remove `has_nested`

4 files changed, 54 insertions, 66 deletions

diff --git a/compiler/rustc_middle/src/traits/select.rs b/compiler/rustc_middle/src/traits/select.rs
index aa2ee756bc5..c498e6b3c83 100644
--- a/compiler/rustc_middle/src/traits/select.rs
+++ b/compiler/rustc_middle/src/traits/select.rs
@@ -97,9 +97,7 @@ pub type EvaluationCache<'tcx, ENV> = Cache<(ENV, ty::PolyTraitPredicate<'tcx>),
 pub enum SelectionCandidate<'tcx> {
     /// A built-in implementation for the `Sized` trait. This is preferred
     /// over all other candidates.
-    SizedCandidate {
-        has_nested: bool,
-    },
+    SizedCandidate,
 
     /// A builtin implementation for some specific traits, used in cases
     /// where we cannot rely an ordinary library implementations.
@@ -107,10 +105,7 @@ pub enum SelectionCandidate<'tcx> {
     /// The most notable examples are `Copy` and `Clone`. This is also
     /// used for the `DiscriminantKind` and `Pointee` trait, both of which have
     /// an associated type.
-    BuiltinCandidate {
-        /// `false` if there are no *further* obligations.
-        has_nested: bool,
-    },
+    BuiltinCandidate,
 
     /// Implementation of transmutability trait.
     TransmutabilityCandidate,
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 81ce58a93fa..9af85f5e3f0 100644
--- a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
@@ -80,11 +80,11 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 }
                 Some(LangItem::DiscriminantKind) => {
                     // `DiscriminantKind` is automatically implemented for every type.
-                    candidates.vec.push(BuiltinCandidate { has_nested: false });
+                    candidates.vec.push(BuiltinCandidate);
                 }
                 Some(LangItem::PointeeTrait) => {
                     // `Pointee` is automatically implemented for every type.
-                    candidates.vec.push(BuiltinCandidate { has_nested: false });
+                    candidates.vec.push(BuiltinCandidate);
                 }
                 Some(LangItem::Sized) => {
                     self.assemble_builtin_sized_candidate(
@@ -365,7 +365,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         {
             debug!(?self_ty, ?obligation, "assemble_fused_iterator_candidates",);
 
-            candidates.vec.push(BuiltinCandidate { has_nested: false });
+            candidates.vec.push(BuiltinCandidate);
         }
     }
 
@@ -810,7 +810,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                         hir::Movability::Movable => {
                             // Movable coroutines are always `Unpin`, so add an
                             // unconditional builtin candidate.
-                            candidates.vec.push(BuiltinCandidate { has_nested: false });
+                            candidates.vec.push(BuiltinCandidate);
                         }
                     }
                 }
@@ -1122,10 +1122,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         sizedness: SizedTraitKind,
     ) {
         match self.sizedness_conditions(obligation, sizedness) {
-            BuiltinImplConditions::Where(nested) => {
-                candidates
-                    .vec
-                    .push(SizedCandidate { has_nested: !nested.skip_binder().is_empty() });
+            BuiltinImplConditions::Where(_nested) => {
+                candidates.vec.push(SizedCandidate);
             }
             BuiltinImplConditions::None => {}
             BuiltinImplConditions::Ambiguous => {
@@ -1143,10 +1141,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         candidates: &mut SelectionCandidateSet<'tcx>,
     ) {
         match conditions {
-            BuiltinImplConditions::Where(nested) => {
-                candidates
-                    .vec
-                    .push(BuiltinCandidate { has_nested: !nested.skip_binder().is_empty() });
+            BuiltinImplConditions::Where(_) => {
+                candidates.vec.push(BuiltinCandidate);
             }
             BuiltinImplConditions::None => {}
             BuiltinImplConditions::Ambiguous => {
@@ -1160,7 +1156,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         _obligation: &PolyTraitObligation<'tcx>,
         candidates: &mut SelectionCandidateSet<'tcx>,
     ) {
-        candidates.vec.push(BuiltinCandidate { has_nested: false });
+        candidates.vec.push(BuiltinCandidate);
     }
 
     fn assemble_candidate_for_tuple(
@@ -1171,7 +1167,7 @@ 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(BuiltinCandidate { has_nested: false });
+                candidates.vec.push(BuiltinCandidate);
             }
             ty::Infer(ty::TyVar(_)) => {
                 candidates.ambiguous = true;
@@ -1215,7 +1211,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         let self_ty = self.infcx.resolve_vars_if_possible(obligation.self_ty());
 
         match self_ty.skip_binder().kind() {
-            ty::FnPtr(..) => candidates.vec.push(BuiltinCandidate { has_nested: false }),
+            ty::FnPtr(..) => candidates.vec.push(BuiltinCandidate),
             ty::Bool
             | ty::Char
             | ty::Int(_)
diff --git a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
index a6b77583fdc..545531f927a 100644
--- a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
@@ -37,13 +37,13 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         candidate: SelectionCandidate<'tcx>,
     ) -> Result<Selection<'tcx>, SelectionError<'tcx>> {
         Ok(match candidate {
-            SizedCandidate { has_nested } => {
-                let data = self.confirm_builtin_candidate(obligation, has_nested);
+            SizedCandidate => {
+                let data = self.confirm_builtin_candidate(obligation);
                 ImplSource::Builtin(BuiltinImplSource::Misc, data)
             }
 
-            BuiltinCandidate { has_nested } => {
-                let data = self.confirm_builtin_candidate(obligation, has_nested);
+            BuiltinCandidate => {
+                let data = self.confirm_builtin_candidate(obligation);
                 ImplSource::Builtin(BuiltinImplSource::Misc, data)
             }
 
@@ -249,50 +249,47 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         }
     }
 
+    #[instrument(level = "debug", skip(self), ret)]
     fn confirm_builtin_candidate(
         &mut self,
         obligation: &PolyTraitObligation<'tcx>,
-        has_nested: bool,
     ) -> PredicateObligations<'tcx> {
-        debug!(?obligation, ?has_nested, "confirm_builtin_candidate");
-
+        debug!(?obligation, "confirm_builtin_candidate");
         let tcx = self.tcx();
-        let obligations = if has_nested {
-            let trait_def = obligation.predicate.def_id();
-            let conditions = match tcx.as_lang_item(trait_def) {
-                Some(LangItem::Sized) => {
-                    self.sizedness_conditions(obligation, SizedTraitKind::Sized)
-                }
-                Some(LangItem::MetaSized) => {
-                    self.sizedness_conditions(obligation, SizedTraitKind::MetaSized)
-                }
-                Some(LangItem::PointeeSized) => {
-                    bug!("`PointeeSized` is removing during lowering");
-                }
-                Some(LangItem::Copy | LangItem::Clone) => self.copy_clone_conditions(obligation),
-                Some(LangItem::FusedIterator) => self.fused_iterator_conditions(obligation),
-                other => bug!("unexpected builtin trait {trait_def:?} ({other:?})"),
-            };
-            let BuiltinImplConditions::Where(types) = conditions else {
-                bug!("obligation {:?} had matched a builtin impl but now doesn't", obligation);
-            };
-            let types = self.infcx.enter_forall_and_leak_universe(types);
-
-            let cause = obligation.derived_cause(ObligationCauseCode::BuiltinDerived);
-            self.collect_predicates_for_types(
-                obligation.param_env,
-                cause,
-                obligation.recursion_depth + 1,
-                trait_def,
-                types,
-            )
-        } else {
-            PredicateObligations::new()
+        let trait_def = obligation.predicate.def_id();
+        let conditions = match tcx.as_lang_item(trait_def) {
+            Some(LangItem::Sized) => self.sizedness_conditions(obligation, SizedTraitKind::Sized),
+            Some(LangItem::MetaSized) => {
+                self.sizedness_conditions(obligation, SizedTraitKind::MetaSized)
+            }
+            Some(LangItem::PointeeSized) => {
+                bug!("`PointeeSized` is removing during lowering");
+            }
+            Some(LangItem::Copy | LangItem::Clone) => self.copy_clone_conditions(obligation),
+            Some(LangItem::FusedIterator) => self.fused_iterator_conditions(obligation),
+            Some(
+                LangItem::Destruct
+                | LangItem::DiscriminantKind
+                | LangItem::FnPtrTrait
+                | LangItem::PointeeTrait
+                | LangItem::Tuple
+                | LangItem::Unpin,
+            ) => BuiltinImplConditions::Where(ty::Binder::dummy(vec![])),
+            other => bug!("unexpected builtin trait {trait_def:?} ({other:?})"),
         };
+        let BuiltinImplConditions::Where(types) = conditions else {
+            bug!("obligation {:?} had matched a builtin impl but now doesn't", obligation);
+        };
+        let types = self.infcx.enter_forall_and_leak_universe(types);
 
-        debug!(?obligations);
-
-        obligations
+        let cause = obligation.derived_cause(ObligationCauseCode::BuiltinDerived);
+        self.collect_predicates_for_types(
+            obligation.param_env,
+            cause,
+            obligation.recursion_depth + 1,
+            trait_def,
+            types,
+        )
     }
 
     #[instrument(level = "debug", skip(self))]
diff --git a/compiler/rustc_trait_selection/src/traits/select/mod.rs b/compiler/rustc_trait_selection/src/traits/select/mod.rs
index af3641c22ed..c9930c69b32 100644
--- a/compiler/rustc_trait_selection/src/traits/select/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/mod.rs
@@ -1834,7 +1834,7 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
 
         // We prefer `Sized` candidates over everything.
         let mut sized_candidates =
-            candidates.iter().filter(|c| matches!(c.candidate, SizedCandidate { has_nested: _ }));
+            candidates.iter().filter(|c| matches!(c.candidate, SizedCandidate));
         if let Some(sized_candidate) = sized_candidates.next() {
             // There should only ever be a single sized candidate
             // as they would otherwise overlap.
@@ -1986,8 +1986,8 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
             // Don't use impl candidates which overlap with other candidates.
             // This should pretty much only ever happen with malformed impls.
             if candidates.iter().all(|c| match c.candidate {
-                SizedCandidate { has_nested: _ }
-                | BuiltinCandidate { has_nested: _ }
+                SizedCandidate
+                | BuiltinCandidate
                 | TransmutabilityCandidate
                 | AutoImplCandidate
                 | ClosureCandidate { .. }