[EXPERIMENT] Check if isolating Matrix::unspecialize dodges regressions

compiler/rustc_abi/src/layout/coroutine.rs

@@ -67,7 +67,7 @@ fn coroutine_saved_local_eligibility<VariantIdx: Idx, FieldIdx: Idx, LocalIdx: I
                         "removing local {:?} in >1 variant ({:?}, {:?})",
                         local, variant_index, idx
                     );
-                    ineligible_locals.insert(*local);
+                    ineligible_locals.insert2(*local);
                     assignments[*local] = Ineligible(None);
                 }
                 Ineligible(_) => {}
@@ -98,7 +98,7 @@ fn coroutine_saved_local_eligibility<VariantIdx: Idx, FieldIdx: Idx, LocalIdx: I
             let conflicts_b = storage_conflicts.count(local_b);
             let (remove, other) =
                 if conflicts_a > conflicts_b { (local_a, local_b) } else { (local_b, local_a) };
-            ineligible_locals.insert(remove);
+            ineligible_locals.insert2(remove);
             assignments[remove] = Ineligible(None);
             trace!("removing local {:?} due to conflict with {:?}", remove, other);
         }
@@ -113,7 +113,7 @@ fn coroutine_saved_local_eligibility<VariantIdx: Idx, FieldIdx: Idx, LocalIdx: I
         let mut used_variants = DenseBitSet::new_empty(variant_fields.len());
         for assignment in &assignments {
             if let Assigned(idx) = assignment {
-                used_variants.insert(*idx);
+                used_variants.insert2(*idx);
             }
         }
         if used_variants.count() < 2 {

compiler/rustc_codegen_ssa/src/mir/analyze.rs

@@ -48,7 +48,7 @@ pub(crate) fn non_ssa_locals<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     let mut non_ssa_locals = DenseBitSet::new_empty(analyzer.locals.len());
     for (local, kind) in analyzer.locals.iter_enumerated() {
         if matches!(kind, LocalKind::Memory) {
-            non_ssa_locals.insert(local);
+            non_ssa_locals.insert2(local);
         }
     }
 

compiler/rustc_codegen_ssa/src/mir/debuginfo.rs

@@ -752,7 +752,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
             // NOTE(eddyb) actually, on second thought, those are always in the
             // function scope, which always exists.
             for var_debug_info in &self.mir.var_debug_info {
-                vars.insert(var_debug_info.source_info.scope);
+                vars.insert2(var_debug_info.source_info.scope);
             }
             Some(vars)
         } else {

compiler/rustc_codegen_ssa/src/mir/mod.rs

@@ -296,7 +296,7 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
         let mut reachable = DenseBitSet::new_empty(mir.basic_blocks.len());
         let mut to_visit = vec![mir::START_BLOCK];
         while let Some(next) = to_visit.pop() {
-            if !reachable.insert(next) {
+            if !reachable.insert2(next) {
                 continue;
             }
 

compiler/rustc_hir_analysis/src/check/check.rs

@@ -2111,7 +2111,7 @@ fn check_type_alias_type_params_are_used<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalD
             && let ty::Param(param) = leaf_ty.kind()
         {
             debug!("found use of ty param {:?}", param);
-            params_used.insert(param.index);
+            params_used.insert2(param.index);
         }
     }
 

compiler/rustc_index/src/bit_set.rs

@@ -161,6 +161,18 @@ impl<T: Idx> DenseBitSet<T> {
         new_word != word
     }
 
+    /// Insert `elem`. Returns whether the set has changed.
+    #[inline]
+    pub fn insert2(&mut self, elem: T) -> bool {
+        assert!(elem.index() < self.domain_size,);
+        let (word_index, mask) = word_index_and_mask(elem);
+        let word_ref = &mut self.words[word_index];
+        let word = *word_ref;
+        let new_word = word | mask;
+        *word_ref = new_word;
+        new_word != word
+    }
+
     #[inline]
     pub fn insert_range(&mut self, elems: impl RangeBounds<T>) {
         let Some((start, end)) = inclusive_start_end(elems, self.domain_size) else {

compiler/rustc_ty_utils/src/representability.rs

@@ -117,7 +117,7 @@ fn params_in_repr_ty<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>, params_in_repr: &mut
         ty::Array(ty, _) => params_in_repr_ty(tcx, ty, params_in_repr),
         ty::Tuple(tys) => tys.iter().for_each(|ty| params_in_repr_ty(tcx, ty, params_in_repr)),
         ty::Param(param) => {
-            params_in_repr.insert(param.index);
+            params_in_repr.insert2(param.index);
         }
         _ => {}
     }

compiler/rustc_ty_utils/src/ty.rs

@@ -329,7 +329,7 @@ fn unsizing_params_for_adt<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId) -> DenseBitSe
     let mut unsizing_params = DenseBitSet::new_empty(num_params);
     for arg in tcx.type_of(tail_field.did).instantiate_identity().skip_norm_wip().walk() {
         if let Some(i) = maybe_unsizing_param_idx(arg) {
-            unsizing_params.insert(i);
+            unsizing_params.insert2(i);
         }
     }