Use a plain bitset for liveness analyses.

compiler/rustc_mir_dataflow/src/impls/initialized.rs

@@ -308,7 +308,10 @@ impl<'a, 'tcx> DefinitelyInitializedPlaces<'a, 'tcx> {
 }
 
 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 {
@@ -444,6 +447,8 @@ impl<'tcx> GenKillAnalysis<'tcx> for MaybeInitializedPlaces<'_, 'tcx> {
 }
 
 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";
@@ -649,6 +654,8 @@ impl<'tcx> GenKillAnalysis<'tcx> for DefinitelyInitializedPlaces<'_, 'tcx> {
 }
 
 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,
@@ -27,14 +27,14 @@ use crate::{Analysis, AnalysisDomain, Backward, GenKill, GenKillAnalysis};
 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) {
@@ -234,14 +234,14 @@ impl<'a> MaybeTransitiveLiveLocals<'a> {
 }
 
 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/rustc_peek.rs

@@ -1,7 +1,7 @@
 use rustc_span::symbol::sym;
 use rustc_span::Span;
 
-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};
@@ -264,7 +264,7 @@ impl<'tcx> RustcPeekAt<'tcx> for MaybeLiveLocals {
         &self,
         tcx: TyCtxt<'tcx>,
         place: mir::Place<'tcx>,
-        flow_state: &ChunkedBitSet<Local>,
+        flow_state: &BitSet<Local>,
         call: PeekCall,
     ) {
         info!(?place, "peek_at");