has_suggestion = false;
+
+            for (((expected, found), param_hir), arg_hir) in expected.iter()
+                .zip(found.iter())
+                .zip(params.iter())
+                .zip(fn_decl.inputs.iter()) {
+                if is_first {
+                    is_first = false;
+                } else {
+                    suggestion += ", ";
+                }
+
+                if let ty::Ref(expected_region, _, _) = expected.kind() &&
+                    let ty::Ref(found_region, _, _) = found.kind() &&
+                    expected_region.is_late_bound() &&
+                    !found_region.is_late_bound() &&
+                    let hir::TyKind::Infer = arg_hir.kind {
+                    // If the expected region is late bound, the found region is not, and users are asking compiler
+                    // to infer the type, we can suggest adding `: &_`.
+                    if param_hir.pat.span == param_hir.ty_span {
+                        // for `|x|`, `|_|`, `|x: impl Foo|`
+                        let Ok(pat) = self.tcx.sess.source_map().span_to_snippet(param_hir.pat.span) else { return; };
+                        suggestion += &format!("{}: &_", pat);
+                    } else {
+                        // for `|x: ty|`, `|_: ty|`
+                        let Ok(pat) = self.tcx.sess.source_map().span_to_snippet(param_hir.pat.span) else { return; };
+                        let Ok(ty) = self.tcx.sess.source_map().span_to_snippet(param_hir.ty_span) else { return; };
+                        suggestion += &format!("{}: &{}", pat, ty);
+                    }
+                    has_suggestion = true;
+                } else {
+                    let Ok(arg) = self.tcx.sess.source_map().span_to_snippet(param_hir.span) else { return; };
+                    // Otherwise, keep it as-is.
+                    suggestion += &arg;
+                }
+            }
+            suggestion += "|";
+
+            if has_suggestion {
+                diag.span_suggestion_verbose(
+                    span,
+                    "consider specifying the type of the closure parameters",
+                    suggestion,
+                    Applicability::MaybeIncorrect,
+                );
+            }
+        }
+    }
 }
 
 impl<'tcx> TypeErrCtxt<'_, 'tcx> {
diff --git a/compiler/rustc_infer/src/infer/freshen.rs b/compiler/rustc_infer/src/infer/freshen.rs
index d89f63e5c..0219167f6 100644
--- a/compiler/rustc_infer/src/infer/freshen.rs
+++ b/compiler/rustc_infer/src/infer/freshen.rs
@@ -127,7 +127,7 @@ impl<'a, 'tcx> TypeFolder<TyCtxt<'tcx>> for TypeFreshener<'a, 'tcx> {
 
     #[inline]
     fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
-        if !t.needs_infer() && !t.has_erasable_regions() {
+        if !t.has_infer() && !t.has_erasable_regions() {
             t
         } else {
             match *t.kind() {
diff --git a/compiler/rustc_infer/src/infer/generalize.rs b/compiler/rustc_infer/src/infer/generalize.rs
new file mode 100644
index 000000000..d4a1dacde
--- /dev/null
+++ b/compiler/rustc_infer/src/infer/generalize.rs
@@ -0,0 +1,479 @@
+use rustc_data_structures::sso::SsoHashMap;
+use rustc_hir::def_id::DefId;
+use rustc_middle::infer::unify_key::{ConstVarValue, ConstVariableValue};
+use rustc_middle::ty::error::TypeError;
+use rustc_middle::ty::relate::{self, Relate, RelateResult, TypeRelation};
+use rustc_middle::ty::{self, InferConst, Term, Ty, TyCtxt, TypeVisitableExt};
+use rustc_span::Span;
+
+use crate::infer::nll_relate::TypeRelatingDelegate;
+use crate::infer::type_variable::TypeVariableValue;
+use crate::infer::{InferCtxt, RegionVariableOrigin};
+
+/// Attempts to generalize `term` for the type variable `for_vid`.
+/// This checks for cycles -- that is, whether the type `term`
+/// references `for_vid`.
+pub(super) fn generalize<'tcx, D: GeneralizerDelegate<'tcx>, T: Into<Term<'tcx>> + Relate<'tcx>>(
+    infcx: &InferCtxt<'tcx>,
+    delegate: &mut D,
+    term: T,
+    for_vid: impl Into<ty::TermVid<'tcx>>,
+    ambient_variance: ty::Variance,
+) -> RelateResult<'tcx, Generalization<T>> {
+    let (for_universe, root_vid) = match for_vid.into() {
+        ty::TermVid::Ty(ty_vid) => (
+            infcx.probe_ty_var(ty_vid).unwrap_err(),
+            ty::TermVid::Ty(infcx.inner.borrow_mut().type_variables().sub_root_var(ty_vid)),
+        ),
+        ty::TermVid::Const(ct_vid) => (
+            infcx.probe_const_var(ct_vid).unwrap_err(),
+            ty::TermVid::Const(infcx.inner.borrow_mut().const_unification_table().find(ct_vid)),
+        ),
+    };
+
+    let mut generalizer = Generalizer {
+        infcx,
+        delegate,
+        ambient_variance,
+        root_vid,
+        for_universe,
+        root_term: term.into(),
+        needs_wf: false,
+        cache: Default::default(),
+    };
+
+    assert!(!term.has_escaping_bound_vars());
+    let value = generalizer.relate(term, term)?;
+    let needs_wf = generalizer.needs_wf;
+    Ok(Generalization { value, needs_wf })
+}
+
+/// Abstracts the handling of region vars between HIR and MIR/NLL typechecking
+/// in the generalizer code.
+pub trait GeneralizerDelegate<'tcx> {
+    fn param_env(&self) -> ty::ParamEnv<'tcx>;
+
+    fn forbid_inference_vars() -> bool;
+
+    fn generalize_region(&mut self, universe: ty::UniverseIndex) -> ty::Region<'tcx>;
+}
+
+pub struct CombineDelegate<'cx, 'tcx> {
+    pub infcx: &'cx InferCtxt<'tcx>,
+    pub param_env: ty::ParamEnv<'tcx>,
+    pub span: Span,
+}
+
+impl<'tcx> GeneralizerDelegate<'tcx> for CombineDelegate<'_, 'tcx> {
+    fn param_env(&self) -> ty::ParamEnv<'tcx> {
+        self.param_env
+    }
+
+    fn forbid_inference_vars() -> bool {
+        false
+    }
+
+    fn generalize_region(&mut self, universe: ty::UniverseIndex) -> ty::Region<'tcx> {
+        // FIXME: This is non-ideal because we don't give a
+        // very descriptive origin for this region variable.
+        self.infcx
+            .next_region_var_in_universe(RegionVariableOrigin::MiscVariable(self.span), universe)
+    }
+}
+
+impl<'tcx, T> GeneralizerDelegate<'tcx> for T
+where
+    T: TypeRelatingDelegate<'tcx>,
+{
+    fn param_env(&self) -> ty::ParamEnv<'tcx> {
+        <Self as TypeRelatingDelegate<'tcx>>::param_env(self)
+    }
+
+    fn forbid_inference_vars() -> bool {
+        <Self as TypeRelatingDelegate<'tcx>>::forbid_inference_vars()
+    }
+
+    fn generalize_region(&mut self, universe: ty::UniverseIndex) -> ty::Region<'tcx> {
+        <Self as TypeRelatingDelegate<'tcx>>::generalize_existential(self, universe)
+    }
+}
+
+/// The "generalizer" is used when handling inference variables.
+///
+/// The basic strategy for handling a constraint like `?A <: B` is to
+/// apply a "generalization strategy" to the term `B` -- this replaces
+/// all the lifetimes in the term `B` with fresh inference variables.
+/// (You can read more about the strategy in this [blog post].)
+///
+/// As an example, if we had `?A <: &'x u32`, we would generalize `&'x
+/// u32` to `&'0 u32` where `'0` is a fresh variable. This becomes the
+/// value of `A`. Finally, we relate `&'0 u32 <: &'x u32`, which
+/// establishes `'0: 'x` as a constraint.
+///
+/// [blog post]: https://is.gd/0hKvIr
+struct Generalizer<'me, 'tcx, D> {
+    infcx: &'me InferCtxt<'tcx>,
+
+    /// This is used to abstract the behaviors of the three previous
+    /// generalizer-like implementations (`Generalizer`, `TypeGeneralizer`,
+    /// and `ConstInferUnifier`). See [`GeneralizerDelegate`] for more
+    /// information.
+    delegate: &'me mut D,
+
+    /// After we generalize this type, we are going to relate it to
+    /// some other type. What will be the variance at this point?
+    ambient_variance: ty::Variance,
+
+    /// The vid of the type variable that is in the process of being
+    /// instantiated. If we find this within the value we are folding,
+    /// that means we would have created a cyclic value.
+    root_vid: ty::TermVid<'tcx>,
+
+    /// The universe of the type variable that is in the process of being
+    /// instantiated. If we find anything that this universe cannot name,
+    /// we reject the relation.
+    for_universe: ty::UniverseIndex,
+
+    /// The root term (const or type) we're generalizing. Used for cycle errors.
+    root_term: Term<'tcx>,
+
+    cache: SsoHashMap<Ty<'tcx>, Ty<'tcx>>,
+
+    /// See the field `needs_wf` in `Generalization`.
+    needs_wf: bool,
+}
+
+impl<'tcx, D> Generalizer<'_, 'tcx, D> {
+    /// Create an error that corresponds to the term kind in `root_term`
+    fn cyclic_term_error(&self) -> TypeError<'tcx> {
+        match self.root_term.unpack() {
+            ty::TermKind::Ty(ty) => TypeError::CyclicTy(ty),
+            ty::TermKind::Const(ct) => TypeError::CyclicConst(ct),
+        }
+    }
+}
+
+impl<'tcx, D> TypeRelation<'tcx> for Generalizer<'_, 'tcx, D>
+where
+    D: GeneralizerDelegate<'tcx>,
+{
+    fn tcx(&self) -> TyCtxt<'tcx> {
+        self.infcx.tcx
+    }
+
+    fn param_env(&self) -> ty::ParamEnv<'tcx> {
+        self.delegate.param_env()
+    }
+
+    fn tag(&self) -> &'static str {
+        "Generalizer"
+    }
+
+    fn a_is_expected(&self) -> bool {
+        true
+    }
+
+    fn relate_item_substs(
+        &mut self,
+        item_def_id: DefId,
+        a_subst: ty::SubstsRef<'tcx>,
+        b_subst: ty::SubstsRef<'tcx>,
+    ) -> RelateResult<'tcx, ty::SubstsRef<'tcx>> {
+        if self.ambient_variance == ty::Variance::Invariant {
+            // Avoid fetching the variance if we are in an invariant
+            // context; no need, and it can induce dependency cycles
+            // (e.g., #41849).
+            relate::relate_substs(self, a_subst, b_subst)
+        } else {
+            let tcx = self.tcx();
+            let opt_variances = tcx.variances_of(item_def_id);
+            relate::relate_substs_with_variances(
+                self,
+                item_def_id,
+                opt_variances,
+                a_subst,
+                b_subst,
+                true,
+            )
+        }
+    }
+
+    #[instrument(level = "debug", skip(self, variance, b), ret)]
+    fn relate_with_variance<T: Relate<'tcx>>(
+        &mut self,
+        variance: ty::Variance,
+        _info: ty::VarianceDiagInfo<'tcx>,
+        a: T,
+        b: T,
+    ) -> RelateResult<'tcx, T> {
+        let old_ambient_variance = self.ambient_variance;
+        self.ambient_variance = self.ambient_variance.xform(variance);
+        debug!(?self.ambient_variance, "new ambient variance");
+        let r = self.relate(a, b)?;
+        self.ambient_variance = old_ambient_variance;
+        Ok(r)
+    }
+
+    #[instrument(level = "debug", skip(self, t2), ret)]
+    fn tys(&mut self, t: Ty<'tcx>, t2: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
+        assert_eq!(t, t2); // we are misusing TypeRelation here; both LHS and RHS ought to be ==
+
+        if let Some(&result) = self.cache.get(&t) {
+            return Ok(result);
+        }
+
+        // Check to see whether the type we are generalizing references
+        // any other type variable related to `vid` via
+        // subtyping. This is basically our "occurs check", preventing
+        // us from creating infinitely sized types.
+        let g = match *t.kind() {
+            ty::Infer(ty::TyVar(_)) | ty::Infer(ty::IntVar(_)) | ty::Infer(ty::FloatVar(_))
+                if D::forbid_inference_vars() =>
+            {
+                bug!("unexpected inference variable encountered in NLL generalization: {t}");
+            }
+
+            ty::Infer(ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
+                bug!("unexpected infer type: {t}")
+            }
+
+            ty::Infer(ty::TyVar(vid)) => {
+                let mut inner = self.infcx.inner.borrow_mut();
+                let vid = inner.type_variables().root_var(vid);
+                let sub_vid = inner.type_variables().sub_root_var(vid);
+
+                if ty::TermVid::Ty(sub_vid) == self.root_vid {
+                    // If sub-roots are equal, then `root_vid` and
+                    // `vid` are related via subtyping.
+                    Err(self.cyclic_term_error())
+                } else {
+                    let probe = inner.type_variables().probe(vid);
+                    match probe {
+                        TypeVariableValue::Known { value: u } => {
+                            drop(inner);
+                            self.relate(u, u)
+                        }
+                        TypeVariableValue::Unknown { universe } => {
+                            match self.ambient_variance {
+                                // Invariant: no need to make a fresh type variable
+                                // if we can name the universe.
+                                ty::Invariant => {
+                                    if self.for_universe.can_name(universe) {
+                                        return Ok(t);
+                                    }
+                                }
+
+                                // Bivariant: make a fresh var, but we
+                                // may need a WF predicate. See
+                                // comment on `needs_wf` field for
+                                // more info.
+                                ty::Bivariant => self.needs_wf = true,
+
+                                // Co/contravariant: this will be
+                                // sufficiently constrained later on.
+                                ty::Covariant | ty::Contravariant => (),
+                            }
+
+                            let origin = *inner.type_variables().var_origin(vid);
+                            let new_var_id =
+                                inner.type_variables().new_var(self.for_universe, origin);
+                            let u = self.tcx().mk_ty_var(new_var_id);
+
+                            // Record that we replaced `vid` with `new_var_id` as part of a generalization
+                            // operation. This is needed to detect cyclic types. To see why, see the
+                            // docs in the `type_variables` module.
+                            inner.type_variables().sub(vid, new_var_id);
+                            debug!("replacing original vid={:?} with new={:?}", vid, u);
+                            Ok(u)
+                        }
+                    }
+                }
+            }
+
+            ty::Infer(ty::IntVar(_) | ty::FloatVar(_)) => {
+                // No matter what mode we are in,
+                // integer/floating-point types must be equal to be
+                // relatable.
+                Ok(t)
+            }
+
+            ty::Placeholder(placeholder) => {
+                if self.for_universe.can_name(placeholder.universe) {
+                    Ok(t)
+                } else {
+                    debug!(
+                        "root universe {:?} cannot name placeholder in universe {:?}",
+                        self.for_universe, placeholder.universe
+                    );
+                    Err(TypeError::Mismatch)
+                }
+            }
+
+            _ => relate::structurally_relate_tys(self, t, t),
+        }?;
+
+        self.cache.insert(t, g);
+        Ok(g)
+    }
+
+    #[instrument(level = "debug", skip(self, r2), ret)]
+    fn regions(
+        &mut self,
+        r: ty::Region<'tcx>,
+        r2: ty::Region<'tcx>,
+    ) -> RelateResult<'tcx, ty::Region<'tcx>> {
+        assert_eq!(r, r2); // we are misusing TypeRelation here; both LHS and RHS ought to be ==
+
+        match *r {
+            // Never make variables for regions bound within the type itself,
+            // nor for erased regions.
+            ty::ReLateBound(..) | ty::ReErased => {
+                return Ok(r);
+            }
+
+            // It doesn't really matter for correctness if we generalize ReError,
+            // since we're already on a doomed compilation path.
+            ty::ReError(_) => {
+                return Ok(r);
+            }
+
+            ty::RePlaceholder(..)
+            | ty::ReVar(..)
+            | ty::ReStatic
+            | ty::ReEarlyBound(..)
+            | ty::ReFree(..) => {
+                // see common code below
+            }
+        }
+
+        // If we are in an invariant context, we can re-use the region
+        // as is, unless it happens to be in some universe that we
+        // can't name.
+        if let ty::Invariant = self.ambient_variance {
+            let r_universe = self.infcx.universe_of_region(r);
+            if self.for_universe.can_name(r_universe) {
+                return Ok(r);
+            }
+        }
+
+        Ok(self.delegate.generalize_region(self.for_universe))
+    }
+
+    #[instrument(level = "debug", skip(self, c2), ret)]
+    fn consts(
+        &mut self,
+        c: ty::Const<'tcx>,
+        c2: ty::Const<'tcx>,
+    ) -> RelateResult<'tcx, ty::Const<'tcx>> {
+        assert_eq!(c, c2); // we are misusing TypeRelation here; both LHS and RHS ought to be ==
+
+        match c.kind() {
+            ty::ConstKind::Infer(InferConst::Var(_)) if D::forbid_inference_vars() => {
+                bug!("unexpected inference variable encountered in NLL generalization: {:?}", c);
+            }
+            ty::ConstKind::Infer(InferConst::Var(vid)) => {
+                // If root const vids are equal, then `root_vid` and
+                // `vid` are related and we'd be inferring an infinitely
+                // deep const.
+                if ty::TermVid::Const(
+                    self.infcx.inner.borrow_mut().const_unification_table().find(vid),
+                ) == self.root_vid
+                {
+                    return Err(self.cyclic_term_error());
+                }
+
+                let mut inner = self.infcx.inner.borrow_mut();
+                let variable_table = &mut inner.const_unification_table();
+                let var_value = variable_table.probe_value(vid);
+                match var_value.val {
+                    ConstVariableValue::Known { value: u } => {
+                        drop(inner);
+                        self.relate(u, u)
+                    }
+                    ConstVariableValue::Unknown { universe } => {
+                        if self.for_universe.can_name(universe) {
+                            Ok(c)
+                        } else {
+                            let new_var_id = variable_table.new_key(ConstVarValue {
+                                origin: var_value.origin,
+                                val: ConstVariableValue::Unknown { universe: self.for_universe },
+                            });
+                            Ok(self.tcx().mk_const(new_var_id, c.ty()))
+                        }
+                    }
+                }
+            }
+            // FIXME: remove this branch once `structurally_relate_consts` is fully
+            // structural.
+            ty::ConstKind::Unevaluated(ty::UnevaluatedConst { def, substs }) => {
+                let substs = self.relate_with_variance(
+                    ty::Variance::Invariant,
+                    ty::VarianceDiagInfo::default(),
+                    substs,
+                    substs,
+                )?;
+                Ok(self.tcx().mk_const(ty::UnevaluatedConst { def, substs }, c.ty()))
+            }
+            ty::ConstKind::Placeholder(placeholder) => {
+                if self.for_universe.can_name(placeholder.universe) {
+                    Ok(c)
+                } else {
+                    debug!(
+                        "root universe {:?} cannot name placeholder in universe {:?}",
+                        self.for_universe, placeholder.universe
+                    );
+                    Err(TypeError::Mismatch)
+                }
+            }
+            _ => relate::structurally_relate_consts(self, c, c),
+        }
+    }
+
+    #[instrument(level = "debug", skip(self), ret)]
+    fn binders<T>(
+        &mut self,
+        a: ty::Binder<'tcx, T>,
+        _: ty::Binder<'tcx, T>,
+    ) -> RelateResult<'tcx, ty::Binder<'tcx, T>>
+    where
+        T: Relate<'tcx>,
+    {
+        let result = self.relate(a.skip_binder(), a.skip_binder())?;
+        Ok(a.rebind(result))
+    }
+}
+
+/// Result from a generalization operation. This includes
+/// not only the generalized type, but also a bool flag
+/// indicating whether further WF checks are needed.
+#[derive(Debug)]
+pub struct Generalization<T> {
+    pub value: T,
+
+    /// If true, then the generalized type may not be well-formed,
+    /// even if the source type is well-formed, so we should add an
+    /// additional check to enforce that it is. This arises in
+    /// particular around 'bivariant' type parameters that are only
+    /// constrained by a where-clause. As an example, imagine a type:
+    ///
+    ///     struct Foo<A, B> where A: Iterator<Item = B> {
+    ///         data: A
+    ///     }
+    ///
+    /// here, `A` will be covariant, but `B` is
+    /// unconstrained. However, whatever it is, for `Foo` to be WF, it
+    /// must be equal to `A::Item`. If we have an input `Foo<?A, ?B>`,
+    /// then after generalization we will wind up with a type like
+    /// `Foo<?C, ?D>`. When we enforce that `Foo<?A, ?B> <: Foo<?C,
+    /// ?D>` (or `>:`), we will wind up with the requirement that `?A
+    /// <: ?C`, but no particular relationship between `?B` and `?D`
+    /// (after all, we do not know the variance of the normalized form
+    /// of `A::Item` with respect to `A`). If we do nothing else, this
+    /// may mean that `?D` goes unconstrained (as in #41677). So, in
+    /// this scenario where we create a new type variable in a
+    /// bivariant context, we set the `needs_wf` flag to true. This
+    /// will force the calling code to check that `WF(Foo<?C, ?D>)`
+    /// holds, which in turn implies that `?C::Item == ?D`. So once
+    /// `?C` is constrained, that should suffice to restrict `?D`.
+    pub needs_wf: bool,
+}
diff --git a/compiler/rustc_infer/src/infer/higher_ranked/mod.rs b/compiler/rustc_infer/src/infer/higher_ranked/mod.rs
index a63cfbc91..c304cd25c 100644
--- a/compiler/rustc_infer/src/infer/higher_ranked/mod.rs
+++ b/compiler/rustc_infer/src/infer/higher_ranked/mod.rs
@@ -42,7 +42,7 @@ impl<'a, 'tcx> CombineFields<'a, 'tcx> {
 
         // Next, we instantiate each bound region in the subtype
         // with a fresh region variable. These region variables --
-        // but no other pre-existing region variables -- can name
+        // but no other preexisting region variables -- can name
         // the placeholders.
         let sub_prime = self.infcx.instantiate_binder_with_fresh_vars(span, HigherRankedType, sub);
 
diff --git a/compiler/rustc_infer/src/infer/lattice.rs b/compiler/rustc_infer/src/infer/lattice.rs
index 7f4c141b9..7190d33d2 100644
--- a/compiler/rustc_infer/src/infer/lattice.rs
+++ b/compiler/rustc_infer/src/infer/lattice.rs
@@ -108,9 +108,12 @@ where
             &ty::Alias(ty::Opaque, ty::AliasTy { def_id: a_def_id, .. }),
             &ty::Alias(ty::Opaque, ty::AliasTy { def_id: b_def_id, .. }),
         ) if a_def_id == b_def_id => infcx.super_combine_tys(this, a, b),
+
         (&ty::Alias(ty::Opaque, ty::AliasTy { def_id, .. }), _)
         | (_, &ty::Alias(ty::Opaque, ty::AliasTy { def_id, .. }))
-            if this.define_opaque_types() == DefineOpaqueTypes::Yes && def_id.is_local() =>
+            if this.define_opaque_types() == DefineOpaqueTypes::Yes
+                && def_id.is_local()
+                && !this.tcx().trait_solver_next() =>
         {
             this.register_obligations(
                 infcx
diff --git a/compiler/rustc_infer/src/infer/lexical_region_resolve/mod.rs b/compiler/rustc_infer/src/infer/lexical_region_resolve/mod.rs
index f298b95ca..8482ae2aa 100644
--- a/compiler/rustc_infer/src/infer/lexical_region_resolve/mod.rs
+++ b/compiler/rustc_infer/src/infer/lexical_region_resolve/mod.rs
@@ -13,7 +13,7 @@ use rustc_data_structures::graph::implementation::{
     Direction, Graph, NodeIndex, INCOMING, OUTGOING,
 };
 use rustc_data_structures::intern::Interned;
-use rustc_index::vec::{IndexSlice, IndexVec};
+use rustc_index::{IndexSlice, IndexVec};
 use rustc_middle::ty::fold::TypeFoldable;
 use rustc_middle::ty::PlaceholderRegion;
 use rustc_middle::ty::{self, Ty, TyCtxt};
@@ -102,6 +102,17 @@ pub enum RegionResolutionError<'tcx> {
     ),
 }
 
+impl<'tcx> RegionResolutionError<'tcx> {
+    pub fn origin(&self) -> &SubregionOrigin<'tcx> {
+        match self {
+            RegionResolutionError::ConcreteFailure(origin, _, _)
+            | RegionResolutionError::GenericBoundFailure(origin, _, _)
+            | RegionResolutionError::SubSupConflict(_, _, origin, _, _, _, _)
+            | RegionResolutionError::UpperBoundUniverseConflict(_, _, _, origin, _) => origin,
+        }
+    }
+}
+
 struct RegionAndOrigin<'tcx> {
     region: Region<'tcx>,
     origin: SubregionOrigin<'tcx>,
@@ -131,10 +142,9 @@ impl<'cx, 'tcx> LexicalResolver<'cx, 'tcx> {
             self.dump_constraints();
         }
 
-        let graph = self.construct_graph();
         self.expansion(&mut var_data);
         self.collect_errors(&mut var_data, errors);
-        self.collect_var_errors(&var_data, &graph, errors);
+        self.collect_var_errors(&var_data, errors);
         var_data
     }
 
@@ -622,7 +632,6 @@ impl<'cx, 'tcx> LexicalResolver<'cx, 'tcx> {
     fn collect_var_errors(
         &self,
         var_data: &LexicalRegionResolutions<'tcx>,
-        graph: &RegionGraph<'tcx>,
         errors: &mut Vec<RegionResolutionError<'tcx>>,
     ) {
         debug!("collect_var_errors, var_data = {:#?}", var_data.values);
@@ -640,6 +649,10 @@ impl<'cx, 'tcx> LexicalResolver<'cx, 'tcx> {
         // overlapping locations.
         let mut dup_vec = IndexVec::from_elem_n(None, self.num_vars());
 
+        // Only construct the graph when necessary, because it's moderately
+        // expensive.
+        let mut graph = None;
+
         for (node_vid, value) in var_data.values.iter_enumerated() {
             match *value {
                 VarValue::Empty(_) | VarValue::Value(_) => { /* Inference successful */ }
@@ -672,7 +685,8 @@ impl<'cx, 'tcx> LexicalResolver<'cx, 'tcx> {
                     // influence the constraints on this value for
                     // richer diagnostics in `static_impl_trait`.
 
-                    self.collect_error_for_expanding_node(graph, &mut dup_vec, node_vid, errors);
+                    let g = graph.get_or_insert_with(|| self.construct_graph());
+                    self.collect_error_for_expanding_node(g, &mut dup_vec, node_vid, errors);
                 }
             }
         }
@@ -821,7 +835,7 @@ impl<'cx, 'tcx> LexicalResolver<'cx, 'tcx> {
         // resolution errors here; delay ICE in favor of those errors.
         self.tcx().sess.delay_span_bug(
             self.var_infos[node_idx].origin.span(),
-            &format!(
+            format!(
                 "collect_error_for_expanding_node() could not find \
                  error for var {:?} in universe {:?}, lower_bounds={:#?}, \
                  upper_bounds={:#?}",
diff --git a/compiler/rustc_infer/src/infer/mod.rs b/compiler/rustc_infer/src/infer/mod.rs
index 66f51328b..cd99fc312 100644
--- a/compiler/rustc_infer/src/infer/mod.rs
+++ b/compiler/rustc_infer/src/infer/mod.rs
@@ -24,7 +24,7 @@ use rustc_middle::infer::unify_key::{ConstVarValue, ConstVariableValue};
 use rustc_middle::infer::unify_key::{ConstVariableOrigin, ConstVariableOriginKind, ToType};
 use rustc_middle::mir::interpret::{ErrorHandled, EvalToValTreeResult};
 use rustc_middle::mir::ConstraintCategory;
-use rustc_middle::traits::select;
+use rustc_middle::traits::{select, DefiningAnchor};
 use rustc_middle::ty::error::{ExpectedFound, TypeError};
 use rustc_middle::ty::fold::BoundVarReplacerDelegate;
 use rustc_middle::ty::fold::{TypeFoldable, TypeFolder, TypeSuperFoldable};
@@ -39,7 +39,6 @@ use rustc_span::Span;
 
 use std::cell::{Cell, RefCell};
 use std::fmt;
-use std::ops::Drop;
 
 use self::combine::CombineFields;
 use self::error_reporting::TypeErrCtxt;
@@ -59,6 +58,7 @@ pub mod error_reporting;
 pub mod free_regions;
 mod freshen;
 mod fudge;
+mod generalize;
 mod glb;
 mod higher_ranked;
 pub mod lattice;
@@ -231,17 +231,6 @@ impl<'tcx> InferCtxtInner<'tcx> {
     }
 }
 
-#[derive(Clone, Copy, Debug, PartialEq, Eq)]
-pub enum DefiningAnchor {
-    /// `DefId` of the item.
-    Bind(LocalDefId),
-    /// When opaque types are not resolved, we `Bubble` up, meaning
-    /// return the opaque/hidden type pair from query, for caller of query to handle it.
-    Bubble,
-    /// Used to catch type mismatch errors when handling opaque types.
-    Error,
-}
-
 pub struct InferCtxt<'tcx> {
     pub tcx: TyCtxt<'tcx>,
 
@@ -342,11 +331,6 @@ pub struct InferCtxt<'tcx> {
     /// there is no type that the user could *actually name* that
     /// would satisfy it. This avoids crippling inference, basically.
     pub intercrate: bool,
-
-    /// Flag that is set when we enter canonicalization. Used for debugging to ensure
-    /// that we only collect region information for `BorrowckInferCtxt::reg_var_to_origin`
-    /// inside non-canonicalization contexts.
-    inside_canonicalization_ctxt: Cell<bool>,
 }
 
 /// See the `error_reporting` module for more details.
@@ -638,7 +622,6 @@ impl<'tcx> InferCtxtBuilder<'tcx> {
             skip_leak_check: Cell::new(false),
             universe: Cell::new(ty::UniverseIndex::ROOT),
             intercrate,
-            inside_canonicalization_ctxt: Cell::new(false),
         }
     }
 }
@@ -1228,11 +1211,11 @@ impl<'tcx> InferCtxt<'tcx> {
     /// hence that `resolve_regions_and_report_errors` can never be
     /// called. This is used only during NLL processing to "hand off" ownership
     /// of the set of region variables into the NLL region context.
-    pub fn take_region_var_origins(&self) -> VarInfos {
+    pub fn get_region_var_origins(&self) -> VarInfos {
         let mut inner = self.inner.borrow_mut();
         let (var_infos, data) = inner
             .region_constraint_storage
-            .take()
+            .clone()
             .expect("regions already resolved")
             .with_log(&mut inner.undo_log)
             .into_infos_and_data();
@@ -1327,7 +1310,7 @@ impl<'tcx> InferCtxt<'tcx> {
     where
         T: TypeFoldable<TyCtxt<'tcx>>,
     {
-        if !value.needs_infer() {
+        if !value.has_infer() {
             return value; // Avoid duplicated subst-folding.
         }
         let mut r = InferenceLiteralEraser { tcx: self.tcx };
@@ -1365,7 +1348,7 @@ impl<'tcx> InferCtxt<'tcx> {
     pub fn fully_resolve<T: TypeFoldable<TyCtxt<'tcx>>>(&self, value: T) -> FixupResult<'tcx, T> {
         let value = resolve::fully_resolve(self, value);
         assert!(
-            value.as_ref().map_or(true, |value| !value.needs_infer()),
+            value.as_ref().map_or(true, |value| !value.has_infer()),
             "`{value:?}` is not fully resolved"
         );
         value
@@ -1500,7 +1483,7 @@ impl<'tcx> InferCtxt<'tcx> {
             Ok(Some(val)) => Ok(self.tcx.mk_const(val, ty)),
             Ok(None) => {
                 let tcx = self.tcx;
-                let def_id = unevaluated.def.did;
+                let def_id = unevaluated.def;
                 span_bug!(
                     tcx.def_span(def_id),
                     "unable to construct a constant value for the unevaluated constant {:?}",
@@ -1537,18 +1520,18 @@ impl<'tcx> InferCtxt<'tcx> {
         // variables
         let tcx = self.tcx;
         if substs.has_non_region_infer() {
-            if let Some(ct) = tcx.bound_abstract_const(unevaluated.def)? {
+            if let Some(ct) = tcx.thir_abstract_const(unevaluated.def)? {
                 let ct = tcx.expand_abstract_consts(ct.subst(tcx, substs));
                 if let Err(e) = ct.error_reported() {
-                    return Err(ErrorHandled::Reported(e));
+                    return Err(ErrorHandled::Reported(e.into()));
                 } else if ct.has_non_region_infer() || ct.has_non_region_param() {
                     return Err(ErrorHandled::TooGeneric);
                 } else {
                     substs = replace_param_and_infer_substs_with_placeholder(tcx, substs);
                 }
             } else {
-                substs = InternalSubsts::identity_for_item(tcx, unevaluated.def.did);
-                param_env = tcx.param_env(unevaluated.def.did);
+                substs = InternalSubsts::identity_for_item(tcx, unevaluated.def);
+                param_env = tcx.param_env(unevaluated.def);
             }
         }
 
@@ -1577,10 +1560,10 @@ impl<'tcx> InferCtxt<'tcx> {
             (TyOrConstInferVar::Ty(ty_var), Ok(inner)) => {
                 use self::type_variable::TypeVariableValue;
 
-                match inner.try_type_variables_probe_ref(ty_var) {
-                    Some(TypeVariableValue::Unknown { .. }) => true,
-                    _ => false,
-                }
+                matches!(
+                    inner.try_type_variables_probe_ref(ty_var),
+                    Some(TypeVariableValue::Unknown { .. })
+                )
             }
             _ => false,
         };
@@ -1636,31 +1619,6 @@ impl<'tcx> InferCtxt<'tcx> {
             }
         }
     }
-
-    pub fn inside_canonicalization_ctxt(&self) -> bool {
-        self.inside_canonicalization_ctxt.get()
-    }
-
-    pub fn set_canonicalization_ctxt(&self) -> CanonicalizationCtxtGuard<'_, 'tcx> {
-        let prev_ctxt = self.inside_canonicalization_ctxt();
-        self.inside_canonicalization_ctxt.set(true);
-        CanonicalizationCtxtGuard { prev_ctxt, infcx: self }
-    }
-
-    fn set_canonicalization_ctxt_to(&self, ctxt: bool) {
-        self.inside_canonicalization_ctxt.set(ctxt);
-    }
-}
-
-pub struct CanonicalizationCtxtGuard<'cx, 'tcx> {
-    prev_ctxt: bool,
-    infcx: &'cx InferCtxt<'tcx>,
-}
-
-impl<'cx, 'tcx> Drop for CanonicalizationCtxtGuard<'cx, 'tcx> {
-    fn drop(&mut self) {
-        self.infcx.set_canonicalization_ctxt_to(self.prev_ctxt)
-    }
 }
 
 impl<'tcx> TypeErrCtxt<'_, 'tcx> {
diff --git a/compiler/rustc_infer/src/infer/nll_relate/mod.rs b/compiler/rustc_infer/src/infer/nll_relate/mod.rs
index 9f7b26b87..d3fd01b96 100644
--- a/compiler/rustc_infer/src/infer/nll_relate/mod.rs
+++ b/compiler/rustc_infer/src/infer/nll_relate/mod.rs
@@ -21,22 +21,20 @@
 //!   thing we relate in chalk are basically domain goals and their
 //!   constituents)
 
-use crate::infer::InferCtxt;
-use crate::infer::{ConstVarValue, ConstVariableValue};
-use crate::infer::{TypeVariableOrigin, TypeVariableOriginKind};
-use crate::traits::{Obligation, PredicateObligations};
 use rustc_data_structures::fx::FxHashMap;
 use rustc_middle::traits::ObligationCause;
-use rustc_middle::ty::error::TypeError;
 use rustc_middle::ty::fold::FnMutDelegate;
-use rustc_middle::ty::relate::{self, Relate, RelateResult, TypeRelation};
-use rustc_middle::ty::visit::{TypeSuperVisitable, TypeVisitable, TypeVisitableExt, TypeVisitor};
+use rustc_middle::ty::relate::{Relate, RelateResult, TypeRelation};
+use rustc_middle::ty::visit::TypeVisitableExt;
 use rustc_middle::ty::{self, InferConst, Ty, TyCtxt};
 use rustc_span::{Span, Symbol};
 use std::fmt::Debug;
-use std::ops::ControlFlow;
 
-use super::combine::ObligationEmittingRelation;
+use crate::infer::combine::ObligationEmittingRelation;
+use crate::infer::generalize::{self, Generalization};
+use crate::infer::InferCtxt;
+use crate::infer::{TypeVariableOrigin, TypeVariableOriginKind};
+use crate::traits::{Obligation, PredicateObligations};
 
 pub struct TypeRelating<'me, 'tcx, D>
 where
@@ -115,11 +113,6 @@ pub trait TypeRelatingDelegate<'tcx> {
     fn forbid_inference_vars() -> bool;
 }
 
-#[derive(Clone, Debug, Default)]
-struct BoundRegionScope<'tcx> {
-    map: FxHashMap<ty::BoundRegion, ty::Region<'tcx>>,
-}
-
 #[derive(Copy, Clone)]
 struct UniversallyQuantified(bool);
 
@@ -204,7 +197,7 @@ where
             _ => (),
         }
 
-        let generalized_ty = self.generalize_value(value_ty, vid)?;
+        let generalized_ty = self.generalize(value_ty, vid)?;
         debug!("relate_ty_var: generalized_ty = {:?}", generalized_ty);
 
         if D::forbid_inference_vars() {
@@ -223,23 +216,15 @@ where
         result
     }
 
-    fn generalize_value<T: Relate<'tcx>>(
-        &mut self,
-        value: T,
-        for_vid: ty::TyVid,
-    ) -> RelateResult<'tcx, T> {
-        let universe = self.infcx.probe_ty_var(for_vid).unwrap_err();
-
-        let mut generalizer = TypeGeneralizer {
-            infcx: self.infcx,
-            delegate: &mut self.delegate,
-            first_free_index: ty::INNERMOST,
-            ambient_variance: self.ambient_variance,
-            for_vid_sub_root: self.infcx.inner.borrow_mut().type_variables().sub_root_var(for_vid),
-            universe,
-        };
-
-        generalizer.relate(value, value)
+    fn generalize(&mut self, ty: Ty<'tcx>, for_vid: ty::TyVid) -> RelateResult<'tcx, Ty<'tcx>> {
+        let Generalization { value: ty, needs_wf: _ } = generalize::generalize(
+            self.infcx,
+            &mut self.delegate,
+            ty,
+            for_vid,
+            self.ambient_variance,
+        )?;
+        Ok(ty)
     }
 
     fn relate_opaques(&mut self, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
@@ -488,13 +473,7 @@ where
         }
 
         if a == b {
-            // Subtle: if a or b has a bound variable that we are lazily
-            // substituting, then even if a == b, it could be that the values we
-            // will substitute for those bound variables are *not* the same, and
-            // hence returning `Ok(a)` is incorrect.
-            if !a.has_escaping_bound_vars() && !b.has_escaping_bound_vars() {
-                return Ok(a);
-            }
+            return Ok(a);
         }
 
         match (a.kind(), b.kind()) {
@@ -512,16 +491,22 @@ where
             (
                 &ty::Alias(ty::Opaque, ty::AliasTy { def_id: a_def_id, .. }),
                 &ty::Alias(ty::Opaque, ty::AliasTy { def_id: b_def_id, .. }),
-            ) if a_def_id == b_def_id => infcx.super_combine_tys(self, a, b).or_else(|err| {
-                self.tcx().sess.delay_span_bug(
-                    self.delegate.span(),
-                    "failure to relate an opaque to itself should result in an error later on",
-                );
-                if a_def_id.is_local() { self.relate_opaques(a, b) } else { Err(err) }
-            }),
+            ) if a_def_id == b_def_id || infcx.tcx.trait_solver_next() => {
+                infcx.super_combine_tys(self, a, b).or_else(|err| {
+                    // This behavior is only there for the old solver, the new solver
+                    // shouldn't ever fail. Instead, it unconditionally emits an
+                    // alias-relate goal.
+                    assert!(!self.tcx().trait_solver_next());
+                    self.tcx().sess.delay_span_bug(
+                        self.delegate.span(),
+                        "failure to relate an opaque to itself should result in an error later on",
+                    );
+                    if a_def_id.is_local() { self.relate_opaques(a, b) } else { Err(err) }
+                })
+            }
             (&ty::Alias(ty::Opaque, ty::AliasTy { def_id, .. }), _)
             | (_, &ty::Alias(ty::Opaque, ty::AliasTy { def_id, .. }))
-                if def_id.is_local() =>
+                if def_id.is_local() && !self.tcx().trait_solver_next() =>
             {
                 self.relate_opaques(a, b)
             }
@@ -725,287 +710,3 @@ where
         })]);
     }
 }
-
-/// When we encounter a binder like `for<..> fn(..)`, we actually have
-/// to walk the `fn` value to find all the values bound by the `for`
-/// (these are not explicitly present in the ty representation right
-/// now). This visitor handles that: it descends the type, tracking
-/// binder depth, and finds late-bound regions targeting the
-/// `for<..`>. For each of those, it creates an entry in
-/// `bound_region_scope`.
-struct ScopeInstantiator<'me, 'tcx> {
-    next_region: &'me mut dyn FnMut(ty::BoundRegion) -> ty::Region<'tcx>,
-    // The debruijn index of the scope we are instantiating.
-    target_index: ty::DebruijnIndex,
-    bound_region_scope: &'me mut BoundRegionScope<'tcx>,
-}
-
-impl<'me, 'tcx> TypeVisitor<TyCtxt<'tcx>> for ScopeInstantiator<'me, 'tcx> {
-    fn visit_binder<T: TypeVisitable<TyCtxt<'tcx>>>(
-        &mut self,
-        t: &ty::Binder<'tcx, T>,
-    ) -> ControlFlow<Self::BreakTy> {
-        self.target_index.shift_in(1);
-        t.super_visit_with(self);
-        self.target_index.shift_out(1);
-
-        ControlFlow::Continue(())
-    }
-
-    fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
-        let ScopeInstantiator { bound_region_scope, next_region, .. } = self;
-
-        match *r {
-            ty::ReLateBound(debruijn, br) if debruijn == self.target_index => {
-                bound_region_scope.map.entry(br).or_insert_with(|| next_region(br));
-            }
-
-            _ => {}
-        }
-
-        ControlFlow::Continue(())
-    }
-}
-
-/// The "type generalizer" is used when handling inference variables.
-///
-/// The basic strategy for handling a constraint like `?A <: B` is to
-/// apply a "generalization strategy" to the type `B` -- this replaces
-/// all the lifetimes in the type `B` with fresh inference
-/// variables. (You can read more about the strategy in this [blog
-/// post].)
-///
-/// As an example, if we had `?A <: &'x u32`, we would generalize `&'x
-/// u32` to `&'0 u32` where `'0` is a fresh variable. This becomes the
-/// value of `A`. Finally, we relate `&'0 u32 <: &'x u32`, which
-/// establishes `'0: 'x` as a constraint.
-///
-/// As a side-effect of this generalization procedure, we also replace
-/// all the bound regions that we have traversed with concrete values,
-/// so that the resulting generalized type is independent from the
-/// scopes.
-///
-/// [blog post]: https://is.gd/0hKvIr
-struct TypeGeneralizer<'me, 'tcx, D>
-where
-    D: TypeRelatingDelegate<'tcx>,
-{
-    infcx: &'me InferCtxt<'tcx>,
-
-    delegate: &'me mut D,
-
-    /// After we generalize this type, we are going to relate it to
-    /// some other type. What will be the variance at this point?
-    ambient_variance: ty::Variance,
-
-    first_free_index: ty::DebruijnIndex,
-
-    /// The vid of the type variable that is in the process of being
-    /// instantiated. If we find this within the value we are folding,
-    /// that means we would have created a cyclic value.
-    for_vid_sub_root: ty::TyVid,
-
-    /// The universe of the type variable that is in the process of being
-    /// instantiated. If we find anything that this universe cannot name,
-    /// we reject the relation.
-    universe: ty::UniverseIndex,
-}
-
-impl<'tcx, D> TypeRelation<'tcx> for TypeGeneralizer<'_, 'tcx, D>
-where
-    D: TypeRelatingDelegate<'tcx>,
-{
-    fn tcx(&self) -> TyCtxt<'tcx> {
-        self.infcx.tcx
-    }
-
-    fn param_env(&self) -> ty::ParamEnv<'tcx> {
-        self.delegate.param_env()
-    }
-
-    fn tag(&self) -> &'static str {
-        "nll::generalizer"
-    }
-
-    fn a_is_expected(&self) -> bool {
-        true
-    }
-
-    fn relate_with_variance<T: Relate<'tcx>>(
-        &mut self,
-        variance: ty::Variance,
-        _info: ty::VarianceDiagInfo<'tcx>,
-        a: T,
-        b: T,
-    ) -> RelateResult<'tcx, T> {
-        debug!(
-            "TypeGeneralizer::relate_with_variance(variance={:?}, a={:?}, b={:?})",
-            variance, a, b
-        );
-
-        let old_ambient_variance = self.ambient_variance;
-        self.ambient_variance = self.ambient_variance.xform(variance);
-
-        debug!(
-            "TypeGeneralizer::relate_with_variance: ambient_variance = {:?}",
-            self.ambient_variance
-        );
-
-        let r = self.relate(a, b)?;
-
-        self.ambient_variance = old_ambient_variance;
-
-        debug!("TypeGeneralizer::relate_with_variance: r={:?}", r);
-
-        Ok(r)
-    }
-
-    fn tys(&mut self, a: Ty<'tcx>, _: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
-        use crate::infer::type_variable::TypeVariableValue;
-
-        debug!("TypeGeneralizer::tys(a={:?})", a);
-
-        match *a.kind() {
-            ty::Infer(ty::TyVar(_)) | ty::Infer(ty::IntVar(_)) | ty::Infer(ty::FloatVar(_))
-                if D::forbid_inference_vars() =>
-            {
-                bug!("unexpected inference variable encountered in NLL generalization: {:?}", a);
-            }
-
-            ty::Infer(ty::TyVar(vid)) => {
-                let mut inner = self.infcx.inner.borrow_mut();
-                let variables = &mut inner.type_variables();
-                let vid = variables.root_var(vid);
-                let sub_vid = variables.sub_root_var(vid);
-                if sub_vid == self.for_vid_sub_root {
-                    // If sub-roots are equal, then `for_vid` and
-                    // `vid` are related via subtyping.
-                    debug!("TypeGeneralizer::tys: occurs check failed");
-                    Err(TypeError::Mismatch)
-                } else {
-                    match variables.probe(vid) {
-                        TypeVariableValue::Known { value: u } => {
-                            drop(variables);
-                            self.relate(u, u)
-                        }
-                        TypeVariableValue::Unknown { universe: _universe } => {
-                            if self.ambient_variance == ty::Bivariant {
-                                // FIXME: we may need a WF predicate (related to #54105).
-                            }
-
-                            let origin = *variables.var_origin(vid);
-
-                            // Replacing with a new variable in the universe `self.universe`,
-                            // it will be unified later with the original type variable in
-                            // the universe `_universe`.
-                            let new_var_id = variables.new_var(self.universe, origin);
-
-                            let u = self.tcx().mk_ty_var(new_var_id);
-                            debug!("generalize: replacing original vid={:?} with new={:?}", vid, u);
-                            Ok(u)
-                        }
-                    }
-                }
-            }
-
-            ty::Infer(ty::IntVar(_) | ty::FloatVar(_)) => {
-                // No matter what mode we are in,
-                // integer/floating-point types must be equal to be
-                // relatable.
-                Ok(a)
-            }
-
-            ty::Placeholder(placeholder) => {
-                if self.universe.cannot_name(placeholder.universe) {
-                    debug!(
-                        "TypeGeneralizer::tys: root universe {:?} cannot name\
-                         placeholder in universe {:?}",
-                        self.universe, placeholder.universe
-                    );
-                    Err(TypeError::Mismatch)
-                } else {
-                    Ok(a)
-                }
-            }
-
-            _ => relate::super_relate_tys(self, a, a),
-        }
-    }
-
-    fn regions(
-        &mut self,
-        a: ty::Region<'tcx>,
-        _: ty::Region<'tcx>,
-    ) -> RelateResult<'tcx, ty::Region<'tcx>> {
-        debug!("TypeGeneralizer::regions(a={:?})", a);
-
-        if let ty::ReLateBound(debruijn, _) = *a && debruijn < self.first_free_index {
-            return Ok(a);
-        }
-
-        // For now, we just always create a fresh region variable to
-        // replace all the regions in the source type. In the main
-        // type checker, we special case the case where the ambient
-        // variance is `Invariant` and try to avoid creating a fresh
-        // region variable, but since this comes up so much less in
-        // NLL (only when users use `_` etc) it is much less
-        // important.
-        //
-        // As an aside, since these new variables are created in
-        // `self.universe` universe, this also serves to enforce the
-        // universe scoping rules.
-        //
-        // FIXME(#54105) -- if the ambient variance is bivariant,
-        // though, we may however need to check well-formedness or
-        // risk a problem like #41677 again.
-
-        let replacement_region_vid = self.delegate.generalize_existential(self.universe);
-
-        Ok(replacement_region_vid)
-    }
-
-    fn consts(
-        &mut self,
-        a: ty::Const<'tcx>,
-        _: ty::Const<'tcx>,
-    ) -> RelateResult<'tcx, ty::Const<'tcx>> {
-        match a.kind() {
-            ty::ConstKind::Infer(InferConst::Var(_)) if D::forbid_inference_vars() => {
-                bug!("unexpected inference variable encountered in NLL generalization: {:?}", a);
-            }
-            ty::ConstKind::Infer(InferConst::Var(vid)) => {
-                let mut inner = self.infcx.inner.borrow_mut();
-                let variable_table = &mut inner.const_unification_table();
-                let var_value = variable_table.probe_value(vid);
-                match var_value.val.known() {
-                    Some(u) => self.relate(u, u),
-                    None => {
-                        let new_var_id = variable_table.new_key(ConstVarValue {
-                            origin: var_value.origin,
-                            val: ConstVariableValue::Unknown { universe: self.universe },
-                        });
-                        Ok(self.tcx().mk_const(new_var_id, a.ty()))
-                    }
-                }
-            }
-            ty::ConstKind::Unevaluated(..) if self.tcx().lazy_normalization() => Ok(a),
-            _ => relate::super_relate_consts(self, a, a),
-        }
-    }
-
-    fn binders<T>(
-        &mut self,
-        a: ty::Binder<'tcx, T>,
-        _: ty::Binder<'tcx, T>,
-    ) -> RelateResult<'tcx, ty::Binder<'tcx, T>>
-    where
-        T: Relate<'tcx>,
-    {
-        debug!("TypeGeneralizer::binders(a={:?})", a);
-
-        self.first_free_index.shift_in(1);
-        let result = self.relate(a.skip_binder(), a.skip_binder())?;
-        self.first_free_index.shift_out(1);
-        Ok(a.rebind(result))
-    }
-}
diff --git a/compiler/rustc_infer/src/infer/opaque_types.rs b/compiler/rustc_infer/src/infer/opaque_types.rs
index 3a0a0494a..9d5ec228d 100644
--- a/compiler/rustc_infer/src/infer/opaque_types.rs
+++ b/compiler/rustc_infer/src/infer/opaque_types.rs
@@ -1,14 +1,14 @@
 use super::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use super::{DefineOpaqueTypes, InferResult};
 use crate::errors::OpaqueHiddenTypeDiag;
-use crate::infer::{DefiningAnchor, InferCtxt, InferOk};
-use crate::traits;
+use crate::infer::{InferCtxt, InferOk};
+use crate::traits::{self, PredicateObligation};
 use hir::def_id::{DefId, LocalDefId};
 use hir::OpaqueTyOrigin;
 use rustc_data_structures::fx::FxIndexMap;
 use rustc_data_structures::sync::Lrc;
 use rustc_hir as hir;
-use rustc_middle::traits::ObligationCause;
+use rustc_middle::traits::{DefiningAnchor, ObligationCause};
 use rustc_middle::ty::error::{ExpectedFound, TypeError};
 use rustc_middle::ty::fold::BottomUpFolder;
 use rustc_middle::ty::GenericArgKind;
@@ -48,12 +48,18 @@ impl<'tcx> InferCtxt<'tcx> {
         span: Span,
         param_env: ty::ParamEnv<'tcx>,
     ) -> InferOk<'tcx, T> {
+        // We handle opaque types differently in the new solver.
+        if self.tcx.trait_solver_next() {
+            return InferOk { value, obligations: vec![] };
+        }
+
         if !value.has_opaque_types() {
             return InferOk { value, obligations: vec![] };
         }
+
         let mut obligations = vec![];
         let replace_opaque_type = |def_id: DefId| {
-            def_id.as_local().map_or(false, |def_id| self.opaque_type_origin(def_id).is_some())
+            def_id.as_local().is_some_and(|def_id| self.opaque_type_origin(def_id).is_some())
         };
         let value = value.fold_with(&mut BottomUpFolder {
             tcx: self.tcx,
@@ -149,7 +155,7 @@ impl<'tcx> InferCtxt<'tcx> {
                     // no one encounters it in practice.
                     // It does occur however in `fn fut() -> impl Future<Output = i32> { async { 42 } }`,
                     // where it is of no concern, so we only check for TAITs.
-                    if let Some(OpaqueTyOrigin::TyAlias) =
+                    if let Some(OpaqueTyOrigin::TyAlias { .. }) =
                         b_def_id.as_local().and_then(|b_def_id| self.opaque_type_origin(b_def_id))
                     {
                         self.tcx.sess.emit_err(OpaqueHiddenTypeDiag {
@@ -381,8 +387,12 @@ impl<'tcx> InferCtxt<'tcx> {
             // Anonymous `impl Trait`
             hir::OpaqueTyOrigin::FnReturn(parent) => parent == parent_def_id,
             // Named `type Foo = impl Bar;`
-            hir::OpaqueTyOrigin::TyAlias => {
-                may_define_opaque_type(self.tcx, parent_def_id, opaque_hir_id)
+            hir::OpaqueTyOrigin::TyAlias { in_assoc_ty } => {
+                if in_assoc_ty {
+                    self.tcx.opaque_types_defined_by(parent_def_id).contains(&def_id)
+                } else {
+                    may_define_opaque_type(self.tcx, parent_def_id, opaque_hir_id)
+                }
             }
         };
         in_definition_scope.then_some(origin)
@@ -392,12 +402,7 @@ impl<'tcx> InferCtxt<'tcx> {
     /// defining scope.
     #[instrument(skip(self), level = "trace", ret)]
     fn opaque_type_origin_unchecked(&self, def_id: LocalDefId) -> OpaqueTyOrigin {
-        match self.tcx.hir().expect_item(def_id).kind {
-            hir::ItemKind::OpaqueTy(hir::OpaqueTy { origin, .. }) => origin,
-            ref itemkind => {
-                bug!("weird opaque type: {:?}, {:#?}", def_id, itemkind)
-            }
-        }
+        self.tcx.hir().expect_item(def_id).expect_opaque_ty().origin
     }
 }
 
@@ -522,30 +527,69 @@ impl<'tcx> InferCtxt<'tcx> {
         origin: hir::OpaqueTyOrigin,
         a_is_expected: bool,
     ) -> InferResult<'tcx, ()> {
-        let tcx = self.tcx;
-        let OpaqueTypeKey { def_id, substs } = opaque_type_key;
-
         // Ideally, we'd get the span where *this specific `ty` came
         // from*, but right now we just use the span from the overall
         // value being folded. In simple cases like `-> impl Foo`,
         // these are the same span, but not in cases like `-> (impl
         // Foo, impl Bar)`.
         let span = cause.span;
-
-        let mut obligations = vec![];
         let prev = self.inner.borrow_mut().opaque_types().register(
-            OpaqueTypeKey { def_id, substs },
+            opaque_type_key,
             OpaqueHiddenType { ty: hidden_ty, span },
             origin,
         );
-        if let Some(prev) = prev {
-            obligations = self
-                .at(&cause, param_env)
+        let mut obligations = if let Some(prev) = prev {
+            self.at(&cause, param_env)
                 .eq_exp(DefineOpaqueTypes::Yes, a_is_expected, prev, hidden_ty)?
-                .obligations;
-        }
+                .obligations
+        } else {
+            Vec::new()
+        };
+
+        self.add_item_bounds_for_hidden_type(
+            opaque_type_key,
+            cause,
+            param_env,
+            hidden_ty,
+            &mut obligations,
+        );
+
+        Ok(InferOk { value: (), obligations })
+    }
 
-        let item_bounds = tcx.bound_explicit_item_bounds(def_id.to_def_id());
+    /// Registers an opaque's hidden type -- only should be used when the opaque
+    /// can be defined. For something more fallible -- checks the anchors, tries
+    /// to unify opaques in both dirs, etc. -- use `InferCtxt::handle_opaque_type`.
+    pub fn register_hidden_type_in_new_solver(
+        &self,
+        opaque_type_key: OpaqueTypeKey<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+        hidden_ty: Ty<'tcx>,
+    ) -> InferResult<'tcx, ()> {
+        assert!(self.tcx.trait_solver_next());
+        let origin = self
+            .opaque_type_origin(opaque_type_key.def_id)
+            .expect("should be called for defining usages only");
+        self.register_hidden_type(
+            opaque_type_key,
+            ObligationCause::dummy(),
+            param_env,
+            hidden_ty,
+            origin,
+            true,
+        )
+    }
+
+    pub fn add_item_bounds_for_hidden_type(
+        &self,
+        OpaqueTypeKey { def_id, substs }: OpaqueTypeKey<'tcx>,
+        cause: ObligationCause<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+        hidden_ty: Ty<'tcx>,
+        obligations: &mut Vec<PredicateObligation<'tcx>>,
+    ) {
+        let tcx = self.tcx;
+        let item_bounds = tcx.explicit_item_bounds(def_id);
 
         for (predicate, _) in item_bounds.subst_iter_copied(tcx, substs) {
             let predicate = predicate.fold_with(&mut BottomUpFolder {
@@ -554,16 +598,18 @@ impl<'tcx> InferCtxt<'tcx> {
                     // We can't normalize associated types from `rustc_infer`,
                     // but we can eagerly register inference variables for them.
                     // FIXME(RPITIT): Don't replace RPITITs with inference vars.
+                    // FIXME(inherent_associated_types): Extend this to support `ty::Inherent`, too.
                     ty::Alias(ty::Projection, projection_ty)
                         if !projection_ty.has_escaping_bound_vars()
-                            && !tcx.is_impl_trait_in_trait(projection_ty.def_id) =>
+                            && !tcx.is_impl_trait_in_trait(projection_ty.def_id)
+                            && !tcx.trait_solver_next() =>
                     {
                         self.infer_projection(
                             param_env,
                             projection_ty,
                             cause.clone(),
                             0,
-                            &mut obligations,
+                            obligations,
                         )
                     }
                     // Replace all other mentions of the same opaque type with the hidden type,
@@ -574,6 +620,7 @@ impl<'tcx> InferCtxt<'tcx> {
                         hidden_ty
                     }
                     // FIXME(RPITIT): This can go away when we move to associated types
+                    // FIXME(inherent_associated_types): Extend this to support `ty::Inherent`, too.
                     ty::Alias(
                         ty::Projection,
                         ty::AliasTy { def_id: def_id2, substs: substs2, .. },
@@ -588,10 +635,10 @@ impl<'tcx> InferCtxt<'tcx> {
                 predicate.kind().skip_binder()
             {
                 if projection.term.references_error() {
-                    // No point on adding these obligations since there's a type error involved.
-                    return Ok(InferOk { value: (), obligations: vec![] });
+                    // No point on adding any obligations since there's a type error involved.
+                    obligations.clear();
+                    return;
                 }
-                trace!("{:#?}", projection.term);
             }
             // Require that the predicate holds for the concrete type.
             debug!(?predicate);
@@ -602,7 +649,6 @@ impl<'tcx> InferCtxt<'tcx> {
                 predicate,
             ));
         }
-        Ok(InferOk { value: (), obligations })
     }
 }
 
diff --git a/compiler/rustc_infer/src/infer/opaque_types/table.rs b/compiler/rustc_infer/src/infer/opaque_types/table.rs
index ae4b85c87..a0f6d7eca 100644
--- a/compiler/rustc_infer/src/infer/opaque_types/table.rs
+++ b/compiler/rustc_infer/src/infer/opaque_types/table.rs
@@ -42,7 +42,7 @@ impl<'tcx> Drop for OpaqueTypeStorage<'tcx> {
     fn drop(&mut self) {
         if !self.opaque_types.is_empty() {
             ty::tls::with(|tcx| {
-                tcx.sess.delay_span_bug(DUMMY_SP, &format!("{:?}", self.opaque_types))
+                tcx.sess.delay_span_bug(DUMMY_SP, format!("{:?}", self.opaque_types))
             });
         }
     }
diff --git a/compiler/rustc_infer/src/infer/outlives/components.rs b/compiler/rustc_infer/src/infer/outlives/components.rs
index ff23087fe..cb63d2f18 100644
--- a/compiler/rustc_infer/src/infer/outlives/components.rs
+++ b/compiler/rustc_infer/src/infer/outlives/components.rs
@@ -143,7 +143,7 @@ fn compute_components<'tcx>(
                     // through and constrain Pi.
                     let mut subcomponents = smallvec![];
                     let mut subvisited = SsoHashSet::new();
-                    compute_components_recursive(tcx, ty.into(), &mut subcomponents, &mut subvisited);
+                    compute_alias_components_recursive(tcx, ty, &mut subcomponents, &mut subvisited);
                     out.push(Component::EscapingAlias(subcomponents.into_iter().collect()));
                 }
             }
@@ -193,7 +193,43 @@ fn compute_components<'tcx>(
 ///
 /// This should not be used to get the components of `parent` itself.
 /// Use [push_outlives_components] instead.
-pub(super) fn compute_components_recursive<'tcx>(
+pub(super) fn compute_alias_components_recursive<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    alias_ty: Ty<'tcx>,
+    out: &mut SmallVec<[Component<'tcx>; 4]>,
+    visited: &mut SsoHashSet<GenericArg<'tcx>>,
+) {
+    let ty::Alias(kind, alias_ty) = alias_ty.kind() else { bug!() };
+    let opt_variances = if *kind == ty::Opaque { tcx.variances_of(alias_ty.def_id) } else { &[] };
+    for (index, child) in alias_ty.substs.iter().enumerate() {
+        if opt_variances.get(index) == Some(&ty::Bivariant) {
+            continue;
+        }
+        if !visited.insert(child) {
+            continue;
+        }
+        match child.unpack() {
+            GenericArgKind::Type(ty) => {
+                compute_components(tcx, ty, out, visited);
+            }
+            GenericArgKind::Lifetime(lt) => {
+                // Ignore late-bound regions.
+                if !lt.is_late_bound() {
+                    out.push(Component::Region(lt));
+                }
+            }
+            GenericArgKind::Const(_) => {
+                compute_components_recursive(tcx, child, out, visited);
+            }
+        }
+    }
+}
+
+/// Collect [Component]s for *all* the substs of `parent`.
+///
+/// This should not be used to get the components of `parent` itself.
+/// Use [push_outlives_components] instead.
+fn compute_components_recursive<'tcx>(
     tcx: TyCtxt<'tcx>,
     parent: GenericArg<'tcx>,
     out: &mut SmallVec<[Component<'tcx>; 4]>,
diff --git a/compiler/rustc_infer/src/infer/outlives/mod.rs b/compiler/rustc_infer/src/infer/outlives/mod.rs
index 9a9a1696b..8a44d5031 100644
--- a/compiler/rustc_infer/src/infer/outlives/mod.rs
+++ b/compiler/rustc_infer/src/infer/outlives/mod.rs
@@ -61,7 +61,7 @@ impl<'tcx> InferCtxt<'tcx> {
         };
 
         let lexical_region_resolutions = LexicalRegionResolutions {
-            values: rustc_index::vec::IndexVec::from_elem_n(
+            values: rustc_index::IndexVec::from_elem_n(
                 crate::infer::lexical_region_resolve::VarValue::Value(self.tcx.lifetimes.re_erased),
                 var_infos.len(),
             ),
diff --git a/compiler/rustc_infer/src/infer/outlives/obligations.rs b/compiler/rustc_infer/src/infer/outlives/obligations.rs
index ccf11c61b..9c20c814b 100644
--- a/compiler/rustc_infer/src/infer/outlives/obligations.rs
+++ b/compiler/rustc_infer/src/infer/outlives/obligations.rs
@@ -256,7 +256,7 @@ where
                     // this point it never will be
                     self.tcx.sess.delay_span_bug(
                         origin.span(),
-                        &format!("unresolved inference variable in outlives: {:?}", v),
+                        format!("unresolved inference variable in outlives: {:?}", v),
                     );
                 }
             }
@@ -344,12 +344,14 @@ where
         // the problem is to add `T: 'r`, which isn't true. So, if there are no
         // inference variables, we use a verify constraint instead of adding
         // edges, which winds up enforcing the same condition.
+        let is_opaque = alias_ty.kind(self.tcx) == ty::Opaque;
         if approx_env_bounds.is_empty()
             && trait_bounds.is_empty()
-            && (alias_ty.needs_infer() || alias_ty.kind(self.tcx) == ty::Opaque)
+            && (alias_ty.has_infer() || is_opaque)
         {
             debug!("no declared bounds");
-            self.substs_must_outlive(alias_ty.substs, origin, region);
+            let opt_variances = is_opaque.then(|| self.tcx.variances_of(alias_ty.def_id));
+            self.substs_must_outlive(alias_ty.substs, origin, region, opt_variances);
             return;
         }
 
@@ -395,22 +397,31 @@ where
         self.delegate.push_verify(origin, GenericKind::Alias(alias_ty), region, verify_bound);
     }
 
+    #[instrument(level = "debug", skip(self))]
     fn substs_must_outlive(
         &mut self,
         substs: SubstsRef<'tcx>,
         origin: infer::SubregionOrigin<'tcx>,
         region: ty::Region<'tcx>,
+        opt_variances: Option<&[ty::Variance]>,
     ) {
         let constraint = origin.to_constraint_category();
-        for k in substs {
+        for (index, k) in substs.iter().enumerate() {
             match k.unpack() {
                 GenericArgKind::Lifetime(lt) => {
-                    self.delegate.push_sub_region_constraint(
-                        origin.clone(),
-                        region,
-                        lt,
-                        constraint,
-                    );
+                    let variance = if let Some(variances) = opt_variances {
+                        variances[index]
+                    } else {
+                        ty::Invariant
+                    };
+                    if variance == ty::Invariant {
+                        self.delegate.push_sub_region_constraint(
+                            origin.clone(),
+                            region,
+                            lt,
+                            constraint,
+                        );
+                    }
                 }
                 GenericArgKind::Type(ty) => {
                     self.type_must_outlive(origin.clone(), ty, region, constraint);
diff --git a/compiler/rustc_infer/src/infer/outlives/test_type_match.rs b/compiler/rustc_infer/src/infer/outlives/test_type_match.rs
index 01f900f05..cd2462d3c 100644
--- a/compiler/rustc_infer/src/infer/outlives/test_type_match.rs
+++ b/compiler/rustc_infer/src/infer/outlives/test_type_match.rs
@@ -13,9 +13,11 @@ use crate::infer::region_constraints::VerifyIfEq;
 
 /// Given a "verify-if-eq" type test like:
 ///
-///     exists<'a...> {
-///         verify_if_eq(some_type, bound_region)
-///     }
+/// ```rust,ignore (pseudo-Rust)
+/// exists<'a...> {
+///     verify_if_eq(some_type, bound_region)
+/// }
+/// ```
 ///
 /// and the type `test_ty` that the type test is being tested against,
 /// returns:
@@ -185,7 +187,7 @@ impl<'tcx> TypeRelation<'tcx> for Match<'tcx> {
         } else if pattern == value {
             Ok(pattern)
         } else {
-            relate::super_relate_tys(self, pattern, value)
+            relate::structurally_relate_tys(self, pattern, value)
         }
     }
 
@@ -199,7 +201,7 @@ impl<'tcx> TypeRelation<'tcx> for Match<'tcx> {
         if pattern == value {
             Ok(pattern)
         } else {
-            relate::super_relate_consts(self, pattern, value)
+            relate::structurally_relate_consts(self, pattern, value)
         }
     }
 
diff --git a/compiler/rustc_infer/src/infer/outlives/verify.rs b/compiler/rustc_infer/src/infer/outlives/verify.rs
index bae246418..c2bf0f3db 100644
--- a/compiler/rustc_infer/src/infer/outlives/verify.rs
+++ b/compiler/rustc_infer/src/infer/outlives/verify.rs
@@ -1,4 +1,4 @@
-use crate::infer::outlives::components::{compute_components_recursive, Component};
+use crate::infer::outlives::components::{compute_alias_components_recursive, Component};
 use crate::infer::outlives::env::RegionBoundPairs;
 use crate::infer::region_constraints::VerifyIfEq;
 use crate::infer::VerifyBound;
@@ -130,7 +130,12 @@ impl<'cx, 'tcx> VerifyBoundCx<'cx, 'tcx> {
         // see the extensive comment in projection_must_outlive
         let recursive_bound = {
             let mut components = smallvec![];
-            compute_components_recursive(self.tcx, alias_ty_as_ty.into(), &mut components, visited);
+            compute_alias_components_recursive(
+                self.tcx,
+                alias_ty_as_ty.into(),
+                &mut components,
+                visited,
+            );
             self.bound_from_components(&components, visited)
         };
 
@@ -174,7 +179,7 @@ impl<'cx, 'tcx> VerifyBoundCx<'cx, 'tcx> {
                 // this point it never will be
                 self.tcx.sess.delay_span_bug(
                     rustc_span::DUMMY_SP,
-                    &format!("unresolved inference variable in outlives: {:?}", v),
+                    format!("unresolved inference variable in outlives: {:?}", v),
                 );
                 // add a bound that never holds
                 VerifyBound::AnyBound(vec![])
@@ -272,7 +277,7 @@ impl<'cx, 'tcx> VerifyBoundCx<'cx, 'tcx> {
     ///
     /// It will not, however, work for higher-ranked bounds like:
     ///
-    /// ```compile_fail,E0311
+    /// ```ignore(this does compile today, previously was marked as `compile_fail,E0311`)
     /// trait Foo<'a, 'b>
     /// where for<'x> <Self as Foo<'x, 'b>>::Bar: 'x
     /// {
diff --git a/compiler/rustc_infer/src/infer/region_constraints/leak_check.rs b/compiler/rustc_infer/src/infer/region_constraints/leak_check.rs
index b8ba98fc0..89cfc9ea3 100644
--- a/compiler/rustc_infer/src/infer/region_constraints/leak_check.rs
+++ b/compiler/rustc_infer/src/infer/region_constraints/leak_check.rs
@@ -5,7 +5,7 @@ use rustc_data_structures::{
     graph::{scc::Sccs, vec_graph::VecGraph},
     undo_log::UndoLogs,
 };
-use rustc_index::vec::Idx;
+use rustc_index::Idx;
 use rustc_middle::ty::error::TypeError;
 use rustc_middle::ty::relate::RelateResult;
 
diff --git a/compiler/rustc_infer/src/infer/region_constraints/mod.rs b/compiler/rustc_infer/src/infer/region_constraints/mod.rs
index 7b272dfd2..c7a307b89 100644
--- a/compiler/rustc_infer/src/infer/region_constraints/mod.rs
+++ b/compiler/rustc_infer/src/infer/region_constraints/mod.rs
@@ -12,7 +12,7 @@ use rustc_data_structures::intern::Interned;
 use rustc_data_structures::sync::Lrc;
 use rustc_data_structures::undo_log::UndoLogs;
 use rustc_data_structures::unify as ut;
-use rustc_index::vec::IndexVec;
+use rustc_index::IndexVec;
 use rustc_middle::infer::unify_key::{RegionVidKey, UnifiedRegion};
 use rustc_middle::ty::ReStatic;
 use rustc_middle::ty::{self, Ty, TyCtxt};
@@ -217,7 +217,7 @@ pub enum VerifyBound<'tcx> {
 /// and supplies a bound if it ended up being relevant. It's used in situations
 /// like this:
 ///
-/// ```rust
+/// ```rust,ignore (pseudo-Rust)
 /// fn foo<'a, 'b, T: SomeTrait<'a>>
 /// where
 ///    <T as SomeTrait<'a>>::Item: 'b
@@ -232,7 +232,7 @@ pub enum VerifyBound<'tcx> {
 /// In the [`VerifyBound`], this struct is enclosed in `Binder` to account
 /// for cases like
 ///
-/// ```rust
+/// ```rust,ignore (pseudo-Rust)
 /// where for<'a> <T as SomeTrait<'a>::Item: 'a
 /// ```
 ///
diff --git a/compiler/rustc_infer/src/infer/resolve.rs b/compiler/rustc_infer/src/infer/resolve.rs
index 4f49f4165..3c41e8b37 100644
--- a/compiler/rustc_infer/src/infer/resolve.rs
+++ b/compiler/rustc_infer/src/infer/resolve.rs
@@ -213,7 +213,7 @@ impl<'a, 'tcx> FallibleTypeFolder<TyCtxt<'tcx>> for FullTypeResolver<'a, 'tcx> {
     }
 
     fn try_fold_ty(&mut self, t: Ty<'tcx>) -> Result<Ty<'tcx>, Self::Error> {
-        if !t.needs_infer() {
+        if !t.has_infer() {
             Ok(t) // micro-optimize -- if there is nothing in this type that this fold affects...
         } else {
             let t = self.infcx.shallow_resolve(t);
@@ -243,7 +243,7 @@ impl<'a, 'tcx> FallibleTypeFolder<TyCtxt<'tcx>> for FullTypeResolver<'a, 'tcx> {
     }
 
     fn try_fold_const(&mut self, c: ty::Const<'tcx>) -> Result<ty::Const<'tcx>, Self::Error> {
-        if !c.needs_infer() {
+        if !c.has_infer() {
             Ok(c) // micro-optimize -- if there is nothing in this const that this fold affects...
         } else {
             let c = self.infcx.shallow_resolve(c);
diff --git a/compiler/rustc_infer/src/infer/sub.rs b/compiler/rustc_infer/src/infer/sub.rs
index 0dd73a6e9..ceafafb55 100644
--- a/compiler/rustc_infer/src/infer/sub.rs
+++ b/compiler/rustc_infer/src/infer/sub.rs
@@ -1,4 +1,4 @@
-use super::combine::{CombineFields, RelationDir};
+use super::combine::CombineFields;
 use super::{DefineOpaqueTypes, ObligationEmittingRelation, SubregionOrigin};
 
 use crate::traits::{Obligation, PredicateObligations};
@@ -108,11 +108,11 @@ impl<'tcx> TypeRelation<'tcx> for Sub<'_, '_, 'tcx> {
                 Ok(a)
             }
             (&ty::Infer(TyVar(a_id)), _) => {
-                self.fields.instantiate(b, RelationDir::SupertypeOf, a_id, !self.a_is_expected)?;
+                self.fields.instantiate(b, ty::Contravariant, a_id, !self.a_is_expected)?;
                 Ok(a)
             }
             (_, &ty::Infer(TyVar(b_id))) => {
-                self.fields.instantiate(a, RelationDir::SubtypeOf, b_id, self.a_is_expected)?;
+                self.fields.instantiate(a, ty::Covariant, b_id, self.a_is_expected)?;
                 Ok(a)
             }
 
@@ -131,7 +131,8 @@ impl<'tcx> TypeRelation<'tcx> for Sub<'_, '_, 'tcx> {
             (&ty::Alias(ty::Opaque, ty::AliasTy { def_id, .. }), _)
             | (_, &ty::Alias(ty::Opaque, ty::AliasTy { def_id, .. }))
                 if self.fields.define_opaque_types == DefineOpaqueTypes::Yes
-                    && def_id.is_local() =>
+                    && def_id.is_local()
+                    && !self.tcx().trait_solver_next() =>
             {
                 self.fields.obligations.extend(
                     infcx
@@ -210,7 +211,7 @@ impl<'tcx> TypeRelation<'tcx> for Sub<'_, '_, 'tcx> {
     where
         T: Relate<'tcx>,
     {
-        // A binder is always a subtype of itself if it's structually equal to itself
+        // A binder is always a subtype of itself if it's structurally equal to itself
         if a == b {
             return Ok(a);
         }