Merging upstream version 1.70.0+dfsg1.

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

25 files changed, 1130 insertions, 834 deletions

diff --git a/compiler/rustc_trait_selection/src/traits/auto_trait.rs b/compiler/rustc_trait_selection/src/traits/auto_trait.rs
index 1fb8659bb..182d995c4 100644
--- a/compiler/rustc_trait_selection/src/traits/auto_trait.rs
+++ b/compiler/rustc_trait_selection/src/traits/auto_trait.rs
@@ -7,8 +7,8 @@ use crate::errors::UnableToConstructConstantValue;
 use crate::infer::region_constraints::{Constraint, RegionConstraintData};
 use crate::infer::InferCtxt;
 use crate::traits::project::ProjectAndUnifyResult;
+use rustc_infer::infer::DefineOpaqueTypes;
 use rustc_middle::mir::interpret::ErrorHandled;
-use rustc_middle::ty::fold::{TypeFolder, TypeSuperFoldable};
 use rustc_middle::ty::visit::TypeVisitableExt;
 use rustc_middle::ty::{ImplPolarity, Region, RegionVid};
 
@@ -179,13 +179,15 @@ impl<'tcx> AutoTraitFinder<'tcx> {
         // At this point, we already have all of the bounds we need. FulfillmentContext is used
         // to store all of the necessary region/lifetime bounds in the InferContext, as well as
         // an additional sanity check.
-        let errors =
-            super::fully_solve_bound(&infcx, ObligationCause::dummy(), full_env, ty, trait_did);
+        let ocx = ObligationCtxt::new(&infcx);
+        ocx.register_bound(ObligationCause::dummy(), full_env, ty, trait_did);
+        let errors = ocx.select_all_or_error();
         if !errors.is_empty() {
             panic!("Unable to fulfill trait {:?} for '{:?}': {:?}", trait_did, ty, errors);
         }
 
-        infcx.process_registered_region_obligations(&Default::default(), full_env);
+        let outlives_env = OutlivesEnvironment::new(full_env);
+        infcx.process_registered_region_obligations(&outlives_env);
 
         let region_data =
             infcx.inner.borrow_mut().unwrap_region_constraints().region_constraint_data().clone();
@@ -232,7 +234,7 @@ impl<'tcx> AutoTraitFinder<'tcx> {
     /// constructed once for a given type. As part of the construction process, the `ParamEnv` will
     /// have any supertrait bounds normalized -- e.g., if we have a type `struct Foo<T: Copy>`, the
     /// `ParamEnv` will contain `T: Copy` and `T: Clone`, since `Copy: Clone`. When we construct our
-    /// own `ParamEnv`, we need to do this ourselves, through `traits::elaborate_predicates`, or
+    /// own `ParamEnv`, we need to do this ourselves, through `traits::elaborate`, or
     /// else `SelectionContext` will choke on the missing predicates. However, this should never
     /// show up in the final synthesized generics: we don't want our generated docs page to contain
     /// something like `T: Copy + Clone`, as that's redundant. Therefore, we keep track of a
@@ -344,11 +346,8 @@ impl<'tcx> AutoTraitFinder<'tcx> {
                 _ => panic!("Unexpected error for '{:?}': {:?}", ty, result),
             };
 
-            let normalized_preds = elaborate_predicates(
-                tcx,
-                computed_preds.clone().chain(user_computed_preds.iter().cloned()),
-            )
-            .map(|o| o.predicate);
+            let normalized_preds =
+                elaborate(tcx, computed_preds.clone().chain(user_computed_preds.iter().cloned()));
             new_env = ty::ParamEnv::new(
                 tcx.mk_predicates_from_iter(normalized_preds),
                 param_env.reveal(),
@@ -814,7 +813,7 @@ impl<'tcx> AutoTraitFinder<'tcx> {
 
                     match (evaluate(c1), evaluate(c2)) {
                         (Ok(c1), Ok(c2)) => {
-                            match selcx.infcx.at(&obligation.cause, obligation.param_env).eq(c1, c2)
+                            match selcx.infcx.at(&obligation.cause, obligation.param_env).eq(DefineOpaqueTypes::No,c1, c2)
                             {
                                 Ok(_) => (),
                                 Err(_) => return false,
@@ -830,7 +829,7 @@ impl<'tcx> AutoTraitFinder<'tcx> {
                 // the `ParamEnv`.
                 ty::PredicateKind::WellFormed(..)
                 | ty::PredicateKind::Clause(ty::Clause::ConstArgHasType(..))
-                | ty::PredicateKind::AliasEq(..)
+                | ty::PredicateKind::AliasRelate(..)
                 | ty::PredicateKind::ObjectSafe(..)
                 | ty::PredicateKind::ClosureKind(..)
                 | ty::PredicateKind::Subtype(..)
@@ -852,23 +851,3 @@ impl<'tcx> AutoTraitFinder<'tcx> {
         infcx.freshen(p)
     }
 }
-
-/// Replaces all ReVars in a type with ty::Region's, using the provided map
-pub struct RegionReplacer<'a, 'tcx> {
-    vid_to_region: &'a FxHashMap<ty::RegionVid, ty::Region<'tcx>>,
-    tcx: TyCtxt<'tcx>,
-}
-
-impl<'a, 'tcx> TypeFolder<TyCtxt<'tcx>> for RegionReplacer<'a, 'tcx> {
-    fn interner(&self) -> TyCtxt<'tcx> {
-        self.tcx
-    }
-
-    fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
-        (match *r {
-            ty::ReVar(vid) => self.vid_to_region.get(&vid).cloned(),
-            _ => None,
-        })
-        .unwrap_or_else(|| r.super_fold_with(self))
-    }
-}
diff --git a/compiler/rustc_trait_selection/src/traits/chalk_fulfill.rs b/compiler/rustc_trait_selection/src/traits/chalk_fulfill.rs
index b42a49eb4..28967e1cc 100644
--- a/compiler/rustc_trait_selection/src/traits/chalk_fulfill.rs
+++ b/compiler/rustc_trait_selection/src/traits/chalk_fulfill.rs
@@ -40,13 +40,16 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
         self.obligations.insert(obligation);
     }
 
-    fn collect_remaining_errors(&mut self) -> Vec<FulfillmentError<'tcx>> {
+    fn collect_remaining_errors(
+        &mut self,
+        _infcx: &InferCtxt<'tcx>,
+    ) -> Vec<FulfillmentError<'tcx>> {
         // any remaining obligations are errors
         self.obligations
             .iter()
             .map(|obligation| FulfillmentError {
                 obligation: obligation.clone(),
-                code: FulfillmentErrorCode::CodeAmbiguity,
+                code: FulfillmentErrorCode::CodeAmbiguity { overflow: false },
                 // FIXME - does Chalk have a notation of 'root obligation'?
                 // This is just for diagnostics, so it's okay if this is wrong
                 root_obligation: obligation.clone(),
diff --git a/compiler/rustc_trait_selection/src/traits/coherence.rs b/compiler/rustc_trait_selection/src/traits/coherence.rs
index 6b688c322..20c2605f2 100644
--- a/compiler/rustc_trait_selection/src/traits/coherence.rs
+++ b/compiler/rustc_trait_selection/src/traits/coherence.rs
@@ -17,7 +17,7 @@ use crate::traits::{
 use rustc_data_structures::fx::FxIndexSet;
 use rustc_errors::Diagnostic;
 use rustc_hir::def_id::{DefId, CRATE_DEF_ID, LOCAL_CRATE};
-use rustc_infer::infer::{DefiningAnchor, InferCtxt, TyCtxtInferExt};
+use rustc_infer::infer::{DefineOpaqueTypes, DefiningAnchor, InferCtxt, TyCtxtInferExt};
 use rustc_infer::traits::util;
 use rustc_middle::traits::specialization_graph::OverlapMode;
 use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams};
@@ -75,12 +75,13 @@ pub fn overlapping_impls(
     // Before doing expensive operations like entering an inference context, do
     // a quick check via fast_reject to tell if the impl headers could possibly
     // unify.
-    let drcx = DeepRejectCtxt { treat_obligation_params: TreatParams::AsInfer };
+    let drcx = DeepRejectCtxt { treat_obligation_params: TreatParams::AsCandidateKey };
     let impl1_ref = tcx.impl_trait_ref(impl1_def_id);
     let impl2_ref = tcx.impl_trait_ref(impl2_def_id);
     let may_overlap = match (impl1_ref, impl2_ref) {
-        (Some(a), Some(b)) => iter::zip(a.skip_binder().substs, b.skip_binder().substs)
-            .all(|(arg1, arg2)| drcx.generic_args_may_unify(arg1, arg2)),
+        (Some(a), Some(b)) => {
+            drcx.substs_refs_may_unify(a.skip_binder().substs, b.skip_binder().substs)
+        }
         (None, None) => {
             let self_ty1 = tcx.type_of(impl1_def_id).skip_binder();
             let self_ty2 = tcx.type_of(impl2_def_id).skip_binder();
@@ -95,8 +96,11 @@ pub fn overlapping_impls(
         return None;
     }
 
-    let infcx =
-        tcx.infer_ctxt().with_opaque_type_inference(DefiningAnchor::Bubble).intercrate().build();
+    let infcx = tcx
+        .infer_ctxt()
+        .with_opaque_type_inference(DefiningAnchor::Bubble)
+        .intercrate(true)
+        .build();
     let selcx = &mut SelectionContext::new(&infcx);
     let overlaps =
         overlap(selcx, skip_leak_check, impl1_def_id, impl2_def_id, overlap_mode).is_some();
@@ -107,8 +111,11 @@ pub fn overlapping_impls(
     // In the case where we detect an error, run the check again, but
     // this time tracking intercrate ambiguity causes for better
     // diagnostics. (These take time and can lead to false errors.)
-    let infcx =
-        tcx.infer_ctxt().with_opaque_type_inference(DefiningAnchor::Bubble).intercrate().build();
+    let infcx = tcx
+        .infer_ctxt()
+        .with_opaque_type_inference(DefiningAnchor::Bubble)
+        .intercrate(true)
+        .build();
     let selcx = &mut SelectionContext::new(&infcx);
     selcx.enable_tracking_intercrate_ambiguity_causes();
     Some(overlap(selcx, skip_leak_check, impl1_def_id, impl2_def_id, overlap_mode).unwrap())
@@ -181,7 +188,7 @@ fn overlap_within_probe<'cx, 'tcx>(
     let impl1_header = with_fresh_ty_vars(selcx, param_env, impl1_def_id);
     let impl2_header = with_fresh_ty_vars(selcx, param_env, impl2_def_id);
 
-    let obligations = equate_impl_headers(selcx, &impl1_header, &impl2_header)?;
+    let obligations = equate_impl_headers(selcx.infcx, &impl1_header, &impl2_header)?;
     debug!("overlap: unification check succeeded");
 
     if overlap_mode.use_implicit_negative() {
@@ -207,20 +214,25 @@ fn overlap_within_probe<'cx, 'tcx>(
     Some(OverlapResult { impl_header, intercrate_ambiguity_causes, involves_placeholder })
 }
 
-fn equate_impl_headers<'cx, 'tcx>(
-    selcx: &mut SelectionContext<'cx, 'tcx>,
-    impl1_header: &ty::ImplHeader<'tcx>,
-    impl2_header: &ty::ImplHeader<'tcx>,
+#[instrument(level = "debug", skip(infcx), ret)]
+fn equate_impl_headers<'tcx>(
+    infcx: &InferCtxt<'tcx>,
+    impl1: &ty::ImplHeader<'tcx>,
+    impl2: &ty::ImplHeader<'tcx>,
 ) -> Option<PredicateObligations<'tcx>> {
-    // Do `a` and `b` unify? If not, no overlap.
-    debug!("equate_impl_headers(impl1_header={:?}, impl2_header={:?}", impl1_header, impl2_header);
-    selcx
-        .infcx
-        .at(&ObligationCause::dummy(), ty::ParamEnv::empty())
-        .define_opaque_types(true)
-        .eq_impl_headers(impl1_header, impl2_header)
-        .map(|infer_ok| infer_ok.obligations)
-        .ok()
+    let result = match (impl1.trait_ref, impl2.trait_ref) {
+        (Some(impl1_ref), Some(impl2_ref)) => infcx
+            .at(&ObligationCause::dummy(), ty::ParamEnv::empty())
+            .eq(DefineOpaqueTypes::Yes, impl1_ref, impl2_ref),
+        (None, None) => infcx.at(&ObligationCause::dummy(), ty::ParamEnv::empty()).eq(
+            DefineOpaqueTypes::Yes,
+            impl1.self_ty,
+            impl2.self_ty,
+        ),
+        _ => bug!("mk_eq_impl_headers given mismatched impl kinds"),
+    };
+
+    result.map(|infer_ok| infer_ok.obligations).ok()
 }
 
 /// Given impl1 and impl2 check if both impls can be satisfied by a common type (including
@@ -294,7 +306,7 @@ fn negative_impl(tcx: TyCtxt<'_>, impl1_def_id: DefId, impl2_def_id: DefId) -> b
         &infcx,
         ObligationCause::dummy(),
         impl_env,
-        tcx.impl_subject(impl1_def_id),
+        tcx.impl_subject(impl1_def_id).subst_identity(),
     ) {
         Ok(s) => s,
         Err(err) => {
@@ -325,7 +337,7 @@ fn equate<'tcx>(
 ) -> bool {
     // do the impls unify? If not, not disjoint.
     let Ok(InferOk { obligations: more_obligations, .. }) =
-        infcx.at(&ObligationCause::dummy(), impl_env).eq(subject1, subject2)
+        infcx.at(&ObligationCause::dummy(), impl_env).eq(DefineOpaqueTypes::No,subject1, subject2)
     else {
         debug!("explicit_disjoint: {:?} does not unify with {:?}", subject1, subject2);
         return true;
@@ -356,8 +368,8 @@ fn negative_impl_exists<'tcx>(
     }
 
     // Try to prove a negative obligation exists for super predicates
-    for o in util::elaborate_predicates(infcx.tcx, iter::once(o.predicate)) {
-        if resolve_negative_obligation(infcx.fork(), &o, body_def_id) {
+    for pred in util::elaborate(infcx.tcx, iter::once(o.predicate)) {
+        if resolve_negative_obligation(infcx.fork(), &o.with(infcx.tcx, pred), body_def_id) {
             return true;
         }
     }
@@ -378,7 +390,10 @@ fn resolve_negative_obligation<'tcx>(
     };
 
     let param_env = o.param_env;
-    if !super::fully_solve_obligation(&infcx, o).is_empty() {
+    let ocx = ObligationCtxt::new(&infcx);
+    ocx.register_obligation(o);
+    let errors = ocx.select_all_or_error();
+    if !errors.is_empty() {
         return false;
     }
 
@@ -388,20 +403,29 @@ fn resolve_negative_obligation<'tcx>(
     let wf_tys = ocx.assumed_wf_types(param_env, DUMMY_SP, body_def_id);
     let outlives_env = OutlivesEnvironment::with_bounds(
         param_env,
-        Some(&infcx),
         infcx.implied_bounds_tys(param_env, body_def_id, wf_tys),
     );
-
-    infcx.process_registered_region_obligations(outlives_env.region_bound_pairs(), param_env);
-
     infcx.resolve_regions(&outlives_env).is_empty()
 }
 
+/// Returns whether all impls which would apply to the `trait_ref`
+/// e.g. `Ty: Trait<Arg>` are already known in the local crate.
+///
+/// This both checks whether any downstream or sibling crates could
+/// implement it and whether an upstream crate can add this impl
+/// without breaking backwards compatibility.
 #[instrument(level = "debug", skip(tcx), ret)]
 pub fn trait_ref_is_knowable<'tcx>(
     tcx: TyCtxt<'tcx>,
     trait_ref: ty::TraitRef<'tcx>,
 ) -> Result<(), Conflict> {
+    if Some(trait_ref.def_id) == tcx.lang_items().fn_ptr_trait() {
+        // The only types implementing `FnPtr` are function pointers,
+        // so if there's no impl of `FnPtr` in the current crate,
+        // then such an impl will never be added in the future.
+        return Ok(());
+    }
+
     if orphan_check_trait_ref(trait_ref, InCrate::Remote).is_ok() {
         // A downstream or cousin crate is allowed to implement some
         // substitution of this trait-ref.
diff --git a/compiler/rustc_trait_selection/src/traits/engine.rs b/compiler/rustc_trait_selection/src/traits/engine.rs
index b20636174..2beebe94b 100644
--- a/compiler/rustc_trait_selection/src/traits/engine.rs
+++ b/compiler/rustc_trait_selection/src/traits/engine.rs
@@ -6,12 +6,14 @@ use super::{ChalkFulfillmentContext, FulfillmentContext};
 use crate::solve::FulfillmentCtxt as NextFulfillmentCtxt;
 use crate::traits::NormalizeExt;
 use rustc_data_structures::fx::FxIndexSet;
+use rustc_errors::ErrorGuaranteed;
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_infer::infer::at::ToTrace;
 use rustc_infer::infer::canonical::{
     Canonical, CanonicalQueryResponse, CanonicalVarValues, QueryResponse,
 };
-use rustc_infer::infer::{InferCtxt, InferOk};
+use rustc_infer::infer::outlives::env::OutlivesEnvironment;
+use rustc_infer::infer::{DefineOpaqueTypes, InferCtxt, InferOk};
 use rustc_infer::traits::query::Fallible;
 use rustc_infer::traits::{
     FulfillmentError, Obligation, ObligationCause, PredicateObligation, TraitEngineExt as _,
@@ -128,8 +130,7 @@ impl<'a, 'tcx> ObligationCtxt<'a, 'tcx> {
     {
         self.infcx
             .at(cause, param_env)
-            .define_opaque_types(true)
-            .eq_exp(a_is_expected, a, b)
+            .eq_exp(DefineOpaqueTypes::Yes, a_is_expected, a, b)
             .map(|infer_ok| self.register_infer_ok_obligations(infer_ok))
     }
 
@@ -142,8 +143,7 @@ impl<'a, 'tcx> ObligationCtxt<'a, 'tcx> {
     ) -> Result<(), TypeError<'tcx>> {
         self.infcx
             .at(cause, param_env)
-            .define_opaque_types(true)
-            .eq(expected, actual)
+            .eq(DefineOpaqueTypes::Yes, expected, actual)
             .map(|infer_ok| self.register_infer_ok_obligations(infer_ok))
     }
 
@@ -157,8 +157,7 @@ impl<'a, 'tcx> ObligationCtxt<'a, 'tcx> {
     ) -> Result<(), TypeError<'tcx>> {
         self.infcx
             .at(cause, param_env)
-            .define_opaque_types(true)
-            .sup(expected, actual)
+            .sub(DefineOpaqueTypes::Yes, expected, actual)
             .map(|infer_ok| self.register_infer_ok_obligations(infer_ok))
     }
 
@@ -172,19 +171,37 @@ impl<'a, 'tcx> ObligationCtxt<'a, 'tcx> {
     ) -> Result<(), TypeError<'tcx>> {
         self.infcx
             .at(cause, param_env)
-            .define_opaque_types(true)
-            .sup(expected, actual)
+            .sup(DefineOpaqueTypes::Yes, expected, actual)
             .map(|infer_ok| self.register_infer_ok_obligations(infer_ok))
     }
 
+    #[must_use]
     pub fn select_where_possible(&self) -> Vec<FulfillmentError<'tcx>> {
         self.engine.borrow_mut().select_where_possible(self.infcx)
     }
 
+    #[must_use]
     pub fn select_all_or_error(&self) -> Vec<FulfillmentError<'tcx>> {
         self.engine.borrow_mut().select_all_or_error(self.infcx)
     }
 
+    /// Resolves regions and reports errors.
+    ///
+    /// Takes ownership of the context as doing trait solving afterwards
+    /// will result in region constraints getting ignored.
+    pub fn resolve_regions_and_report_errors(
+        self,
+        generic_param_scope: LocalDefId,
+        outlives_env: &OutlivesEnvironment<'tcx>,
+    ) -> Result<(), ErrorGuaranteed> {
+        let errors = self.infcx.resolve_regions(&outlives_env);
+        if errors.is_empty() {
+            Ok(())
+        } else {
+            Err(self.infcx.err_ctxt().report_region_errors(generic_param_scope, &errors))
+        }
+    }
+
     pub fn assumed_wf_types(
         &self,
         param_env: ty::ParamEnv<'tcx>,
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/ambiguity.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/ambiguity.rs
index 84045c4d0..0475f24d8 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/ambiguity.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/ambiguity.rs
@@ -1,6 +1,6 @@
 use rustc_hir::def_id::DefId;
 use rustc_infer::infer::{InferCtxt, LateBoundRegionConversionTime};
-use rustc_infer::traits::util::elaborate_predicates_with_span;
+use rustc_infer::traits::util::elaborate;
 use rustc_infer::traits::{Obligation, ObligationCause, TraitObligation};
 use rustc_middle::ty;
 use rustc_span::{Span, DUMMY_SP};
@@ -82,15 +82,15 @@ pub fn recompute_applicable_impls<'tcx>(
 
     let predicates =
         tcx.predicates_of(obligation.cause.body_id.to_def_id()).instantiate_identity(tcx);
-    for obligation in elaborate_predicates_with_span(tcx, predicates.into_iter()) {
-        let kind = obligation.predicate.kind();
+    for (pred, span) in elaborate(tcx, predicates.into_iter()) {
+        let kind = pred.kind();
         if let ty::PredicateKind::Clause(ty::Clause::Trait(trait_pred)) = kind.skip_binder()
             && param_env_candidate_may_apply(kind.rebind(trait_pred))
         {
             if kind.rebind(trait_pred.trait_ref) == ty::TraitRef::identity(tcx, trait_pred.def_id()) {
                 ambiguities.push(Ambiguity::ParamEnv(tcx.def_span(trait_pred.def_id())))
             } else {
-                ambiguities.push(Ambiguity::ParamEnv(obligation.cause.span))
+                ambiguities.push(Ambiguity::ParamEnv(span))
             }
         }
     }
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/method_chain.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/method_chain.rs
index 1174efdbf..7e1dba4ed 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/method_chain.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/method_chain.rs
@@ -21,10 +21,6 @@ impl<'a, 'tcx> TypeRelation<'tcx> for CollectAllMismatches<'a, 'tcx> {
         self.infcx.tcx
     }
 
-    fn intercrate(&self) -> bool {
-        false
-    }
-
     fn param_env(&self) -> ty::ParamEnv<'tcx> {
         self.param_env
     }
@@ -33,10 +29,6 @@ impl<'a, 'tcx> TypeRelation<'tcx> for CollectAllMismatches<'a, 'tcx> {
         true
     }
 
-    fn mark_ambiguous(&mut self) {
-        bug!()
-    }
-
     fn relate_with_variance<T: Relate<'tcx>>(
         &mut self,
         _: ty::Variance,
@@ -92,6 +84,11 @@ impl<'a, 'tcx> TypeRelation<'tcx> for CollectAllMismatches<'a, 'tcx> {
 }
 
 impl<'tcx> ObligationEmittingRelation<'tcx> for CollectAllMismatches<'_, 'tcx> {
+    fn alias_relate_direction(&self) -> ty::AliasRelationDirection {
+        // FIXME(deferred_projection_equality): We really should get rid of this relation.
+        ty::AliasRelationDirection::Equate
+    }
+
     fn register_obligations(&mut self, _obligations: PredicateObligations<'tcx>) {
         // FIXME(deferred_projection_equality)
     }
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
index a844a1494..1b741b730 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
@@ -24,16 +24,15 @@ use rustc_errors::{
 };
 use rustc_hir as hir;
 use rustc_hir::def::Namespace;
-use rustc_hir::def_id::DefId;
+use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_hir::intravisit::Visitor;
-use rustc_hir::GenericParam;
-use rustc_hir::Item;
-use rustc_hir::Node;
+use rustc_hir::{GenericParam, Item, Node};
 use rustc_infer::infer::error_reporting::TypeErrCtxt;
 use rustc_infer::infer::{InferOk, TypeTrace};
 use rustc_middle::traits::select::OverflowError;
 use rustc_middle::ty::abstract_const::NotConstEvaluatable;
 use rustc_middle::ty::error::{ExpectedFound, TypeError};
+use rustc_middle::ty::fast_reject::TreatProjections;
 use rustc_middle::ty::fold::{TypeFolder, TypeSuperFoldable};
 use rustc_middle::ty::print::{with_forced_trimmed_paths, FmtPrinter, Print};
 use rustc_middle::ty::{
@@ -126,11 +125,9 @@ pub trait TypeErrCtxtExt<'tcx> {
             + Print<'tcx, FmtPrinter<'tcx, 'tcx>, Output = FmtPrinter<'tcx, 'tcx>>,
         <T as Print<'tcx, FmtPrinter<'tcx, 'tcx>>>::Error: std::fmt::Debug;
 
-    fn report_fulfillment_errors(
-        &self,
-        errors: &[FulfillmentError<'tcx>],
-        body_id: Option<hir::BodyId>,
-    ) -> ErrorGuaranteed;
+    fn report_overflow_no_abort(&self, obligation: PredicateObligation<'tcx>) -> ErrorGuaranteed;
+
+    fn report_fulfillment_errors(&self, errors: &[FulfillmentError<'tcx>]) -> ErrorGuaranteed;
 
     fn report_overflow_obligation<T>(
         &self,
@@ -388,11 +385,7 @@ impl<'tcx> InferCtxtExt<'tcx> for InferCtxt<'tcx> {
 }
 
 impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
-    fn report_fulfillment_errors(
-        &self,
-        errors: &[FulfillmentError<'tcx>],
-        body_id: Option<hir::BodyId>,
-    ) -> ErrorGuaranteed {
+    fn report_fulfillment_errors(&self, errors: &[FulfillmentError<'tcx>]) -> ErrorGuaranteed {
         #[derive(Debug)]
         struct ErrorDescriptor<'tcx> {
             predicate: ty::Predicate<'tcx>,
@@ -469,7 +462,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
         for from_expansion in [false, true] {
             for (error, suppressed) in iter::zip(errors, &is_suppressed) {
                 if !suppressed && error.obligation.cause.span.from_expansion() == from_expansion {
-                    self.report_fulfillment_error(error, body_id);
+                    self.report_fulfillment_error(error);
                 }
             }
         }
@@ -611,6 +604,14 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
         );
     }
 
+    fn report_overflow_no_abort(&self, obligation: PredicateObligation<'tcx>) -> ErrorGuaranteed {
+        let obligation = self.resolve_vars_if_possible(obligation);
+        let mut err = self.build_overflow_error(&obligation.predicate, obligation.cause.span, true);
+        self.note_obligation_cause(&mut err, &obligation);
+        self.point_at_returns_when_relevant(&mut err, &obligation);
+        err.emit()
+    }
+
     fn report_selection_error(
         &self,
         mut obligation: PredicateObligation<'tcx>,
@@ -672,6 +673,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                             return;
                         }
                         let trait_ref = trait_predicate.to_poly_trait_ref();
+
                         let (post_message, pre_message, type_def) = self
                             .get_parent_trait_ref(obligation.cause.code())
                             .map(|(t, s)| {
@@ -711,33 +713,45 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                             (message, note, append_const_msg)
                         };
 
-                        let mut err = struct_span_err!(
-                            self.tcx.sess,
-                            span,
-                            E0277,
-                            "{}",
-                            message
-                                .and_then(|cannot_do_this| {
-                                    match (predicate_is_const, append_const_msg) {
-                                        // do nothing if predicate is not const
-                                        (false, _) => Some(cannot_do_this),
-                                        // suggested using default post message
-                                        (true, Some(None)) => {
-                                            Some(format!("{cannot_do_this} in const contexts"))
-                                        }
-                                        // overridden post message
-                                        (true, Some(Some(post_message))) => {
-                                            Some(format!("{cannot_do_this}{post_message}"))
-                                        }
-                                        // fallback to generic message
-                                        (true, None) => None,
+                        let err_msg = message
+                            .and_then(|cannot_do_this| {
+                                match (predicate_is_const, append_const_msg) {
+                                    // do nothing if predicate is not const
+                                    (false, _) => Some(cannot_do_this),
+                                    // suggested using default post message
+                                    (true, Some(None)) => {
+                                        Some(format!("{cannot_do_this} in const contexts"))
                                     }
-                                })
-                                .unwrap_or_else(|| format!(
+                                    // overridden post message
+                                    (true, Some(Some(post_message))) => {
+                                        Some(format!("{cannot_do_this}{post_message}"))
+                                    }
+                                    // fallback to generic message
+                                    (true, None) => None,
+                                }
+                            })
+                            .unwrap_or_else(|| {
+                                format!(
                                     "the trait bound `{}` is not satisfied{}",
                                     trait_predicate, post_message,
-                                ))
-                        );
+                                )
+                            });
+
+                        let (err_msg, safe_transmute_explanation) = if Some(trait_ref.def_id())
+                            == self.tcx.lang_items().transmute_trait()
+                        {
+                            // Recompute the safe transmute reason and use that for the error reporting
+                            self.get_safe_transmute_error_and_reason(
+                                trait_predicate,
+                                obligation.clone(),
+                                trait_ref,
+                                span,
+                            )
+                        } else {
+                            (err_msg, None)
+                        };
+
+                        let mut err = struct_span_err!(self.tcx.sess, span, E0277, "{}", err_msg);
 
                         if is_try_conversion && let Some(ret_span) = self.return_type_span(&obligation) {
                             err.span_label(
@@ -827,6 +841,8 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                                 // at the type param with a label to suggest constraining it.
                                 err.help(&explanation);
                             }
+                        } else if let Some(custom_explanation) = safe_transmute_explanation {
+                            err.span_label(span, custom_explanation);
                         } else {
                             err.span_label(span, explanation);
                         }
@@ -955,8 +971,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                             );
                         }
 
-                        let body_hir_id =
-                            self.tcx.hir().local_def_id_to_hir_id(obligation.cause.body_id);
+                        let body_def_id = obligation.cause.body_id;
                         // Try to report a help message
                         if is_fn_trait
                             && let Ok((implemented_kind, params)) = self.type_implements_fn_trait(
@@ -1035,9 +1050,9 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                             // Can't show anything else useful, try to find similar impls.
                             let impl_candidates = self.find_similar_impl_candidates(trait_predicate);
                             if !self.report_similar_impl_candidates(
-                                impl_candidates,
+                                &impl_candidates,
                                 trait_ref,
-                                body_hir_id,
+                                body_def_id,
                                 &mut err,
                                 true,
                             ) {
@@ -1071,14 +1086,21 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                                     let impl_candidates =
                                         self.find_similar_impl_candidates(trait_pred);
                                     self.report_similar_impl_candidates(
-                                        impl_candidates,
+                                        &impl_candidates,
                                         trait_ref,
-                                        body_hir_id,
+                                        body_def_id,
                                         &mut err,
                                         true,
                                     );
                                 }
                             }
+
+                            self.maybe_suggest_convert_to_slice(
+                                &mut err,
+                                trait_ref,
+                                impl_candidates.as_slice(),
+                                span,
+                            );
                         }
 
                         // Changing mutability doesn't make a difference to whether we have
@@ -1279,16 +1301,26 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                         "TypeWellFormedFromEnv predicate should only exist in the environment"
                     ),
 
-                    ty::PredicateKind::AliasEq(..) => span_bug!(
+                    ty::PredicateKind::AliasRelate(..) => span_bug!(
                         span,
-                        "AliasEq predicate should never be the predicate cause of a SelectionError"
+                        "AliasRelate predicate should never be the predicate cause of a SelectionError"
                     ),
 
                     ty::PredicateKind::Clause(ty::Clause::ConstArgHasType(ct, ty)) => {
-                        self.tcx.sess.struct_span_err(
+                        let mut diag = self.tcx.sess.struct_span_err(
                             span,
                             &format!("the constant `{}` is not of type `{}`", ct, ty),
-                        )
+                        );
+                        self.note_type_err(
+                            &mut diag,
+                            &obligation.cause,
+                            None,
+                            None,
+                            TypeError::Sorts(ty::error::ExpectedFound::new(true, ty, ct.ty())),
+                            false,
+                            false,
+                        );
+                        diag
                     }
                 }
             }
@@ -1494,11 +1526,7 @@ trait InferCtxtPrivExt<'tcx> {
     // `error` occurring implies that `cond` occurs.
     fn error_implies(&self, cond: ty::Predicate<'tcx>, error: ty::Predicate<'tcx>) -> bool;
 
-    fn report_fulfillment_error(
-        &self,
-        error: &FulfillmentError<'tcx>,
-        body_id: Option<hir::BodyId>,
-    );
+    fn report_fulfillment_error(&self, error: &FulfillmentError<'tcx>);
 
     fn report_projection_error(
         &self,
@@ -1529,9 +1557,9 @@ trait InferCtxtPrivExt<'tcx> {
 
     fn report_similar_impl_candidates(
         &self,
-        impl_candidates: Vec<ImplCandidate<'tcx>>,
+        impl_candidates: &[ImplCandidate<'tcx>],
         trait_ref: ty::PolyTraitRef<'tcx>,
-        body_id: hir::HirId,
+        body_def_id: LocalDefId,
         err: &mut Diagnostic,
         other: bool,
     ) -> bool;
@@ -1561,11 +1589,7 @@ trait InferCtxtPrivExt<'tcx> {
         trait_ref_and_ty: ty::Binder<'tcx, (ty::TraitPredicate<'tcx>, Ty<'tcx>)>,
     ) -> PredicateObligation<'tcx>;
 
-    fn maybe_report_ambiguity(
-        &self,
-        obligation: &PredicateObligation<'tcx>,
-        body_id: Option<hir::BodyId>,
-    );
+    fn maybe_report_ambiguity(&self, obligation: &PredicateObligation<'tcx>);
 
     fn predicate_can_apply(
         &self,
@@ -1602,6 +1626,14 @@ trait InferCtxtPrivExt<'tcx> {
         obligated_types: &mut Vec<Ty<'tcx>>,
         cause_code: &ObligationCauseCode<'tcx>,
     ) -> bool;
+
+    fn get_safe_transmute_error_and_reason(
+        &self,
+        trait_predicate: ty::Binder<'tcx, ty::TraitPredicate<'tcx>>,
+        obligation: Obligation<'tcx, ty::Predicate<'tcx>>,
+        trait_ref: ty::Binder<'tcx, ty::TraitRef<'tcx>>,
+        span: Span,
+    ) -> (String, Option<String>);
 }
 
 impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
@@ -1625,8 +1657,8 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
             }
         };
 
-        for obligation in super::elaborate_predicates(self.tcx, std::iter::once(cond)) {
-            let bound_predicate = obligation.predicate.kind();
+        for pred in super::elaborate(self.tcx, std::iter::once(cond)) {
+            let bound_predicate = pred.kind();
             if let ty::PredicateKind::Clause(ty::Clause::Trait(implication)) =
                 bound_predicate.skip_binder()
             {
@@ -1647,11 +1679,7 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
     }
 
     #[instrument(skip(self), level = "debug")]
-    fn report_fulfillment_error(
-        &self,
-        error: &FulfillmentError<'tcx>,
-        body_id: Option<hir::BodyId>,
-    ) {
+    fn report_fulfillment_error(&self, error: &FulfillmentError<'tcx>) {
         match error.code {
             FulfillmentErrorCode::CodeSelectionError(ref selection_error) => {
                 self.report_selection_error(
@@ -1663,8 +1691,11 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
             FulfillmentErrorCode::CodeProjectionError(ref e) => {
                 self.report_projection_error(&error.obligation, e);
             }
-            FulfillmentErrorCode::CodeAmbiguity => {
-                self.maybe_report_ambiguity(&error.obligation, body_id);
+            FulfillmentErrorCode::CodeAmbiguity { overflow: false } => {
+                self.maybe_report_ambiguity(&error.obligation);
+            }
+            FulfillmentErrorCode::CodeAmbiguity { overflow: true } => {
+                self.report_overflow_no_abort(error.obligation.clone());
             }
             FulfillmentErrorCode::CodeSubtypeError(ref expected_found, ref err) => {
                 self.report_mismatched_types(
@@ -1768,7 +1799,10 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
 
                 // constrain inference variables a bit more to nested obligations from normalize so
                 // we can have more helpful errors.
-                ocx.select_where_possible();
+                //
+                // we intentionally drop errors from normalization here,
+                // since the normalization is just done to improve the error message.
+                let _ = ocx.select_where_possible();
 
                 if let Err(new_err) = ocx.eq_exp(
                     &obligation.cause,
@@ -1815,12 +1849,17 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                     })
                     .and_then(|(trait_assoc_item, id)| {
                         let trait_assoc_ident = trait_assoc_item.ident(self.tcx);
-                        self.tcx.find_map_relevant_impl(id, proj.projection_ty.self_ty(), |did| {
-                            self.tcx
-                                .associated_items(did)
-                                .in_definition_order()
-                                .find(|assoc| assoc.ident(self.tcx) == trait_assoc_ident)
-                        })
+                        self.tcx.find_map_relevant_impl(
+                            id,
+                            proj.projection_ty.self_ty(),
+                            TreatProjections::ForLookup,
+                            |did| {
+                                self.tcx
+                                    .associated_items(did)
+                                    .in_definition_order()
+                                    .find(|assoc| assoc.ident(self.tcx) == trait_assoc_ident)
+                            },
+                        )
                     })
                     .and_then(|item| match self.tcx.hir().get_if_local(item.def_id) {
                         Some(
@@ -2027,9 +2066,9 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
 
     fn report_similar_impl_candidates(
         &self,
-        impl_candidates: Vec<ImplCandidate<'tcx>>,
+        impl_candidates: &[ImplCandidate<'tcx>],
         trait_ref: ty::PolyTraitRef<'tcx>,
-        body_id: hir::HirId,
+        body_def_id: LocalDefId,
         err: &mut Diagnostic,
         other: bool,
     ) -> bool {
@@ -2120,9 +2159,7 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                         // FIXME(compiler-errors): This could be generalized, both to
                         // be more granular, and probably look past other `#[fundamental]`
                         // types, too.
-                        self.tcx
-                            .visibility(def.did())
-                            .is_accessible_from(body_id.owner.def_id, self.tcx)
+                        self.tcx.visibility(def.did()).is_accessible_from(body_def_id, self.tcx)
                     } else {
                         true
                     }
@@ -2138,7 +2175,8 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
         // Prefer more similar candidates first, then sort lexicographically
         // by their normalized string representation.
         let mut normalized_impl_candidates_and_similarities = impl_candidates
-            .into_iter()
+            .iter()
+            .copied()
             .map(|ImplCandidate { trait_ref, similarity }| {
                 // FIXME(compiler-errors): This should be using `NormalizeExt::normalize`
                 let normalized = self
@@ -2193,7 +2231,12 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
         trait_ref: &ty::PolyTraitRef<'tcx>,
     ) -> bool {
         let get_trait_impl = |trait_def_id| {
-            self.tcx.find_map_relevant_impl(trait_def_id, trait_ref.skip_binder().self_ty(), Some)
+            self.tcx.find_map_relevant_impl(
+                trait_def_id,
+                trait_ref.skip_binder().self_ty(),
+                TreatProjections::ForLookup,
+                Some,
+            )
         };
         let required_trait_path = self.tcx.def_path_str(trait_ref.def_id());
         let traits_with_same_path: std::collections::BTreeSet<_> = self
@@ -2231,11 +2274,7 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
     }
 
     #[instrument(skip(self), level = "debug")]
-    fn maybe_report_ambiguity(
-        &self,
-        obligation: &PredicateObligation<'tcx>,
-        body_id: Option<hir::BodyId>,
-    ) {
+    fn maybe_report_ambiguity(&self, obligation: &PredicateObligation<'tcx>) {
         // Unable to successfully determine, probably means
         // insufficient type information, but could mean
         // ambiguous impls. The latter *ought* to be a
@@ -2277,7 +2316,7 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                 if self.tcx.lang_items().sized_trait() == Some(trait_ref.def_id()) {
                     if let None = self.tainted_by_errors() {
                         self.emit_inference_failure_err(
-                            body_id,
+                            obligation.cause.body_id,
                             span,
                             trait_ref.self_ty().skip_binder().into(),
                             ErrorCode::E0282,
@@ -2304,7 +2343,13 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                 let subst = data.trait_ref.substs.iter().find(|s| s.has_non_region_infer());
 
                 let mut err = if let Some(subst) = subst {
-                    self.emit_inference_failure_err(body_id, span, subst, ErrorCode::E0283, true)
+                    self.emit_inference_failure_err(
+                        obligation.cause.body_id,
+                        span,
+                        subst,
+                        ErrorCode::E0283,
+                        true,
+                    )
                 } else {
                     struct_span_err!(
                         self.tcx.sess,
@@ -2348,12 +2393,10 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                                 predicate.to_opt_poly_trait_pred().unwrap(),
                             );
                             if impl_candidates.len() < 10 {
-                                let hir =
-                                    self.tcx.hir().local_def_id_to_hir_id(obligation.cause.body_id);
                                 self.report_similar_impl_candidates(
-                                    impl_candidates,
+                                    impl_candidates.as_slice(),
                                     trait_ref,
-                                    body_id.map(|id| id.hir_id).unwrap_or(hir),
+                                    obligation.cause.body_id,
                                     &mut err,
                                     false,
                                 );
@@ -2375,8 +2418,8 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                     self.suggest_fully_qualified_path(&mut err, def_id, span, trait_ref.def_id());
                 }
 
-                if let (Some(body_id), Some(ty::subst::GenericArgKind::Type(_))) =
-                    (body_id, subst.map(|subst| subst.unpack()))
+                if let Some(ty::subst::GenericArgKind::Type(_)) = subst.map(|subst| subst.unpack())
+                    && let Some(body_id) = self.tcx.hir().maybe_body_owned_by(obligation.cause.body_id)
                 {
                     let mut expr_finder = FindExprBySpan::new(span);
                     expr_finder.visit_expr(&self.tcx.hir().body(body_id).value);
@@ -2473,7 +2516,13 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                     return;
                 }
 
-                self.emit_inference_failure_err(body_id, span, arg, ErrorCode::E0282, false)
+                self.emit_inference_failure_err(
+                    obligation.cause.body_id,
+                    span,
+                    arg,
+                    ErrorCode::E0282,
+                    false,
+                )
             }
 
             ty::PredicateKind::Subtype(data) => {
@@ -2487,7 +2536,13 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                 let SubtypePredicate { a_is_expected: _, a, b } = data;
                 // both must be type variables, or the other would've been instantiated
                 assert!(a.is_ty_var() && b.is_ty_var());
-                self.emit_inference_failure_err(body_id, span, a.into(), ErrorCode::E0282, true)
+                self.emit_inference_failure_err(
+                    obligation.cause.body_id,
+                    span,
+                    a.into(),
+                    ErrorCode::E0282,
+                    true,
+                )
             }
             ty::PredicateKind::Clause(ty::Clause::Projection(data)) => {
                 if predicate.references_error() || self.tainted_by_errors().is_some() {
@@ -2501,7 +2556,7 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                     .find(|g| g.has_non_region_infer());
                 if let Some(subst) = subst {
                     let mut err = self.emit_inference_failure_err(
-                        body_id,
+                        obligation.cause.body_id,
                         span,
                         subst,
                         ErrorCode::E0284,
@@ -2530,7 +2585,7 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                 let subst = data.walk().find(|g| g.is_non_region_infer());
                 if let Some(subst) = subst {
                     let err = self.emit_inference_failure_err(
-                        body_id,
+                        obligation.cause.body_id,
                         span,
                         subst,
                         ErrorCode::E0284,
@@ -2863,6 +2918,63 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
         }
         false
     }
+
+    fn get_safe_transmute_error_and_reason(
+        &self,
+        trait_predicate: ty::Binder<'tcx, ty::TraitPredicate<'tcx>>,
+        obligation: Obligation<'tcx, ty::Predicate<'tcx>>,
+        trait_ref: ty::Binder<'tcx, ty::TraitRef<'tcx>>,
+        span: Span,
+    ) -> (String, Option<String>) {
+        let src_and_dst = trait_predicate.map_bound(|p| rustc_transmute::Types {
+            dst: p.trait_ref.substs.type_at(0),
+            src: p.trait_ref.substs.type_at(1),
+        });
+        let scope = trait_ref.skip_binder().substs.type_at(2);
+        let Some(assume) =
+            rustc_transmute::Assume::from_const(self.infcx.tcx, obligation.param_env, trait_ref.skip_binder().substs.const_at(3)) else {
+                span_bug!(span, "Unable to construct rustc_transmute::Assume where it was previously possible");
+            };
+        match rustc_transmute::TransmuteTypeEnv::new(self.infcx).is_transmutable(
+            obligation.cause,
+            src_and_dst,
+            scope,
+            assume,
+        ) {
+            rustc_transmute::Answer::No(reason) => {
+                let dst = trait_ref.skip_binder().substs.type_at(0);
+                let src = trait_ref.skip_binder().substs.type_at(1);
+                let custom_err_msg = format!("`{src}` cannot be safely transmuted into `{dst}` in the defining scope of `{scope}`").to_string();
+                let reason_msg = match reason {
+                    rustc_transmute::Reason::SrcIsUnspecified => {
+                        format!("`{src}` does not have a well-specified layout").to_string()
+                    }
+                    rustc_transmute::Reason::DstIsUnspecified => {
+                        format!("`{dst}` does not have a well-specified layout").to_string()
+                    }
+                    rustc_transmute::Reason::DstIsBitIncompatible => {
+                        format!("At least one value of `{src}` isn't a bit-valid value of `{dst}`")
+                            .to_string()
+                    }
+                    rustc_transmute::Reason::DstIsPrivate => format!(
+                        "`{dst}` is or contains a type or field that is not visible in that scope"
+                    )
+                    .to_string(),
+                    // FIXME(bryangarza): Include the number of bytes of src and dst
+                    rustc_transmute::Reason::DstIsTooBig => {
+                        format!("The size of `{src}` is smaller than the size of `{dst}`")
+                    }
+                };
+                (custom_err_msg, Some(reason_msg))
+            }
+            // Should never get a Yes at this point! We already ran it before, and did not get a Yes.
+            rustc_transmute::Answer::Yes => span_bug!(
+                span,
+                "Inconsistent rustc_transmute::is_transmutable(...) result, got Yes",
+            ),
+            _ => span_bug!(span, "Unsupported rustc_transmute::Reason variant"),
+        }
+    }
 }
 
 /// Crude way of getting back an `Expr` from a `Span`.
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/on_unimplemented.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/on_unimplemented.rs
index b3bf9ad59..a9c4e1268 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/on_unimplemented.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/on_unimplemented.rs
@@ -149,9 +149,17 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
             .unwrap_or_else(|| (trait_ref.def_id(), trait_ref.skip_binder().substs));
         let trait_ref = trait_ref.skip_binder();
 
-        let body_hir = self.tcx.hir().local_def_id_to_hir_id(obligation.cause.body_id);
-        let mut flags =
-            vec![(sym::ItemContext, self.describe_enclosure(body_hir).map(|s| s.to_owned()))];
+        let mut flags = vec![];
+        // FIXME(-Zlower-impl-trait-in-trait-to-assoc-ty): HIR is not present for RPITITs,
+        // but I guess we could synthesize one here. We don't see any errors that rely on
+        // that yet, though.
+        let enclosure =
+            if let Some(body_hir) = self.tcx.opt_local_def_id_to_hir_id(obligation.cause.body_id) {
+                self.describe_enclosure(body_hir).map(|s| s.to_owned())
+            } else {
+                None
+            };
+        flags.push((sym::ItemContext, enclosure));
 
         match obligation.cause.code() {
             ObligationCauseCode::BuiltinDerivedObligation(..)
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
index 66d74fd05..fb75ec767 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs
@@ -1,7 +1,7 @@
 // ignore-tidy-filelength
 
 use super::{
-    DefIdOrName, FindExprBySpan, Obligation, ObligationCause, ObligationCauseCode,
+    DefIdOrName, FindExprBySpan, ImplCandidate, Obligation, ObligationCause, ObligationCauseCode,
     PredicateObligation,
 };
 
@@ -30,7 +30,7 @@ use rustc_middle::hir::map;
 use rustc_middle::ty::error::TypeError::{self, Sorts};
 use rustc_middle::ty::relate::TypeRelation;
 use rustc_middle::ty::{
-    self, suggest_arbitrary_trait_bound, suggest_constraining_type_param, AdtKind, DefIdTree,
+    self, suggest_arbitrary_trait_bound, suggest_constraining_type_param, AdtKind,
     GeneratorDiagnosticData, GeneratorInteriorTypeCause, Infer, InferTy, InternalSubsts,
     IsSuggestable, ToPredicate, Ty, TyCtxt, TypeAndMut, TypeFoldable, TypeFolder,
     TypeSuperFoldable, TypeVisitableExt, TypeckResults,
@@ -212,7 +212,7 @@ pub trait TypeErrCtxtExt<'tcx> {
 
     fn extract_callable_info(
         &self,
-        hir_id: HirId,
+        body_id: LocalDefId,
         param_env: ty::ParamEnv<'tcx>,
         found: Ty<'tcx>,
     ) -> Option<(DefIdOrName, Ty<'tcx>, Vec<Ty<'tcx>>)>;
@@ -382,6 +382,14 @@ pub trait TypeErrCtxtExt<'tcx> {
         body_id: hir::HirId,
         param_env: ty::ParamEnv<'tcx>,
     ) -> Vec<Option<(Span, (DefId, Ty<'tcx>))>>;
+
+    fn maybe_suggest_convert_to_slice(
+        &self,
+        err: &mut Diagnostic,
+        trait_ref: ty::Binder<'tcx, ty::TraitRef<'tcx>>,
+        candidate_impls: &[ImplCandidate<'tcx>],
+        span: Span,
+    );
 }
 
 fn predicate_constraint(generics: &hir::Generics<'_>, pred: ty::Predicate<'_>) -> (Span, String) {
@@ -412,6 +420,7 @@ fn suggest_restriction<'tcx>(
 ) {
     if hir_generics.where_clause_span.from_expansion()
         || hir_generics.where_clause_span.desugaring_kind().is_some()
+        || projection.map_or(false, |projection| tcx.opt_rpitit_info(projection.def_id).is_some())
     {
         return;
     }
@@ -900,9 +909,8 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
             trait_pred.self_ty(),
         );
 
-        let body_hir_id = self.tcx.hir().local_def_id_to_hir_id(obligation.cause.body_id);
         let Some((def_id_or_name, output, inputs)) = self.extract_callable_info(
-            body_hir_id,
+            obligation.cause.body_id,
             obligation.param_env,
             self_ty,
         ) else { return false; };
@@ -1104,10 +1112,9 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
     /// Extracts information about a callable type for diagnostics. This is a
     /// heuristic -- it doesn't necessarily mean that a type is always callable,
     /// because the callable type must also be well-formed to be called.
-    // FIXME(vincenzopalazzo): move the HirId to a LocalDefId
     fn extract_callable_info(
         &self,
-        hir_id: HirId,
+        body_id: LocalDefId,
         param_env: ty::ParamEnv<'tcx>,
         found: Ty<'tcx>,
     ) -> Option<(DefIdOrName, Ty<'tcx>, Vec<Ty<'tcx>>)> {
@@ -1159,7 +1166,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                     })
                 }
                 ty::Param(param) => {
-                    let generics = self.tcx.generics_of(hir_id.owner.to_def_id());
+                    let generics = self.tcx.generics_of(body_id);
                     let name = if generics.count() > param.index as usize
                         && let def = generics.param_at(param.index as usize, self.tcx)
                         && matches!(def.kind, ty::GenericParamDefKind::Type { .. })
@@ -1349,6 +1356,31 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                             Applicability::MaybeIncorrect,
                         );
                     } else {
+                        let is_mut = mut_ref_self_ty_satisfies_pred || ref_inner_ty_mut;
+                        let sugg_prefix = format!("&{}", if is_mut { "mut " } else { "" });
+                        let sugg_msg = &format!(
+                            "consider{} borrowing here",
+                            if is_mut { " mutably" } else { "" }
+                        );
+
+                        // Issue #109436, we need to add parentheses properly for method calls
+                        // for example, `foo.into()` should be `(&foo).into()`
+                        if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(
+                            self.tcx.sess.source_map().span_look_ahead(span, Some("."), Some(50)),
+                        ) {
+                            if snippet == "." {
+                                err.multipart_suggestion_verbose(
+                                    sugg_msg,
+                                    vec![
+                                        (span.shrink_to_lo(), format!("({}", sugg_prefix)),
+                                        (span.shrink_to_hi(), ")".to_string()),
+                                    ],
+                                    Applicability::MaybeIncorrect,
+                                );
+                                return true;
+                            }
+                        }
+
                         // Issue #104961, we need to add parentheses properly for compond expressions
                         // for example, `x.starts_with("hi".to_string() + "you")`
                         // should be `x.starts_with(&("hi".to_string() + "you"))`
@@ -1365,14 +1397,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                             _ => false,
                         };
 
-                        let is_mut = mut_ref_self_ty_satisfies_pred || ref_inner_ty_mut;
                         let span = if needs_parens { span } else { span.shrink_to_lo() };
-                        let sugg_prefix = format!("&{}", if is_mut { "mut " } else { "" });
-                        let sugg_msg = &format!(
-                            "consider{} borrowing here",
-                            if is_mut { " mutably" } else { "" }
-                        );
-
                         let suggestions = if !needs_parens {
                             vec![(span.shrink_to_lo(), format!("{}", sugg_prefix))]
                         } else {
@@ -2220,7 +2245,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
         // - `BuiltinDerivedObligation` with a generator witness (A)
         // - `BuiltinDerivedObligation` with a generator (A)
         // - `BuiltinDerivedObligation` with `impl std::future::Future` (A)
-        // - `BindingObligation` with `impl_send (Send requirement)
+        // - `BindingObligation` with `impl_send` (Send requirement)
         //
         // The first obligation in the chain is the most useful and has the generator that captured
         // the type. The last generator (`outer_generator` below) has information about where the
@@ -2936,9 +2961,6 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
             ObligationCauseCode::SizedYieldType => {
                 err.note("the yield type of a generator must have a statically known size");
             }
-            ObligationCauseCode::SizedBoxType => {
-                err.note("the type of a box expression must have a statically known size");
-            }
             ObligationCauseCode::AssignmentLhsSized => {
                 err.note("the left-hand-side of an assignment must have a statically known size");
             }
@@ -3025,8 +3047,6 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                     }
                 };
 
-                let identity_future = tcx.require_lang_item(LangItem::IdentityFuture, None);
-
                 // Don't print the tuple of capture types
                 'print: {
                     if !is_upvar_tys_infer_tuple {
@@ -3039,12 +3059,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                                 None => err.note(&msg),
                             },
                             ty::Alias(ty::Opaque, ty::AliasTy { def_id, .. }) => {
-                                // Avoid printing the future from `core::future::identity_future`, it's not helpful
-                                if tcx.parent(*def_id) == identity_future {
-                                    break 'print;
-                                }
-
-                                // If the previous type is `identity_future`, this is the future generated by the body of an async function.
+                                // If the previous type is async fn, this is the future generated by the body of an async function.
                                 // Avoid printing it twice (it was already printed in the `ty::Generator` arm below).
                                 let is_future = tcx.ty_is_opaque_future(ty);
                                 debug!(
@@ -3826,6 +3841,72 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
         }
         assocs_in_this_method
     }
+
+    /// If the type that failed selection is an array or a reference to an array,
+    /// but the trait is implemented for slices, suggest that the user converts
+    /// the array into a slice.
+    fn maybe_suggest_convert_to_slice(
+        &self,
+        err: &mut Diagnostic,
+        trait_ref: ty::Binder<'tcx, ty::TraitRef<'tcx>>,
+        candidate_impls: &[ImplCandidate<'tcx>],
+        span: Span,
+    ) {
+        // Three cases where we can make a suggestion:
+        // 1. `[T; _]` (array of T)
+        // 2. `&[T; _]` (reference to array of T)
+        // 3. `&mut [T; _]` (mutable reference to array of T)
+        let (element_ty, mut mutability) = match *trait_ref.skip_binder().self_ty().kind() {
+            ty::Array(element_ty, _) => (element_ty, None),
+
+            ty::Ref(_, pointee_ty, mutability) => match *pointee_ty.kind() {
+                ty::Array(element_ty, _) => (element_ty, Some(mutability)),
+                _ => return,
+            },
+
+            _ => return,
+        };
+
+        // Go through all the candidate impls to see if any of them is for
+        // slices of `element_ty` with `mutability`.
+        let mut is_slice = |candidate: Ty<'tcx>| match *candidate.kind() {
+            ty::RawPtr(ty::TypeAndMut { ty: t, mutbl: m }) | ty::Ref(_, t, m) => {
+                if matches!(*t.kind(), ty::Slice(e) if e == element_ty)
+                    && m == mutability.unwrap_or(m)
+                {
+                    // Use the candidate's mutability going forward.
+                    mutability = Some(m);
+                    true
+                } else {
+                    false
+                }
+            }
+            _ => false,
+        };
+
+        // Grab the first candidate that matches, if any, and make a suggestion.
+        if let Some(slice_ty) = candidate_impls
+            .iter()
+            .map(|trait_ref| trait_ref.trait_ref.self_ty())
+            .find(|t| is_slice(*t))
+        {
+            let msg = &format!("convert the array to a `{}` slice instead", slice_ty);
+
+            if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span) {
+                let mut suggestions = vec![];
+                if snippet.starts_with('&') {
+                } else if let Some(hir::Mutability::Mut) = mutability {
+                    suggestions.push((span.shrink_to_lo(), "&mut ".into()));
+                } else {
+                    suggestions.push((span.shrink_to_lo(), "&".into()));
+                }
+                suggestions.push((span.shrink_to_hi(), "[..]".into()));
+                err.multipart_suggestion_verbose(msg, suggestions, Applicability::MaybeIncorrect);
+            } else {
+                err.span_help(span, msg);
+            }
+        }
+    }
 }
 
 /// Add a hint to add a missing borrow or remove an unnecessary one.
@@ -3854,7 +3935,7 @@ fn hint_missing_borrow<'tcx>(
     // This could be a variant constructor, for example.
     let Some(fn_decl) = found_node.fn_decl() else { return; };
 
-    let args = fn_decl.inputs.iter().map(|ty| ty);
+    let args = fn_decl.inputs.iter();
 
     fn get_deref_type_and_refs(mut ty: Ty<'_>) -> (Ty<'_>, Vec<hir::Mutability>) {
         let mut refs = vec![];
diff --git a/compiler/rustc_trait_selection/src/traits/fulfill.rs b/compiler/rustc_trait_selection/src/traits/fulfill.rs
index 944436ab8..26cadab3e 100644
--- a/compiler/rustc_trait_selection/src/traits/fulfill.rs
+++ b/compiler/rustc_trait_selection/src/traits/fulfill.rs
@@ -2,6 +2,7 @@ use crate::infer::{InferCtxt, TyOrConstInferVar};
 use rustc_data_structures::obligation_forest::ProcessResult;
 use rustc_data_structures::obligation_forest::{Error, ForestObligation, Outcome};
 use rustc_data_structures::obligation_forest::{ObligationForest, ObligationProcessor};
+use rustc_infer::infer::DefineOpaqueTypes;
 use rustc_infer::traits::ProjectionCacheKey;
 use rustc_infer::traits::{SelectionError, TraitEngine, TraitObligation};
 use rustc_middle::mir::interpret::ErrorHandled;
@@ -132,8 +133,15 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
             .register_obligation(PendingPredicateObligation { obligation, stalled_on: vec![] });
     }
 
-    fn collect_remaining_errors(&mut self) -> Vec<FulfillmentError<'tcx>> {
-        self.predicates.to_errors(CodeAmbiguity).into_iter().map(to_fulfillment_error).collect()
+    fn collect_remaining_errors(
+        &mut self,
+        _infcx: &InferCtxt<'tcx>,
+    ) -> Vec<FulfillmentError<'tcx>> {
+        self.predicates
+            .to_errors(CodeAmbiguity { overflow: false })
+            .into_iter()
+            .map(to_fulfillment_error)
+            .collect()
     }
 
     fn select_where_possible(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<FulfillmentError<'tcx>> {
@@ -210,6 +218,29 @@ impl<'a, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'tcx> {
     type Error = FulfillmentErrorCode<'tcx>;
     type OUT = Outcome<Self::Obligation, Self::Error>;
 
+    /// Compared to `needs_process_obligation` this and its callees
+    /// contain some optimizations that come at the price of false negatives.
+    ///
+    /// They
+    /// - reduce branching by covering only the most common case
+    /// - take a read-only view of the unification tables which allows skipping undo_log
+    ///   construction.
+    /// - bail out on value-cache misses in ena to avoid pointer chasing
+    /// - hoist RefCell locking out of the loop
+    #[inline]
+    fn skippable_obligations<'b>(
+        &'b self,
+        it: impl Iterator<Item = &'b Self::Obligation>,
+    ) -> usize {
+        let is_unchanged = self.selcx.infcx.is_ty_infer_var_definitely_unchanged();
+
+        it.take_while(|o| match o.stalled_on.as_slice() {
+            [o] => is_unchanged(*o),
+            _ => false,
+        })
+        .count()
+    }
+
     /// Identifies whether a predicate obligation needs processing.
     ///
     /// This is always inlined because it has a single callsite and it is
@@ -337,8 +368,8 @@ impl<'a, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'tcx> {
                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
                     bug!("TypeWellFormedFromEnv is only used for Chalk")
                 }
-                ty::PredicateKind::AliasEq(..) => {
-                    bug!("AliasEq is only used for new solver")
+                ty::PredicateKind::AliasRelate(..) => {
+                    bug!("AliasRelate is only used for new solver")
                 }
             },
             Some(pred) => match pred {
@@ -515,7 +546,7 @@ impl<'a, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'tcx> {
                                 if let Ok(new_obligations) = infcx
                                     .at(&obligation.cause, obligation.param_env)
                                     .trace(c1, c2)
-                                    .eq(a.substs, b.substs)
+                                    .eq(DefineOpaqueTypes::No, a.substs, b.substs)
                                 {
                                     return ProcessResult::Changed(mk_pending(
                                         new_obligations.into_obligations(),
@@ -524,8 +555,9 @@ impl<'a, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'tcx> {
                             }
                             (_, Unevaluated(_)) | (Unevaluated(_), _) => (),
                             (_, _) => {
-                                if let Ok(new_obligations) =
-                                    infcx.at(&obligation.cause, obligation.param_env).eq(c1, c2)
+                                if let Ok(new_obligations) = infcx
+                                    .at(&obligation.cause, obligation.param_env)
+                                    .eq(DefineOpaqueTypes::No, c1, c2)
                                 {
                                     return ProcessResult::Changed(mk_pending(
                                         new_obligations.into_obligations(),
@@ -565,12 +597,11 @@ impl<'a, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'tcx> {
 
                     match (evaluate(c1), evaluate(c2)) {
                         (Ok(c1), Ok(c2)) => {
-                            match self
-                                .selcx
-                                .infcx
-                                .at(&obligation.cause, obligation.param_env)
-                                .eq(c1, c2)
-                            {
+                            match self.selcx.infcx.at(&obligation.cause, obligation.param_env).eq(
+                                DefineOpaqueTypes::No,
+                                c1,
+                                c2,
+                            ) {
                                 Ok(inf_ok) => {
                                     ProcessResult::Changed(mk_pending(inf_ok.into_obligations()))
                                 }
@@ -606,16 +637,15 @@ impl<'a, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'tcx> {
                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
                     bug!("TypeWellFormedFromEnv is only used for Chalk")
                 }
-                ty::PredicateKind::AliasEq(..) => {
-                    bug!("AliasEq is only used for new solver")
+                ty::PredicateKind::AliasRelate(..) => {
+                    bug!("AliasRelate is only used for new solver")
                 }
                 ty::PredicateKind::Clause(ty::Clause::ConstArgHasType(ct, ty)) => {
-                    match self
-                        .selcx
-                        .infcx
-                        .at(&obligation.cause, obligation.param_env)
-                        .eq(ct.ty(), ty)
-                    {
+                    match self.selcx.infcx.at(&obligation.cause, obligation.param_env).eq(
+                        DefineOpaqueTypes::No,
+                        ct.ty(),
+                        ty,
+                    ) {
                         Ok(inf_ok) => ProcessResult::Changed(mk_pending(inf_ok.into_obligations())),
                         Err(_) => ProcessResult::Error(FulfillmentErrorCode::CodeSelectionError(
                             SelectionError::Unimplemented,
diff --git a/compiler/rustc_trait_selection/src/traits/misc.rs b/compiler/rustc_trait_selection/src/traits/misc.rs
index b94346b09..af567c074 100644
--- a/compiler/rustc_trait_selection/src/traits/misc.rs
+++ b/compiler/rustc_trait_selection/src/traits/misc.rs
@@ -87,7 +87,12 @@ pub fn type_allowed_to_implement_copy<'tcx>(
             };
             let ty = ocx.normalize(&normalization_cause, param_env, unnormalized_ty);
             let normalization_errors = ocx.select_where_possible();
-            if !normalization_errors.is_empty() {
+
+            // NOTE: The post-normalization type may also reference errors,
+            // such as when we project to a missing type or we have a mismatch
+            // between expected and found const-generic types. Don't report an
+            // additional copy error here, since it's not typically useful.
+            if !normalization_errors.is_empty() || ty.references_error() {
                 tcx.sess.delay_span_bug(field_span, format!("couldn't normalize struct field `{unnormalized_ty}` when checking Copy implementation"));
                 continue;
             }
@@ -106,17 +111,12 @@ pub fn type_allowed_to_implement_copy<'tcx>(
             // Check regions assuming the self type of the impl is WF
             let outlives_env = OutlivesEnvironment::with_bounds(
                 param_env,
-                Some(&infcx),
                 infcx.implied_bounds_tys(
                     param_env,
                     parent_cause.body_id,
                     FxIndexSet::from_iter([self_type]),
                 ),
             );
-            infcx.process_registered_region_obligations(
-                outlives_env.region_bound_pairs(),
-                param_env,
-            );
             let errors = infcx.resolve_regions(&outlives_env);
             if !errors.is_empty() {
                 infringing.push((field, ty, InfringingFieldsReason::Regions(errors)));
diff --git a/compiler/rustc_trait_selection/src/traits/mod.rs b/compiler/rustc_trait_selection/src/traits/mod.rs
index 4e30108be..8a203dec8 100644
--- a/compiler/rustc_trait_selection/src/traits/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/mod.rs
@@ -4,7 +4,7 @@
 
 pub mod auto_trait;
 mod chalk_fulfill;
-mod coherence;
+pub(crate) mod coherence;
 pub mod const_evaluatable;
 mod engine;
 pub mod error_reporting;
@@ -28,9 +28,9 @@ use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
 use rustc_errors::ErrorGuaranteed;
 use rustc_middle::ty::fold::TypeFoldable;
 use rustc_middle::ty::visit::{TypeVisitable, TypeVisitableExt};
-use rustc_middle::ty::{self, DefIdTree, ToPredicate, Ty, TyCtxt, TypeSuperVisitable};
+use rustc_middle::ty::{self, ToPredicate, Ty, TyCtxt, TypeSuperVisitable};
 use rustc_middle::ty::{InternalSubsts, SubstsRef};
-use rustc_span::def_id::{DefId, CRATE_DEF_ID};
+use rustc_span::def_id::DefId;
 use rustc_span::Span;
 
 use std::fmt::Debug;
@@ -58,17 +58,12 @@ pub use self::specialize::{specialization_graph, translate_substs, OverlapError}
 pub use self::structural_match::{
     search_for_adt_const_param_violation, search_for_structural_match_violation,
 };
-pub use self::util::{
-    elaborate_obligations, elaborate_predicates, elaborate_predicates_with_span,
-    elaborate_trait_ref, elaborate_trait_refs,
-};
+pub use self::util::elaborate;
 pub use self::util::{expand_trait_aliases, TraitAliasExpander};
-pub use self::util::{
-    get_vtable_index_of_object_method, impl_item_is_final, predicate_for_trait_def, upcast_choices,
-};
+pub use self::util::{get_vtable_index_of_object_method, impl_item_is_final, upcast_choices};
 pub use self::util::{
     supertrait_def_ids, supertraits, transitive_bounds, transitive_bounds_that_define_assoc_type,
-    SupertraitDefIds, Supertraits,
+    SupertraitDefIds,
 };
 
 pub use self::chalk_fulfill::FulfillmentContext as ChalkFulfillmentContext;
@@ -131,29 +126,23 @@ pub fn type_known_to_meet_bound_modulo_regions<'tcx>(
     param_env: ty::ParamEnv<'tcx>,
     ty: Ty<'tcx>,
     def_id: DefId,
-    span: Span,
 ) -> bool {
     let trait_ref = ty::Binder::dummy(infcx.tcx.mk_trait_ref(def_id, [ty]));
-    pred_known_to_hold_modulo_regions(infcx, param_env, trait_ref.without_const(), span)
+    pred_known_to_hold_modulo_regions(infcx, param_env, trait_ref.without_const())
 }
 
-#[instrument(level = "debug", skip(infcx, param_env, span, pred), ret)]
+/// FIXME(@lcnr): this function doesn't seem right and shouldn't exist?
+///
+/// Ping me on zulip if you want to use this method and need help with finding
+/// an appropriate replacement.
+#[instrument(level = "debug", skip(infcx, param_env, pred), ret)]
 fn pred_known_to_hold_modulo_regions<'tcx>(
     infcx: &InferCtxt<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     pred: impl ToPredicate<'tcx> + TypeVisitable<TyCtxt<'tcx>>,
-    span: Span,
 ) -> bool {
     let has_non_region_infer = pred.has_non_region_infer();
-    let obligation = Obligation {
-        param_env,
-        // We can use a dummy node-id here because we won't pay any mind
-        // to region obligations that arise (there shouldn't really be any
-        // anyhow).
-        cause: ObligationCause::misc(span, CRATE_DEF_ID),
-        recursion_depth: 0,
-        predicate: pred.to_predicate(infcx.tcx),
-    };
+    let obligation = Obligation::new(infcx.tcx, ObligationCause::dummy(), param_env, pred);
 
     let result = infcx.evaluate_obligation_no_overflow(&obligation);
     debug!(?result);
@@ -166,14 +155,13 @@ fn pred_known_to_hold_modulo_regions<'tcx>(
         // this function's result remains infallible, we must confirm
         // that guess. While imperfect, I believe this is sound.
 
-        // FIXME(@lcnr): this function doesn't seem right.
-        //
         // The handling of regions in this area of the code is terrible,
         // see issue #29149. We should be able to improve on this with
         // NLL.
-        let errors = fully_solve_obligation(infcx, obligation);
-
-        match &errors[..] {
+        let ocx = ObligationCtxt::new(infcx);
+        ocx.register_obligation(obligation);
+        let errors = ocx.select_all_or_error();
+        match errors.as_slice() {
             [] => true,
             errors => {
                 debug!(?errors);
@@ -210,7 +198,7 @@ fn do_normalize_predicates<'tcx>(
     let predicates = match fully_normalize(&infcx, cause, elaborated_env, predicates) {
         Ok(predicates) => predicates,
         Err(errors) => {
-            let reported = infcx.err_ctxt().report_fulfillment_errors(&errors, None);
+            let reported = infcx.err_ctxt().report_fulfillment_errors(&errors);
             return Err(reported);
         }
     };
@@ -276,9 +264,7 @@ pub fn normalize_param_env_or_error<'tcx>(
     // and errors will get reported then; so outside of type inference we
     // can be sure that no errors should occur.
     let mut predicates: Vec<_> =
-        util::elaborate_predicates(tcx, unnormalized_env.caller_bounds().into_iter())
-            .map(|obligation| obligation.predicate)
-            .collect();
+        util::elaborate(tcx, unnormalized_env.caller_bounds().into_iter()).collect();
 
     debug!("normalize_param_env_or_error: elaborated-predicates={:?}", predicates);
 
@@ -389,43 +375,6 @@ where
     Ok(resolved_value)
 }
 
-/// Process an obligation (and any nested obligations that come from it) to
-/// completion, returning any errors
-pub fn fully_solve_obligation<'tcx>(
-    infcx: &InferCtxt<'tcx>,
-    obligation: PredicateObligation<'tcx>,
-) -> Vec<FulfillmentError<'tcx>> {
-    fully_solve_obligations(infcx, [obligation])
-}
-
-/// Process a set of obligations (and any nested obligations that come from them)
-/// to completion
-pub fn fully_solve_obligations<'tcx>(
-    infcx: &InferCtxt<'tcx>,
-    obligations: impl IntoIterator<Item = PredicateObligation<'tcx>>,
-) -> Vec<FulfillmentError<'tcx>> {
-    let ocx = ObligationCtxt::new(infcx);
-    ocx.register_obligations(obligations);
-    ocx.select_all_or_error()
-}
-
-/// Process a bound (and any nested obligations that come from it) to completion.
-/// This is a convenience function for traits that have no generic arguments, such
-/// as auto traits, and builtin traits like Copy or Sized.
-pub fn fully_solve_bound<'tcx>(
-    infcx: &InferCtxt<'tcx>,
-    cause: ObligationCause<'tcx>,
-    param_env: ty::ParamEnv<'tcx>,
-    ty: Ty<'tcx>,
-    bound: DefId,
-) -> Vec<FulfillmentError<'tcx>> {
-    let tcx = infcx.tcx;
-    let trait_ref = tcx.mk_trait_ref(bound, [ty]);
-    let obligation = Obligation::new(tcx, cause, param_env, ty::Binder::dummy(trait_ref));
-
-    fully_solve_obligation(infcx, obligation)
-}
-
 /// Normalizes the predicates and checks whether they hold in an empty environment. If this
 /// returns true, then either normalize encountered an error or one of the predicates did not
 /// hold. Used when creating vtables to check for unsatisfiable methods.
diff --git a/compiler/rustc_trait_selection/src/traits/object_safety.rs b/compiler/rustc_trait_selection/src/traits/object_safety.rs
index 4eacb5211..b8ad1925e 100644
--- a/compiler/rustc_trait_selection/src/traits/object_safety.rs
+++ b/compiler/rustc_trait_selection/src/traits/object_safety.rs
@@ -8,20 +8,19 @@
 //!   - not reference the erased type `Self` except for in this receiver;
 //!   - not have generic type parameters.
 
-use super::{elaborate_predicates, elaborate_trait_ref};
+use super::elaborate;
 
 use crate::infer::TyCtxtInferExt;
 use crate::traits::query::evaluate_obligation::InferCtxtExt;
 use crate::traits::{self, Obligation, ObligationCause};
-use hir::def::DefKind;
 use rustc_errors::{DelayDm, FatalError, MultiSpan};
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_middle::ty::subst::{GenericArg, InternalSubsts};
-use rustc_middle::ty::ToPredicate;
 use rustc_middle::ty::{
     self, EarlyBinder, Ty, TyCtxt, TypeSuperVisitable, TypeVisitable, TypeVisitor,
 };
+use rustc_middle::ty::{ToPredicate, TypeVisitableExt};
 use rustc_session::lint::builtin::WHERE_CLAUSES_OBJECT_SAFETY;
 use rustc_span::symbol::Symbol;
 use rustc_span::Span;
@@ -140,6 +139,10 @@ fn object_safety_violations_for_trait(
     if !spans.is_empty() {
         violations.push(ObjectSafetyViolation::SupertraitSelf(spans));
     }
+    let spans = super_predicates_have_non_lifetime_binders(tcx, trait_def_id);
+    if !spans.is_empty() {
+        violations.push(ObjectSafetyViolation::SupertraitNonLifetimeBinder(spans));
+    }
 
     violations.extend(
         tcx.associated_items(trait_def_id)
@@ -157,6 +160,7 @@ fn object_safety_violations_for_trait(
                 .in_definition_order()
                 .filter(|item| item.kind == ty::AssocKind::Type)
                 .filter(|item| !tcx.generics_of(item.def_id).params.is_empty())
+                .filter(|item| tcx.opt_rpitit_info(item.def_id).is_none())
                 .map(|item| {
                     let ident = item.ident(tcx);
                     ObjectSafetyViolation::GAT(ident.name, ident.span)
@@ -331,7 +335,7 @@ fn predicate_references_self<'tcx>(
             has_self_ty(&ty.into()).then_some(sp)
         }
 
-        ty::PredicateKind::AliasEq(..) => bug!("`AliasEq` not allowed as assumption"),
+        ty::PredicateKind::AliasRelate(..) => bug!("`AliasRelate` not allowed as assumption"),
 
         ty::PredicateKind::WellFormed(..)
         | ty::PredicateKind::ObjectSafe(..)
@@ -348,6 +352,21 @@ fn predicate_references_self<'tcx>(
     }
 }
 
+fn super_predicates_have_non_lifetime_binders(
+    tcx: TyCtxt<'_>,
+    trait_def_id: DefId,
+) -> SmallVec<[Span; 1]> {
+    // If non_lifetime_binders is disabled, then exit early
+    if !tcx.features().non_lifetime_binders {
+        return SmallVec::new();
+    }
+    tcx.super_predicates_of(trait_def_id)
+        .predicates
+        .iter()
+        .filter_map(|(pred, span)| pred.has_non_region_late_bound().then_some(*span))
+        .collect()
+}
+
 fn trait_has_sized_self(tcx: TyCtxt<'_>, trait_def_id: DefId) -> bool {
     generics_require_sized_self(tcx, trait_def_id)
 }
@@ -360,26 +379,24 @@ fn generics_require_sized_self(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
     // Search for a predicate like `Self : Sized` amongst the trait bounds.
     let predicates = tcx.predicates_of(def_id);
     let predicates = predicates.instantiate_identity(tcx).predicates;
-    elaborate_predicates(tcx, predicates.into_iter()).any(|obligation| {
-        match obligation.predicate.kind().skip_binder() {
-            ty::PredicateKind::Clause(ty::Clause::Trait(ref trait_pred)) => {
-                trait_pred.def_id() == sized_def_id && trait_pred.self_ty().is_param(0)
-            }
-            ty::PredicateKind::Clause(ty::Clause::Projection(..))
-            | ty::PredicateKind::Clause(ty::Clause::ConstArgHasType(..))
-            | ty::PredicateKind::Subtype(..)
-            | ty::PredicateKind::Coerce(..)
-            | ty::PredicateKind::Clause(ty::Clause::RegionOutlives(..))
-            | ty::PredicateKind::WellFormed(..)
-            | ty::PredicateKind::ObjectSafe(..)
-            | ty::PredicateKind::ClosureKind(..)
-            | ty::PredicateKind::Clause(ty::Clause::TypeOutlives(..))
-            | ty::PredicateKind::ConstEvaluatable(..)
-            | ty::PredicateKind::ConstEquate(..)
-            | ty::PredicateKind::AliasEq(..)
-            | ty::PredicateKind::Ambiguous
-            | ty::PredicateKind::TypeWellFormedFromEnv(..) => false,
+    elaborate(tcx, predicates.into_iter()).any(|pred| match pred.kind().skip_binder() {
+        ty::PredicateKind::Clause(ty::Clause::Trait(ref trait_pred)) => {
+            trait_pred.def_id() == sized_def_id && trait_pred.self_ty().is_param(0)
         }
+        ty::PredicateKind::Clause(ty::Clause::Projection(..))
+        | ty::PredicateKind::Clause(ty::Clause::ConstArgHasType(..))
+        | ty::PredicateKind::Subtype(..)
+        | ty::PredicateKind::Coerce(..)
+        | ty::PredicateKind::Clause(ty::Clause::RegionOutlives(..))
+        | ty::PredicateKind::WellFormed(..)
+        | ty::PredicateKind::ObjectSafe(..)
+        | ty::PredicateKind::ClosureKind(..)
+        | ty::PredicateKind::Clause(ty::Clause::TypeOutlives(..))
+        | ty::PredicateKind::ConstEvaluatable(..)
+        | ty::PredicateKind::ConstEquate(..)
+        | ty::PredicateKind::AliasRelate(..)
+        | ty::PredicateKind::Ambiguous
+        | ty::PredicateKind::TypeWellFormedFromEnv(..) => false,
     })
 }
 
@@ -649,10 +666,11 @@ fn object_ty_for_trait<'tcx>(
     });
     debug!(?trait_predicate);
 
-    let mut elaborated_predicates: Vec<_> = elaborate_trait_ref(tcx, trait_ref)
-        .filter_map(|obligation| {
-            debug!(?obligation);
-            let pred = obligation.predicate.to_opt_poly_projection_pred()?;
+    let pred: ty::Predicate<'tcx> = trait_ref.to_predicate(tcx);
+    let mut elaborated_predicates: Vec<_> = elaborate(tcx, [pred])
+        .filter_map(|pred| {
+            debug!(?pred);
+            let pred = pred.to_opt_poly_projection_pred()?;
             Some(pred.map_bound(|p| {
                 ty::ExistentialPredicate::Projection(ty::ExistentialProjection::erase_self_ty(
                     tcx, p,
@@ -854,7 +872,7 @@ fn contains_illegal_self_type_reference<'tcx, T: TypeVisitable<TyCtxt<'tcx>>>(
                     }
                 }
                 ty::Alias(ty::Projection, ref data)
-                    if self.tcx.def_kind(data.def_id) == DefKind::ImplTraitPlaceholder =>
+                    if self.tcx.is_impl_trait_in_trait(data.def_id) =>
                 {
                     // We'll deny these later in their own pass
                     ControlFlow::Continue(())
@@ -921,7 +939,7 @@ pub fn contains_illegal_impl_trait_in_trait<'tcx>(
     ty.skip_binder().walk().find_map(|arg| {
         if let ty::GenericArgKind::Type(ty) = arg.unpack()
             && let ty::Alias(ty::Projection, proj) = ty.kind()
-            && tcx.def_kind(proj.def_id) == DefKind::ImplTraitPlaceholder
+            && tcx.is_impl_trait_in_trait(proj.def_id)
         {
             Some(MethodViolationCode::ReferencesImplTraitInTrait(tcx.def_span(proj.def_id)))
         } else {
diff --git a/compiler/rustc_trait_selection/src/traits/outlives_bounds.rs b/compiler/rustc_trait_selection/src/traits/outlives_bounds.rs
index 6cb64ad57..cff3d277a 100644
--- a/compiler/rustc_trait_selection/src/traits/outlives_bounds.rs
+++ b/compiler/rustc_trait_selection/src/traits/outlives_bounds.rs
@@ -1,9 +1,10 @@
 use crate::infer::InferCtxt;
 use crate::traits::query::type_op::{self, TypeOp, TypeOpOutput};
 use crate::traits::query::NoSolution;
-use crate::traits::ObligationCause;
+use crate::traits::{ObligationCause, ObligationCtxt};
 use rustc_data_structures::fx::FxIndexSet;
-use rustc_middle::ty::{self, ParamEnv, Ty};
+use rustc_infer::infer::resolve::OpportunisticRegionResolver;
+use rustc_middle::ty::{self, ParamEnv, Ty, TypeFolder, TypeVisitableExt};
 use rustc_span::def_id::LocalDefId;
 
 pub use rustc_middle::traits::query::OutlivesBound;
@@ -52,6 +53,10 @@ impl<'a, 'tcx: 'a> InferCtxtExt<'a, 'tcx> for InferCtxt<'tcx> {
         body_id: LocalDefId,
         ty: Ty<'tcx>,
     ) -> Vec<OutlivesBound<'tcx>> {
+        let ty = self.resolve_vars_if_possible(ty);
+        let ty = OpportunisticRegionResolver::new(self).fold_ty(ty);
+        assert!(!ty.needs_infer());
+
         let span = self.tcx.def_span(body_id);
         let result = param_env
             .and(type_op::implied_outlives_bounds::ImpliedOutlivesBounds { ty })
@@ -71,22 +76,23 @@ impl<'a, 'tcx: 'a> InferCtxtExt<'a, 'tcx> for InferCtxt<'tcx> {
 
         if let Some(constraints) = constraints {
             debug!(?constraints);
+            if !constraints.member_constraints.is_empty() {
+                span_bug!(span, "{:#?}", constraints.member_constraints);
+            }
+
             // Instantiation may have produced new inference variables and constraints on those
             // variables. Process these constraints.
+            let ocx = ObligationCtxt::new(self);
             let cause = ObligationCause::misc(span, body_id);
-            let errors = super::fully_solve_obligations(
-                self,
-                constraints.outlives.iter().map(|constraint| {
-                    self.query_outlives_constraint_to_obligation(
-                        *constraint,
-                        cause.clone(),
-                        param_env,
-                    )
-                }),
-            );
-            if !constraints.member_constraints.is_empty() {
-                span_bug!(span, "{:#?}", constraints.member_constraints);
+            for &constraint in &constraints.outlives {
+                ocx.register_obligation(self.query_outlives_constraint_to_obligation(
+                    constraint,
+                    cause.clone(),
+                    param_env,
+                ));
             }
+
+            let errors = ocx.select_all_or_error();
             if !errors.is_empty() {
                 self.tcx.sess.delay_span_bug(
                     span,
@@ -104,11 +110,6 @@ impl<'a, 'tcx: 'a> InferCtxtExt<'a, 'tcx> for InferCtxt<'tcx> {
         body_id: LocalDefId,
         tys: FxIndexSet<Ty<'tcx>>,
     ) -> Bounds<'a, 'tcx> {
-        tys.into_iter()
-            .map(move |ty| {
-                let ty = self.resolve_vars_if_possible(ty);
-                self.implied_outlives_bounds(param_env, body_id, ty)
-            })
-            .flatten()
+        tys.into_iter().flat_map(move |ty| self.implied_outlives_bounds(param_env, body_id, ty))
     }
 }
diff --git a/compiler/rustc_trait_selection/src/traits/project.rs b/compiler/rustc_trait_selection/src/traits/project.rs
index 870ecc2a9..826fc63ca 100644
--- a/compiler/rustc_trait_selection/src/traits/project.rs
+++ b/compiler/rustc_trait_selection/src/traits/project.rs
@@ -28,11 +28,11 @@ use rustc_hir::def::DefKind;
 use rustc_hir::lang_items::LangItem;
 use rustc_infer::infer::at::At;
 use rustc_infer::infer::resolve::OpportunisticRegionResolver;
+use rustc_infer::infer::DefineOpaqueTypes;
 use rustc_infer::traits::ImplSourceBuiltinData;
 use rustc_middle::traits::select::OverflowError;
 use rustc_middle::ty::fold::{TypeFoldable, TypeFolder, TypeSuperFoldable};
 use rustc_middle::ty::visit::{MaxUniverse, TypeVisitable, TypeVisitableExt};
-use rustc_middle::ty::DefIdTree;
 use rustc_middle::ty::{self, Term, ToPredicate, Ty, TyCtxt};
 use rustc_span::symbol::sym;
 
@@ -286,12 +286,12 @@ fn project_and_unify_type<'cx, 'tcx>(
         );
     obligations.extend(new);
 
-    match infcx
-        .at(&obligation.cause, obligation.param_env)
-        // This is needed to support nested opaque types like `impl Fn() -> impl Trait`
-        .define_opaque_types(true)
-        .eq(normalized, actual)
-    {
+    // Need to define opaque types to support nested opaque types like `impl Fn() -> impl Trait`
+    match infcx.at(&obligation.cause, obligation.param_env).eq(
+        DefineOpaqueTypes::Yes,
+        normalized,
+        actual,
+    ) {
         Ok(InferOk { obligations: inferred_obligations, value: () }) => {
             obligations.extend(inferred_obligations);
             ProjectAndUnifyResult::Holds(obligations)
@@ -468,6 +468,11 @@ impl<'a, 'b, 'tcx> TypeFolder<TyCtxt<'tcx>> for AssocTypeNormalizer<'a, 'b, 'tcx
             return ty;
         }
 
+        let (kind, data) = match *ty.kind() {
+            ty::Alias(kind, alias_ty) => (kind, alias_ty),
+            _ => return ty.super_fold_with(self),
+        };
+
         // We try to be a little clever here as a performance optimization in
         // cases where there are nested projections under binders.
         // For example:
@@ -491,13 +496,11 @@ impl<'a, 'b, 'tcx> TypeFolder<TyCtxt<'tcx>> for AssocTypeNormalizer<'a, 'b, 'tcx
         // replace bound vars if the current type is a `Projection` and we need
         // to make sure we don't forget to fold the substs regardless.
 
-        match *ty.kind() {
+        match kind {
             // This is really important. While we *can* handle this, this has
             // severe performance implications for large opaque types with
             // late-bound regions. See `issue-88862` benchmark.
-            ty::Alias(ty::Opaque, ty::AliasTy { def_id, substs, .. })
-                if !substs.has_escaping_bound_vars() =>
-            {
+            ty::Opaque if !data.substs.has_escaping_bound_vars() => {
                 // Only normalize `impl Trait` outside of type inference, usually in codegen.
                 match self.param_env.reveal() {
                     Reveal::UserFacing => ty.super_fold_with(self),
@@ -513,8 +516,8 @@ impl<'a, 'b, 'tcx> TypeFolder<TyCtxt<'tcx>> for AssocTypeNormalizer<'a, 'b, 'tcx
                             );
                         }
 
-                        let substs = substs.fold_with(self);
-                        let generic_ty = self.interner().type_of(def_id);
+                        let substs = data.substs.fold_with(self);
+                        let generic_ty = self.interner().type_of(data.def_id);
                         let concrete_ty = generic_ty.subst(self.interner(), substs);
                         self.depth += 1;
                         let folded_ty = self.fold_ty(concrete_ty);
@@ -523,8 +526,9 @@ impl<'a, 'b, 'tcx> TypeFolder<TyCtxt<'tcx>> for AssocTypeNormalizer<'a, 'b, 'tcx
                     }
                 }
             }
+            ty::Opaque => ty.super_fold_with(self),
 
-            ty::Alias(ty::Projection, data) if !data.has_escaping_bound_vars() => {
+            ty::Projection if !data.has_escaping_bound_vars() => {
                 // This branch is *mostly* just an optimization: when we don't
                 // have escaping bound vars, we don't need to replace them with
                 // placeholders (see branch below). *Also*, we know that we can
@@ -563,7 +567,7 @@ impl<'a, 'b, 'tcx> TypeFolder<TyCtxt<'tcx>> for AssocTypeNormalizer<'a, 'b, 'tcx
                 normalized_ty.ty().unwrap()
             }
 
-            ty::Alias(ty::Projection, data) => {
+            ty::Projection => {
                 // If there are escaping bound vars, we temporarily replace the
                 // bound vars with placeholders. Note though, that in the case
                 // that we still can't project for whatever reason (e.g. self
@@ -612,8 +616,6 @@ impl<'a, 'b, 'tcx> TypeFolder<TyCtxt<'tcx>> for AssocTypeNormalizer<'a, 'b, 'tcx
                 );
                 normalized_ty
             }
-
-            _ => ty.super_fold_with(self),
         }
     }
 
@@ -770,7 +772,7 @@ impl<'tcx> TypeFolder<TyCtxt<'tcx>> for BoundVarReplacer<'_, 'tcx> {
             }
             ty::ReLateBound(debruijn, br) if debruijn >= self.current_index => {
                 let universe = self.universe_for(debruijn);
-                let p = ty::PlaceholderRegion { universe, name: br.kind };
+                let p = ty::PlaceholderRegion { universe, bound: br };
                 self.mapped_regions.insert(p, br);
                 self.infcx.tcx.mk_re_placeholder(p)
             }
@@ -788,7 +790,7 @@ impl<'tcx> TypeFolder<TyCtxt<'tcx>> for BoundVarReplacer<'_, 'tcx> {
             }
             ty::Bound(debruijn, bound_ty) if debruijn >= self.current_index => {
                 let universe = self.universe_for(debruijn);
-                let p = ty::PlaceholderType { universe, name: bound_ty.kind };
+                let p = ty::PlaceholderType { universe, bound: bound_ty };
                 self.mapped_types.insert(p, bound_ty);
                 self.infcx.tcx.mk_placeholder(p)
             }
@@ -807,7 +809,7 @@ impl<'tcx> TypeFolder<TyCtxt<'tcx>> for BoundVarReplacer<'_, 'tcx> {
             }
             ty::ConstKind::Bound(debruijn, bound_const) if debruijn >= self.current_index => {
                 let universe = self.universe_for(debruijn);
-                let p = ty::PlaceholderConst { universe, name: bound_const };
+                let p = ty::PlaceholderConst { universe, bound: bound_const };
                 self.mapped_consts.insert(p, bound_const);
                 self.infcx.tcx.mk_const(p, ct.ty())
             }
@@ -871,12 +873,12 @@ impl<'tcx> TypeFolder<TyCtxt<'tcx>> for PlaceholderReplacer<'_, 'tcx> {
 
     fn fold_region(&mut self, r0: ty::Region<'tcx>) -> ty::Region<'tcx> {
         let r1 = match *r0 {
-            ty::ReVar(_) => self
+            ty::ReVar(vid) => self
                 .infcx
                 .inner
                 .borrow_mut()
                 .unwrap_region_constraints()
-                .opportunistic_resolve_region(self.infcx.tcx, r0),
+                .opportunistic_resolve_var(self.infcx.tcx, vid),
             _ => r0,
         };
 
@@ -1296,7 +1298,7 @@ fn assemble_candidate_for_impl_trait_in_trait<'cx, 'tcx>(
 ) {
     let tcx = selcx.tcx();
     if tcx.def_kind(obligation.predicate.def_id) == DefKind::ImplTraitPlaceholder {
-        let trait_fn_def_id = tcx.impl_trait_in_trait_parent(obligation.predicate.def_id);
+        let trait_fn_def_id = tcx.impl_trait_in_trait_parent_fn(obligation.predicate.def_id);
 
         let trait_def_id = tcx.parent(trait_fn_def_id);
         let trait_substs =
@@ -2065,7 +2067,11 @@ fn confirm_param_env_candidate<'cx, 'tcx>(
 
     debug!(?cache_projection, ?obligation_projection);
 
-    match infcx.at(cause, param_env).eq(cache_projection, obligation_projection) {
+    match infcx.at(cause, param_env).eq(
+        DefineOpaqueTypes::No,
+        cache_projection,
+        obligation_projection,
+    ) {
         Ok(InferOk { value: _, obligations }) => {
             nested_obligations.extend(obligations);
             assoc_ty_own_obligations(selcx, obligation, &mut nested_obligations);
@@ -2194,13 +2200,14 @@ fn confirm_impl_trait_in_trait_candidate<'tcx>(
     let tcx = selcx.tcx();
     let mut obligations = data.nested;
 
-    let trait_fn_def_id = tcx.impl_trait_in_trait_parent(obligation.predicate.def_id);
+    let trait_fn_def_id = tcx.impl_trait_in_trait_parent_fn(obligation.predicate.def_id);
     let leaf_def = match specialization_graph::assoc_def(tcx, data.impl_def_id, trait_fn_def_id) {
         Ok(assoc_ty) => assoc_ty,
         Err(guar) => return Progress::error(tcx, guar),
     };
     // We don't support specialization for RPITITs anyways... yet.
-    if !leaf_def.is_final() {
+    // Also don't try to project to an RPITIT that has no value
+    if !leaf_def.is_final() || !leaf_def.item.defaultness(tcx).has_value() {
         return Progress { term: tcx.ty_error_misc().into(), obligations };
     }
 
diff --git a/compiler/rustc_trait_selection/src/traits/query/evaluate_obligation.rs b/compiler/rustc_trait_selection/src/traits/query/evaluate_obligation.rs
index f183248f2..edbe2de81 100644
--- a/compiler/rustc_trait_selection/src/traits/query/evaluate_obligation.rs
+++ b/compiler/rustc_trait_selection/src/traits/query/evaluate_obligation.rs
@@ -1,9 +1,8 @@
+use rustc_infer::traits::{TraitEngine, TraitEngineExt};
 use rustc_middle::ty;
-use rustc_session::config::TraitSolver;
 
 use crate::infer::canonical::OriginalQueryValues;
 use crate::infer::InferCtxt;
-use crate::solve::{Certainty, Goal, InferCtxtEvalExt, MaybeCause};
 use crate::traits::{EvaluationResult, OverflowError, PredicateObligation, SelectionContext};
 
 pub trait InferCtxtExt<'tcx> {
@@ -79,37 +78,30 @@ impl<'tcx> InferCtxtExt<'tcx> for InferCtxt<'tcx> {
             _ => obligation.param_env.without_const(),
         };
 
-        if self.tcx.sess.opts.unstable_opts.trait_solver != TraitSolver::Next {
+        if self.tcx.trait_solver_next() {
+            self.probe(|snapshot| {
+                let mut fulfill_cx = crate::solve::FulfillmentCtxt::new();
+                fulfill_cx.register_predicate_obligation(self, obligation.clone());
+                // True errors
+                // FIXME(-Ztrait-solver=next): Overflows are reported as ambig here, is that OK?
+                if !fulfill_cx.select_where_possible(self).is_empty() {
+                    Ok(EvaluationResult::EvaluatedToErr)
+                } else if !fulfill_cx.select_all_or_error(self).is_empty() {
+                    Ok(EvaluationResult::EvaluatedToAmbig)
+                } else if self.opaque_types_added_in_snapshot(snapshot) {
+                    Ok(EvaluationResult::EvaluatedToOkModuloOpaqueTypes)
+                } else if self.region_constraints_added_in_snapshot(snapshot).is_some() {
+                    Ok(EvaluationResult::EvaluatedToOkModuloRegions)
+                } else {
+                    Ok(EvaluationResult::EvaluatedToOk)
+                }
+            })
+        } else {
             let c_pred = self.canonicalize_query_keep_static(
                 param_env.and(obligation.predicate),
                 &mut _orig_values,
             );
             self.tcx.at(obligation.cause.span()).evaluate_obligation(c_pred)
-        } else {
-            self.probe(|snapshot| {
-                if let Ok((_, certainty)) =
-                    self.evaluate_root_goal(Goal::new(self.tcx, param_env, obligation.predicate))
-                {
-                    match certainty {
-                        Certainty::Yes => {
-                            if self.opaque_types_added_in_snapshot(snapshot) {
-                                Ok(EvaluationResult::EvaluatedToOkModuloOpaqueTypes)
-                            } else if self.region_constraints_added_in_snapshot(snapshot).is_some()
-                            {
-                                Ok(EvaluationResult::EvaluatedToOkModuloRegions)
-                            } else {
-                                Ok(EvaluationResult::EvaluatedToOk)
-                            }
-                        }
-                        Certainty::Maybe(MaybeCause::Ambiguity) => {
-                            Ok(EvaluationResult::EvaluatedToAmbig)
-                        }
-                        Certainty::Maybe(MaybeCause::Overflow) => Err(OverflowError::Canonical),
-                    }
-                } else {
-                    Ok(EvaluationResult::EvaluatedToErr)
-                }
-            })
         }
     }
 
diff --git a/compiler/rustc_trait_selection/src/traits/query/normalize.rs b/compiler/rustc_trait_selection/src/traits/query/normalize.rs
index b0cec3ce7..a986a9b6a 100644
--- a/compiler/rustc_trait_selection/src/traits/query/normalize.rs
+++ b/compiler/rustc_trait_selection/src/traits/query/normalize.rs
@@ -197,23 +197,30 @@ impl<'cx, 'tcx> FallibleTypeFolder<TyCtxt<'tcx>> for QueryNormalizer<'cx, 'tcx>
             return Ok(*ty);
         }
 
+        let (kind, data) = match *ty.kind() {
+            ty::Alias(kind, data) => (kind, data),
+            _ => {
+                let res = ty.try_super_fold_with(self)?;
+                self.cache.insert(ty, res);
+                return Ok(res);
+            }
+        };
+
         // See note in `rustc_trait_selection::traits::project` about why we
         // wait to fold the substs.
 
         // Wrap this in a closure so we don't accidentally return from the outer function
-        let res = match *ty.kind() {
+        let res = match kind {
             // This is really important. While we *can* handle this, this has
             // severe performance implications for large opaque types with
             // late-bound regions. See `issue-88862` benchmark.
-            ty::Alias(ty::Opaque, ty::AliasTy { def_id, substs, .. })
-                if !substs.has_escaping_bound_vars() =>
-            {
+            ty::Opaque if !data.substs.has_escaping_bound_vars() => {
                 // Only normalize `impl Trait` outside of type inference, usually in codegen.
                 match self.param_env.reveal() {
                     Reveal::UserFacing => ty.try_super_fold_with(self)?,
 
                     Reveal::All => {
-                        let substs = substs.try_fold_with(self)?;
+                        let substs = data.substs.try_fold_with(self)?;
                         let recursion_limit = self.interner().recursion_limit();
                         if !recursion_limit.value_within_limit(self.anon_depth) {
                             // A closure or generator may have itself as in its upvars.
@@ -228,7 +235,7 @@ impl<'cx, 'tcx> FallibleTypeFolder<TyCtxt<'tcx>> for QueryNormalizer<'cx, 'tcx>
                             return ty.try_super_fold_with(self);
                         }
 
-                        let generic_ty = self.interner().type_of(def_id);
+                        let generic_ty = self.interner().type_of(data.def_id);
                         let concrete_ty = generic_ty.subst(self.interner(), substs);
                         self.anon_depth += 1;
                         if concrete_ty == ty {
@@ -248,62 +255,22 @@ impl<'cx, 'tcx> FallibleTypeFolder<TyCtxt<'tcx>> for QueryNormalizer<'cx, 'tcx>
                 }
             }
 
-            ty::Alias(ty::Projection, data) if !data.has_escaping_bound_vars() => {
-                // This branch is just an optimization: when we don't have escaping bound vars,
-                // we don't need to replace them with placeholders (see branch below).
-
-                let tcx = self.infcx.tcx;
-                let data = data.try_fold_with(self)?;
-
-                let mut orig_values = OriginalQueryValues::default();
-                // HACK(matthewjasper) `'static` is special-cased in selection,
-                // so we cannot canonicalize it.
-                let c_data = self
-                    .infcx
-                    .canonicalize_query_keep_static(self.param_env.and(data), &mut orig_values);
-                debug!("QueryNormalizer: c_data = {:#?}", c_data);
-                debug!("QueryNormalizer: orig_values = {:#?}", orig_values);
-                let result = tcx.normalize_projection_ty(c_data)?;
-                // We don't expect ambiguity.
-                if result.is_ambiguous() {
-                    // Rustdoc normalizes possibly not well-formed types, so only
-                    // treat this as a bug if we're not in rustdoc.
-                    if !tcx.sess.opts.actually_rustdoc {
-                        tcx.sess.delay_span_bug(
-                            DUMMY_SP,
-                            format!("unexpected ambiguity: {:?} {:?}", c_data, result),
-                        );
-                    }
-                    return Err(NoSolution);
-                }
-                let InferOk { value: result, obligations } =
-                    self.infcx.instantiate_query_response_and_region_obligations(
-                        self.cause,
-                        self.param_env,
-                        &orig_values,
-                        result,
-                    )?;
-                debug!("QueryNormalizer: result = {:#?}", result);
-                debug!("QueryNormalizer: obligations = {:#?}", obligations);
-                self.obligations.extend(obligations);
-
-                let res = result.normalized_ty;
-                // `tcx.normalize_projection_ty` may normalize to a type that still has
-                // unevaluated consts, so keep normalizing here if that's the case.
-                if res != ty && res.has_type_flags(ty::TypeFlags::HAS_CT_PROJECTION) {
-                    res.try_super_fold_with(self)?
-                } else {
-                    res
-                }
-            }
+            ty::Opaque => ty.try_super_fold_with(self)?,
 
-            ty::Alias(ty::Projection, data) => {
+            ty::Projection => {
                 // See note in `rustc_trait_selection::traits::project`
 
                 let tcx = self.infcx.tcx;
                 let infcx = self.infcx;
-                let (data, mapped_regions, mapped_types, mapped_consts) =
-                    BoundVarReplacer::replace_bound_vars(infcx, &mut self.universes, data);
+                // Just an optimization: When we don't have escaping bound vars,
+                // we don't need to replace them with placeholders.
+                let (data, maps) = if data.has_escaping_bound_vars() {
+                    let (data, mapped_regions, mapped_types, mapped_consts) =
+                        BoundVarReplacer::replace_bound_vars(infcx, &mut self.universes, data);
+                    (data, Some((mapped_regions, mapped_types, mapped_consts)))
+                } else {
+                    (data, None)
+                };
                 let data = data.try_fold_with(self)?;
 
                 let mut orig_values = OriginalQueryValues::default();
@@ -337,14 +304,18 @@ impl<'cx, 'tcx> FallibleTypeFolder<TyCtxt<'tcx>> for QueryNormalizer<'cx, 'tcx>
                 debug!("QueryNormalizer: result = {:#?}", result);
                 debug!("QueryNormalizer: obligations = {:#?}", obligations);
                 self.obligations.extend(obligations);
-                let res = PlaceholderReplacer::replace_placeholders(
-                    infcx,
-                    mapped_regions,
-                    mapped_types,
-                    mapped_consts,
-                    &self.universes,
-                    result.normalized_ty,
-                );
+                let res = if let Some((mapped_regions, mapped_types, mapped_consts)) = maps {
+                    PlaceholderReplacer::replace_placeholders(
+                        infcx,
+                        mapped_regions,
+                        mapped_types,
+                        mapped_consts,
+                        &self.universes,
+                        result.normalized_ty,
+                    )
+                } else {
+                    result.normalized_ty
+                };
                 // `tcx.normalize_projection_ty` may normalize to a type that still has
                 // unevaluated consts, so keep normalizing here if that's the case.
                 if res != ty && res.has_type_flags(ty::TypeFlags::HAS_CT_PROJECTION) {
@@ -353,8 +324,6 @@ impl<'cx, 'tcx> FallibleTypeFolder<TyCtxt<'tcx>> for QueryNormalizer<'cx, 'tcx>
                     res
                 }
             }
-
-            _ => ty.try_super_fold_with(self)?,
         };
 
         self.cache.insert(ty, res);
diff --git a/compiler/rustc_trait_selection/src/traits/query/type_op/custom.rs b/compiler/rustc_trait_selection/src/traits/query/type_op/custom.rs
index 6bf3ed0d0..8f1b05c11 100644
--- a/compiler/rustc_trait_selection/src/traits/query/type_op/custom.rs
+++ b/compiler/rustc_trait_selection/src/traits/query/type_op/custom.rs
@@ -1,8 +1,8 @@
 use crate::infer::canonical::query_response;
 use crate::infer::{InferCtxt, InferOk};
-use crate::traits;
 use crate::traits::query::type_op::TypeOpOutput;
 use crate::traits::query::Fallible;
+use crate::traits::ObligationCtxt;
 use rustc_infer::infer::region_constraints::RegionConstraintData;
 use rustc_span::source_map::DUMMY_SP;
 
@@ -73,7 +73,9 @@ pub fn scrape_region_constraints<'tcx, Op: super::TypeOp<'tcx, Output = R>, R>(
     );
 
     let InferOk { value, obligations } = infcx.commit_if_ok(|_| op())?;
-    let errors = traits::fully_solve_obligations(infcx, obligations);
+    let ocx = ObligationCtxt::new(infcx);
+    ocx.register_obligations(obligations);
+    let errors = ocx.select_all_or_error();
     if !errors.is_empty() {
         infcx.tcx.sess.diagnostic().delay_span_bug(
             DUMMY_SP,
@@ -82,9 +84,7 @@ pub fn scrape_region_constraints<'tcx, Op: super::TypeOp<'tcx, Output = R>, R>(
     }
 
     let region_obligations = infcx.take_registered_region_obligations();
-
     let region_constraint_data = infcx.take_and_reset_region_constraints();
-
     let region_constraints = query_response::make_query_region_constraints(
         infcx.tcx,
         region_obligations
diff --git a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
index e91057356..1f5bbc178 100644
--- a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
@@ -5,12 +5,14 @@
 //! candidates. See the [rustc dev guide] for more details.
 //!
 //! [rustc dev guide]:https://rustc-dev-guide.rust-lang.org/traits/resolution.html#candidate-assembly
+
+use hir::def_id::DefId;
 use hir::LangItem;
 use rustc_hir as hir;
 use rustc_infer::traits::ObligationCause;
 use rustc_infer::traits::{Obligation, SelectionError, TraitObligation};
+use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams, TreatProjections};
 use rustc_middle::ty::{self, Ty, TypeVisitableExt};
-use rustc_target::spec::abi::Abi;
 
 use crate::traits;
 use crate::traits::query::evaluate_obligation::InferCtxtExt;
@@ -95,7 +97,9 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             } else if lang_items.tuple_trait() == Some(def_id) {
                 self.assemble_candidate_for_tuple(obligation, &mut candidates);
             } else if lang_items.pointer_like() == Some(def_id) {
-                self.assemble_candidate_for_ptr_sized(obligation, &mut candidates);
+                self.assemble_candidate_for_pointer_like(obligation, &mut candidates);
+            } else if lang_items.fn_ptr_trait() == Some(def_id) {
+                self.assemble_candidates_for_fn_ptr_trait(obligation, &mut candidates);
             } else {
                 if lang_items.clone_trait() == Some(def_id) {
                     // Same builtin conditions as `Copy`, i.e., every type which has builtin support
@@ -290,6 +294,9 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             return;
         }
 
+        // Keep this function in sync with extract_tupled_inputs_and_output_from_callable
+        // until the old solver (and thus this function) is removed.
+
         // Okay to skip binder because what we are inspecting doesn't involve bound regions.
         let self_ty = obligation.self_ty().skip_binder();
         match *self_ty.kind() {
@@ -298,31 +305,19 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 candidates.ambiguous = true; // Could wind up being a fn() type.
             }
             // Provide an impl, but only for suitable `fn` pointers.
-            ty::FnPtr(_) => {
-                if let ty::FnSig {
-                    unsafety: hir::Unsafety::Normal,
-                    abi: Abi::Rust,
-                    c_variadic: false,
-                    ..
-                } = self_ty.fn_sig(self.tcx()).skip_binder()
-                {
+            ty::FnPtr(sig) => {
+                if sig.is_fn_trait_compatible() {
                     candidates.vec.push(FnPointerCandidate { is_const: false });
                 }
             }
             // Provide an impl for suitable functions, rejecting `#[target_feature]` functions (RFC 2396).
             ty::FnDef(def_id, _) => {
-                if let ty::FnSig {
-                    unsafety: hir::Unsafety::Normal,
-                    abi: Abi::Rust,
-                    c_variadic: false,
-                    ..
-                } = self_ty.fn_sig(self.tcx()).skip_binder()
+                if self.tcx().fn_sig(def_id).skip_binder().is_fn_trait_compatible()
+                    && self.tcx().codegen_fn_attrs(def_id).target_features.is_empty()
                 {
-                    if self.tcx().codegen_fn_attrs(def_id).target_features.is_empty() {
-                        candidates
-                            .vec
-                            .push(FnPointerCandidate { is_const: self.tcx().is_const_fn(def_id) });
-                    }
+                    candidates
+                        .vec
+                        .push(FnPointerCandidate { is_const: self.tcx().is_const_fn(def_id) });
                 }
             }
             _ => {}
@@ -330,13 +325,12 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     }
 
     /// Searches for impls that might apply to `obligation`.
+    #[instrument(level = "debug", skip(self, candidates))]
     fn assemble_candidates_from_impls(
         &mut self,
         obligation: &TraitObligation<'tcx>,
         candidates: &mut SelectionCandidateSet<'tcx>,
     ) {
-        debug!(?obligation, "assemble_candidates_from_impls");
-
         // Essentially any user-written impl will match with an error type,
         // so creating `ImplCandidates` isn't useful. However, we might
         // end up finding a candidate elsewhere (e.g. a `BuiltinCandidate` for `Sized`)
@@ -350,6 +344,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             return;
         }
 
+        let drcx = DeepRejectCtxt { treat_obligation_params: TreatParams::ForLookup };
+        let obligation_substs = obligation.predicate.skip_binder().trait_ref.substs;
         self.tcx().for_each_relevant_impl(
             obligation.predicate.def_id(),
             obligation.predicate.skip_binder().trait_ref.self_ty(),
@@ -358,7 +354,14 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 // consider a "quick reject". This avoids creating more types
                 // and so forth that we need to.
                 let impl_trait_ref = self.tcx().impl_trait_ref(impl_def_id).unwrap();
-                if self.fast_reject_trait_refs(obligation, &impl_trait_ref.0) {
+                if !drcx.substs_refs_may_unify(obligation_substs, impl_trait_ref.0.substs) {
+                    return;
+                }
+                if self.reject_fn_ptr_impls(
+                    impl_def_id,
+                    obligation,
+                    impl_trait_ref.skip_binder().self_ty(),
+                ) {
                     return;
                 }
 
@@ -371,6 +374,99 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         );
     }
 
+    /// The various `impl<T: FnPtr> Trait for T` in libcore are more like builtin impls for all function items
+    /// and function pointers and less like blanket impls. Rejecting them when they can't possibly apply (because
+    /// the obligation's self-type does not implement `FnPtr`) avoids reporting that the self type does not implement
+    /// `FnPtr`, when we wanted to report that it doesn't implement `Trait`.
+    #[instrument(level = "trace", skip(self), ret)]
+    fn reject_fn_ptr_impls(
+        &self,
+        impl_def_id: DefId,
+        obligation: &TraitObligation<'tcx>,
+        impl_self_ty: Ty<'tcx>,
+    ) -> bool {
+        // Let `impl<T: FnPtr> Trait for Vec<T>` go through the normal rejection path.
+        if !matches!(impl_self_ty.kind(), ty::Param(..)) {
+            return false;
+        }
+        let Some(fn_ptr_trait) = self.tcx().lang_items().fn_ptr_trait() else {
+            return false;
+        };
+
+        for &(predicate, _) in self.tcx().predicates_of(impl_def_id).predicates {
+            let ty::PredicateKind::Clause(ty::Clause::Trait(pred))
+                = predicate.kind().skip_binder() else { continue };
+            if fn_ptr_trait != pred.trait_ref.def_id {
+                continue;
+            }
+            trace!(?pred);
+            // Not the bound we're looking for
+            if pred.self_ty() != impl_self_ty {
+                continue;
+            }
+
+            match obligation.self_ty().skip_binder().kind() {
+                // Fast path to avoid evaluating an obligation that trivially holds.
+                // There may be more bounds, but these are checked by the regular path.
+                ty::FnPtr(..) => return false,
+                // These may potentially implement `FnPtr`
+                ty::Placeholder(..)
+                | ty::Dynamic(_, _, _)
+                | ty::Alias(_, _)
+                | ty::Infer(_)
+                | ty::Param(..) => {}
+
+                ty::Bound(_, _) => span_bug!(
+                    obligation.cause.span(),
+                    "cannot have escaping bound var in self type of {obligation:#?}"
+                ),
+                // These can't possibly implement `FnPtr` as they are concrete types
+                // and not `FnPtr`
+                ty::Bool
+                | ty::Char
+                | ty::Int(_)
+                | ty::Uint(_)
+                | ty::Float(_)
+                | ty::Adt(_, _)
+                | ty::Foreign(_)
+                | ty::Str
+                | ty::Array(_, _)
+                | ty::Slice(_)
+                | ty::RawPtr(_)
+                | ty::Ref(_, _, _)
+                | ty::Closure(_, _)
+                | ty::Generator(_, _, _)
+                | ty::GeneratorWitness(_)
+                | ty::GeneratorWitnessMIR(_, _)
+                | ty::Never
+                | ty::Tuple(_)
+                | ty::Error(_) => return true,
+                // FIXME: Function definitions could actually implement `FnPtr` by
+                // casting the ZST function def to a function pointer.
+                ty::FnDef(_, _) => return true,
+            }
+
+            // Generic params can implement `FnPtr` if the predicate
+            // holds within its own environment.
+            let obligation = Obligation::new(
+                self.tcx(),
+                obligation.cause.clone(),
+                obligation.param_env,
+                self.tcx().mk_predicate(obligation.predicate.map_bound(|mut pred| {
+                    pred.trait_ref =
+                        self.tcx().mk_trait_ref(fn_ptr_trait, [pred.trait_ref.self_ty()]);
+                    ty::PredicateKind::Clause(ty::Clause::Trait(pred))
+                })),
+            );
+            if let Ok(r) = self.infcx.evaluate_obligation(&obligation) {
+                if !r.may_apply() {
+                    return true;
+                }
+            }
+        }
+        false
+    }
+
     fn assemble_candidates_from_auto_impls(
         &mut self,
         obligation: &TraitObligation<'tcx>,
@@ -783,6 +879,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 let relevant_impl = self.tcx().find_map_relevant_impl(
                     self.tcx().require_lang_item(LangItem::Drop, None),
                     obligation.predicate.skip_binder().trait_ref.self_ty(),
+                    TreatProjections::ForLookup,
                     Some,
                 );
 
@@ -845,15 +942,15 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         }
     }
 
-    fn assemble_candidate_for_ptr_sized(
+    fn assemble_candidate_for_pointer_like(
         &mut self,
         obligation: &TraitObligation<'tcx>,
         candidates: &mut SelectionCandidateSet<'tcx>,
     ) {
         // The regions of a type don't affect the size of the type
-        let self_ty = self
-            .tcx()
-            .erase_regions(self.tcx().erase_late_bound_regions(obligation.predicate.self_ty()));
+        let tcx = self.tcx();
+        let self_ty =
+            tcx.erase_regions(tcx.erase_late_bound_regions(obligation.predicate.self_ty()));
 
         // But if there are inference variables, we have to wait until it's resolved.
         if self_ty.has_non_region_infer() {
@@ -861,13 +958,55 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             return;
         }
 
-        let usize_layout =
-            self.tcx().layout_of(ty::ParamEnv::empty().and(self.tcx().types.usize)).unwrap().layout;
-        if let Ok(layout) = self.tcx().layout_of(obligation.param_env.and(self_ty))
-            && layout.layout.size() == usize_layout.size()
-            && layout.layout.align().abi == usize_layout.align().abi
+        if let Ok(layout) = tcx.layout_of(obligation.param_env.and(self_ty))
+            && layout.layout.is_pointer_like(&tcx.data_layout)
         {
             candidates.vec.push(BuiltinCandidate { has_nested: false });
         }
     }
+
+    fn assemble_candidates_for_fn_ptr_trait(
+        &mut self,
+        obligation: &TraitObligation<'tcx>,
+        candidates: &mut SelectionCandidateSet<'tcx>,
+    ) {
+        let self_ty = self.infcx.shallow_resolve(obligation.self_ty());
+        match self_ty.skip_binder().kind() {
+            ty::FnPtr(_) => candidates.vec.push(BuiltinCandidate { has_nested: false }),
+            ty::Bool
+            | ty::Char
+            | ty::Int(_)
+            | ty::Uint(_)
+            | ty::Float(_)
+            | ty::Adt(..)
+            | ty::Foreign(..)
+            | ty::Str
+            | ty::Array(..)
+            | ty::Slice(_)
+            | ty::RawPtr(_)
+            | ty::Ref(..)
+            | ty::FnDef(..)
+            | ty::Placeholder(..)
+            | ty::Dynamic(..)
+            | ty::Closure(..)
+            | ty::Generator(..)
+            | ty::GeneratorWitness(..)
+            | ty::GeneratorWitnessMIR(..)
+            | ty::Never
+            | ty::Tuple(..)
+            | ty::Alias(..)
+            | ty::Param(..)
+            | ty::Bound(..)
+            | ty::Error(_)
+            | ty::Infer(
+                ty::InferTy::IntVar(_)
+                | ty::InferTy::FloatVar(_)
+                | ty::InferTy::FreshIntTy(_)
+                | ty::InferTy::FreshFloatTy(_),
+            ) => {}
+            ty::Infer(ty::InferTy::TyVar(_) | ty::InferTy::FreshTy(_)) => {
+                candidates.ambiguous = true;
+            }
+        }
+    }
 }
diff --git a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
index 21c158fd0..88121f865 100644
--- a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
@@ -8,8 +8,8 @@
 //! https://rustc-dev-guide.rust-lang.org/traits/resolution.html#confirmation
 use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_hir::lang_items::LangItem;
-use rustc_infer::infer::InferOk;
 use rustc_infer::infer::LateBoundRegionConversionTime::HigherRankedType;
+use rustc_infer::infer::{DefineOpaqueTypes, InferOk};
 use rustc_middle::ty::{
     self, Binder, GenericParamDefKind, InternalSubsts, SubstsRef, ToPolyTraitRef, ToPredicate,
     TraitRef, Ty, TyCtxt, TypeVisitableExt,
@@ -18,7 +18,7 @@ use rustc_session::config::TraitSolver;
 use rustc_span::def_id::DefId;
 
 use crate::traits::project::{normalize_with_depth, normalize_with_depth_to};
-use crate::traits::util::{self, closure_trait_ref_and_return_type, predicate_for_trait_def};
+use crate::traits::util::{self, closure_trait_ref_and_return_type};
 use crate::traits::vtable::{
     count_own_vtable_entries, prepare_vtable_segments, vtable_trait_first_method_offset,
     VtblSegment,
@@ -131,6 +131,12 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             }
         };
 
+        // The obligations returned by confirmation are recursively evaluated
+        // so we need to make sure they have the correct depth.
+        for subobligation in impl_src.borrow_nested_obligations_mut() {
+            subobligation.set_depth_from_parent(obligation.recursion_depth);
+        }
+
         if !obligation.predicate.is_const_if_const() {
             // normalize nested predicates according to parent predicate's constness.
             impl_src = impl_src.map(|mut o| {
@@ -177,7 +183,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         obligations.extend(self.infcx.commit_if_ok(|_| {
             self.infcx
                 .at(&obligation.cause, obligation.param_env)
-                .sup(placeholder_trait_predicate, candidate)
+                .sup(DefineOpaqueTypes::No, placeholder_trait_predicate, candidate)
                 .map(|InferOk { obligations, .. }| obligations)
                 .map_err(|_| Unimplemented)
         })?);
@@ -253,15 +259,13 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             };
 
             let cause = obligation.derived_cause(BuiltinDerivedObligation);
-            ensure_sufficient_stack(|| {
-                self.collect_predicates_for_types(
-                    obligation.param_env,
-                    cause,
-                    obligation.recursion_depth + 1,
-                    trait_def,
-                    nested,
-                )
-            })
+            self.collect_predicates_for_types(
+                obligation.param_env,
+                cause,
+                obligation.recursion_depth + 1,
+                trait_def,
+                nested,
+            )
         } else {
             vec![]
         };
@@ -462,7 +466,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         nested.extend(self.infcx.commit_if_ok(|_| {
             self.infcx
                 .at(&obligation.cause, obligation.param_env)
-                .sup(obligation_trait_ref, upcast_trait_ref)
+                .sup(DefineOpaqueTypes::No, obligation_trait_ref, upcast_trait_ref)
                 .map(|InferOk { obligations, .. }| obligations)
                 .map_err(|_| Unimplemented)
         })?);
@@ -601,10 +605,18 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         debug!(?obligation, "confirm_fn_pointer_candidate");
 
         let tcx = self.tcx();
-        let self_ty = self
+
+        let Some(self_ty) = self
             .infcx
-            .shallow_resolve(obligation.self_ty().no_bound_vars())
-            .expect("fn pointer should not capture bound vars from predicate");
+            .shallow_resolve(obligation.self_ty().no_bound_vars()) else
+        {
+            // FIXME: Ideally we'd support `for<'a> fn(&'a ()): Fn(&'a ())`,
+            // but we do not currently. Luckily, such a bound is not
+            // particularly useful, so we don't expect users to write
+            // them often.
+            return Err(SelectionError::Unimplemented);
+        };
+
         let sig = self_ty.fn_sig(tcx);
         let trait_ref = closure_trait_ref_and_return_type(
             tcx,
@@ -819,11 +831,10 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 )
             });
 
+        // needed to define opaque types for tests/ui/type-alias-impl-trait/assoc-projection-ice.rs
         self.infcx
             .at(&obligation.cause, obligation.param_env)
-            // needed for tests/ui/type-alias-impl-trait/assoc-projection-ice.rs
-            .define_opaque_types(true)
-            .sup(obligation_trait_ref, expected_trait_ref)
+            .sup(DefineOpaqueTypes::Yes, obligation_trait_ref, expected_trait_ref)
             .map(|InferOk { mut obligations, .. }| {
                 obligations.extend(nested);
                 obligations
@@ -888,7 +899,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 let InferOk { obligations, .. } = self
                     .infcx
                     .at(&obligation.cause, obligation.param_env)
-                    .sup(target, source_trait)
+                    .sup(DefineOpaqueTypes::No, target, source_trait)
                     .map_err(|_| Unimplemented)?;
                 nested.extend(obligations);
 
@@ -987,7 +998,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 let InferOk { obligations, .. } = self
                     .infcx
                     .at(&obligation.cause, obligation.param_env)
-                    .sup(target, source_trait)
+                    .sup(DefineOpaqueTypes::No, target, source_trait)
                     .map_err(|_| Unimplemented)?;
                 nested.extend(obligations);
 
@@ -1058,7 +1069,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 let InferOk { obligations, .. } = self
                     .infcx
                     .at(&obligation.cause, obligation.param_env)
-                    .eq(b, a)
+                    .eq(DefineOpaqueTypes::No, b, a)
                     .map_err(|_| Unimplemented)?;
                 nested.extend(obligations);
             }
@@ -1073,6 +1084,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 let tail_field = def
                     .non_enum_variant()
                     .fields
+                    .raw
                     .last()
                     .expect("expected unsized ADT to have a tail field");
                 let tail_field_ty = tcx.type_of(tail_field.did);
@@ -1106,19 +1118,16 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 let InferOk { obligations, .. } = self
                     .infcx
                     .at(&obligation.cause, obligation.param_env)
-                    .eq(target, new_struct)
+                    .eq(DefineOpaqueTypes::No, target, new_struct)
                     .map_err(|_| Unimplemented)?;
                 nested.extend(obligations);
 
                 // Construct the nested `TailField<T>: Unsize<TailField<U>>` predicate.
-                nested.push(predicate_for_trait_def(
+                let tail_unsize_obligation = obligation.with(
                     tcx,
-                    obligation.param_env,
-                    obligation.cause.clone(),
-                    obligation.predicate.def_id(),
-                    obligation.recursion_depth + 1,
-                    [source_tail, target_tail],
-                ));
+                    tcx.mk_trait_ref(obligation.predicate.def_id(), [source_tail, target_tail]),
+                );
+                nested.push(tail_unsize_obligation);
             }
 
             // `(.., T)` -> `(.., U)`
@@ -1136,21 +1145,14 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 let InferOk { obligations, .. } = self
                     .infcx
                     .at(&obligation.cause, obligation.param_env)
-                    .eq(target, new_tuple)
+                    .eq(DefineOpaqueTypes::No, target, new_tuple)
                     .map_err(|_| Unimplemented)?;
                 nested.extend(obligations);
 
-                // Construct the nested `T: Unsize<U>` predicate.
-                nested.push(ensure_sufficient_stack(|| {
-                    predicate_for_trait_def(
-                        tcx,
-                        obligation.param_env,
-                        obligation.cause.clone(),
-                        obligation.predicate.def_id(),
-                        obligation.recursion_depth + 1,
-                        [a_last, b_last],
-                    )
-                }));
+                // Add a nested `T: Unsize<U>` predicate.
+                let last_unsize_obligation = obligation
+                    .with(tcx, tcx.mk_trait_ref(obligation.predicate.def_id(), [a_last, b_last]));
+                nested.push(last_unsize_obligation);
             }
 
             _ => bug!("source: {source}, target: {target}"),
diff --git a/compiler/rustc_trait_selection/src/traits/select/mod.rs b/compiler/rustc_trait_selection/src/traits/select/mod.rs
index 7f454fbb3..6bb53418b 100644
--- a/compiler/rustc_trait_selection/src/traits/select/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/mod.rs
@@ -2,12 +2,6 @@
 //!
 //! [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/traits/resolution.html#selection
 
-// FIXME: The `map` field in ProvisionalEvaluationCache should be changed to
-// a `FxIndexMap` to avoid query instability, but right now it causes a perf regression. This would be
-// fixed or at least lightened by the addition of the `drain_filter` method to `FxIndexMap`
-// Relevant: https://github.com/rust-lang/rust/pull/103723 and https://github.com/bluss/indexmap/issues/242
-#![allow(rustc::potential_query_instability)]
-
 use self::EvaluationResult::*;
 use self::SelectionCandidate::*;
 
@@ -17,7 +11,7 @@ use super::project;
 use super::project::normalize_with_depth_to;
 use super::project::ProjectionTyObligation;
 use super::util;
-use super::util::{closure_trait_ref_and_return_type, predicate_for_trait_def};
+use super::util::closure_trait_ref_and_return_type;
 use super::wf;
 use super::{
     ErrorReporting, ImplDerivedObligation, ImplDerivedObligationCause, Normalized, Obligation,
@@ -32,25 +26,23 @@ use crate::traits::project::ProjectAndUnifyResult;
 use crate::traits::project::ProjectionCacheKeyExt;
 use crate::traits::ProjectionCacheKey;
 use crate::traits::Unimplemented;
-use rustc_data_structures::fx::FxHashMap;
-use rustc_data_structures::fx::{FxHashSet, FxIndexSet};
+use rustc_data_structures::fx::{FxHashSet, FxIndexMap, FxIndexSet};
 use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_errors::Diagnostic;
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
+use rustc_infer::infer::DefineOpaqueTypes;
 use rustc_infer::infer::LateBoundRegionConversionTime;
 use rustc_infer::traits::TraitEngine;
 use rustc_infer::traits::TraitEngineExt;
 use rustc_middle::dep_graph::{DepKind, DepNodeIndex};
 use rustc_middle::mir::interpret::ErrorHandled;
 use rustc_middle::ty::abstract_const::NotConstEvaluatable;
-use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams};
 use rustc_middle::ty::fold::BottomUpFolder;
 use rustc_middle::ty::relate::TypeRelation;
 use rustc_middle::ty::SubstsRef;
 use rustc_middle::ty::{self, EarlyBinder, PolyProjectionPredicate, ToPolyTraitRef, ToPredicate};
 use rustc_middle::ty::{Ty, TyCtxt, TypeFoldable, TypeVisitableExt};
-use rustc_session::config::TraitSolver;
 use rustc_span::symbol::sym;
 
 use std::cell::{Cell, RefCell};
@@ -178,14 +170,14 @@ struct TraitObligationStack<'prev, 'tcx> {
 }
 
 struct SelectionCandidateSet<'tcx> {
-    // A list of candidates that definitely apply to the current
-    // obligation (meaning: types unify).
+    /// A list of candidates that definitely apply to the current
+    /// obligation (meaning: types unify).
     vec: Vec<SelectionCandidate<'tcx>>,
 
-    // If `true`, then there were candidates that might or might
-    // not have applied, but we couldn't tell. This occurs when some
-    // of the input types are type variables, in which case there are
-    // various "builtin" rules that might or might not trigger.
+    /// If `true`, then there were candidates that might or might
+    /// not have applied, but we couldn't tell. This occurs when some
+    /// of the input types are type variables, in which case there are
+    /// various "builtin" rules that might or might not trigger.
     ambiguous: bool,
 }
 
@@ -211,7 +203,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     pub fn new(infcx: &'cx InferCtxt<'tcx>) -> SelectionContext<'cx, 'tcx> {
         SelectionContext {
             infcx,
-            freshener: infcx.freshener_keep_static(),
+            freshener: infcx.freshener(),
             intercrate_ambiguity_causes: None,
             query_mode: TraitQueryMode::Standard,
         }
@@ -465,14 +457,14 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         if candidates.len() > 1 {
             let mut i = 0;
             while i < candidates.len() {
-                let is_dup = (0..candidates.len()).filter(|&j| i != j).any(|j| {
+                let should_drop_i = (0..candidates.len()).filter(|&j| i != j).any(|j| {
                     self.candidate_should_be_dropped_in_favor_of(
                         &candidates[i],
                         &candidates[j],
                         needs_infer,
-                    )
+                    ) == DropVictim::Yes
                 });
-                if is_dup {
+                if should_drop_i {
                     debug!(candidate = ?candidates[i], "Dropping candidate #{}/{}", i, candidates.len());
                     candidates.swap_remove(i);
                 } else {
@@ -545,13 +537,13 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         obligation: &PredicateObligation<'tcx>,
     ) -> Result<EvaluationResult, OverflowError> {
         self.evaluation_probe(|this| {
-            if this.tcx().sess.opts.unstable_opts.trait_solver != TraitSolver::Next {
+            if this.tcx().trait_solver_next() {
+                this.evaluate_predicates_recursively_in_new_solver([obligation.clone()])
+            } else {
                 this.evaluate_predicate_recursively(
                     TraitObligationStackList::empty(&ProvisionalEvaluationCache::default()),
                     obligation.clone(),
                 )
-            } else {
-                this.evaluate_predicates_recursively_in_new_solver([obligation.clone()])
             }
         })
     }
@@ -591,9 +583,12 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     where
         I: IntoIterator<Item = PredicateObligation<'tcx>> + std::fmt::Debug,
     {
-        if self.tcx().sess.opts.unstable_opts.trait_solver != TraitSolver::Next {
+        if self.tcx().trait_solver_next() {
+            self.evaluate_predicates_recursively_in_new_solver(predicates)
+        } else {
             let mut result = EvaluatedToOk;
-            for obligation in predicates {
+            for mut obligation in predicates {
+                obligation.set_depth_from_parent(stack.depth());
                 let eval = self.evaluate_predicate_recursively(stack, obligation.clone())?;
                 if let EvaluatedToErr = eval {
                     // fast-path - EvaluatedToErr is the top of the lattice,
@@ -604,8 +599,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 }
             }
             Ok(result)
-        } else {
-            self.evaluate_predicates_recursively_in_new_solver(predicates)
         }
     }
 
@@ -617,6 +610,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         let mut fulfill_cx = crate::solve::FulfillmentCtxt::new();
         fulfill_cx.register_predicate_obligations(self.infcx, predicates);
         // True errors
+        // FIXME(-Ztrait-solver=next): Overflows are reported as ambig here, is that OK?
         if !fulfill_cx.select_where_possible(self.infcx).is_empty() {
             return Ok(EvaluatedToErr);
         }
@@ -661,12 +655,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     let p = bound_predicate.rebind(p);
                     // Does this code ever run?
                     match self.infcx.subtype_predicate(&obligation.cause, obligation.param_env, p) {
-                        Ok(Ok(InferOk { mut obligations, .. })) => {
-                            self.add_depth(obligations.iter_mut(), obligation.recursion_depth);
-                            self.evaluate_predicates_recursively(
-                                previous_stack,
-                                obligations.into_iter(),
-                            )
+                        Ok(Ok(InferOk { obligations, .. })) => {
+                            self.evaluate_predicates_recursively(previous_stack, obligations)
                         }
                         Ok(Err(_)) => Ok(EvaluatedToErr),
                         Err(..) => Ok(EvaluatedToAmbig),
@@ -677,12 +667,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     let p = bound_predicate.rebind(p);
                     // Does this code ever run?
                     match self.infcx.coerce_predicate(&obligation.cause, obligation.param_env, p) {
-                        Ok(Ok(InferOk { mut obligations, .. })) => {
-                            self.add_depth(obligations.iter_mut(), obligation.recursion_depth);
-                            self.evaluate_predicates_recursively(
-                                previous_stack,
-                                obligations.into_iter(),
-                            )
+                        Ok(Ok(InferOk { obligations, .. })) => {
+                            self.evaluate_predicates_recursively(previous_stack, obligations)
                         }
                         Ok(Err(_)) => Ok(EvaluatedToErr),
                         Err(..) => Ok(EvaluatedToAmbig),
@@ -755,9 +741,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                         arg,
                         obligation.cause.span,
                     ) {
-                        Some(mut obligations) => {
-                            self.add_depth(obligations.iter_mut(), obligation.recursion_depth);
-
+                        Some(obligations) => {
                             cache.wf_args.borrow_mut().push((arg, previous_stack.depth()));
                             let result =
                                 self.evaluate_predicates_recursively(previous_stack, obligations);
@@ -778,14 +762,16 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 }
 
                 ty::PredicateKind::Clause(ty::Clause::TypeOutlives(pred)) => {
-                    // A global type with no late-bound regions can only
-                    // contain the "'static" lifetime (any other lifetime
-                    // would either be late-bound or local), so it is guaranteed
-                    // to outlive any other lifetime
-                    if pred.0.is_global() && !pred.0.has_late_bound_vars() {
-                        Ok(EvaluatedToOk)
-                    } else {
+                    // A global type with no free lifetimes or generic parameters
+                    // outlives anything.
+                    if pred.0.has_free_regions()
+                        || pred.0.has_late_bound_regions()
+                        || pred.0.has_non_region_infer()
+                        || pred.0.has_non_region_infer()
+                    {
                         Ok(EvaluatedToOkModuloRegions)
+                    } else {
+                        Ok(EvaluatedToOk)
                     }
                 }
 
@@ -826,10 +812,14 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                                     }
                                 }
 
-                                self.add_depth(
-                                    subobligations.iter_mut(),
-                                    obligation.recursion_depth,
-                                );
+                                // Need to explicitly set the depth of nested goals here as
+                                // projection obligations can cycle by themselves and in
+                                // `evaluate_predicates_recursively` we only add the depth
+                                // for parent trait goals because only these get added to the
+                                // `TraitObligationStackList`.
+                                for subobligation in subobligations.iter_mut() {
+                                    subobligation.set_depth_from_parent(obligation.recursion_depth);
+                                }
                                 let res = self.evaluate_predicates_recursively(
                                     previous_stack,
                                     subobligations,
@@ -909,38 +899,28 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                                 if a.def.did == b.def.did
                                     && tcx.def_kind(a.def.did) == DefKind::AssocConst =>
                             {
-                                if let Ok(new_obligations) = self
+                                if let Ok(InferOk { obligations, value: () }) = self
                                     .infcx
                                     .at(&obligation.cause, obligation.param_env)
                                     .trace(c1, c2)
-                                    .eq(a.substs, b.substs)
+                                    .eq(DefineOpaqueTypes::No, a.substs, b.substs)
                                 {
-                                    let mut obligations = new_obligations.obligations;
-                                    self.add_depth(
-                                        obligations.iter_mut(),
-                                        obligation.recursion_depth,
-                                    );
                                     return self.evaluate_predicates_recursively(
                                         previous_stack,
-                                        obligations.into_iter(),
+                                        obligations,
                                     );
                                 }
                             }
                             (_, Unevaluated(_)) | (Unevaluated(_), _) => (),
                             (_, _) => {
-                                if let Ok(new_obligations) = self
+                                if let Ok(InferOk { obligations, value: () }) = self
                                     .infcx
                                     .at(&obligation.cause, obligation.param_env)
-                                    .eq(c1, c2)
+                                    .eq(DefineOpaqueTypes::No, c1, c2)
                                 {
-                                    let mut obligations = new_obligations.obligations;
-                                    self.add_depth(
-                                        obligations.iter_mut(),
-                                        obligation.recursion_depth,
-                                    );
                                     return self.evaluate_predicates_recursively(
                                         previous_stack,
-                                        obligations.into_iter(),
+                                        obligations,
                                     );
                                 }
                             }
@@ -965,8 +945,11 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
                     match (evaluate(c1), evaluate(c2)) {
                         (Ok(c1), Ok(c2)) => {
-                            match self.infcx.at(&obligation.cause, obligation.param_env).eq(c1, c2)
-                            {
+                            match self.infcx.at(&obligation.cause, obligation.param_env).eq(
+                                DefineOpaqueTypes::No,
+                                c1,
+                                c2,
+                            ) {
                                 Ok(inf_ok) => self.evaluate_predicates_recursively(
                                     previous_stack,
                                     inf_ok.into_obligations(),
@@ -989,12 +972,16 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
                     bug!("TypeWellFormedFromEnv is only used for chalk")
                 }
-                ty::PredicateKind::AliasEq(..) => {
-                    bug!("AliasEq is only used for new solver")
+                ty::PredicateKind::AliasRelate(..) => {
+                    bug!("AliasRelate is only used for new solver")
                 }
                 ty::PredicateKind::Ambiguous => Ok(EvaluatedToAmbig),
                 ty::PredicateKind::Clause(ty::Clause::ConstArgHasType(ct, ty)) => {
-                    match self.infcx.at(&obligation.cause, obligation.param_env).eq(ct.ty(), ty) {
+                    match self.infcx.at(&obligation.cause, obligation.param_env).eq(
+                        DefineOpaqueTypes::No,
+                        ct.ty(),
+                        ty,
+                    ) {
                         Ok(inf_ok) => self.evaluate_predicates_recursively(
                             previous_stack,
                             inf_ok.into_obligations(),
@@ -1359,24 +1346,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         self.infcx.evaluation_cache.insert((param_env, trait_pred), dep_node, result);
     }
 
-    /// For various reasons, it's possible for a subobligation
-    /// to have a *lower* recursion_depth than the obligation used to create it.
-    /// Projection sub-obligations may be returned from the projection cache,
-    /// which results in obligations with an 'old' `recursion_depth`.
-    /// Additionally, methods like `InferCtxt.subtype_predicate` produce
-    /// subobligations without taking in a 'parent' depth, causing the
-    /// generated subobligations to have a `recursion_depth` of `0`.
-    ///
-    /// To ensure that obligation_depth never decreases, we force all subobligations
-    /// to have at least the depth of the original obligation.
-    fn add_depth<T: 'cx, I: Iterator<Item = &'cx mut Obligation<'tcx, T>>>(
-        &self,
-        it: I,
-        min_depth: usize,
-    ) {
-        it.for_each(|o| o.recursion_depth = cmp::max(min_depth, o.recursion_depth) + 1);
-    }
-
     fn check_recursion_depth<T>(
         &self,
         depth: usize,
@@ -1752,7 +1721,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         });
         self.infcx
             .at(&obligation.cause, obligation.param_env)
-            .sup(ty::Binder::dummy(placeholder_trait_ref), trait_bound)
+            .sup(DefineOpaqueTypes::No, ty::Binder::dummy(placeholder_trait_ref), trait_bound)
             .map(|InferOk { obligations: _, value: () }| {
                 // This method is called within a probe, so we can't have
                 // inference variables and placeholders escape.
@@ -1814,7 +1783,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         let is_match = self
             .infcx
             .at(&obligation.cause, obligation.param_env)
-            .sup(obligation.predicate, infer_projection)
+            .sup(DefineOpaqueTypes::No, obligation.predicate, infer_projection)
             .map_or(false, |InferOk { obligations, value: () }| {
                 self.evaluate_predicates_recursively(
                     TraitObligationStackList::empty(&ProvisionalEvaluationCache::default()),
@@ -1842,16 +1811,28 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             ProjectionMatchesProjection::No
         }
     }
+}
 
-    ///////////////////////////////////////////////////////////////////////////
-    // WINNOW
-    //
-    // Winnowing is the process of attempting to resolve ambiguity by
-    // probing further. During the winnowing process, we unify all
-    // type variables and then we also attempt to evaluate recursive
-    // bounds to see if they are satisfied.
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+enum DropVictim {
+    Yes,
+    No,
+}
+
+impl DropVictim {
+    fn drop_if(should_drop: bool) -> DropVictim {
+        if should_drop { DropVictim::Yes } else { DropVictim::No }
+    }
+}
 
-    /// Returns `true` if `victim` should be dropped in favor of
+/// ## Winnowing
+///
+/// Winnowing is the process of attempting to resolve ambiguity by
+/// probing further. During the winnowing process, we unify all
+/// type variables and then we also attempt to evaluate recursive
+/// bounds to see if they are satisfied.
+impl<'tcx> SelectionContext<'_, 'tcx> {
+    /// Returns `DropVictim::Yes` if `victim` should be dropped in favor of
     /// `other`. Generally speaking we will drop duplicate
     /// candidates and prefer where-clause candidates.
     ///
@@ -1861,9 +1842,9 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         victim: &EvaluatedCandidate<'tcx>,
         other: &EvaluatedCandidate<'tcx>,
         needs_infer: bool,
-    ) -> bool {
+    ) -> DropVictim {
         if victim.candidate == other.candidate {
-            return true;
+            return DropVictim::Yes;
         }
 
         // Check if a bound would previously have been removed when normalizing
@@ -1887,11 +1868,15 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             }
 
             // FIXME(@jswrenn): this should probably be more sophisticated
-            (TransmutabilityCandidate, _) | (_, TransmutabilityCandidate) => false,
+            (TransmutabilityCandidate, _) | (_, TransmutabilityCandidate) => DropVictim::No,
 
             // (*)
-            (BuiltinCandidate { has_nested: false } | ConstDestructCandidate(_), _) => true,
-            (_, BuiltinCandidate { has_nested: false } | ConstDestructCandidate(_)) => false,
+            (BuiltinCandidate { has_nested: false } | ConstDestructCandidate(_), _) => {
+                DropVictim::Yes
+            }
+            (_, BuiltinCandidate { has_nested: false } | ConstDestructCandidate(_)) => {
+                DropVictim::No
+            }
 
             (ParamCandidate(other), ParamCandidate(victim)) => {
                 let same_except_bound_vars = other.skip_binder().trait_ref
@@ -1905,28 +1890,23 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     // or the current one if tied (they should both evaluate to the same answer). This is
                     // probably best characterized as a "hack", since we might prefer to just do our
                     // best to *not* create essentially duplicate candidates in the first place.
-                    other.bound_vars().len() <= victim.bound_vars().len()
+                    DropVictim::drop_if(other.bound_vars().len() <= victim.bound_vars().len())
                 } else if other.skip_binder().trait_ref == victim.skip_binder().trait_ref
                     && victim.skip_binder().constness == ty::BoundConstness::NotConst
                     && other.skip_binder().polarity == victim.skip_binder().polarity
                 {
                     // Drop otherwise equivalent non-const candidates in favor of const candidates.
-                    true
+                    DropVictim::Yes
                 } else {
-                    false
+                    DropVictim::No
                 }
             }
 
             // Drop otherwise equivalent non-const fn pointer candidates
-            (FnPointerCandidate { .. }, FnPointerCandidate { is_const: false }) => true,
+            (FnPointerCandidate { .. }, FnPointerCandidate { is_const: false }) => DropVictim::Yes,
 
-            // Global bounds from the where clause should be ignored
-            // here (see issue #50825). Otherwise, we have a where
-            // clause so don't go around looking for impls.
-            // Arbitrarily give param candidates priority
-            // over projection and object candidates.
             (
-                ParamCandidate(ref cand),
+                ParamCandidate(ref other_cand),
                 ImplCandidate(..)
                 | ClosureCandidate { .. }
                 | GeneratorCandidate
@@ -1939,11 +1919,19 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 | TraitAliasCandidate
                 | ObjectCandidate(_)
                 | ProjectionCandidate(..),
-            ) => !is_global(cand),
-            (ObjectCandidate(_) | ProjectionCandidate(..), ParamCandidate(ref cand)) => {
+            ) => {
+                // We have a where clause so don't go around looking
+                // for impls. Arbitrarily give param candidates priority
+                // over projection and object candidates.
+                //
+                // Global bounds from the where clause should be ignored
+                // here (see issue #50825).
+                DropVictim::drop_if(!is_global(other_cand))
+            }
+            (ObjectCandidate(_) | ProjectionCandidate(..), ParamCandidate(ref victim_cand)) => {
                 // Prefer these to a global where-clause bound
                 // (see issue #50825).
-                is_global(cand)
+                if is_global(victim_cand) { DropVictim::Yes } else { DropVictim::No }
             }
             (
                 ImplCandidate(_)
@@ -1956,18 +1944,20 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 | TraitUpcastingUnsizeCandidate(_)
                 | BuiltinCandidate { has_nested: true }
                 | TraitAliasCandidate,
-                ParamCandidate(ref cand),
+                ParamCandidate(ref victim_cand),
             ) => {
                 // Prefer these to a global where-clause bound
                 // (see issue #50825).
-                is_global(cand) && other.evaluation.must_apply_modulo_regions()
+                DropVictim::drop_if(
+                    is_global(victim_cand) && other.evaluation.must_apply_modulo_regions(),
+                )
             }
 
             (ProjectionCandidate(i, _), ProjectionCandidate(j, _))
             | (ObjectCandidate(i), ObjectCandidate(j)) => {
                 // Arbitrarily pick the lower numbered candidate for backwards
                 // compatibility reasons. Don't let this affect inference.
-                i < j && !needs_infer
+                DropVictim::drop_if(i < j && !needs_infer)
             }
             (ObjectCandidate(_), ProjectionCandidate(..))
             | (ProjectionCandidate(..), ObjectCandidate(_)) => {
@@ -1987,7 +1977,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 | TraitUpcastingUnsizeCandidate(_)
                 | BuiltinCandidate { .. }
                 | TraitAliasCandidate,
-            ) => true,
+            ) => DropVictim::Yes,
 
             (
                 ImplCandidate(..)
@@ -2001,7 +1991,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 | BuiltinCandidate { .. }
                 | TraitAliasCandidate,
                 ObjectCandidate(_) | ProjectionCandidate(..),
-            ) => false,
+            ) => DropVictim::No,
 
             (&ImplCandidate(other_def), &ImplCandidate(victim_def)) => {
                 // See if we can toss out `victim` based on specialization.
@@ -2014,59 +2004,69 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 let tcx = self.tcx();
                 if other.evaluation.must_apply_modulo_regions() {
                     if tcx.specializes((other_def, victim_def)) {
-                        return true;
+                        return DropVictim::Yes;
                     }
                 }
 
-                if other.evaluation.must_apply_considering_regions() {
-                    match tcx.impls_are_allowed_to_overlap(other_def, victim_def) {
-                        Some(ty::ImplOverlapKind::Permitted { marker: true }) => {
-                            // Subtle: If the predicate we are evaluating has inference
-                            // variables, do *not* allow discarding candidates due to
-                            // marker trait impls.
-                            //
-                            // Without this restriction, we could end up accidentally
-                            // constraining inference variables based on an arbitrarily
-                            // chosen trait impl.
-                            //
-                            // Imagine we have the following code:
-                            //
-                            // ```rust
-                            // #[marker] trait MyTrait {}
-                            // impl MyTrait for u8 {}
-                            // impl MyTrait for bool {}
-                            // ```
-                            //
-                            // And we are evaluating the predicate `<_#0t as MyTrait>`.
-                            //
-                            // During selection, we will end up with one candidate for each
-                            // impl of `MyTrait`. If we were to discard one impl in favor
-                            // of the other, we would be left with one candidate, causing
-                            // us to "successfully" select the predicate, unifying
-                            // _#0t with (for example) `u8`.
-                            //
-                            // However, we have no reason to believe that this unification
-                            // is correct - we've essentially just picked an arbitrary
-                            // *possibility* for _#0t, and required that this be the *only*
-                            // possibility.
-                            //
-                            // Eventually, we will either:
-                            // 1) Unify all inference variables in the predicate through
-                            // some other means (e.g. type-checking of a function). We will
-                            // then be in a position to drop marker trait candidates
-                            // without constraining inference variables (since there are
-                            // none left to constrain)
-                            // 2) Be left with some unconstrained inference variables. We
-                            // will then correctly report an inference error, since the
-                            // existence of multiple marker trait impls tells us nothing
-                            // about which one should actually apply.
-                            !needs_infer
-                        }
-                        Some(_) => true,
-                        None => false,
+                match tcx.impls_are_allowed_to_overlap(other_def, victim_def) {
+                    // For #33140 the impl headers must be exactly equal, the trait must not have
+                    // any associated items and there are no where-clauses.
+                    //
+                    // We can just arbitrarily drop one of the impls.
+                    Some(ty::ImplOverlapKind::Issue33140) => {
+                        assert_eq!(other.evaluation, victim.evaluation);
+                        DropVictim::Yes
                     }
-                } else {
-                    false
+                    // For candidates which already reference errors it doesn't really
+                    // matter what we do 🤷
+                    Some(ty::ImplOverlapKind::Permitted { marker: false }) => {
+                        DropVictim::drop_if(other.evaluation.must_apply_considering_regions())
+                    }
+                    Some(ty::ImplOverlapKind::Permitted { marker: true }) => {
+                        // Subtle: If the predicate we are evaluating has inference
+                        // variables, do *not* allow discarding candidates due to
+                        // marker trait impls.
+                        //
+                        // Without this restriction, we could end up accidentally
+                        // constraining inference variables based on an arbitrarily
+                        // chosen trait impl.
+                        //
+                        // Imagine we have the following code:
+                        //
+                        // ```rust
+                        // #[marker] trait MyTrait {}
+                        // impl MyTrait for u8 {}
+                        // impl MyTrait for bool {}
+                        // ```
+                        //
+                        // And we are evaluating the predicate `<_#0t as MyTrait>`.
+                        //
+                        // During selection, we will end up with one candidate for each
+                        // impl of `MyTrait`. If we were to discard one impl in favor
+                        // of the other, we would be left with one candidate, causing
+                        // us to "successfully" select the predicate, unifying
+                        // _#0t with (for example) `u8`.
+                        //
+                        // However, we have no reason to believe that this unification
+                        // is correct - we've essentially just picked an arbitrary
+                        // *possibility* for _#0t, and required that this be the *only*
+                        // possibility.
+                        //
+                        // Eventually, we will either:
+                        // 1) Unify all inference variables in the predicate through
+                        // some other means (e.g. type-checking of a function). We will
+                        // then be in a position to drop marker trait candidates
+                        // without constraining inference variables (since there are
+                        // none left to constrain)
+                        // 2) Be left with some unconstrained inference variables. We
+                        // will then correctly report an inference error, since the
+                        // existence of multiple marker trait impls tells us nothing
+                        // about which one should actually apply.
+                        DropVictim::drop_if(
+                            !needs_infer && other.evaluation.must_apply_considering_regions(),
+                        )
+                    }
+                    None => DropVictim::No,
                 }
             }
 
@@ -2092,10 +2092,12 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 | TraitUpcastingUnsizeCandidate(_)
                 | BuiltinCandidate { has_nested: true }
                 | TraitAliasCandidate,
-            ) => false,
+            ) => DropVictim::No,
         }
     }
+}
 
+impl<'tcx> SelectionContext<'_, 'tcx> {
     fn sized_conditions(
         &mut self,
         obligation: &TraitObligation<'tcx>,
@@ -2149,7 +2151,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             ty::Alias(..) | ty::Param(_) | ty::Placeholder(..) => None,
             ty::Infer(ty::TyVar(_)) => Ambiguous,
 
-            // We can make this an ICE if/once we actually instantiate the trait obligation.
+            // We can make this an ICE if/once we actually instantiate the trait obligation eagerly.
             ty::Bound(..) => None,
 
             ty::Infer(ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
@@ -2257,7 +2259,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 }
             }
 
-            ty::Adt(..) | ty::Alias(..) | ty::Param(..) => {
+            ty::Adt(..) | ty::Alias(..) | ty::Param(..) | ty::Placeholder(..) => {
                 // Fallback to whatever user-defined impls exist in this case.
                 None
             }
@@ -2269,9 +2271,10 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 Ambiguous
             }
 
-            ty::Placeholder(..)
-            | ty::Bound(..)
-            | ty::Infer(ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
+            // We can make this an ICE if/once we actually instantiate the trait obligation eagerly.
+            ty::Bound(..) => None,
+
+            ty::Infer(ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
                 bug!("asked to assemble builtin bounds of unexpected type: {:?}", self_ty);
             }
         }
@@ -2405,15 +2408,14 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                             placeholder_ty,
                         )
                     });
-                let placeholder_obligation = predicate_for_trait_def(
+
+                let obligation = Obligation::new(
                     self.tcx(),
-                    param_env,
                     cause.clone(),
-                    trait_def_id,
-                    recursion_depth,
-                    [normalized_ty],
+                    param_env,
+                    self.tcx().mk_trait_ref(trait_def_id, [normalized_ty]),
                 );
-                obligations.push(placeholder_obligation);
+                obligations.push(obligation);
                 obligations
             })
             .collect()
@@ -2507,7 +2509,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         let InferOk { obligations, .. } = self
             .infcx
             .at(&cause, obligation.param_env)
-            .eq(placeholder_obligation_trait_ref, impl_trait_ref)
+            .eq(DefineOpaqueTypes::No, placeholder_obligation_trait_ref, impl_trait_ref)
             .map_err(|e| {
                 debug!("match_impl: failed eq_trait_refs due to `{}`", e.to_string(self.tcx()))
             })?;
@@ -2523,19 +2525,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         Ok(Normalized { value: impl_substs, obligations: nested_obligations })
     }
 
-    fn fast_reject_trait_refs(
-        &mut self,
-        obligation: &TraitObligation<'tcx>,
-        impl_trait_ref: &ty::TraitRef<'tcx>,
-    ) -> bool {
-        // We can avoid creating type variables and doing the full
-        // substitution if we find that any of the input types, when
-        // simplified, do not match.
-        let drcx = DeepRejectCtxt { treat_obligation_params: TreatParams::AsPlaceholder };
-        iter::zip(obligation.predicate.skip_binder().trait_ref.substs, impl_trait_ref.substs)
-            .any(|(obl, imp)| !drcx.generic_args_may_unify(obl, imp))
-    }
-
     /// Normalize `where_clause_trait_ref` and try to match it against
     /// `obligation`. If successful, return any predicates that
     /// result from the normalization.
@@ -2557,7 +2546,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     ) -> Result<Vec<PredicateObligation<'tcx>>, ()> {
         self.infcx
             .at(&obligation.cause, obligation.param_env)
-            .sup(obligation.predicate.to_poly_trait_ref(), poly_trait_ref)
+            .sup(DefineOpaqueTypes::No, obligation.predicate.to_poly_trait_ref(), poly_trait_ref)
             .map(|InferOk { obligations, .. }| obligations)
             .map_err(|_| ())
     }
@@ -2786,7 +2775,7 @@ struct ProvisionalEvaluationCache<'tcx> {
     /// - then we determine that `E` is in error -- we will then clear
     ///   all cache values whose DFN is >= 4 -- in this case, that
     ///   means the cached value for `F`.
-    map: RefCell<FxHashMap<ty::PolyTraitPredicate<'tcx>, ProvisionalEvaluation>>,
+    map: RefCell<FxIndexMap<ty::PolyTraitPredicate<'tcx>, ProvisionalEvaluation>>,
 
     /// The stack of args that we assume to be true because a `WF(arg)` predicate
     /// is on the stack above (and because of wellformedness is coinductive).
@@ -2934,12 +2923,13 @@ impl<'tcx> ProvisionalEvaluationCache<'tcx> {
     /// have a performance impact in practice.
     fn on_completion(&self, dfn: usize) {
         debug!(?dfn, "on_completion");
-
-        for (fresh_trait_pred, eval) in
-            self.map.borrow_mut().drain_filter(|_k, eval| eval.from_dfn >= dfn)
-        {
-            debug!(?fresh_trait_pred, ?eval, "on_completion");
-        }
+        self.map.borrow_mut().retain(|fresh_trait_pred, eval| {
+            if eval.from_dfn >= dfn {
+                debug!(?fresh_trait_pred, ?eval, "on_completion");
+                return false;
+            }
+            true
+        });
     }
 }
 
@@ -3021,7 +3011,7 @@ fn bind_generator_hidden_types_above<'tcx>(
                     if let ty::ReErased = r.kind() {
                         let br = ty::BoundRegion {
                             var: ty::BoundVar::from_u32(counter),
-                            kind: ty::BrAnon(counter, None),
+                            kind: ty::BrAnon(None),
                         };
                         counter += 1;
                         r = tcx.mk_re_late_bound(current_depth, br);
@@ -3037,7 +3027,7 @@ fn bind_generator_hidden_types_above<'tcx>(
         debug_assert!(!hidden_types.has_erased_regions());
     }
     let bound_vars = tcx.mk_bound_variable_kinds_from_iter(bound_vars.iter().chain(
-        (num_bound_variables..counter).map(|i| ty::BoundVariableKind::Region(ty::BrAnon(i, None))),
+        (num_bound_variables..counter).map(|_| ty::BoundVariableKind::Region(ty::BrAnon(None))),
     ));
     ty::Binder::bind_with_vars(hidden_types, bound_vars)
 }
diff --git a/compiler/rustc_trait_selection/src/traits/specialize/mod.rs b/compiler/rustc_trait_selection/src/traits/specialize/mod.rs
index d1d6a7a90..8546bbe52 100644
--- a/compiler/rustc_trait_selection/src/traits/specialize/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/specialize/mod.rs
@@ -10,6 +10,7 @@
 //! [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/traits/specialization.html
 
 pub mod specialization_graph;
+use rustc_infer::infer::DefineOpaqueTypes;
 use specialization_graph::GraphExt;
 
 use crate::errors::NegativePositiveConflict;
@@ -21,7 +22,7 @@ use crate::traits::{
 use rustc_data_structures::fx::FxIndexSet;
 use rustc_errors::{error_code, DelayDm, Diagnostic};
 use rustc_hir::def_id::{DefId, LocalDefId};
-use rustc_middle::ty::{self, ImplSubject, Ty, TyCtxt};
+use rustc_middle::ty::{self, ImplSubject, Ty, TyCtxt, TypeVisitableExt};
 use rustc_middle::ty::{InternalSubsts, SubstsRef};
 use rustc_session::lint::builtin::COHERENCE_LEAK_CHECK;
 use rustc_session::lint::builtin::ORDER_DEPENDENT_TRAIT_OBJECTS;
@@ -99,10 +100,10 @@ pub fn translate_substs<'tcx>(
             }
 
             fulfill_implication(infcx, param_env, source_trait_ref, target_impl).unwrap_or_else(
-                |_| {
+                |()| {
                     bug!(
-                        "When translating substitutions for specialization, the expected \
-                         specialization failed to hold"
+                        "When translating substitutions from {source_impl:?} to {target_impl:?}, \
+                        the expected specialization failed to hold"
                     )
                 },
             )
@@ -193,7 +194,7 @@ fn fulfill_implication<'tcx>(
 
     // do the impls unify? If not, no specialization.
     let Ok(InferOk { obligations: more_obligations, .. }) =
-        infcx.at(&ObligationCause::dummy(), param_env).eq(source_trait, target_trait)
+        infcx.at(&ObligationCause::dummy(), param_env, ).eq(DefineOpaqueTypes::No,source_trait, target_trait)
     else {
         debug!(
             "fulfill_implication: {:?} does not unify with {:?}",
@@ -349,6 +350,10 @@ fn report_conflicting_impls<'tcx>(
         impl_span: Span,
         err: &mut Diagnostic,
     ) {
+        if (overlap.trait_ref, overlap.self_ty).references_error() {
+            err.downgrade_to_delayed_bug();
+        }
+
         match tcx.span_of_impl(overlap.with_impl) {
             Ok(span) => {
                 err.span_label(span, "first implementation here");
diff --git a/compiler/rustc_trait_selection/src/traits/specialize/specialization_graph.rs b/compiler/rustc_trait_selection/src/traits/specialize/specialization_graph.rs
index 61ed9ef2e..aa5c624f4 100644
--- a/compiler/rustc_trait_selection/src/traits/specialize/specialization_graph.rs
+++ b/compiler/rustc_trait_selection/src/traits/specialize/specialization_graph.rs
@@ -21,6 +21,7 @@ pub struct FutureCompatOverlapError<'tcx> {
 }
 
 /// The result of attempting to insert an impl into a group of children.
+#[derive(Debug)]
 enum Inserted<'tcx> {
     /// The impl was inserted as a new child in this group of children.
     BecameNewSibling(Option<FutureCompatOverlapError<'tcx>>),
@@ -49,7 +50,8 @@ impl<'tcx> ChildrenExt<'tcx> for Children {
     /// Insert an impl into this set of children without comparing to any existing impls.
     fn insert_blindly(&mut self, tcx: TyCtxt<'tcx>, impl_def_id: DefId) {
         let trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap().skip_binder();
-        if let Some(st) = fast_reject::simplify_type(tcx, trait_ref.self_ty(), TreatParams::AsInfer)
+        if let Some(st) =
+            fast_reject::simplify_type(tcx, trait_ref.self_ty(), TreatParams::AsCandidateKey)
         {
             debug!("insert_blindly: impl_def_id={:?} st={:?}", impl_def_id, st);
             self.non_blanket_impls.entry(st).or_default().push(impl_def_id)
@@ -65,7 +67,8 @@ impl<'tcx> ChildrenExt<'tcx> for Children {
     fn remove_existing(&mut self, tcx: TyCtxt<'tcx>, impl_def_id: DefId) {
         let trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap().skip_binder();
         let vec: &mut Vec<DefId>;
-        if let Some(st) = fast_reject::simplify_type(tcx, trait_ref.self_ty(), TreatParams::AsInfer)
+        if let Some(st) =
+            fast_reject::simplify_type(tcx, trait_ref.self_ty(), TreatParams::AsCandidateKey)
         {
             debug!("remove_existing: impl_def_id={:?} st={:?}", impl_def_id, st);
             vec = self.non_blanket_impls.get_mut(&st).unwrap();
@@ -80,6 +83,7 @@ impl<'tcx> ChildrenExt<'tcx> for Children {
 
     /// Attempt to insert an impl into this set of children, while comparing for
     /// specialization relationships.
+    #[instrument(level = "debug", skip(self, tcx), ret)]
     fn insert(
         &mut self,
         tcx: TyCtxt<'tcx>,
@@ -90,18 +94,13 @@ impl<'tcx> ChildrenExt<'tcx> for Children {
         let mut last_lint = None;
         let mut replace_children = Vec::new();
 
-        debug!("insert(impl_def_id={:?}, simplified_self={:?})", impl_def_id, simplified_self,);
-
         let possible_siblings = match simplified_self {
             Some(st) => PotentialSiblings::Filtered(filtered_children(self, st)),
             None => PotentialSiblings::Unfiltered(iter_children(self)),
         };
 
         for possible_sibling in possible_siblings {
-            debug!(
-                "insert: impl_def_id={:?}, simplified_self={:?}, possible_sibling={:?}",
-                impl_def_id, simplified_self, possible_sibling,
-            );
+            debug!(?possible_sibling);
 
             let create_overlap_error = |overlap: traits::coherence::OverlapResult<'tcx>| {
                 let trait_ref = overlap.impl_header.trait_ref.unwrap();
@@ -302,7 +301,8 @@ impl<'tcx> GraphExt<'tcx> for Graph {
 
         let mut parent = trait_def_id;
         let mut last_lint = None;
-        let simplified = fast_reject::simplify_type(tcx, trait_ref.self_ty(), TreatParams::AsInfer);
+        let simplified =
+            fast_reject::simplify_type(tcx, trait_ref.self_ty(), TreatParams::AsCandidateKey);
 
         // Descend the specialization tree, where `parent` is the current parent node.
         loop {
diff --git a/compiler/rustc_trait_selection/src/traits/util.rs b/compiler/rustc_trait_selection/src/traits/util.rs
index bcf63d5a6..20357d4d2 100644
--- a/compiler/rustc_trait_selection/src/traits/util.rs
+++ b/compiler/rustc_trait_selection/src/traits/util.rs
@@ -1,15 +1,14 @@
-use rustc_errors::Diagnostic;
-use rustc_span::Span;
-use smallvec::SmallVec;
-
+use super::NormalizeExt;
+use super::{ObligationCause, PredicateObligation, SelectionContext};
 use rustc_data_structures::fx::FxHashSet;
+use rustc_errors::Diagnostic;
 use rustc_hir::def_id::DefId;
+use rustc_infer::infer::InferOk;
+use rustc_middle::ty::SubstsRef;
 use rustc_middle::ty::{self, ImplSubject, ToPredicate, Ty, TyCtxt, TypeVisitableExt};
-use rustc_middle::ty::{GenericArg, SubstsRef};
+use rustc_span::Span;
+use smallvec::SmallVec;
 
-use super::NormalizeExt;
-use super::{Obligation, ObligationCause, PredicateObligation, SelectionContext};
-use rustc_infer::infer::InferOk;
 pub use rustc_infer::traits::{self, util::*};
 
 ///////////////////////////////////////////////////////////////////////////
@@ -116,7 +115,7 @@ impl<'tcx> TraitAliasExpander<'tcx> {
         }
 
         // Get components of trait alias.
-        let predicates = tcx.super_predicates_of(trait_ref.def_id());
+        let predicates = tcx.implied_predicates_of(trait_ref.def_id());
         debug!(?predicates);
 
         let items = predicates.predicates.iter().rev().filter_map(|(pred, span)| {
@@ -199,8 +198,9 @@ pub fn impl_subject_and_oblig<'a, 'tcx>(
     impl_def_id: DefId,
     impl_substs: SubstsRef<'tcx>,
 ) -> (ImplSubject<'tcx>, impl Iterator<Item = PredicateObligation<'tcx>>) {
-    let subject = selcx.tcx().bound_impl_subject(impl_def_id);
+    let subject = selcx.tcx().impl_subject(impl_def_id);
     let subject = subject.subst(selcx.tcx(), impl_substs);
+
     let InferOk { value: subject, obligations: normalization_obligations1 } =
         selcx.infcx.at(&ObligationCause::dummy(), param_env).normalize(subject);
 
@@ -218,33 +218,6 @@ pub fn impl_subject_and_oblig<'a, 'tcx>(
     (subject, impl_obligations)
 }
 
-pub fn predicate_for_trait_ref<'tcx>(
-    tcx: TyCtxt<'tcx>,
-    cause: ObligationCause<'tcx>,
-    param_env: ty::ParamEnv<'tcx>,
-    trait_ref: ty::TraitRef<'tcx>,
-    recursion_depth: usize,
-) -> PredicateObligation<'tcx> {
-    Obligation {
-        cause,
-        param_env,
-        recursion_depth,
-        predicate: ty::Binder::dummy(trait_ref).without_const().to_predicate(tcx),
-    }
-}
-
-pub fn predicate_for_trait_def<'tcx>(
-    tcx: TyCtxt<'tcx>,
-    param_env: ty::ParamEnv<'tcx>,
-    cause: ObligationCause<'tcx>,
-    trait_def_id: DefId,
-    recursion_depth: usize,
-    params: impl IntoIterator<Item: Into<GenericArg<'tcx>>>,
-) -> PredicateObligation<'tcx> {
-    let trait_ref = tcx.mk_trait_ref(trait_def_id, params);
-    predicate_for_trait_ref(tcx, cause, param_env, trait_ref, recursion_depth)
-}
-
 /// Casts a trait reference into a reference to one of its super
 /// traits; returns `None` if `target_trait_def_id` is not a
 /// supertrait.
diff --git a/compiler/rustc_trait_selection/src/traits/wf.rs b/compiler/rustc_trait_selection/src/traits/wf.rs
index d498af359..3d026506a 100644
--- a/compiler/rustc_trait_selection/src/traits/wf.rs
+++ b/compiler/rustc_trait_selection/src/traits/wf.rs
@@ -191,8 +191,8 @@ pub fn predicate_obligations<'tcx>(
         ty::PredicateKind::TypeWellFormedFromEnv(..) => {
             bug!("TypeWellFormedFromEnv is only used for Chalk")
         }
-        ty::PredicateKind::AliasEq(..) => {
-            bug!("We should only wf check where clauses and `AliasEq` is not a `Clause`")
+        ty::PredicateKind::AliasRelate(..) => {
+            bug!("We should only wf check where clauses and `AliasRelate` is not a `Clause`")
         }
     }
 
@@ -364,7 +364,7 @@ impl<'tcx> WfPredicates<'tcx> {
         };
 
         if let Elaborate::All = elaborate {
-            let implied_obligations = traits::util::elaborate_obligations(tcx, obligations);
+            let implied_obligations = traits::util::elaborate(tcx, obligations);
             let implied_obligations = implied_obligations.map(extend);
             self.out.extend(implied_obligations);
         } else {
@@ -920,10 +920,10 @@ pub(crate) fn required_region_bounds<'tcx>(
 ) -> Vec<ty::Region<'tcx>> {
     assert!(!erased_self_ty.has_escaping_bound_vars());
 
-    traits::elaborate_predicates(tcx, predicates)
-        .filter_map(|obligation| {
-            debug!(?obligation);
-            match obligation.predicate.kind().skip_binder() {
+    traits::elaborate(tcx, predicates)
+        .filter_map(|pred| {
+            debug!(?pred);
+            match pred.kind().skip_binder() {
                 ty::PredicateKind::Clause(ty::Clause::Projection(..))
                 | ty::PredicateKind::Clause(ty::Clause::Trait(..))
                 | ty::PredicateKind::Clause(ty::Clause::ConstArgHasType(..))
@@ -936,7 +936,7 @@ pub(crate) fn required_region_bounds<'tcx>(
                 | ty::PredicateKind::ConstEvaluatable(..)
                 | ty::PredicateKind::ConstEquate(..)
                 | ty::PredicateKind::Ambiguous
-                | ty::PredicateKind::AliasEq(..)
+                | ty::PredicateKind::AliasRelate(..)
                 | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
                 ty::PredicateKind::Clause(ty::Clause::TypeOutlives(ty::OutlivesPredicate(
                     ref t,