Use a plain bitset for liveness analyses.

compiler/rustc_mir_dataflow/src/impls/initialized.rs

@@ -305,7 +305,10 @@ fn update_bits(
 }
 
 impl<'tcx> AnalysisDomain<'tcx> for MaybeInitializedPlaces<'_, 'tcx> {
+    /// There can be many more `MovePathIndex` than there are locals in a MIR body.
+    /// We use a chunked bitset to avoid paying too high a memory footprint.
     type Domain = MaybeReachable<ChunkedBitSet<MovePathIndex>>;
+
     const NAME: &'static str = "maybe_init";
 
     fn bottom_value(&self, _: &mir::Body<'tcx>) -> Self::Domain {
@@ -437,6 +440,8 @@ fn switch_int_edge_effects<G: GenKill<Self::Idx>>(
 }
 
 impl<'tcx> AnalysisDomain<'tcx> for MaybeUninitializedPlaces<'_, 'tcx> {
+    /// There can be many more `MovePathIndex` than there are locals in a MIR body.
+    /// We use a chunked bitset to avoid paying too high a memory footprint.
     type Domain = ChunkedBitSet<MovePathIndex>;
 
     const NAME: &'static str = "maybe_uninit";
@@ -636,6 +641,8 @@ fn call_return_effect(
 }
 
 impl<'tcx> AnalysisDomain<'tcx> for EverInitializedPlaces<'_, 'tcx> {
+    /// There can be many more `InitIndex` than there are locals in a MIR body.
+    /// We use a chunked bitset to avoid paying too high a memory footprint.
     type Domain = ChunkedBitSet<InitIndex>;
 
     const NAME: &'static str = "ever_init";

compiler/rustc_mir_dataflow/src/impls/liveness.rs

@@ -1,4 +1,4 @@
-use rustc_index::bit_set::{BitSet, ChunkedBitSet};
+use rustc_index::bit_set::BitSet;
 use rustc_middle::mir::visit::{MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor};
 use rustc_middle::mir::{
     self, CallReturnPlaces, Local, Location, Place, StatementKind, TerminatorEdges,
@@ -26,14 +26,14 @@
 pub struct MaybeLiveLocals;
 
 impl<'tcx> AnalysisDomain<'tcx> for MaybeLiveLocals {
-    type Domain = ChunkedBitSet<Local>;
+    type Domain = BitSet<Local>;
     type Direction = Backward;
 
     const NAME: &'static str = "liveness";
 
     fn bottom_value(&self, body: &mir::Body<'tcx>) -> Self::Domain {
         // bottom = not live
-        ChunkedBitSet::new_empty(body.local_decls.len())
+        BitSet::new_empty(body.local_decls.len())
     }
 
     fn initialize_start_block(&self, _: &mir::Body<'tcx>, _: &mut Self::Domain) {
@@ -233,14 +233,14 @@ pub fn new(always_live: &'a BitSet<Local>) -> Self {
 }
 
 impl<'a, 'tcx> AnalysisDomain<'tcx> for MaybeTransitiveLiveLocals<'a> {
-    type Domain = ChunkedBitSet<Local>;
+    type Domain = BitSet<Local>;
     type Direction = Backward;
 
     const NAME: &'static str = "transitive liveness";
 
     fn bottom_value(&self, body: &mir::Body<'tcx>) -> Self::Domain {
         // bottom = not live
-        ChunkedBitSet::new_empty(body.local_decls.len())
+        BitSet::new_empty(body.local_decls.len())
     }
 
     fn initialize_start_block(&self, _: &mir::Body<'tcx>, _: &mut Self::Domain) {

compiler/rustc_mir_dataflow/src/points.rs

@@ -1,5 +1,5 @@
 use crate::framework::{visit_results, ResultsVisitable, ResultsVisitor};
-use rustc_index::bit_set::ChunkedBitSet;
+use rustc_index::bit_set::BitSet;
 use rustc_index::interval::SparseIntervalMatrix;
 use rustc_index::Idx;
 use rustc_index::IndexVec;
@@ -102,7 +102,7 @@ pub fn save_as_intervals<'tcx, N, R>(
 ) -> SparseIntervalMatrix<N, PointIndex>
 where
     N: Idx,
-    R: ResultsVisitable<'tcx, FlowState = ChunkedBitSet<N>>,
+    R: ResultsVisitable<'tcx, FlowState = BitSet<N>>,
 {
     let values = SparseIntervalMatrix::new(elements.num_points());
     let mut visitor = Visitor { elements, values };
@@ -124,7 +124,7 @@ impl<'mir, 'tcx, R, N> ResultsVisitor<'mir, 'tcx, R> for Visitor<'_, N>
 where
     N: Idx,
 {
-    type FlowState = ChunkedBitSet<N>;
+    type FlowState = BitSet<N>;
 
     fn visit_statement_after_primary_effect(
         &mut self,

compiler/rustc_mir_dataflow/src/rustc_peek.rs

@@ -12,7 +12,7 @@
 use crate::{Analysis, JoinSemiLattice, ResultsCursor};
 use rustc_ast::MetaItem;
 use rustc_hir::def_id::DefId;
-use rustc_index::bit_set::ChunkedBitSet;
+use rustc_index::bit_set::BitSet;
 use rustc_middle::mir::MirPass;
 use rustc_middle::mir::{self, Body, Local, Location};
 use rustc_middle::ty::{self, Ty, TyCtxt};
@@ -275,7 +275,7 @@ fn peek_at(
         &self,
         tcx: TyCtxt<'tcx>,
         place: mir::Place<'tcx>,
-        flow_state: &ChunkedBitSet<Local>,
+        flow_state: &BitSet<Local>,
         call: PeekCall,
     ) {
         info!(?place, "peek_at");