Merging upstream version 1.66.0+dfsg1.

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

3 files changed, 120 insertions, 130 deletions

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 a80527f63..4c5bc3339 100644
--- a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
@@ -6,6 +6,7 @@
 //!
 //! [rustc dev guide]:https://rustc-dev-guide.rust-lang.org/traits/resolution.html#candidate-assembly
 use hir::LangItem;
+use rustc_errors::DelayDm;
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_infer::traits::ObligationCause;
@@ -173,7 +174,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
         debug!(?stack, ?candidates, "winnowed to {} candidates", candidates.len());
 
-        let needs_infer = stack.obligation.predicate.has_infer_types_or_consts();
+        let needs_infer = stack.obligation.predicate.has_non_region_infer();
 
         // If there are STILL multiple candidates, we can further
         // reduce the list by dropping duplicates -- including
@@ -362,7 +363,9 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             .infcx
             .probe(|_| self.match_projection_obligation_against_definition_bounds(obligation));
 
-        candidates.vec.extend(result.into_iter().map(ProjectionCandidate));
+        candidates
+            .vec
+            .extend(result.into_iter().map(|(idx, constness)| ProjectionCandidate(idx, constness)));
     }
 
     /// Given an obligation like `<SomeTrait for T>`, searches the obligations that the caller
@@ -622,7 +625,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     }
                 }
 
-                _ => candidates.vec.push(AutoImplCandidate(def_id)),
+                _ => candidates.vec.push(AutoImplCandidate),
             }
         }
     }
@@ -823,13 +826,11 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                                     DEREF_INTO_DYN_SUPERTRAIT,
                                     obligation.cause.body_id,
                                     obligation.cause.span,
-                                    |lint| {
-                                        lint.build(&format!(
-                                            "`{}` implements `Deref` with supertrait `{}` as output",
-                                            source,
-                                            deref_output_ty
-                                        )).emit();
-                                    },
+                                    DelayDm(|| format!(
+                                        "`{}` implements `Deref` with supertrait `{}` as output",
+                                        source, deref_output_ty
+                                    )),
+                                    |lint| lint,
                                 );
                                 return;
                             }
@@ -888,11 +889,11 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         obligation: &TraitObligation<'tcx>,
         candidates: &mut SelectionCandidateSet<'tcx>,
     ) {
-        if obligation.has_param_types_or_consts() {
+        if obligation.has_non_region_param() {
             return;
         }
 
-        if obligation.has_infer_types_or_consts() {
+        if obligation.has_non_region_infer() {
             candidates.ambiguous = true;
             return;
         }
@@ -913,7 +914,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         let def_id = obligation.predicate.def_id();
 
         if self.tcx().is_trait_alias(def_id) {
-            candidates.vec.push(TraitAliasCandidate(def_id));
+            candidates.vec.push(TraitAliasCandidate);
         }
     }
 
@@ -1020,7 +1021,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         let self_ty = self.infcx().shallow_resolve(obligation.self_ty().skip_binder());
         match self_ty.kind() {
             ty::Tuple(_) => {
-                candidates.vec.push(TupleCandidate);
+                candidates.vec.push(BuiltinCandidate { has_nested: false });
             }
             ty::Infer(ty::TyVar(_)) => {
                 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 d1deef784..ed22058c6 100644
--- a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
@@ -11,9 +11,10 @@ use rustc_hir::lang_items::LangItem;
 use rustc_index::bit_set::GrowableBitSet;
 use rustc_infer::infer::InferOk;
 use rustc_infer::infer::LateBoundRegionConversionTime::HigherRankedType;
-use rustc_middle::ty::subst::{GenericArg, GenericArgKind, InternalSubsts, Subst, SubstsRef};
-use rustc_middle::ty::{self, GenericParamDefKind, Ty, TyCtxt};
-use rustc_middle::ty::{ToPolyTraitRef, ToPredicate};
+use rustc_middle::ty::{
+    self, GenericArg, GenericArgKind, GenericParamDefKind, InternalSubsts, SubstsRef,
+    ToPolyTraitRef, ToPredicate, Ty, TyCtxt,
+};
 use rustc_span::def_id::DefId;
 
 use crate::traits::project::{normalize_with_depth, normalize_with_depth_to};
@@ -63,15 +64,14 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 ImplSource::UserDefined(self.confirm_impl_candidate(obligation, impl_def_id))
             }
 
-            AutoImplCandidate(trait_def_id) => {
-                let data = self.confirm_auto_impl_candidate(obligation, trait_def_id);
+            AutoImplCandidate => {
+                let data = self.confirm_auto_impl_candidate(obligation);
                 ImplSource::AutoImpl(data)
             }
 
-            ProjectionCandidate(idx) => {
+            ProjectionCandidate(idx, constness) => {
                 let obligations = self.confirm_projection_candidate(obligation, idx)?;
-                // FIXME(jschievink): constness
-                ImplSource::Param(obligations, ty::BoundConstness::NotConst)
+                ImplSource::Param(obligations, constness)
             }
 
             ObjectCandidate(idx) => {
@@ -100,8 +100,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
             PointeeCandidate => ImplSource::Pointee(ImplSourcePointeeData),
 
-            TraitAliasCandidate(alias_def_id) => {
-                let data = self.confirm_trait_alias_candidate(obligation, alias_def_id);
+            TraitAliasCandidate => {
+                let data = self.confirm_trait_alias_candidate(obligation);
                 ImplSource::TraitAlias(data)
             }
 
@@ -126,8 +126,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 let data = self.confirm_const_destruct_candidate(obligation, def_id)?;
                 ImplSource::ConstDestruct(data)
             }
-
-            TupleCandidate => ImplSource::Tuple,
         };
 
         if !obligation.predicate.is_const_if_const() {
@@ -290,8 +288,10 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
         let scope = type_at(2).skip_binder();
 
-        let assume =
-            rustc_transmute::Assume::from_const(self.infcx.tcx, obligation.param_env, const_at(3));
+        let Some(assume) =
+            rustc_transmute::Assume::from_const(self.infcx.tcx, obligation.param_env, const_at(3)) else {
+                return Err(Unimplemented);
+            };
 
         let cause = obligation.cause.clone();
 
@@ -315,13 +315,12 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     fn confirm_auto_impl_candidate(
         &mut self,
         obligation: &TraitObligation<'tcx>,
-        trait_def_id: DefId,
     ) -> ImplSourceAutoImplData<PredicateObligation<'tcx>> {
-        debug!(?obligation, ?trait_def_id, "confirm_auto_impl_candidate");
+        debug!(?obligation, "confirm_auto_impl_candidate");
 
         let self_ty = self.infcx.shallow_resolve(obligation.predicate.self_ty());
         let types = self.constituent_types_for_ty(self_ty);
-        self.vtable_auto_impl(obligation, trait_def_id, types)
+        self.vtable_auto_impl(obligation, obligation.predicate.def_id(), types)
     }
 
     /// See `confirm_auto_impl_candidate`.
@@ -619,17 +618,47 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         )
         .map_bound(|(trait_ref, _)| trait_ref);
 
-        let nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
+        let mut nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
+
+        // Confirm the `type Output: Sized;` bound that is present on `FnOnce`
+        let cause = obligation.derived_cause(BuiltinDerivedObligation);
+        // The binder on the Fn obligation is "less" important than the one on
+        // the signature, as evidenced by how we treat it during projection.
+        // The safe thing to do here is to liberate it, though, which should
+        // have no worse effect than skipping the binder here.
+        let liberated_fn_ty =
+            self.infcx.replace_bound_vars_with_placeholders(obligation.predicate.rebind(self_ty));
+        let output_ty = self
+            .infcx
+            .replace_bound_vars_with_placeholders(liberated_fn_ty.fn_sig(self.tcx()).output());
+        let output_ty = normalize_with_depth_to(
+            self,
+            obligation.param_env,
+            cause.clone(),
+            obligation.recursion_depth,
+            output_ty,
+            &mut nested,
+        );
+        let tr = ty::Binder::dummy(ty::TraitRef::new(
+            self.tcx().require_lang_item(LangItem::Sized, None),
+            self.tcx().mk_substs_trait(output_ty, &[]),
+        ));
+        nested.push(Obligation::new(
+            cause,
+            obligation.param_env,
+            tr.to_poly_trait_predicate().to_predicate(self.tcx()),
+        ));
+
         Ok(ImplSourceFnPointerData { fn_ty: self_ty, nested })
     }
 
     fn confirm_trait_alias_candidate(
         &mut self,
         obligation: &TraitObligation<'tcx>,
-        alias_def_id: DefId,
     ) -> ImplSourceTraitAliasData<'tcx, PredicateObligation<'tcx>> {
-        debug!(?obligation, ?alias_def_id, "confirm_trait_alias_candidate");
+        debug!(?obligation, "confirm_trait_alias_candidate");
 
+        let alias_def_id = obligation.predicate.def_id();
         let predicate = self.infcx().replace_bound_vars_with_placeholders(obligation.predicate);
         let trait_ref = predicate.trait_ref;
         let trait_def_id = trait_ref.def_id;
@@ -1196,6 +1225,9 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 | ty::Never
                 | ty::Foreign(_) => {}
 
+                // `ManuallyDrop` is trivially drop
+                ty::Adt(def, _) if Some(def.did()) == tcx.lang_items().manually_drop() => {}
+
                 // These types are built-in, so we can fast-track by registering
                 // nested predicates for their constituent type(s)
                 ty::Array(ty, _) | ty::Slice(ty) => {
diff --git a/compiler/rustc_trait_selection/src/traits/select/mod.rs b/compiler/rustc_trait_selection/src/traits/select/mod.rs
index 75bd2c89f..9ebff4892 100644
--- a/compiler/rustc_trait_selection/src/traits/select/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/mod.rs
@@ -20,7 +20,7 @@ use super::{
 };
 
 use crate::infer::{InferCtxt, InferOk, TypeFreshener};
-use crate::traits::error_reporting::InferCtxtExt;
+use crate::traits::error_reporting::TypeErrCtxtExt;
 use crate::traits::project::ProjectAndUnifyResult;
 use crate::traits::project::ProjectionCacheKeyExt;
 use crate::traits::ProjectionCacheKey;
@@ -35,9 +35,8 @@ 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::print::with_no_trimmed_paths;
 use rustc_middle::ty::relate::TypeRelation;
-use rustc_middle::ty::subst::{Subst, SubstsRef};
+use rustc_middle::ty::SubstsRef;
 use rustc_middle::ty::{self, EarlyBinder, PolyProjectionPredicate, ToPolyTraitRef, ToPredicate};
 use rustc_middle::ty::{Ty, TyCtxt, TypeFoldable, TypeVisitable};
 use rustc_span::symbol::sym;
@@ -94,7 +93,7 @@ impl IntercrateAmbiguityCause {
 }
 
 pub struct SelectionContext<'cx, 'tcx> {
-    infcx: &'cx InferCtxt<'cx, 'tcx>,
+    infcx: &'cx InferCtxt<'tcx>,
 
     /// Freshener used specifically for entries on the obligation
     /// stack. This ensures that all entries on the stack at one time
@@ -215,7 +214,7 @@ enum BuiltinImplConditions<'tcx> {
 }
 
 impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
-    pub fn new(infcx: &'cx InferCtxt<'cx, 'tcx>) -> SelectionContext<'cx, 'tcx> {
+    pub fn new(infcx: &'cx InferCtxt<'tcx>) -> SelectionContext<'cx, 'tcx> {
         SelectionContext {
             infcx,
             freshener: infcx.freshener_keep_static(),
@@ -225,28 +224,16 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         }
     }
 
-    pub fn intercrate(infcx: &'cx InferCtxt<'cx, 'tcx>) -> SelectionContext<'cx, 'tcx> {
-        SelectionContext {
-            infcx,
-            freshener: infcx.freshener_keep_static(),
-            intercrate: true,
-            intercrate_ambiguity_causes: None,
-            query_mode: TraitQueryMode::Standard,
-        }
+    pub fn intercrate(infcx: &'cx InferCtxt<'tcx>) -> SelectionContext<'cx, 'tcx> {
+        SelectionContext { intercrate: true, ..SelectionContext::new(infcx) }
     }
 
     pub fn with_query_mode(
-        infcx: &'cx InferCtxt<'cx, 'tcx>,
+        infcx: &'cx InferCtxt<'tcx>,
         query_mode: TraitQueryMode,
     ) -> SelectionContext<'cx, 'tcx> {
         debug!(?query_mode, "with_query_mode");
-        SelectionContext {
-            infcx,
-            freshener: infcx.freshener_keep_static(),
-            intercrate: false,
-            intercrate_ambiguity_causes: None,
-            query_mode,
-        }
+        SelectionContext { query_mode, ..SelectionContext::new(infcx) }
     }
 
     /// Enables tracking of intercrate ambiguity causes. See
@@ -266,7 +253,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         self.intercrate_ambiguity_causes.take().unwrap_or_default()
     }
 
-    pub fn infcx(&self) -> &'cx InferCtxt<'cx, 'tcx> {
+    pub fn infcx(&self) -> &'cx InferCtxt<'tcx> {
         self.infcx
     }
 
@@ -689,19 +676,21 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 }
 
                 ty::PredicateKind::ConstEquate(c1, c2) => {
+                    assert!(
+                        self.tcx().features().generic_const_exprs,
+                        "`ConstEquate` without a feature gate: {c1:?} {c2:?}",
+                    );
                     debug!(?c1, ?c2, "evaluate_predicate_recursively: equating consts");
 
-                    if self.tcx().features().generic_const_exprs {
-                        // FIXME: we probably should only try to unify abstract constants
-                        // if the constants depend on generic parameters.
-                        //
-                        // Let's just see where this breaks :shrug:
-                        if let (ty::ConstKind::Unevaluated(a), ty::ConstKind::Unevaluated(b)) =
-                            (c1.kind(), c2.kind())
-                        {
-                            if self.infcx.try_unify_abstract_consts(a, b, obligation.param_env) {
-                                return Ok(EvaluatedToOk);
-                            }
+                    // FIXME: we probably should only try to unify abstract constants
+                    // if the constants depend on generic parameters.
+                    //
+                    // Let's just see where this breaks :shrug:
+                    if let (ty::ConstKind::Unevaluated(a), ty::ConstKind::Unevaluated(b)) =
+                        (c1.kind(), c2.kind())
+                    {
+                        if self.infcx.try_unify_abstract_consts(a, b, obligation.param_env) {
+                            return Ok(EvaluatedToOk);
                         }
                     }
 
@@ -741,7 +730,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                             )
                         }
                         (Err(ErrorHandled::TooGeneric), _) | (_, Err(ErrorHandled::TooGeneric)) => {
-                            if c1.has_infer_types_or_consts() || c2.has_infer_types_or_consts() {
+                            if c1.has_non_region_infer() || c2.has_non_region_infer() {
                                 Ok(EvaluatedToAmbig)
                             } else {
                                 // Two different constants using generic parameters ~> error.
@@ -839,7 +828,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     /// must be met of course). One obvious case this comes up is
     /// marker traits like `Send`. Think of a linked list:
     ///
-    ///    struct List<T> { data: T, next: Option<Box<List<T>>> }
+    ///     struct List<T> { data: T, next: Option<Box<List<T>>> }
     ///
     /// `Box<List<T>>` will be `Send` if `T` is `Send` and
     /// `Option<Box<List<T>>>` is `Send`, and in turn
@@ -926,38 +915,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         let unbound_input_types =
             stack.fresh_trait_pred.skip_binder().trait_ref.substs.types().any(|ty| ty.is_fresh());
 
-        if stack.obligation.polarity() != ty::ImplPolarity::Negative {
-            // This check was an imperfect workaround for a bug in the old
-            // intercrate mode; it should be removed when that goes away.
-            if unbound_input_types && self.intercrate {
-                debug!("evaluate_stack --> unbound argument, intercrate -->  ambiguous",);
-                // Heuristics: show the diagnostics when there are no candidates in crate.
-                if self.intercrate_ambiguity_causes.is_some() {
-                    debug!("evaluate_stack: intercrate_ambiguity_causes is some");
-                    if let Ok(candidate_set) = self.assemble_candidates(stack) {
-                        if !candidate_set.ambiguous && candidate_set.vec.is_empty() {
-                            let trait_ref = stack.obligation.predicate.skip_binder().trait_ref;
-                            let self_ty = trait_ref.self_ty();
-                            let cause = with_no_trimmed_paths!({
-                                IntercrateAmbiguityCause::DownstreamCrate {
-                                    trait_desc: trait_ref.print_only_trait_path().to_string(),
-                                    self_desc: if self_ty.has_concrete_skeleton() {
-                                        Some(self_ty.to_string())
-                                    } else {
-                                        None
-                                    },
-                                }
-                            });
-
-                            debug!(?cause, "evaluate_stack: pushing cause");
-                            self.intercrate_ambiguity_causes.as_mut().unwrap().insert(cause);
-                        }
-                    }
-                }
-                return Ok(EvaluatedToAmbig);
-            }
-        }
-
         if unbound_input_types
             && stack.iter().skip(1).any(|prev| {
                 stack.obligation.param_env == prev.obligation.param_env
@@ -1140,7 +1097,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                             ErrorGuaranteed::unchecked_claim_error_was_emitted(),
                         ));
                     }
-                    self.infcx.report_overflow_error(error_obligation, true);
+                    self.infcx.err_ctxt().report_overflow_error(error_obligation, true);
                 }
                 TraitQueryMode::Canonical => {
                     return Err(OverflowError::Canonical);
@@ -1192,8 +1149,10 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     ImplCandidate(def_id) if tcx.constness(def_id) == hir::Constness::Const => {}
                     // const param
                     ParamCandidate(trait_pred) if trait_pred.is_const_if_const() => {}
+                    // const projection
+                    ProjectionCandidate(_, ty::BoundConstness::ConstIfConst) => {}
                     // auto trait impl
-                    AutoImplCandidate(..) => {}
+                    AutoImplCandidate => {}
                     // generator, this will raise error in other places
                     // or ignore error with const_async_blocks feature
                     GeneratorCandidate => {}
@@ -1399,7 +1358,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     fn match_projection_obligation_against_definition_bounds(
         &mut self,
         obligation: &TraitObligation<'tcx>,
-    ) -> smallvec::SmallVec<[usize; 2]> {
+    ) -> smallvec::SmallVec<[(usize, ty::BoundConstness); 2]> {
         let poly_trait_predicate = self.infcx().resolve_vars_if_possible(obligation.predicate);
         let placeholder_trait_predicate =
             self.infcx().replace_bound_vars_with_placeholders(poly_trait_predicate);
@@ -1447,7 +1406,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                             _ => false,
                         }
                     }) {
-                        return Some(idx);
+                        return Some((idx, pred.constness));
                     }
                 }
                 None
@@ -1563,7 +1522,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             if !generics.params.is_empty()
                 && obligation.predicate.substs[generics.parent_count..]
                     .iter()
-                    .any(|&p| p.has_infer_types_or_consts() && self.infcx.shallow_resolve(p) != p)
+                    .any(|&p| p.has_non_region_infer() && self.infcx.shallow_resolve(p) != p)
             {
                 ProjectionMatchesProjection::Ambiguous
             } else {
@@ -1605,13 +1564,13 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         };
 
         // (*) Prefer `BuiltinCandidate { has_nested: false }`, `PointeeCandidate`,
-        // `DiscriminantKindCandidate`, `ConstDestructCandidate`, and `TupleCandidate`
+        // `DiscriminantKindCandidate`, `ConstDestructCandidate`
         // to anything else.
         //
         // This is a fix for #53123 and prevents winnowing from accidentally extending the
         // lifetime of a variable.
         match (&other.candidate, &victim.candidate) {
-            (_, AutoImplCandidate(..)) | (AutoImplCandidate(..), _) => {
+            (_, AutoImplCandidate) | (AutoImplCandidate, _) => {
                 bug!(
                     "default implementations shouldn't be recorded \
                     when there are other valid candidates"
@@ -1626,8 +1585,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 BuiltinCandidate { has_nested: false }
                 | DiscriminantKindCandidate
                 | PointeeCandidate
-                | ConstDestructCandidate(_)
-                | TupleCandidate,
+                | ConstDestructCandidate(_),
                 _,
             ) => true,
             (
@@ -1635,8 +1593,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 BuiltinCandidate { has_nested: false }
                 | DiscriminantKindCandidate
                 | PointeeCandidate
-                | ConstDestructCandidate(_)
-                | TupleCandidate,
+                | ConstDestructCandidate(_),
             ) => false,
 
             (ParamCandidate(other), ParamCandidate(victim)) => {
@@ -1681,11 +1638,11 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 | BuiltinUnsizeCandidate
                 | TraitUpcastingUnsizeCandidate(_)
                 | BuiltinCandidate { .. }
-                | TraitAliasCandidate(..)
+                | TraitAliasCandidate
                 | ObjectCandidate(_)
-                | ProjectionCandidate(_),
+                | ProjectionCandidate(..),
             ) => !is_global(cand),
-            (ObjectCandidate(_) | ProjectionCandidate(_), ParamCandidate(ref cand)) => {
+            (ObjectCandidate(_) | ProjectionCandidate(..), ParamCandidate(ref cand)) => {
                 // Prefer these to a global where-clause bound
                 // (see issue #50825).
                 is_global(cand)
@@ -1699,7 +1656,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 | BuiltinUnsizeCandidate
                 | TraitUpcastingUnsizeCandidate(_)
                 | BuiltinCandidate { has_nested: true }
-                | TraitAliasCandidate(..),
+                | TraitAliasCandidate,
                 ParamCandidate(ref cand),
             ) => {
                 // Prefer these to a global where-clause bound
@@ -1707,20 +1664,20 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 is_global(cand) && other.evaluation.must_apply_modulo_regions()
             }
 
-            (ProjectionCandidate(i), ProjectionCandidate(j))
+            (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
             }
-            (ObjectCandidate(_), ProjectionCandidate(_))
-            | (ProjectionCandidate(_), ObjectCandidate(_)) => {
+            (ObjectCandidate(_), ProjectionCandidate(..))
+            | (ProjectionCandidate(..), ObjectCandidate(_)) => {
                 bug!("Have both object and projection candidate")
             }
 
             // Arbitrarily give projection and object candidates priority.
             (
-                ObjectCandidate(_) | ProjectionCandidate(_),
+                ObjectCandidate(_) | ProjectionCandidate(..),
                 ImplCandidate(..)
                 | ClosureCandidate
                 | GeneratorCandidate
@@ -1729,7 +1686,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 | BuiltinUnsizeCandidate
                 | TraitUpcastingUnsizeCandidate(_)
                 | BuiltinCandidate { .. }
-                | TraitAliasCandidate(..),
+                | TraitAliasCandidate,
             ) => true,
 
             (
@@ -1741,18 +1698,18 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 | BuiltinUnsizeCandidate
                 | TraitUpcastingUnsizeCandidate(_)
                 | BuiltinCandidate { .. }
-                | TraitAliasCandidate(..),
-                ObjectCandidate(_) | ProjectionCandidate(_),
+                | TraitAliasCandidate,
+                ObjectCandidate(_) | ProjectionCandidate(..),
             ) => false,
 
             (&ImplCandidate(other_def), &ImplCandidate(victim_def)) => {
                 // See if we can toss out `victim` based on specialization.
-                // This requires us to know *for sure* that the `other` impl applies
-                // i.e., `EvaluatedToOk`.
+                // While this requires us to know *for sure* that the `other` impl applies
+                // we still use modulo regions here.
                 //
-                // FIXME(@lcnr): Using `modulo_regions` here seems kind of scary
-                // to me but is required for `std` to compile, so I didn't change it
-                // for now.
+                // This is fine as specialization currently assumes that specializing
+                // impls have to be always applicable, meaning that the only allowed
+                // region constraints may be constraints also present on the default impl.
                 let tcx = self.tcx();
                 if other.evaluation.must_apply_modulo_regions() {
                     if tcx.specializes((other_def, victim_def)) {
@@ -1822,7 +1779,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 | BuiltinUnsizeCandidate
                 | TraitUpcastingUnsizeCandidate(_)
                 | BuiltinCandidate { has_nested: true }
-                | TraitAliasCandidate(..),
+                | TraitAliasCandidate,
                 ImplCandidate(_)
                 | ClosureCandidate
                 | GeneratorCandidate
@@ -1831,7 +1788,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 | BuiltinUnsizeCandidate
                 | TraitUpcastingUnsizeCandidate(_)
                 | BuiltinCandidate { has_nested: true }
-                | TraitAliasCandidate(..),
+                | TraitAliasCandidate,
             ) => false,
         }
     }