rename `BitSet` to `DenseBitSet`

This should make it clearer that this bitset is dense, with the
advantages and disadvantages that it entails.

67 files changed, 367 insertions, 356 deletions

diff --git a/compiler/rustc_borrowck/src/borrow_set.rs b/compiler/rustc_borrowck/src/borrow_set.rs
index a29833464fb..303fa469332 100644
--- a/compiler/rustc_borrowck/src/borrow_set.rs
+++ b/compiler/rustc_borrowck/src/borrow_set.rs
@@ -2,7 +2,7 @@ use std::fmt;
 use std::ops::Index;
 
 use rustc_data_structures::fx::{FxIndexMap, FxIndexSet};
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::mir::visit::{MutatingUseContext, NonUseContext, PlaceContext, Visitor};
 use rustc_middle::mir::{self, Body, Local, Location, traversal};
 use rustc_middle::span_bug;
@@ -131,7 +131,7 @@ impl<'tcx> fmt::Display for BorrowData<'tcx> {
 
 pub enum LocalsStateAtExit {
     AllAreInvalidated,
-    SomeAreInvalidated { has_storage_dead_or_moved: BitSet<Local> },
+    SomeAreInvalidated { has_storage_dead_or_moved: DenseBitSet<Local> },
 }
 
 impl LocalsStateAtExit {
@@ -140,7 +140,7 @@ impl LocalsStateAtExit {
         body: &Body<'tcx>,
         move_data: &MoveData<'tcx>,
     ) -> Self {
-        struct HasStorageDead(BitSet<Local>);
+        struct HasStorageDead(DenseBitSet<Local>);
 
         impl<'tcx> Visitor<'tcx> for HasStorageDead {
             fn visit_local(&mut self, local: Local, ctx: PlaceContext, _: Location) {
@@ -153,7 +153,8 @@ impl LocalsStateAtExit {
         if locals_are_invalidated_at_exit {
             LocalsStateAtExit::AllAreInvalidated
         } else {
-            let mut has_storage_dead = HasStorageDead(BitSet::new_empty(body.local_decls.len()));
+            let mut has_storage_dead =
+                HasStorageDead(DenseBitSet::new_empty(body.local_decls.len()));
             has_storage_dead.visit_body(body);
             let mut has_storage_dead_or_moved = has_storage_dead.0;
             for move_out in &move_data.moves {
diff --git a/compiler/rustc_borrowck/src/dataflow.rs b/compiler/rustc_borrowck/src/dataflow.rs
index abe4d4f20ec..a7a6f2da509 100644
--- a/compiler/rustc_borrowck/src/dataflow.rs
+++ b/compiler/rustc_borrowck/src/dataflow.rs
@@ -2,7 +2,7 @@ use std::fmt;
 
 use rustc_data_structures::fx::FxIndexMap;
 use rustc_data_structures::graph;
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::mir::{
     self, BasicBlock, Body, CallReturnPlaces, Location, Place, TerminatorEdges,
 };
@@ -180,7 +180,7 @@ pub struct Borrows<'a, 'tcx> {
 }
 
 struct OutOfScopePrecomputer<'a, 'tcx> {
-    visited: BitSet<mir::BasicBlock>,
+    visited: DenseBitSet<mir::BasicBlock>,
     visit_stack: Vec<mir::BasicBlock>,
     body: &'a Body<'tcx>,
     regioncx: &'a RegionInferenceContext<'tcx>,
@@ -190,7 +190,7 @@ struct OutOfScopePrecomputer<'a, 'tcx> {
 impl<'a, 'tcx> OutOfScopePrecomputer<'a, 'tcx> {
     fn new(body: &'a Body<'tcx>, regioncx: &'a RegionInferenceContext<'tcx>) -> Self {
         OutOfScopePrecomputer {
-            visited: BitSet::new_empty(body.basic_blocks.len()),
+            visited: DenseBitSet::new_empty(body.basic_blocks.len()),
             visit_stack: vec![],
             body,
             regioncx,
@@ -292,7 +292,7 @@ pub fn calculate_borrows_out_of_scope_at_location<'tcx>(
 }
 
 struct PoloniusOutOfScopePrecomputer<'a, 'tcx> {
-    visited: BitSet<mir::BasicBlock>,
+    visited: DenseBitSet<mir::BasicBlock>,
     visit_stack: Vec<mir::BasicBlock>,
     body: &'a Body<'tcx>,
     regioncx: &'a RegionInferenceContext<'tcx>,
@@ -303,7 +303,7 @@ struct PoloniusOutOfScopePrecomputer<'a, 'tcx> {
 impl<'a, 'tcx> PoloniusOutOfScopePrecomputer<'a, 'tcx> {
     fn new(body: &'a Body<'tcx>, regioncx: &'a RegionInferenceContext<'tcx>) -> Self {
         Self {
-            visited: BitSet::new_empty(body.basic_blocks.len()),
+            visited: DenseBitSet::new_empty(body.basic_blocks.len()),
             visit_stack: vec![],
             body,
             regioncx,
@@ -559,7 +559,7 @@ impl<'a, 'tcx> Borrows<'a, 'tcx> {
     }
 }
 
-type BorrowsDomain = BitSet<BorrowIndex>;
+type BorrowsDomain = DenseBitSet<BorrowIndex>;
 
 /// Forward dataflow computation of the set of borrows that are in scope at a particular location.
 /// - we gen the introduced loans
@@ -575,7 +575,7 @@ impl<'tcx> rustc_mir_dataflow::Analysis<'tcx> for Borrows<'_, 'tcx> {
 
     fn bottom_value(&self, _: &mir::Body<'tcx>) -> Self::Domain {
         // bottom = nothing is reserved or activated yet;
-        BitSet::new_empty(self.borrow_set.len())
+        DenseBitSet::new_empty(self.borrow_set.len())
     }
 
     fn initialize_start_block(&self, _: &mir::Body<'tcx>, _: &mut Self::Domain) {
diff --git a/compiler/rustc_borrowck/src/lib.rs b/compiler/rustc_borrowck/src/lib.rs
index 4f46a2cef7c..45764f9c6b8 100644
--- a/compiler/rustc_borrowck/src/lib.rs
+++ b/compiler/rustc_borrowck/src/lib.rs
@@ -28,7 +28,7 @@ use rustc_errors::LintDiagnostic;
 use rustc_hir as hir;
 use rustc_hir::CRATE_HIR_ID;
 use rustc_hir::def_id::LocalDefId;
-use rustc_index::bit_set::{BitSet, MixedBitSet};
+use rustc_index::bit_set::{DenseBitSet, MixedBitSet};
 use rustc_index::{IndexSlice, IndexVec};
 use rustc_infer::infer::{
     InferCtxt, NllRegionVariableOrigin, RegionVariableOrigin, TyCtxtInferExt,
@@ -1017,11 +1017,11 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, '_, 'tcx> {
         &self,
         location: Location,
         state: &'s BorrowckDomain,
-    ) -> Cow<'s, BitSet<BorrowIndex>> {
+    ) -> Cow<'s, DenseBitSet<BorrowIndex>> {
         if let Some(polonius) = &self.polonius_output {
             // Use polonius output if it has been enabled.
             let location = self.location_table.start_index(location);
-            let mut polonius_output = BitSet::new_empty(self.borrow_set.len());
+            let mut polonius_output = DenseBitSet::new_empty(self.borrow_set.len());
             for &idx in polonius.errors_at(location) {
                 polonius_output.insert(idx);
             }
diff --git a/compiler/rustc_borrowck/src/type_check/liveness/trace.rs b/compiler/rustc_borrowck/src/type_check/liveness/trace.rs
index 7fce49fd16b..4c0d3138f2d 100644
--- a/compiler/rustc_borrowck/src/type_check/liveness/trace.rs
+++ b/compiler/rustc_borrowck/src/type_check/liveness/trace.rs
@@ -1,5 +1,5 @@
 use rustc_data_structures::fx::{FxIndexMap, FxIndexSet};
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_index::interval::IntervalSet;
 use rustc_infer::infer::canonical::QueryRegionConstraints;
 use rustc_infer::infer::outlives::for_liveness;
@@ -129,7 +129,7 @@ struct LivenessResults<'a, 'typeck, 'b, 'tcx> {
     cx: LivenessContext<'a, 'typeck, 'b, 'tcx>,
 
     /// Set of points that define the current local.
-    defs: BitSet<PointIndex>,
+    defs: DenseBitSet<PointIndex>,
 
     /// Points where the current variable is "use live" -- meaning
     /// that there is a future "full use" that may use its value.
@@ -152,7 +152,7 @@ impl<'a, 'typeck, 'b, 'tcx> LivenessResults<'a, 'typeck, 'b, 'tcx> {
         let num_points = cx.location_map.num_points();
         LivenessResults {
             cx,
-            defs: BitSet::new_empty(num_points),
+            defs: DenseBitSet::new_empty(num_points),
             use_live_at: IntervalSet::new(num_points),
             drop_live_at: IntervalSet::new(num_points),
             drop_locations: vec![],
diff --git a/compiler/rustc_codegen_llvm/src/debuginfo/create_scope_map.rs b/compiler/rustc_codegen_llvm/src/debuginfo/create_scope_map.rs
index 07bd0f4d1c1..e545ce386ed 100644
--- a/compiler/rustc_codegen_llvm/src/debuginfo/create_scope_map.rs
+++ b/compiler/rustc_codegen_llvm/src/debuginfo/create_scope_map.rs
@@ -4,7 +4,7 @@ use rustc_codegen_ssa::mir::debuginfo::{DebugScope, FunctionDebugContext};
 use rustc_codegen_ssa::traits::*;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_index::Idx;
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::mir::{Body, SourceScope};
 use rustc_middle::ty::layout::{FnAbiOf, HasTypingEnv};
 use rustc_middle::ty::{self, Instance};
@@ -27,7 +27,7 @@ pub(crate) fn compute_mir_scopes<'ll, 'tcx>(
 ) {
     // Find all scopes with variables defined in them.
     let variables = if cx.sess().opts.debuginfo == DebugInfo::Full {
-        let mut vars = BitSet::new_empty(mir.source_scopes.len());
+        let mut vars = DenseBitSet::new_empty(mir.source_scopes.len());
         // FIXME(eddyb) take into account that arguments always have debuginfo,
         // irrespective of their name (assuming full debuginfo is enabled).
         // NOTE(eddyb) actually, on second thought, those are always in the
@@ -40,7 +40,7 @@ pub(crate) fn compute_mir_scopes<'ll, 'tcx>(
         // Nothing to emit, of course.
         None
     };
-    let mut instantiated = BitSet::new_empty(mir.source_scopes.len());
+    let mut instantiated = DenseBitSet::new_empty(mir.source_scopes.len());
     let mut discriminators = FxHashMap::default();
     // Instantiate all scopes.
     for idx in 0..mir.source_scopes.len() {
@@ -63,9 +63,9 @@ fn make_mir_scope<'ll, 'tcx>(
     cx: &CodegenCx<'ll, 'tcx>,
     instance: Instance<'tcx>,
     mir: &Body<'tcx>,
-    variables: &Option<BitSet<SourceScope>>,
+    variables: &Option<DenseBitSet<SourceScope>>,
     debug_context: &mut FunctionDebugContext<'tcx, &'ll DIScope, &'ll DILocation>,
-    instantiated: &mut BitSet<SourceScope>,
+    instantiated: &mut DenseBitSet<SourceScope>,
     discriminators: &mut FxHashMap<BytePos, u32>,
     scope: SourceScope,
 ) {
diff --git a/compiler/rustc_codegen_ssa/src/mir/analyze.rs b/compiler/rustc_codegen_ssa/src/mir/analyze.rs
index 43b8230c679..23baab3124e 100644
--- a/compiler/rustc_codegen_ssa/src/mir/analyze.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/analyze.rs
@@ -2,7 +2,7 @@
 //! which do not.
 
 use rustc_data_structures::graph::dominators::Dominators;
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_index::{IndexSlice, IndexVec};
 use rustc_middle::mir::visit::{MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor};
 use rustc_middle::mir::{self, DefLocation, Location, TerminatorKind, traversal};
@@ -16,7 +16,7 @@ use crate::traits::*;
 pub(crate) fn non_ssa_locals<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     fx: &FunctionCx<'a, 'tcx, Bx>,
     traversal_order: &[mir::BasicBlock],
-) -> BitSet<mir::Local> {
+) -> DenseBitSet<mir::Local> {
     let mir = fx.mir;
     let dominators = mir.basic_blocks.dominators();
     let locals = mir
@@ -44,7 +44,7 @@ pub(crate) fn non_ssa_locals<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
         analyzer.visit_basic_block_data(bb, data);
     }
 
-    let mut non_ssa_locals = BitSet::new_empty(analyzer.locals.len());
+    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);
diff --git a/compiler/rustc_codegen_ssa/src/mir/mod.rs b/compiler/rustc_codegen_ssa/src/mir/mod.rs
index 62f69af3f2f..3a896071bc6 100644
--- a/compiler/rustc_codegen_ssa/src/mir/mod.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/mod.rs
@@ -1,7 +1,7 @@
 use std::iter;
 
 use rustc_index::IndexVec;
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags;
 use rustc_middle::mir::{UnwindTerminateReason, traversal};
 use rustc_middle::ty::layout::{FnAbiOf, HasTyCtxt, HasTypingEnv, TyAndLayout};
@@ -293,7 +293,7 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     // So drop the builder of `start_llbb` to avoid having two at the same time.
     drop(start_bx);
 
-    let mut unreached_blocks = BitSet::new_filled(mir.basic_blocks.len());
+    let mut unreached_blocks = DenseBitSet::new_filled(mir.basic_blocks.len());
     // Codegen the body of each reachable block using our reverse postorder list.
     for bb in traversal_order {
         fx.codegen_block(bb);
@@ -316,7 +316,7 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
 fn arg_local_refs<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     bx: &mut Bx,
     fx: &mut FunctionCx<'a, 'tcx, Bx>,
-    memory_locals: &BitSet<mir::Local>,
+    memory_locals: &DenseBitSet<mir::Local>,
 ) -> Vec<LocalRef<'tcx, Bx::Value>> {
     let mir = fx.mir;
     let mut idx = 0;
diff --git a/compiler/rustc_const_eval/src/check_consts/check.rs b/compiler/rustc_const_eval/src/check_consts/check.rs
index 015e1671682..7a5f6c17268 100644
--- a/compiler/rustc_const_eval/src/check_consts/check.rs
+++ b/compiler/rustc_const_eval/src/check_consts/check.rs
@@ -10,7 +10,7 @@ use rustc_attr_parsing::{ConstStability, StabilityLevel};
 use rustc_errors::{Diag, ErrorGuaranteed};
 use rustc_hir::def_id::DefId;
 use rustc_hir::{self as hir, LangItem};
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_infer::infer::TyCtxtInferExt;
 use rustc_middle::mir::visit::Visitor;
 use rustc_middle::mir::*;
@@ -172,7 +172,7 @@ pub struct Checker<'mir, 'tcx> {
 
     /// A set that stores for each local whether it is "transient", i.e. guaranteed to be dead
     /// when this MIR body returns.
-    transient_locals: Option<BitSet<Local>>,
+    transient_locals: Option<DenseBitSet<Local>>,
 
     error_emitted: Option<ErrorGuaranteed>,
     secondary_errors: Vec<Diag<'tcx>>,
@@ -242,7 +242,7 @@ impl<'mir, 'tcx> Checker<'mir, 'tcx> {
 
                 // And then check all `Return` in the MIR, and if a local is "maybe live" at a
                 // `Return` then it is definitely not transient.
-                let mut transient = BitSet::new_filled(ccx.body.local_decls.len());
+                let mut transient = DenseBitSet::new_filled(ccx.body.local_decls.len());
                 // Make sure to only visit reachable blocks, the dataflow engine can ICE otherwise.
                 for (bb, data) in traversal::reachable(&ccx.body) {
                     if matches!(data.terminator().kind, TerminatorKind::Return) {
diff --git a/compiler/rustc_data_structures/src/graph/implementation/mod.rs b/compiler/rustc_data_structures/src/graph/implementation/mod.rs
index 43fdfe6ee0d..7724e9347d8 100644
--- a/compiler/rustc_data_structures/src/graph/implementation/mod.rs
+++ b/compiler/rustc_data_structures/src/graph/implementation/mod.rs
@@ -22,7 +22,7 @@
 
 use std::fmt::Debug;
 
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use tracing::debug;
 
 #[cfg(test)]
@@ -214,7 +214,7 @@ impl<N: Debug, E: Debug> Graph<N, E> {
         direction: Direction,
         entry_node: NodeIndex,
     ) -> Vec<NodeIndex> {
-        let mut visited = BitSet::new_empty(self.len_nodes());
+        let mut visited = DenseBitSet::new_empty(self.len_nodes());
         let mut stack = vec![];
         let mut result = Vec::with_capacity(self.len_nodes());
         let mut push_node = |stack: &mut Vec<_>, node: NodeIndex| {
@@ -287,7 +287,7 @@ impl<'g, N: Debug, E: Debug> Iterator for AdjacentEdges<'g, N, E> {
 pub struct DepthFirstTraversal<'g, N, E> {
     graph: &'g Graph<N, E>,
     stack: Vec<NodeIndex>,
-    visited: BitSet<usize>,
+    visited: DenseBitSet<usize>,
     direction: Direction,
 }
 
@@ -297,7 +297,7 @@ impl<'g, N: Debug, E: Debug> DepthFirstTraversal<'g, N, E> {
         start_node: NodeIndex,
         direction: Direction,
     ) -> Self {
-        let mut visited = BitSet::new_empty(graph.len_nodes());
+        let mut visited = DenseBitSet::new_empty(graph.len_nodes());
         visited.insert(start_node.node_id());
         DepthFirstTraversal { graph, stack: vec![start_node], visited, direction }
     }
diff --git a/compiler/rustc_data_structures/src/graph/iterate/mod.rs b/compiler/rustc_data_structures/src/graph/iterate/mod.rs
index cbc6664d853..ecda7d3fba8 100644
--- a/compiler/rustc_data_structures/src/graph/iterate/mod.rs
+++ b/compiler/rustc_data_structures/src/graph/iterate/mod.rs
@@ -1,6 +1,6 @@
 use std::ops::ControlFlow;
 
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_index::{IndexSlice, IndexVec};
 
 use super::{DirectedGraph, StartNode, Successors};
@@ -78,7 +78,7 @@ where
 {
     graph: G,
     stack: Vec<G::Node>,
-    visited: BitSet<G::Node>,
+    visited: DenseBitSet<G::Node>,
 }
 
 impl<G> DepthFirstSearch<G>
@@ -86,7 +86,7 @@ where
     G: DirectedGraph + Successors,
 {
     pub fn new(graph: G) -> Self {
-        Self { stack: vec![], visited: BitSet::new_empty(graph.num_nodes()), graph }
+        Self { stack: vec![], visited: DenseBitSet::new_empty(graph.num_nodes()), graph }
     }
 
     /// Version of `push_start_node` that is convenient for chained
@@ -207,8 +207,8 @@ where
 {
     graph: &'graph G,
     stack: Vec<Event<G::Node>>,
-    visited: BitSet<G::Node>,
-    settled: BitSet<G::Node>,
+    visited: DenseBitSet<G::Node>,
+    settled: DenseBitSet<G::Node>,
 }
 
 impl<'graph, G> TriColorDepthFirstSearch<'graph, G>
@@ -219,8 +219,8 @@ where
         TriColorDepthFirstSearch {
             graph,
             stack: vec![],
-            visited: BitSet::new_empty(graph.num_nodes()),
-            settled: BitSet::new_empty(graph.num_nodes()),
+            visited: DenseBitSet::new_empty(graph.num_nodes()),
+            settled: DenseBitSet::new_empty(graph.num_nodes()),
         }
     }
 
diff --git a/compiler/rustc_data_structures/src/stable_hasher.rs b/compiler/rustc_data_structures/src/stable_hasher.rs
index 0872bd2c9ac..9cd0cc499ca 100644
--- a/compiler/rustc_data_structures/src/stable_hasher.rs
+++ b/compiler/rustc_data_structures/src/stable_hasher.rs
@@ -3,7 +3,7 @@ use std::marker::PhantomData;
 use std::mem;
 use std::num::NonZero;
 
-use rustc_index::bit_set::{self, BitSet};
+use rustc_index::bit_set::{self, DenseBitSet};
 use rustc_index::{Idx, IndexSlice, IndexVec};
 use smallvec::SmallVec;
 
@@ -544,7 +544,7 @@ where
     }
 }
 
-impl<I: Idx, CTX> HashStable<CTX> for BitSet<I> {
+impl<I: Idx, CTX> HashStable<CTX> for DenseBitSet<I> {
     fn hash_stable(&self, _ctx: &mut CTX, hasher: &mut StableHasher) {
         ::std::hash::Hash::hash(self, hasher);
     }
diff --git a/compiler/rustc_data_structures/src/stable_hasher/tests.rs b/compiler/rustc_data_structures/src/stable_hasher/tests.rs
index aab50a13af0..635f241847c 100644
--- a/compiler/rustc_data_structures/src/stable_hasher/tests.rs
+++ b/compiler/rustc_data_structures/src/stable_hasher/tests.rs
@@ -17,9 +17,9 @@ fn hash<T: HashStable<()>>(t: &T) -> Hash128 {
 // Check that bit set hash includes the domain size.
 #[test]
 fn test_hash_bit_set() {
-    use rustc_index::bit_set::BitSet;
-    let a: BitSet<usize> = BitSet::new_empty(1);
-    let b: BitSet<usize> = BitSet::new_empty(2);
+    use rustc_index::bit_set::DenseBitSet;
+    let a: DenseBitSet<usize> = DenseBitSet::new_empty(1);
+    let b: DenseBitSet<usize> = DenseBitSet::new_empty(2);
     assert_ne!(a, b);
     assert_ne!(hash(&a), hash(&b));
 }
diff --git a/compiler/rustc_data_structures/src/work_queue.rs b/compiler/rustc_data_structures/src/work_queue.rs
index ca052e2eac6..815756edfeb 100644
--- a/compiler/rustc_data_structures/src/work_queue.rs
+++ b/compiler/rustc_data_structures/src/work_queue.rs
@@ -1,7 +1,7 @@
 use std::collections::VecDeque;
 
 use rustc_index::Idx;
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 
 /// A work queue is a handy data structure for tracking work left to
 /// do. (For example, basic blocks left to process.) It is basically a
@@ -11,14 +11,14 @@ use rustc_index::bit_set::BitSet;
 /// and also use a bit set to track occupancy.
 pub struct WorkQueue<T: Idx> {
     deque: VecDeque<T>,
-    set: BitSet<T>,
+    set: DenseBitSet<T>,
 }
 
 impl<T: Idx> WorkQueue<T> {
     /// Creates a new work queue that starts empty, where elements range from (0..len).
     #[inline]
     pub fn with_none(len: usize) -> Self {
-        WorkQueue { deque: VecDeque::with_capacity(len), set: BitSet::new_empty(len) }
+        WorkQueue { deque: VecDeque::with_capacity(len), set: DenseBitSet::new_empty(len) }
     }
 
     /// Attempt to enqueue `element` in the work queue. Returns false if it was already present.
diff --git a/compiler/rustc_hir_analysis/src/check/check.rs b/compiler/rustc_hir_analysis/src/check/check.rs
index 8c6059d49a8..821829455d9 100644
--- a/compiler/rustc_hir_analysis/src/check/check.rs
+++ b/compiler/rustc_hir_analysis/src/check/check.rs
@@ -1666,7 +1666,7 @@ fn check_type_alias_type_params_are_used<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalD
             .collect::<FxIndexMap<_, _>>()
     });
 
-    let mut params_used = BitSet::new_empty(generics.own_params.len());
+    let mut params_used = DenseBitSet::new_empty(generics.own_params.len());
     for leaf in ty.walk() {
         if let GenericArgKind::Type(leaf_ty) = leaf.unpack()
             && let ty::Param(param) = leaf_ty.kind()
diff --git a/compiler/rustc_hir_analysis/src/check/mod.rs b/compiler/rustc_hir_analysis/src/check/mod.rs
index 0b0c92a726d..92b18c80fd8 100644
--- a/compiler/rustc_hir_analysis/src/check/mod.rs
+++ b/compiler/rustc_hir_analysis/src/check/mod.rs
@@ -79,7 +79,7 @@ use rustc_data_structures::fx::{FxHashSet, FxIndexMap};
 use rustc_errors::{Diag, ErrorGuaranteed, pluralize, struct_span_code_err};
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_hir::intravisit::Visitor;
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_infer::infer::outlives::env::OutlivesEnvironment;
 use rustc_infer::infer::{self, TyCtxtInferExt as _};
 use rustc_infer::traits::ObligationCause;
diff --git a/compiler/rustc_index/src/bit_set.rs b/compiler/rustc_index/src/bit_set.rs
index 38e2dbbde7d..73dd040a69c 100644
--- a/compiler/rustc_index/src/bit_set.rs
+++ b/compiler/rustc_index/src/bit_set.rs
@@ -108,33 +108,33 @@ macro_rules! bit_relations_inherent_impls {
 ///
 #[cfg_attr(feature = "nightly", derive(Decodable_Generic, Encodable_Generic))]
 #[derive(Eq, PartialEq, Hash)]
-pub struct BitSet<T> {
+pub struct DenseBitSet<T> {
     domain_size: usize,
     words: SmallVec<[Word; 2]>,
     marker: PhantomData<T>,
 }
 
-impl<T> BitSet<T> {
+impl<T> DenseBitSet<T> {
     /// Gets the domain size.
     pub fn domain_size(&self) -> usize {
         self.domain_size
     }
 }
 
-impl<T: Idx> BitSet<T> {
+impl<T: Idx> DenseBitSet<T> {
     /// Creates a new, empty bitset with a given `domain_size`.
     #[inline]
-    pub fn new_empty(domain_size: usize) -> BitSet<T> {
+    pub fn new_empty(domain_size: usize) -> DenseBitSet<T> {
         let num_words = num_words(domain_size);
-        BitSet { domain_size, words: smallvec![0; num_words], marker: PhantomData }
+        DenseBitSet { domain_size, words: smallvec![0; num_words], marker: PhantomData }
     }
 
     /// Creates a new, filled bitset with a given `domain_size`.
     #[inline]
-    pub fn new_filled(domain_size: usize) -> BitSet<T> {
+    pub fn new_filled(domain_size: usize) -> DenseBitSet<T> {
         let num_words = num_words(domain_size);
         let mut result =
-            BitSet { domain_size, words: smallvec![!0; num_words], marker: PhantomData };
+            DenseBitSet { domain_size, words: smallvec![!0; num_words], marker: PhantomData };
         result.clear_excess_bits();
         result
     }
@@ -165,7 +165,7 @@ impl<T: Idx> BitSet<T> {
 
     /// Is `self` is a (non-strict) superset of `other`?
     #[inline]
-    pub fn superset(&self, other: &BitSet<T>) -> bool {
+    pub fn superset(&self, other: &DenseBitSet<T>) -> bool {
         assert_eq!(self.domain_size, other.domain_size);
         self.words.iter().zip(&other.words).all(|(a, b)| (a & b) == *b)
     }
@@ -278,32 +278,36 @@ impl<T: Idx> BitSet<T> {
 }
 
 // dense REL dense
-impl<T: Idx> BitRelations<BitSet<T>> for BitSet<T> {
-    fn union(&mut self, other: &BitSet<T>) -> bool {
+impl<T: Idx> BitRelations<DenseBitSet<T>> for DenseBitSet<T> {
+    fn union(&mut self, other: &DenseBitSet<T>) -> bool {
         assert_eq!(self.domain_size, other.domain_size);
         bitwise(&mut self.words, &other.words, |a, b| a | b)
     }
 
-    fn subtract(&mut self, other: &BitSet<T>) -> bool {
+    fn subtract(&mut self, other: &DenseBitSet<T>) -> bool {
         assert_eq!(self.domain_size, other.domain_size);
         bitwise(&mut self.words, &other.words, |a, b| a & !b)
     }
 
-    fn intersect(&mut self, other: &BitSet<T>) -> bool {
+    fn intersect(&mut self, other: &DenseBitSet<T>) -> bool {
         assert_eq!(self.domain_size, other.domain_size);
         bitwise(&mut self.words, &other.words, |a, b| a & b)
     }
 }
 
-impl<T: Idx> From<GrowableBitSet<T>> for BitSet<T> {
+impl<T: Idx> From<GrowableBitSet<T>> for DenseBitSet<T> {
     fn from(bit_set: GrowableBitSet<T>) -> Self {
         bit_set.bit_set
     }
 }
 
-impl<T> Clone for BitSet<T> {
+impl<T> Clone for DenseBitSet<T> {
     fn clone(&self) -> Self {
-        BitSet { domain_size: self.domain_size, words: self.words.clone(), marker: PhantomData }
+        DenseBitSet {
+            domain_size: self.domain_size,
+            words: self.words.clone(),
+            marker: PhantomData,
+        }
     }
 
     fn clone_from(&mut self, from: &Self) {
@@ -312,13 +316,13 @@ impl<T> Clone for BitSet<T> {
     }
 }
 
-impl<T: Idx> fmt::Debug for BitSet<T> {
+impl<T: Idx> fmt::Debug for DenseBitSet<T> {
     fn fmt(&self, w: &mut fmt::Formatter<'_>) -> fmt::Result {
         w.debug_list().entries(self.iter()).finish()
     }
 }
 
-impl<T: Idx> ToString for BitSet<T> {
+impl<T: Idx> ToString for DenseBitSet<T> {
     fn to_string(&self) -> String {
         let mut result = String::new();
         let mut sep = '[';
@@ -902,7 +906,7 @@ impl<T: Idx> BitRelations<ChunkedBitSet<T>> for ChunkedBitSet<T> {
     }
 }
 
-impl<T: Idx> BitRelations<ChunkedBitSet<T>> for BitSet<T> {
+impl<T: Idx> BitRelations<ChunkedBitSet<T>> for DenseBitSet<T> {
     fn union(&mut self, other: &ChunkedBitSet<T>) -> bool {
         sequential_update(|elem| self.insert(elem), other.iter())
     }
@@ -1114,10 +1118,10 @@ where
     false
 }
 
-/// A bitset with a mixed representation, using `BitSet` for small and medium
-/// bitsets, and `ChunkedBitSet` for large bitsets, i.e. those with enough bits
-/// for at least two chunks. This is a good choice for many bitsets that can
-/// have large domain sizes (e.g. 5000+).
+/// A bitset with a mixed representation, using `DenseBitSet` for small and
+/// medium bitsets, and `ChunkedBitSet` for large bitsets, i.e. those with
+/// enough bits for at least two chunks. This is a good choice for many bitsets
+/// that can have large domain sizes (e.g. 5000+).
 ///
 /// `T` is an index type, typically a newtyped `usize` wrapper, but it can also
 /// just be `usize`.
@@ -1127,7 +1131,7 @@ where
 /// will panic if the bitsets have differing domain sizes.
 #[derive(PartialEq, Eq)]
 pub enum MixedBitSet<T> {
-    Small(BitSet<T>),
+    Small(DenseBitSet<T>),
     Large(ChunkedBitSet<T>),
 }
 
@@ -1144,7 +1148,7 @@ impl<T: Idx> MixedBitSet<T> {
     #[inline]
     pub fn new_empty(domain_size: usize) -> MixedBitSet<T> {
         if domain_size <= CHUNK_BITS {
-            MixedBitSet::Small(BitSet::new_empty(domain_size))
+            MixedBitSet::Small(DenseBitSet::new_empty(domain_size))
         } else {
             MixedBitSet::Large(ChunkedBitSet::new_empty(domain_size))
         }
@@ -1283,7 +1287,7 @@ impl<'a, T: Idx> Iterator for MixedBitIter<'a, T> {
 /// to or greater than the domain size.
 #[derive(Clone, Debug, PartialEq)]
 pub struct GrowableBitSet<T: Idx> {
-    bit_set: BitSet<T>,
+    bit_set: DenseBitSet<T>,
 }
 
 impl<T: Idx> Default for GrowableBitSet<T> {
@@ -1306,11 +1310,11 @@ impl<T: Idx> GrowableBitSet<T> {
     }
 
     pub fn new_empty() -> GrowableBitSet<T> {
-        GrowableBitSet { bit_set: BitSet::new_empty(0) }
+        GrowableBitSet { bit_set: DenseBitSet::new_empty(0) }
     }
 
     pub fn with_capacity(capacity: usize) -> GrowableBitSet<T> {
-        GrowableBitSet { bit_set: BitSet::new_empty(capacity) }
+        GrowableBitSet { bit_set: DenseBitSet::new_empty(capacity) }
     }
 
     /// Returns `true` if the set has changed.
@@ -1349,8 +1353,8 @@ impl<T: Idx> GrowableBitSet<T> {
     }
 }
 
-impl<T: Idx> From<BitSet<T>> for GrowableBitSet<T> {
-    fn from(bit_set: BitSet<T>) -> Self {
+impl<T: Idx> From<DenseBitSet<T>> for GrowableBitSet<T> {
+    fn from(bit_set: DenseBitSet<T>) -> Self {
         Self { bit_set }
     }
 }
@@ -1386,7 +1390,7 @@ impl<R: Idx, C: Idx> BitMatrix<R, C> {
     }
 
     /// Creates a new matrix, with `row` used as the value for every row.
-    pub fn from_row_n(row: &BitSet<C>, num_rows: usize) -> BitMatrix<R, C> {
+    pub fn from_row_n(row: &DenseBitSet<C>, num_rows: usize) -> BitMatrix<R, C> {
         let num_columns = row.domain_size();
         let words_per_row = num_words(num_columns);
         assert_eq!(words_per_row, row.words.len());
@@ -1484,7 +1488,7 @@ impl<R: Idx, C: Idx> BitMatrix<R, C> {
 
     /// Adds the bits from `with` to the bits from row `write`, and
     /// returns `true` if anything changed.
-    pub fn union_row_with(&mut self, with: &BitSet<C>, write: R) -> bool {
+    pub fn union_row_with(&mut self, with: &DenseBitSet<C>, write: R) -> bool {
         assert!(write.index() < self.num_rows);
         assert_eq!(with.domain_size(), self.num_columns);
         let (write_start, write_end) = self.range(write);
@@ -1541,8 +1545,8 @@ impl<R: Idx, C: Idx> fmt::Debug for BitMatrix<R, C> {
 /// A fixed-column-size, variable-row-size 2D bit matrix with a moderately
 /// sparse representation.
 ///
-/// Initially, every row has no explicit representation. If any bit within a
-/// row is set, the entire row is instantiated as `Some(<BitSet>)`.
+/// Initially, every row has no explicit representation. If any bit within a row
+/// is set, the entire row is instantiated as `Some(<DenseBitSet>)`.
 /// Furthermore, any previously uninstantiated rows prior to it will be
 /// instantiated as `None`. Those prior rows may themselves become fully
 /// instantiated later on if any of their bits are set.
@@ -1556,7 +1560,7 @@ where
     C: Idx,
 {
     num_columns: usize,
-    rows: IndexVec<R, Option<BitSet<C>>>,
+    rows: IndexVec<R, Option<DenseBitSet<C>>>,
 }
 
 impl<R: Idx, C: Idx> SparseBitMatrix<R, C> {
@@ -1565,10 +1569,10 @@ impl<R: Idx, C: Idx> SparseBitMatrix<R, C> {
         Self { num_columns, rows: IndexVec::new() }
     }
 
-    fn ensure_row(&mut self, row: R) -> &mut BitSet<C> {
-        // Instantiate any missing rows up to and including row `row` with an empty `BitSet`.
-        // Then replace row `row` with a full `BitSet` if necessary.
-        self.rows.get_or_insert_with(row, || BitSet::new_empty(self.num_columns))
+    fn ensure_row(&mut self, row: R) -> &mut DenseBitSet<C> {
+        // Instantiate any missing rows up to and including row `row` with an empty `DenseBitSet`.
+        // Then replace row `row` with a full `DenseBitSet` if necessary.
+        self.rows.get_or_insert_with(row, || DenseBitSet::new_empty(self.num_columns))
     }
 
     /// Sets the cell at `(row, column)` to true. Put another way, insert
@@ -1642,17 +1646,17 @@ impl<R: Idx, C: Idx> SparseBitMatrix<R, C> {
         self.row(row).into_iter().flat_map(|r| r.iter())
     }
 
-    pub fn row(&self, row: R) -> Option<&BitSet<C>> {
+    pub fn row(&self, row: R) -> Option<&DenseBitSet<C>> {
         self.rows.get(row)?.as_ref()
     }
 
-    /// Intersects `row` with `set`. `set` can be either `BitSet` or
+    /// Intersects `row` with `set`. `set` can be either `DenseBitSet` or
     /// `ChunkedBitSet`. Has no effect if `row` does not exist.
     ///
     /// Returns true if the row was changed.
     pub fn intersect_row<Set>(&mut self, row: R, set: &Set) -> bool
     where
-        BitSet<C>: BitRelations<Set>,
+        DenseBitSet<C>: BitRelations<Set>,
     {
         match self.rows.get_mut(row) {
             Some(Some(row)) => row.intersect(set),
@@ -1660,13 +1664,13 @@ impl<R: Idx, C: Idx> SparseBitMatrix<R, C> {
         }
     }
 
-    /// Subtracts `set` from `row`. `set` can be either `BitSet` or
+    /// Subtracts `set` from `row`. `set` can be either `DenseBitSet` or
     /// `ChunkedBitSet`. Has no effect if `row` does not exist.
     ///
     /// Returns true if the row was changed.
     pub fn subtract_row<Set>(&mut self, row: R, set: &Set) -> bool
     where
-        BitSet<C>: BitRelations<Set>,
+        DenseBitSet<C>: BitRelations<Set>,
     {
         match self.rows.get_mut(row) {
             Some(Some(row)) => row.subtract(set),
@@ -1674,13 +1678,13 @@ impl<R: Idx, C: Idx> SparseBitMatrix<R, C> {
         }
     }
 
-    /// Unions `row` with `set`. `set` can be either `BitSet` or
+    /// Unions `row` with `set`. `set` can be either `DenseBitSet` or
     /// `ChunkedBitSet`.
     ///
     /// Returns true if the row was changed.
     pub fn union_row<Set>(&mut self, row: R, set: &Set) -> bool
     where
-        BitSet<C>: BitRelations<Set>,
+        DenseBitSet<C>: BitRelations<Set>,
     {
         self.ensure_row(row).union(set)
     }
diff --git a/compiler/rustc_index/src/bit_set/tests.rs b/compiler/rustc_index/src/bit_set/tests.rs
index f6142323979..0350740aa81 100644
--- a/compiler/rustc_index/src/bit_set/tests.rs
+++ b/compiler/rustc_index/src/bit_set/tests.rs
@@ -8,7 +8,7 @@ use test::Bencher;
 #[test]
 fn test_new_filled() {
     for i in 0..128 {
-        let idx_buf = BitSet::new_filled(i);
+        let idx_buf = DenseBitSet::new_filled(i);
         let elems: Vec<usize> = idx_buf.iter().collect();
         let expected: Vec<usize> = (0..i).collect();
         assert_eq!(elems, expected);
@@ -17,7 +17,7 @@ fn test_new_filled() {
 
 #[test]
 fn bitset_iter_works() {
-    let mut bitset: BitSet<usize> = BitSet::new_empty(100);
+    let mut bitset: DenseBitSet<usize> = DenseBitSet::new_empty(100);
     bitset.insert(1);
     bitset.insert(10);
     bitset.insert(19);
@@ -32,7 +32,7 @@ fn bitset_iter_works() {
 
 #[test]
 fn bitset_iter_works_2() {
-    let mut bitset: BitSet<usize> = BitSet::new_empty(320);
+    let mut bitset: DenseBitSet<usize> = DenseBitSet::new_empty(320);
     bitset.insert(0);
     bitset.insert(127);
     bitset.insert(191);
@@ -43,25 +43,25 @@ fn bitset_iter_works_2() {
 
 #[test]
 fn bitset_clone_from() {
-    let mut a: BitSet<usize> = BitSet::new_empty(10);
+    let mut a: DenseBitSet<usize> = DenseBitSet::new_empty(10);
     a.insert(4);
     a.insert(7);
     a.insert(9);
 
-    let mut b = BitSet::new_empty(2);
+    let mut b = DenseBitSet::new_empty(2);
     b.clone_from(&a);
     assert_eq!(b.domain_size(), 10);
     assert_eq!(b.iter().collect::<Vec<_>>(), [4, 7, 9]);
 
-    b.clone_from(&BitSet::new_empty(40));
+    b.clone_from(&DenseBitSet::new_empty(40));
     assert_eq!(b.domain_size(), 40);
     assert_eq!(b.iter().collect::<Vec<_>>(), []);
 }
 
 #[test]
 fn union_two_sets() {
-    let mut set1: BitSet<usize> = BitSet::new_empty(65);
-    let mut set2: BitSet<usize> = BitSet::new_empty(65);
+    let mut set1: DenseBitSet<usize> = DenseBitSet::new_empty(65);
+    let mut set2: DenseBitSet<usize> = DenseBitSet::new_empty(65);
     assert!(set1.insert(3));
     assert!(!set1.insert(3));
     assert!(set2.insert(5));
@@ -268,8 +268,8 @@ fn with_elements_chunked(elements: &[usize], domain_size: usize) -> ChunkedBitSe
     s
 }
 
-fn with_elements_standard(elements: &[usize], domain_size: usize) -> BitSet<usize> {
-    let mut s = BitSet::new_empty(domain_size);
+fn with_elements_standard(elements: &[usize], domain_size: usize) -> DenseBitSet<usize> {
+    let mut s = DenseBitSet::new_empty(domain_size);
     for &e in elements {
         assert!(s.insert(e));
     }
@@ -503,15 +503,15 @@ fn sparse_matrix_operations() {
     matrix.insert(2, 99);
     matrix.insert(4, 0);
 
-    let mut disjoint: BitSet<usize> = BitSet::new_empty(100);
+    let mut disjoint: DenseBitSet<usize> = DenseBitSet::new_empty(100);
     disjoint.insert(33);
 
-    let mut superset = BitSet::new_empty(100);
+    let mut superset = DenseBitSet::new_empty(100);
     superset.insert(22);
     superset.insert(75);
     superset.insert(33);
 
-    let mut subset = BitSet::new_empty(100);
+    let mut subset = DenseBitSet::new_empty(100);
     subset.insert(22);
 
     // SparseBitMatrix::remove
@@ -568,7 +568,7 @@ fn dense_insert_range() {
     where
         R: RangeBounds<usize> + Clone + IntoIterator<Item = usize> + std::fmt::Debug,
     {
-        let mut set = BitSet::new_empty(domain);
+        let mut set = DenseBitSet::new_empty(domain);
         set.insert_range(range.clone());
         for i in set.iter() {
             assert!(range.contains(&i));
@@ -609,7 +609,7 @@ fn dense_insert_range() {
 
 #[test]
 fn dense_last_set_before() {
-    fn easy(set: &BitSet<usize>, needle: impl RangeBounds<usize>) -> Option<usize> {
+    fn easy(set: &DenseBitSet<usize>, needle: impl RangeBounds<usize>) -> Option<usize> {
         let mut last_leq = None;
         for e in set.iter() {
             if needle.contains(&e) {
@@ -620,7 +620,7 @@ fn dense_last_set_before() {
     }
 
     #[track_caller]
-    fn cmp(set: &BitSet<usize>, needle: impl RangeBounds<usize> + Clone + std::fmt::Debug) {
+    fn cmp(set: &DenseBitSet<usize>, needle: impl RangeBounds<usize> + Clone + std::fmt::Debug) {
         assert_eq!(
             set.last_set_in(needle.clone()),
             easy(set, needle.clone()),
@@ -629,7 +629,7 @@ fn dense_last_set_before() {
             set
         );
     }
-    let mut set = BitSet::new_empty(300);
+    let mut set = DenseBitSet::new_empty(300);
     cmp(&set, 50..=50);
     set.insert(WORD_BITS);
     cmp(&set, WORD_BITS..=WORD_BITS);
@@ -645,7 +645,7 @@ fn dense_last_set_before() {
     for i in 0..=WORD_BITS * 2 {
         for j in i..=WORD_BITS * 2 {
             for k in 0..WORD_BITS * 2 {
-                let mut set = BitSet::new_empty(300);
+                let mut set = DenseBitSet::new_empty(300);
                 cmp(&set, i..j);
                 cmp(&set, i..=j);
                 set.insert(k);
@@ -658,7 +658,7 @@ fn dense_last_set_before() {
 
 #[bench]
 fn bench_insert(b: &mut Bencher) {
-    let mut bs = BitSet::new_filled(99999usize);
+    let mut bs = DenseBitSet::new_filled(99999usize);
     b.iter(|| {
         black_box(bs.insert(black_box(100u32)));
     });
@@ -666,7 +666,7 @@ fn bench_insert(b: &mut Bencher) {
 
 #[bench]
 fn bench_remove(b: &mut Bencher) {
-    let mut bs = BitSet::new_filled(99999usize);
+    let mut bs = DenseBitSet::new_filled(99999usize);
     b.iter(|| {
         black_box(bs.remove(black_box(100u32)));
     });
@@ -674,7 +674,7 @@ fn bench_remove(b: &mut Bencher) {
 
 #[bench]
 fn bench_iter(b: &mut Bencher) {
-    let bs = BitSet::new_filled(99999usize);
+    let bs = DenseBitSet::new_filled(99999usize);
     b.iter(|| {
         bs.iter().map(|b: usize| black_box(b)).for_each(drop);
     });
@@ -682,8 +682,8 @@ fn bench_iter(b: &mut Bencher) {
 
 #[bench]
 fn bench_intersect(b: &mut Bencher) {
-    let mut ba: BitSet<u32> = BitSet::new_filled(99999usize);
-    let bb = BitSet::new_filled(99999usize);
+    let mut ba: DenseBitSet<u32> = DenseBitSet::new_filled(99999usize);
+    let bb = DenseBitSet::new_filled(99999usize);
     b.iter(|| {
         ba.intersect(black_box(&bb));
     });
diff --git a/compiler/rustc_metadata/src/rmeta/mod.rs b/compiler/rustc_metadata/src/rmeta/mod.rs
index 4fe8f73efd6..4f9cdc9a474 100644
--- a/compiler/rustc_metadata/src/rmeta/mod.rs
+++ b/compiler/rustc_metadata/src/rmeta/mod.rs
@@ -15,7 +15,7 @@ use rustc_hir::def_id::{CrateNum, DefId, DefIdMap, DefIndex, DefPathHash, Stable
 use rustc_hir::definitions::DefKey;
 use rustc_hir::lang_items::LangItem;
 use rustc_index::IndexVec;
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_macros::{
     Decodable, Encodable, MetadataDecodable, MetadataEncodable, TyDecodable, TyEncodable,
 };
@@ -450,7 +450,7 @@ define_tables! {
     trait_item_def_id: Table<DefIndex, RawDefId>,
     expn_that_defined: Table<DefIndex, LazyValue<ExpnId>>,
     default_fields: Table<DefIndex, LazyValue<DefId>>,
-    params_in_repr: Table<DefIndex, LazyValue<BitSet<u32>>>,
+    params_in_repr: Table<DefIndex, LazyValue<DenseBitSet<u32>>>,
     repr_options: Table<DefIndex, LazyValue<ReprOptions>>,
     // `def_keys` and `def_path_hashes` represent a lazy version of a
     // `DefPathTable`. This allows us to avoid deserializing an entire
diff --git a/compiler/rustc_middle/src/mir/coverage.rs b/compiler/rustc_middle/src/mir/coverage.rs
index 29f26180c97..cd6b2d65bf1 100644
--- a/compiler/rustc_middle/src/mir/coverage.rs
+++ b/compiler/rustc_middle/src/mir/coverage.rs
@@ -3,7 +3,7 @@
 use std::fmt::{self, Debug, Formatter};
 
 use rustc_index::IndexVec;
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_macros::{HashStable, TyDecodable, TyEncodable, TypeFoldable, TypeVisitable};
 use rustc_span::Span;
 
@@ -303,8 +303,8 @@ pub struct MCDCDecisionSpan {
 /// Used by the `coverage_ids_info` query.
 #[derive(Clone, TyEncodable, TyDecodable, Debug, HashStable)]
 pub struct CoverageIdsInfo {
-    pub counters_seen: BitSet<CounterId>,
-    pub zero_expressions: BitSet<ExpressionId>,
+    pub counters_seen: DenseBitSet<CounterId>,
+    pub zero_expressions: DenseBitSet<ExpressionId>,
 }
 
 impl CoverageIdsInfo {
diff --git a/compiler/rustc_middle/src/mir/mod.rs b/compiler/rustc_middle/src/mir/mod.rs
index 98ef7d58a50..bbb8bdce4a0 100644
--- a/compiler/rustc_middle/src/mir/mod.rs
+++ b/compiler/rustc_middle/src/mir/mod.rs
@@ -21,7 +21,7 @@ use rustc_hir::def_id::{CRATE_DEF_ID, DefId};
 use rustc_hir::{
     self as hir, BindingMode, ByRef, CoroutineDesugaring, CoroutineKind, HirId, ImplicitSelfKind,
 };
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_index::{Idx, IndexSlice, IndexVec};
 use rustc_macros::{HashStable, TyDecodable, TyEncodable, TypeFoldable, TypeVisitable};
 use rustc_serialize::{Decodable, Encodable};
diff --git a/compiler/rustc_middle/src/mir/traversal.rs b/compiler/rustc_middle/src/mir/traversal.rs
index b8b74da401c..0e7dcc24daf 100644
--- a/compiler/rustc_middle/src/mir/traversal.rs
+++ b/compiler/rustc_middle/src/mir/traversal.rs
@@ -21,7 +21,7 @@ use super::*;
 #[derive(Clone)]
 pub struct Preorder<'a, 'tcx> {
     body: &'a Body<'tcx>,
-    visited: BitSet<BasicBlock>,
+    visited: DenseBitSet<BasicBlock>,
     worklist: Vec<BasicBlock>,
     root_is_start_block: bool,
 }
@@ -32,7 +32,7 @@ impl<'a, 'tcx> Preorder<'a, 'tcx> {
 
         Preorder {
             body,
-            visited: BitSet::new_empty(body.basic_blocks.len()),
+            visited: DenseBitSet::new_empty(body.basic_blocks.len()),
             worklist,
             root_is_start_block: root == START_BLOCK,
         }
@@ -106,7 +106,7 @@ impl<'a, 'tcx> Iterator for Preorder<'a, 'tcx> {
 /// A Postorder traversal of this graph is `D B C A` or `D C B A`
 pub struct Postorder<'a, 'tcx, C> {
     basic_blocks: &'a IndexSlice<BasicBlock, BasicBlockData<'tcx>>,
-    visited: BitSet<BasicBlock>,
+    visited: DenseBitSet<BasicBlock>,
     visit_stack: Vec<(BasicBlock, Successors<'a>)>,
     root_is_start_block: bool,
     extra: C,
@@ -123,7 +123,7 @@ where
     ) -> Postorder<'a, 'tcx, C> {
         let mut po = Postorder {
             basic_blocks,
-            visited: BitSet::new_empty(basic_blocks.len()),
+            visited: DenseBitSet::new_empty(basic_blocks.len()),
             visit_stack: Vec::new(),
             root_is_start_block: root == START_BLOCK,
             extra,
@@ -285,8 +285,8 @@ pub fn reachable<'a, 'tcx>(
     preorder(body)
 }
 
-/// Returns a `BitSet` containing all basic blocks reachable from the `START_BLOCK`.
-pub fn reachable_as_bitset(body: &Body<'_>) -> BitSet<BasicBlock> {
+/// Returns a `DenseBitSet` containing all basic blocks reachable from the `START_BLOCK`.
+pub fn reachable_as_bitset(body: &Body<'_>) -> DenseBitSet<BasicBlock> {
     let mut iter = preorder(body);
     while let Some(_) = iter.next() {}
     iter.visited
@@ -340,13 +340,13 @@ pub fn mono_reachable<'a, 'tcx>(
     MonoReachable::new(body, tcx, instance)
 }
 
-/// [`MonoReachable`] internally accumulates a [`BitSet`] of visited blocks. This is just a
+/// [`MonoReachable`] internally accumulates a [`DenseBitSet`] of visited blocks. This is just a
 /// convenience function to run that traversal then extract its set of reached blocks.
 pub fn mono_reachable_as_bitset<'a, 'tcx>(
     body: &'a Body<'tcx>,
     tcx: TyCtxt<'tcx>,
     instance: Instance<'tcx>,
-) -> BitSet<BasicBlock> {
+) -> DenseBitSet<BasicBlock> {
     let mut iter = mono_reachable(body, tcx, instance);
     while let Some(_) = iter.next() {}
     iter.visited
@@ -356,11 +356,11 @@ pub struct MonoReachable<'a, 'tcx> {
     body: &'a Body<'tcx>,
     tcx: TyCtxt<'tcx>,
     instance: Instance<'tcx>,
-    visited: BitSet<BasicBlock>,
+    visited: DenseBitSet<BasicBlock>,
     // Other traversers track their worklist in a Vec. But we don't care about order, so we can
-    // store ours in a BitSet and thus save allocations because BitSet has a small size
+    // store ours in a DenseBitSet and thus save allocations because DenseBitSet has a small size
     // optimization.
-    worklist: BitSet<BasicBlock>,
+    worklist: DenseBitSet<BasicBlock>,
 }
 
 impl<'a, 'tcx> MonoReachable<'a, 'tcx> {
@@ -369,13 +369,13 @@ impl<'a, 'tcx> MonoReachable<'a, 'tcx> {
         tcx: TyCtxt<'tcx>,
         instance: Instance<'tcx>,
     ) -> MonoReachable<'a, 'tcx> {
-        let mut worklist = BitSet::new_empty(body.basic_blocks.len());
+        let mut worklist = DenseBitSet::new_empty(body.basic_blocks.len());
         worklist.insert(START_BLOCK);
         MonoReachable {
             body,
             tcx,
             instance,
-            visited: BitSet::new_empty(body.basic_blocks.len()),
+            visited: DenseBitSet::new_empty(body.basic_blocks.len()),
             worklist,
         }
     }
diff --git a/compiler/rustc_middle/src/query/mod.rs b/compiler/rustc_middle/src/query/mod.rs
index 283675573d4..7d334847d44 100644
--- a/compiler/rustc_middle/src/query/mod.rs
+++ b/compiler/rustc_middle/src/query/mod.rs
@@ -297,7 +297,7 @@ rustc_queries! {
         separate_provide_extern
     }
 
-    query unsizing_params_for_adt(key: DefId) -> &'tcx rustc_index::bit_set::BitSet<u32>
+    query unsizing_params_for_adt(key: DefId) -> &'tcx rustc_index::bit_set::DenseBitSet<u32>
     {
         arena_cache
         desc { |tcx|
@@ -494,7 +494,7 @@ rustc_queries! {
     }
 
     /// Set of param indexes for type params that are in the type's representation
-    query params_in_repr(key: DefId) -> &'tcx rustc_index::bit_set::BitSet<u32> {
+    query params_in_repr(key: DefId) -> &'tcx rustc_index::bit_set::DenseBitSet<u32> {
         desc { "finding type parameters in the representation" }
         arena_cache
         no_hash
diff --git a/compiler/rustc_middle/src/ty/context.rs b/compiler/rustc_middle/src/ty/context.rs
index d26c007d227..24f10b4fbe7 100644
--- a/compiler/rustc_middle/src/ty/context.rs
+++ b/compiler/rustc_middle/src/ty/context.rs
@@ -613,7 +613,7 @@ impl<'tcx> Interner for TyCtxt<'tcx> {
         self.coroutine_is_async_gen(coroutine_def_id)
     }
 
-    type UnsizingParams = &'tcx rustc_index::bit_set::BitSet<u32>;
+    type UnsizingParams = &'tcx rustc_index::bit_set::DenseBitSet<u32>;
     fn unsizing_params_for_adt(self, adt_def_id: DefId) -> Self::UnsizingParams {
         self.unsizing_params_for_adt(adt_def_id)
     }
diff --git a/compiler/rustc_middle/src/ty/parameterized.rs b/compiler/rustc_middle/src/ty/parameterized.rs
index 86a95827e84..6b6c6f3c72f 100644
--- a/compiler/rustc_middle/src/ty/parameterized.rs
+++ b/compiler/rustc_middle/src/ty/parameterized.rs
@@ -96,7 +96,7 @@ trivially_parameterized_over_tcx! {
     rustc_hir::def_id::DefIndex,
     rustc_hir::definitions::DefKey,
     rustc_hir::OpaqueTyOrigin<rustc_hir::def_id::DefId>,
-    rustc_index::bit_set::BitSet<u32>,
+    rustc_index::bit_set::DenseBitSet<u32>,
     rustc_index::bit_set::FiniteBitSet<u32>,
     rustc_session::cstore::ForeignModule,
     rustc_session::cstore::LinkagePreference,
diff --git a/compiler/rustc_mir_dataflow/src/debuginfo.rs b/compiler/rustc_mir_dataflow/src/debuginfo.rs
index fd5e8cf2955..0d25ce91c9a 100644
--- a/compiler/rustc_mir_dataflow/src/debuginfo.rs
+++ b/compiler/rustc_mir_dataflow/src/debuginfo.rs
@@ -1,17 +1,17 @@
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::mir::visit::*;
 use rustc_middle::mir::*;
 
 /// Return the set of locals that appear in debuginfo.
-pub fn debuginfo_locals(body: &Body<'_>) -> BitSet<Local> {
-    let mut visitor = DebuginfoLocals(BitSet::new_empty(body.local_decls.len()));
+pub fn debuginfo_locals(body: &Body<'_>) -> DenseBitSet<Local> {
+    let mut visitor = DebuginfoLocals(DenseBitSet::new_empty(body.local_decls.len()));
     for debuginfo in body.var_debug_info.iter() {
         visitor.visit_var_debug_info(debuginfo);
     }
     visitor.0
 }
 
-struct DebuginfoLocals(BitSet<Local>);
+struct DebuginfoLocals(DenseBitSet<Local>);
 
 impl Visitor<'_> for DebuginfoLocals {
     fn visit_local(&mut self, local: Local, _: PlaceContext, _: Location) {
diff --git a/compiler/rustc_mir_dataflow/src/framework/cursor.rs b/compiler/rustc_mir_dataflow/src/framework/cursor.rs
index 89ff93d9943..c46ae9775cf 100644
--- a/compiler/rustc_mir_dataflow/src/framework/cursor.rs
+++ b/compiler/rustc_mir_dataflow/src/framework/cursor.rs
@@ -4,7 +4,7 @@ use std::cmp::Ordering;
 use std::ops::{Deref, DerefMut};
 
 #[cfg(debug_assertions)]
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::mir::{self, BasicBlock, Location};
 
 use super::{Analysis, Direction, Effect, EffectIndex, Results};
@@ -71,7 +71,7 @@ where
     state_needs_reset: bool,
 
     #[cfg(debug_assertions)]
-    reachable_blocks: BitSet<BasicBlock>,
+    reachable_blocks: DenseBitSet<BasicBlock>,
 }
 
 impl<'mir, 'tcx, A> ResultsCursor<'mir, 'tcx, A>
diff --git a/compiler/rustc_mir_dataflow/src/framework/fmt.rs b/compiler/rustc_mir_dataflow/src/framework/fmt.rs
index faf2c411dde..38599cd0949 100644
--- a/compiler/rustc_mir_dataflow/src/framework/fmt.rs
+++ b/compiler/rustc_mir_dataflow/src/framework/fmt.rs
@@ -4,7 +4,7 @@
 use std::fmt;
 
 use rustc_index::Idx;
-use rustc_index::bit_set::{BitSet, ChunkedBitSet, MixedBitSet};
+use rustc_index::bit_set::{ChunkedBitSet, DenseBitSet, MixedBitSet};
 
 use super::lattice::MaybeReachable;
 
@@ -73,7 +73,7 @@ where
 
 // Impls
 
-impl<T, C> DebugWithContext<C> for BitSet<T>
+impl<T, C> DebugWithContext<C> for DenseBitSet<T>
 where
     T: Idx + DebugWithContext<C>,
 {
diff --git a/compiler/rustc_mir_dataflow/src/framework/graphviz.rs b/compiler/rustc_mir_dataflow/src/framework/graphviz.rs
index 60c97710c7f..e457b514936 100644
--- a/compiler/rustc_mir_dataflow/src/framework/graphviz.rs
+++ b/compiler/rustc_mir_dataflow/src/framework/graphviz.rs
@@ -9,7 +9,7 @@ use std::{io, ops, str};
 
 use regex::Regex;
 use rustc_hir::def_id::DefId;
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::mir::{
     self, BasicBlock, Body, Location, create_dump_file, dump_enabled, graphviz_safe_def_name,
     traversal,
@@ -205,7 +205,7 @@ where
     // the operations that involve the mutation, i.e. within the `borrow_mut`.
     cursor: RefCell<ResultsCursor<'mir, 'tcx, A>>,
     style: OutputStyle,
-    reachable: BitSet<BasicBlock>,
+    reachable: DenseBitSet<BasicBlock>,
 }
 
 impl<'mir, 'tcx, A> Formatter<'mir, 'tcx, A>
diff --git a/compiler/rustc_mir_dataflow/src/framework/lattice.rs b/compiler/rustc_mir_dataflow/src/framework/lattice.rs
index cb8159ce37b..919e346bda7 100644
--- a/compiler/rustc_mir_dataflow/src/framework/lattice.rs
+++ b/compiler/rustc_mir_dataflow/src/framework/lattice.rs
@@ -39,7 +39,7 @@
 //! [poset]: https://en.wikipedia.org/wiki/Partially_ordered_set
 
 use rustc_index::Idx;
-use rustc_index::bit_set::{BitSet, MixedBitSet};
+use rustc_index::bit_set::{DenseBitSet, MixedBitSet};
 
 use crate::framework::BitSetExt;
 
@@ -68,10 +68,10 @@ pub trait HasTop {
     const TOP: Self;
 }
 
-/// A `BitSet` represents the lattice formed by the powerset of all possible values of
-/// the index type `T` ordered by inclusion. Equivalently, it is a tuple of "two-point" lattices,
-/// one for each possible value of `T`.
-impl<T: Idx> JoinSemiLattice for BitSet<T> {
+/// A `DenseBitSet` represents the lattice formed by the powerset of all possible values of the
+/// index type `T` ordered by inclusion. Equivalently, it is a tuple of "two-point" lattices, one
+/// for each possible value of `T`.
+impl<T: Idx> JoinSemiLattice for DenseBitSet<T> {
     fn join(&mut self, other: &Self) -> bool {
         self.union(other)
     }
diff --git a/compiler/rustc_mir_dataflow/src/framework/mod.rs b/compiler/rustc_mir_dataflow/src/framework/mod.rs
index 3de2c6e3f47..60c5cb0cae8 100644
--- a/compiler/rustc_mir_dataflow/src/framework/mod.rs
+++ b/compiler/rustc_mir_dataflow/src/framework/mod.rs
@@ -35,7 +35,7 @@
 use std::cmp::Ordering;
 
 use rustc_data_structures::work_queue::WorkQueue;
-use rustc_index::bit_set::{BitSet, MixedBitSet};
+use rustc_index::bit_set::{DenseBitSet, MixedBitSet};
 use rustc_index::{Idx, IndexVec};
 use rustc_middle::bug;
 use rustc_middle::mir::{self, BasicBlock, CallReturnPlaces, Location, TerminatorEdges, traversal};
@@ -65,7 +65,7 @@ pub trait BitSetExt<T> {
     fn contains(&self, elem: T) -> bool;
 }
 
-impl<T: Idx> BitSetExt<T> for BitSet<T> {
+impl<T: Idx> BitSetExt<T> for DenseBitSet<T> {
     fn contains(&self, elem: T) -> bool {
         self.contains(elem)
     }
@@ -334,7 +334,7 @@ pub trait GenKill<T> {
     }
 }
 
-impl<T: Idx> GenKill<T> for BitSet<T> {
+impl<T: Idx> GenKill<T> for DenseBitSet<T> {
     fn gen_(&mut self, elem: T) {
         self.insert(elem);
     }
diff --git a/compiler/rustc_mir_dataflow/src/framework/tests.rs b/compiler/rustc_mir_dataflow/src/framework/tests.rs
index 8e7d4ab0fa3..5b3a9ccba69 100644
--- a/compiler/rustc_mir_dataflow/src/framework/tests.rs
+++ b/compiler/rustc_mir_dataflow/src/framework/tests.rs
@@ -84,13 +84,13 @@ impl<D: Direction> MockAnalysis<'_, D> {
 
     /// The entry set for each `BasicBlock` is the ID of that block offset by a fixed amount to
     /// avoid colliding with the statement/terminator effects.
-    fn mock_entry_set(&self, bb: BasicBlock) -> BitSet<usize> {
+    fn mock_entry_set(&self, bb: BasicBlock) -> DenseBitSet<usize> {
         let mut ret = self.bottom_value(self.body);
         ret.insert(Self::BASIC_BLOCK_OFFSET + bb.index());
         ret
     }
 
-    fn mock_entry_states(&self) -> IndexVec<BasicBlock, BitSet<usize>> {
+    fn mock_entry_states(&self) -> IndexVec<BasicBlock, DenseBitSet<usize>> {
         let empty = self.bottom_value(self.body);
         let mut ret = IndexVec::from_elem(empty, &self.body.basic_blocks);
 
@@ -121,7 +121,7 @@ impl<D: Direction> MockAnalysis<'_, D> {
     /// For example, the expected state when calling
     /// `seek_before_primary_effect(Location { block: 2, statement_index: 2 })`
     /// would be `[102, 0, 1, 2, 3, 4]`.
-    fn expected_state_at_target(&self, target: SeekTarget) -> BitSet<usize> {
+    fn expected_state_at_target(&self, target: SeekTarget) -> DenseBitSet<usize> {
         let block = target.block();
         let mut ret = self.bottom_value(self.body);
         ret.insert(Self::BASIC_BLOCK_OFFSET + block.index());
@@ -155,13 +155,13 @@ impl<D: Direction> MockAnalysis<'_, D> {
 }
 
 impl<'tcx, D: Direction> Analysis<'tcx> for MockAnalysis<'tcx, D> {
-    type Domain = BitSet<usize>;
+    type Domain = DenseBitSet<usize>;
     type Direction = D;
 
     const NAME: &'static str = "mock";
 
     fn bottom_value(&self, body: &mir::Body<'tcx>) -> Self::Domain {
-        BitSet::new_empty(Self::BASIC_BLOCK_OFFSET + body.basic_blocks.len())
+        DenseBitSet::new_empty(Self::BASIC_BLOCK_OFFSET + body.basic_blocks.len())
     }
 
     fn initialize_start_block(&self, _: &mir::Body<'tcx>, _: &mut Self::Domain) {
diff --git a/compiler/rustc_mir_dataflow/src/impls/borrowed_locals.rs b/compiler/rustc_mir_dataflow/src/impls/borrowed_locals.rs
index 217594b3238..f2ef5018c49 100644
--- a/compiler/rustc_mir_dataflow/src/impls/borrowed_locals.rs
+++ b/compiler/rustc_mir_dataflow/src/impls/borrowed_locals.rs
@@ -1,4 +1,4 @@
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::mir::visit::Visitor;
 use rustc_middle::mir::*;
 
@@ -21,12 +21,12 @@ impl MaybeBorrowedLocals {
 }
 
 impl<'tcx> Analysis<'tcx> for MaybeBorrowedLocals {
-    type Domain = BitSet<Local>;
+    type Domain = DenseBitSet<Local>;
     const NAME: &'static str = "maybe_borrowed_locals";
 
     fn bottom_value(&self, body: &Body<'tcx>) -> Self::Domain {
         // bottom = unborrowed
-        BitSet::new_empty(body.local_decls().len())
+        DenseBitSet::new_empty(body.local_decls().len())
     }
 
     fn initialize_start_block(&self, _: &Body<'tcx>, _: &mut Self::Domain) {
@@ -137,8 +137,8 @@ where
 }
 
 /// The set of locals that are borrowed at some point in the MIR body.
-pub fn borrowed_locals(body: &Body<'_>) -> BitSet<Local> {
-    struct Borrowed(BitSet<Local>);
+pub fn borrowed_locals(body: &Body<'_>) -> DenseBitSet<Local> {
+    struct Borrowed(DenseBitSet<Local>);
 
     impl GenKill<Local> for Borrowed {
         #[inline]
@@ -151,7 +151,7 @@ pub fn borrowed_locals(body: &Body<'_>) -> BitSet<Local> {
         }
     }
 
-    let mut borrowed = Borrowed(BitSet::new_empty(body.local_decls.len()));
+    let mut borrowed = Borrowed(DenseBitSet::new_empty(body.local_decls.len()));
     TransferFunction { trans: &mut borrowed }.visit_body(body);
     borrowed.0
 }
diff --git a/compiler/rustc_mir_dataflow/src/impls/initialized.rs b/compiler/rustc_mir_dataflow/src/impls/initialized.rs
index 769f9c7cfc3..760f94af52d 100644
--- a/compiler/rustc_mir_dataflow/src/impls/initialized.rs
+++ b/compiler/rustc_mir_dataflow/src/impls/initialized.rs
@@ -2,7 +2,7 @@ use std::assert_matches::assert_matches;
 
 use rustc_abi::VariantIdx;
 use rustc_index::Idx;
-use rustc_index::bit_set::{BitSet, MixedBitSet};
+use rustc_index::bit_set::{DenseBitSet, MixedBitSet};
 use rustc_middle::bug;
 use rustc_middle::mir::{self, Body, CallReturnPlaces, Location, TerminatorEdges};
 use rustc_middle::ty::util::Discr;
@@ -207,7 +207,7 @@ pub struct MaybeUninitializedPlaces<'a, 'tcx> {
     move_data: &'a MoveData<'tcx>,
 
     mark_inactive_variants_as_uninit: bool,
-    skip_unreachable_unwind: BitSet<mir::BasicBlock>,
+    skip_unreachable_unwind: DenseBitSet<mir::BasicBlock>,
 }
 
 impl<'a, 'tcx> MaybeUninitializedPlaces<'a, 'tcx> {
@@ -217,7 +217,7 @@ impl<'a, 'tcx> MaybeUninitializedPlaces<'a, 'tcx> {
             body,
             move_data,
             mark_inactive_variants_as_uninit: false,
-            skip_unreachable_unwind: BitSet::new_empty(body.basic_blocks.len()),
+            skip_unreachable_unwind: DenseBitSet::new_empty(body.basic_blocks.len()),
         }
     }
 
@@ -233,7 +233,7 @@ impl<'a, 'tcx> MaybeUninitializedPlaces<'a, 'tcx> {
 
     pub fn skipping_unreachable_unwind(
         mut self,
-        unreachable_unwind: BitSet<mir::BasicBlock>,
+        unreachable_unwind: DenseBitSet<mir::BasicBlock>,
     ) -> Self {
         self.skip_unreachable_unwind = unreachable_unwind;
         self
diff --git a/compiler/rustc_mir_dataflow/src/impls/liveness.rs b/compiler/rustc_mir_dataflow/src/impls/liveness.rs
index b2050a6adf9..6ec1b03a34e 100644
--- a/compiler/rustc_mir_dataflow/src/impls/liveness.rs
+++ b/compiler/rustc_mir_dataflow/src/impls/liveness.rs
@@ -1,4 +1,4 @@
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::mir::visit::{MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor};
 use rustc_middle::mir::{
     self, CallReturnPlaces, Local, Location, Place, StatementKind, TerminatorEdges,
@@ -26,14 +26,14 @@ use crate::{Analysis, Backward, GenKill};
 pub struct MaybeLiveLocals;
 
 impl<'tcx> Analysis<'tcx> for MaybeLiveLocals {
-    type Domain = BitSet<Local>;
+    type Domain = DenseBitSet<Local>;
     type Direction = Backward;
 
     const NAME: &'static str = "liveness";
 
     fn bottom_value(&self, body: &mir::Body<'tcx>) -> Self::Domain {
         // bottom = not live
-        BitSet::new_empty(body.local_decls.len())
+        DenseBitSet::new_empty(body.local_decls.len())
     }
 
     fn initialize_start_block(&self, _: &mir::Body<'tcx>, _: &mut Self::Domain) {
@@ -81,7 +81,7 @@ impl<'tcx> Analysis<'tcx> for MaybeLiveLocals {
     }
 }
 
-pub struct TransferFunction<'a>(pub &'a mut BitSet<Local>);
+pub struct TransferFunction<'a>(pub &'a mut DenseBitSet<Local>);
 
 impl<'tcx> Visitor<'tcx> for TransferFunction<'_> {
     fn visit_place(&mut self, place: &mir::Place<'tcx>, context: PlaceContext, location: Location) {
@@ -117,7 +117,7 @@ impl<'tcx> Visitor<'tcx> for TransferFunction<'_> {
     }
 }
 
-struct YieldResumeEffect<'a>(&'a mut BitSet<Local>);
+struct YieldResumeEffect<'a>(&'a mut DenseBitSet<Local>);
 
 impl<'tcx> Visitor<'tcx> for YieldResumeEffect<'_> {
     fn visit_place(&mut self, place: &mir::Place<'tcx>, context: PlaceContext, location: Location) {
@@ -137,7 +137,7 @@ enum DefUse {
 }
 
 impl DefUse {
-    fn apply(state: &mut BitSet<Local>, place: Place<'_>, context: PlaceContext) {
+    fn apply(state: &mut DenseBitSet<Local>, place: Place<'_>, context: PlaceContext) {
         match DefUse::for_place(place, context) {
             Some(DefUse::Def) => state.kill(place.local),
             Some(DefUse::Use) => state.gen_(place.local),
@@ -204,7 +204,7 @@ impl DefUse {
 ///
 /// All of the caveats of `MaybeLiveLocals` apply.
 pub struct MaybeTransitiveLiveLocals<'a> {
-    always_live: &'a BitSet<Local>,
+    always_live: &'a DenseBitSet<Local>,
 }
 
 impl<'a> MaybeTransitiveLiveLocals<'a> {
@@ -212,20 +212,20 @@ impl<'a> MaybeTransitiveLiveLocals<'a> {
     /// considered live.
     ///
     /// This should include at least all locals that are ever borrowed.
-    pub fn new(always_live: &'a BitSet<Local>) -> Self {
+    pub fn new(always_live: &'a DenseBitSet<Local>) -> Self {
         MaybeTransitiveLiveLocals { always_live }
     }
 }
 
 impl<'a, 'tcx> Analysis<'tcx> for MaybeTransitiveLiveLocals<'a> {
-    type Domain = BitSet<Local>;
+    type Domain = DenseBitSet<Local>;
     type Direction = Backward;
 
     const NAME: &'static str = "transitive liveness";
 
     fn bottom_value(&self, body: &mir::Body<'tcx>) -> Self::Domain {
         // bottom = not live
-        BitSet::new_empty(body.local_decls.len())
+        DenseBitSet::new_empty(body.local_decls.len())
     }
 
     fn initialize_start_block(&self, _: &mir::Body<'tcx>, _: &mut Self::Domain) {
diff --git a/compiler/rustc_mir_dataflow/src/impls/storage_liveness.rs b/compiler/rustc_mir_dataflow/src/impls/storage_liveness.rs
index 65b480d3a5e..e3aa8f5a620 100644
--- a/compiler/rustc_mir_dataflow/src/impls/storage_liveness.rs
+++ b/compiler/rustc_mir_dataflow/src/impls/storage_liveness.rs
@@ -1,6 +1,6 @@
 use std::borrow::Cow;
 
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::mir::visit::{NonMutatingUseContext, PlaceContext, Visitor};
 use rustc_middle::mir::*;
 
@@ -10,8 +10,8 @@ use crate::{Analysis, GenKill, ResultsCursor};
 /// The set of locals in a MIR body that do not have `StorageLive`/`StorageDead` annotations.
 ///
 /// These locals have fixed storage for the duration of the body.
-pub fn always_storage_live_locals(body: &Body<'_>) -> BitSet<Local> {
-    let mut always_live_locals = BitSet::new_filled(body.local_decls.len());
+pub fn always_storage_live_locals(body: &Body<'_>) -> DenseBitSet<Local> {
+    let mut always_live_locals = DenseBitSet::new_filled(body.local_decls.len());
 
     for block in &*body.basic_blocks {
         for statement in &block.statements {
@@ -25,23 +25,23 @@ pub fn always_storage_live_locals(body: &Body<'_>) -> BitSet<Local> {
 }
 
 pub struct MaybeStorageLive<'a> {
-    always_live_locals: Cow<'a, BitSet<Local>>,
+    always_live_locals: Cow<'a, DenseBitSet<Local>>,
 }
 
 impl<'a> MaybeStorageLive<'a> {
-    pub fn new(always_live_locals: Cow<'a, BitSet<Local>>) -> Self {
+    pub fn new(always_live_locals: Cow<'a, DenseBitSet<Local>>) -> Self {
         MaybeStorageLive { always_live_locals }
     }
 }
 
 impl<'a, 'tcx> Analysis<'tcx> for MaybeStorageLive<'a> {
-    type Domain = BitSet<Local>;
+    type Domain = DenseBitSet<Local>;
 
     const NAME: &'static str = "maybe_storage_live";
 
     fn bottom_value(&self, body: &Body<'tcx>) -> Self::Domain {
         // bottom = dead
-        BitSet::new_empty(body.local_decls.len())
+        DenseBitSet::new_empty(body.local_decls.len())
     }
 
     fn initialize_start_block(&self, body: &Body<'tcx>, state: &mut Self::Domain) {
@@ -67,23 +67,23 @@ impl<'a, 'tcx> Analysis<'tcx> for MaybeStorageLive<'a> {
 }
 
 pub struct MaybeStorageDead<'a> {
-    always_live_locals: Cow<'a, BitSet<Local>>,
+    always_live_locals: Cow<'a, DenseBitSet<Local>>,
 }
 
 impl<'a> MaybeStorageDead<'a> {
-    pub fn new(always_live_locals: Cow<'a, BitSet<Local>>) -> Self {
+    pub fn new(always_live_locals: Cow<'a, DenseBitSet<Local>>) -> Self {
         MaybeStorageDead { always_live_locals }
     }
 }
 
 impl<'a, 'tcx> Analysis<'tcx> for MaybeStorageDead<'a> {
-    type Domain = BitSet<Local>;
+    type Domain = DenseBitSet<Local>;
 
     const NAME: &'static str = "maybe_storage_dead";
 
     fn bottom_value(&self, body: &Body<'tcx>) -> Self::Domain {
         // bottom = live
-        BitSet::new_empty(body.local_decls.len())
+        DenseBitSet::new_empty(body.local_decls.len())
     }
 
     fn initialize_start_block(&self, body: &Body<'tcx>, state: &mut Self::Domain) {
@@ -125,13 +125,13 @@ impl<'mir, 'tcx> MaybeRequiresStorage<'mir, 'tcx> {
 }
 
 impl<'tcx> Analysis<'tcx> for MaybeRequiresStorage<'_, 'tcx> {
-    type Domain = BitSet<Local>;
+    type Domain = DenseBitSet<Local>;
 
     const NAME: &'static str = "requires_storage";
 
     fn bottom_value(&self, body: &Body<'tcx>) -> Self::Domain {
         // bottom = dead
-        BitSet::new_empty(body.local_decls.len())
+        DenseBitSet::new_empty(body.local_decls.len())
     }
 
     fn initialize_start_block(&self, body: &Body<'tcx>, state: &mut Self::Domain) {
@@ -304,7 +304,7 @@ impl<'tcx> MaybeRequiresStorage<'_, 'tcx> {
 
 struct MoveVisitor<'a, 'mir, 'tcx> {
     borrowed_locals: &'a mut BorrowedLocalsResults<'mir, 'tcx>,
-    state: &'a mut BitSet<Local>,
+    state: &'a mut DenseBitSet<Local>,
 }
 
 impl<'tcx> Visitor<'tcx> for MoveVisitor<'_, '_, 'tcx> {
diff --git a/compiler/rustc_mir_dataflow/src/points.rs b/compiler/rustc_mir_dataflow/src/points.rs
index 74209da876a..5d2a78acbf5 100644
--- a/compiler/rustc_mir_dataflow/src/points.rs
+++ b/compiler/rustc_mir_dataflow/src/points.rs
@@ -1,4 +1,4 @@
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_index::interval::SparseIntervalMatrix;
 use rustc_index::{Idx, IndexVec};
 use rustc_middle::mir::{self, BasicBlock, Body, Location};
@@ -102,7 +102,7 @@ pub fn save_as_intervals<'tcx, N, A>(
 ) -> SparseIntervalMatrix<N, PointIndex>
 where
     N: Idx,
-    A: Analysis<'tcx, Domain = BitSet<N>>,
+    A: Analysis<'tcx, Domain = DenseBitSet<N>>,
 {
     let values = SparseIntervalMatrix::new(elements.num_points());
     let mut visitor = Visitor { elements, values };
@@ -122,7 +122,7 @@ struct Visitor<'a, N: Idx> {
 
 impl<'mir, 'tcx, A, N> ResultsVisitor<'mir, 'tcx, A> for Visitor<'_, N>
 where
-    A: Analysis<'tcx, Domain = BitSet<N>>,
+    A: Analysis<'tcx, Domain = DenseBitSet<N>>,
     N: Idx,
 {
     fn visit_after_primary_statement_effect(
diff --git a/compiler/rustc_mir_dataflow/src/value_analysis.rs b/compiler/rustc_mir_dataflow/src/value_analysis.rs
index 9328870c7ae..a51af8c40fd 100644
--- a/compiler/rustc_mir_dataflow/src/value_analysis.rs
+++ b/compiler/rustc_mir_dataflow/src/value_analysis.rs
@@ -6,7 +6,7 @@ use rustc_data_structures::captures::Captures;
 use rustc_data_structures::fx::{FxHashMap, FxIndexSet, StdEntry};
 use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_index::IndexVec;
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::mir::tcx::PlaceTy;
 use rustc_middle::mir::visit::{MutatingUseContext, PlaceContext, Visitor};
 use rustc_middle::mir::*;
@@ -399,7 +399,7 @@ impl<'tcx> Map<'tcx> {
         &mut self,
         tcx: TyCtxt<'tcx>,
         body: &Body<'tcx>,
-        exclude: BitSet<Local>,
+        exclude: DenseBitSet<Local>,
         value_limit: Option<usize>,
     ) {
         // Start by constructing the places for each bare local.
@@ -912,9 +912,9 @@ pub fn iter_fields<'tcx>(
 }
 
 /// Returns all locals with projections that have their reference or address taken.
-pub fn excluded_locals(body: &Body<'_>) -> BitSet<Local> {
+pub fn excluded_locals(body: &Body<'_>) -> DenseBitSet<Local> {
     struct Collector {
-        result: BitSet<Local>,
+        result: DenseBitSet<Local>,
     }
 
     impl<'tcx> Visitor<'tcx> for Collector {
@@ -932,7 +932,7 @@ pub fn excluded_locals(body: &Body<'_>) -> BitSet<Local> {
         }
     }
 
-    let mut collector = Collector { result: BitSet::new_empty(body.local_decls.len()) };
+    let mut collector = Collector { result: DenseBitSet::new_empty(body.local_decls.len()) };
     collector.visit_body(body);
     collector.result
 }
diff --git a/compiler/rustc_mir_transform/src/copy_prop.rs b/compiler/rustc_mir_transform/src/copy_prop.rs
index 9b3443d3209..f149bf97cde 100644
--- a/compiler/rustc_mir_transform/src/copy_prop.rs
+++ b/compiler/rustc_mir_transform/src/copy_prop.rs
@@ -1,5 +1,5 @@
 use rustc_index::IndexSlice;
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::mir::visit::*;
 use rustc_middle::mir::*;
 use rustc_middle::ty::TyCtxt;
@@ -34,7 +34,7 @@ impl<'tcx> crate::MirPass<'tcx> for CopyProp {
         let fully_moved = fully_moved_locals(&ssa, body);
         debug!(?fully_moved);
 
-        let mut storage_to_remove = BitSet::new_empty(fully_moved.domain_size());
+        let mut storage_to_remove = DenseBitSet::new_empty(fully_moved.domain_size());
         for (local, &head) in ssa.copy_classes().iter_enumerated() {
             if local != head {
                 storage_to_remove.insert(head);
@@ -68,8 +68,8 @@ impl<'tcx> crate::MirPass<'tcx> for CopyProp {
 /// This means that replacing it by a copy of `_a` if ok, since this copy happens before `_c` is
 /// moved, and therefore that `_d` is moved.
 #[instrument(level = "trace", skip(ssa, body))]
-fn fully_moved_locals(ssa: &SsaLocals, body: &Body<'_>) -> BitSet<Local> {
-    let mut fully_moved = BitSet::new_filled(body.local_decls.len());
+fn fully_moved_locals(ssa: &SsaLocals, body: &Body<'_>) -> DenseBitSet<Local> {
+    let mut fully_moved = DenseBitSet::new_filled(body.local_decls.len());
 
     for (_, rvalue, _) in ssa.assignments(body) {
         let (Rvalue::Use(Operand::Copy(place) | Operand::Move(place))
@@ -96,9 +96,9 @@ fn fully_moved_locals(ssa: &SsaLocals, body: &Body<'_>) -> BitSet<Local> {
 /// Utility to help performing substitution of `*pattern` by `target`.
 struct Replacer<'a, 'tcx> {
     tcx: TyCtxt<'tcx>,
-    fully_moved: BitSet<Local>,
-    storage_to_remove: BitSet<Local>,
-    borrowed_locals: &'a BitSet<Local>,
+    fully_moved: DenseBitSet<Local>,
+    storage_to_remove: DenseBitSet<Local>,
+    borrowed_locals: &'a DenseBitSet<Local>,
     copy_classes: &'a IndexSlice<Local, Local>,
 }
 
diff --git a/compiler/rustc_mir_transform/src/coroutine.rs b/compiler/rustc_mir_transform/src/coroutine.rs
index f6536d78761..a3715b5d485 100644
--- a/compiler/rustc_mir_transform/src/coroutine.rs
+++ b/compiler/rustc_mir_transform/src/coroutine.rs
@@ -60,7 +60,7 @@ use rustc_errors::pluralize;
 use rustc_hir as hir;
 use rustc_hir::lang_items::LangItem;
 use rustc_hir::{CoroutineDesugaring, CoroutineKind};
-use rustc_index::bit_set::{BitMatrix, BitSet, GrowableBitSet};
+use rustc_index::bit_set::{BitMatrix, DenseBitSet, GrowableBitSet};
 use rustc_index::{Idx, IndexVec};
 use rustc_middle::mir::visit::{MutVisitor, PlaceContext, Visitor};
 use rustc_middle::mir::*;
@@ -185,13 +185,13 @@ struct TransformVisitor<'tcx> {
     remap: IndexVec<Local, Option<(Ty<'tcx>, VariantIdx, FieldIdx)>>,
 
     // A map from a suspension point in a block to the locals which have live storage at that point
-    storage_liveness: IndexVec<BasicBlock, Option<BitSet<Local>>>,
+    storage_liveness: IndexVec<BasicBlock, Option<DenseBitSet<Local>>>,
 
     // A list of suspension points, generated during the transform
     suspension_points: Vec<SuspensionPoint<'tcx>>,
 
     // The set of locals that have no `StorageLive`/`StorageDead` annotations.
-    always_live_locals: BitSet<Local>,
+    always_live_locals: DenseBitSet<Local>,
 
     // The original RETURN_PLACE local
     old_ret_local: Local,
@@ -633,7 +633,7 @@ struct LivenessInfo {
     saved_locals: CoroutineSavedLocals,
 
     /// The set of saved locals live at each suspension point.
-    live_locals_at_suspension_points: Vec<BitSet<CoroutineSavedLocal>>,
+    live_locals_at_suspension_points: Vec<DenseBitSet<CoroutineSavedLocal>>,
 
     /// Parallel vec to the above with SourceInfo for each yield terminator.
     source_info_at_suspension_points: Vec<SourceInfo>,
@@ -645,7 +645,7 @@ struct LivenessInfo {
 
     /// For every suspending block, the locals which are storage-live across
     /// that suspension point.
-    storage_liveness: IndexVec<BasicBlock, Option<BitSet<Local>>>,
+    storage_liveness: IndexVec<BasicBlock, Option<DenseBitSet<Local>>>,
 }
 
 /// Computes which locals have to be stored in the state-machine for the
@@ -659,7 +659,7 @@ struct LivenessInfo {
 fn locals_live_across_suspend_points<'tcx>(
     tcx: TyCtxt<'tcx>,
     body: &Body<'tcx>,
-    always_live_locals: &BitSet<Local>,
+    always_live_locals: &DenseBitSet<Local>,
     movable: bool,
 ) -> LivenessInfo {
     // Calculate when MIR locals have live storage. This gives us an upper bound of their
@@ -688,7 +688,7 @@ fn locals_live_across_suspend_points<'tcx>(
     let mut storage_liveness_map = IndexVec::from_elem(None, &body.basic_blocks);
     let mut live_locals_at_suspension_points = Vec::new();
     let mut source_info_at_suspension_points = Vec::new();
-    let mut live_locals_at_any_suspension_point = BitSet::new_empty(body.local_decls.len());
+    let mut live_locals_at_any_suspension_point = DenseBitSet::new_empty(body.local_decls.len());
 
     for (block, data) in body.basic_blocks.iter_enumerated() {
         if let TerminatorKind::Yield { .. } = data.terminator().kind {
@@ -768,7 +768,7 @@ fn locals_live_across_suspend_points<'tcx>(
 /// `CoroutineSavedLocal` is indexed in terms of the elements in this set;
 /// i.e. `CoroutineSavedLocal::new(1)` corresponds to the second local
 /// included in this set.
-struct CoroutineSavedLocals(BitSet<Local>);
+struct CoroutineSavedLocals(DenseBitSet<Local>);
 
 impl CoroutineSavedLocals {
     /// Returns an iterator over each `CoroutineSavedLocal` along with the `Local` it corresponds
@@ -777,11 +777,11 @@ impl CoroutineSavedLocals {
         self.iter().enumerate().map(|(i, l)| (CoroutineSavedLocal::from(i), l))
     }
 
-    /// Transforms a `BitSet<Local>` that contains only locals saved across yield points to the
-    /// equivalent `BitSet<CoroutineSavedLocal>`.
-    fn renumber_bitset(&self, input: &BitSet<Local>) -> BitSet<CoroutineSavedLocal> {
+    /// Transforms a `DenseBitSet<Local>` that contains only locals saved across yield points to the
+    /// equivalent `DenseBitSet<CoroutineSavedLocal>`.
+    fn renumber_bitset(&self, input: &DenseBitSet<Local>) -> DenseBitSet<CoroutineSavedLocal> {
         assert!(self.superset(input), "{:?} not a superset of {:?}", self.0, input);
-        let mut out = BitSet::new_empty(self.count());
+        let mut out = DenseBitSet::new_empty(self.count());
         for (saved_local, local) in self.iter_enumerated() {
             if input.contains(local) {
                 out.insert(saved_local);
@@ -801,7 +801,7 @@ impl CoroutineSavedLocals {
 }
 
 impl ops::Deref for CoroutineSavedLocals {
-    type Target = BitSet<Local>;
+    type Target = DenseBitSet<Local>;
 
     fn deref(&self) -> &Self::Target {
         &self.0
@@ -815,7 +815,7 @@ impl ops::Deref for CoroutineSavedLocals {
 fn compute_storage_conflicts<'mir, 'tcx>(
     body: &'mir Body<'tcx>,
     saved_locals: &'mir CoroutineSavedLocals,
-    always_live_locals: BitSet<Local>,
+    always_live_locals: DenseBitSet<Local>,
     mut requires_storage: Results<'tcx, MaybeRequiresStorage<'mir, 'tcx>>,
 ) -> BitMatrix<CoroutineSavedLocal, CoroutineSavedLocal> {
     assert_eq!(body.local_decls.len(), saved_locals.domain_size());
@@ -833,7 +833,7 @@ fn compute_storage_conflicts<'mir, 'tcx>(
         body,
         saved_locals,
         local_conflicts: BitMatrix::from_row_n(&ineligible_locals, body.local_decls.len()),
-        eligible_storage_live: BitSet::new_empty(body.local_decls.len()),
+        eligible_storage_live: DenseBitSet::new_empty(body.local_decls.len()),
     };
 
     requires_storage.visit_reachable_with(body, &mut visitor);
@@ -871,7 +871,7 @@ struct StorageConflictVisitor<'a, 'tcx> {
     // benchmarks for coroutines.
     local_conflicts: BitMatrix<Local, Local>,
     // We keep this bitset as a buffer to avoid reallocating memory.
-    eligible_storage_live: BitSet<Local>,
+    eligible_storage_live: DenseBitSet<Local>,
 }
 
 impl<'a, 'tcx> ResultsVisitor<'a, 'tcx, MaybeRequiresStorage<'a, 'tcx>>
@@ -880,7 +880,7 @@ impl<'a, 'tcx> ResultsVisitor<'a, 'tcx, MaybeRequiresStorage<'a, 'tcx>>
     fn visit_after_early_statement_effect(
         &mut self,
         _results: &mut Results<'tcx, MaybeRequiresStorage<'a, 'tcx>>,
-        state: &BitSet<Local>,
+        state: &DenseBitSet<Local>,
         _statement: &'a Statement<'tcx>,
         loc: Location,
     ) {
@@ -890,7 +890,7 @@ impl<'a, 'tcx> ResultsVisitor<'a, 'tcx, MaybeRequiresStorage<'a, 'tcx>>
     fn visit_after_early_terminator_effect(
         &mut self,
         _results: &mut Results<'tcx, MaybeRequiresStorage<'a, 'tcx>>,
-        state: &BitSet<Local>,
+        state: &DenseBitSet<Local>,
         _terminator: &'a Terminator<'tcx>,
         loc: Location,
     ) {
@@ -899,7 +899,7 @@ impl<'a, 'tcx> ResultsVisitor<'a, 'tcx, MaybeRequiresStorage<'a, 'tcx>>
 }
 
 impl StorageConflictVisitor<'_, '_> {
-    fn apply_state(&mut self, state: &BitSet<Local>, loc: Location) {
+    fn apply_state(&mut self, state: &DenseBitSet<Local>, loc: Location) {
         // Ignore unreachable blocks.
         if let TerminatorKind::Unreachable = self.body.basic_blocks[loc.block].terminator().kind {
             return;
@@ -924,7 +924,7 @@ fn compute_layout<'tcx>(
 ) -> (
     IndexVec<Local, Option<(Ty<'tcx>, VariantIdx, FieldIdx)>>,
     CoroutineLayout<'tcx>,
-    IndexVec<BasicBlock, Option<BitSet<Local>>>,
+    IndexVec<BasicBlock, Option<DenseBitSet<Local>>>,
 ) {
     let LivenessInfo {
         saved_locals,
diff --git a/compiler/rustc_mir_transform/src/coverage/counters.rs b/compiler/rustc_mir_transform/src/coverage/counters.rs
index 9e80f1f1c4a..a9111a5ef9b 100644
--- a/compiler/rustc_mir_transform/src/coverage/counters.rs
+++ b/compiler/rustc_mir_transform/src/coverage/counters.rs
@@ -5,7 +5,7 @@ use rustc_data_structures::captures::Captures;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::graph::DirectedGraph;
 use rustc_index::IndexVec;
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::mir::coverage::{CounterId, CovTerm, Expression, ExpressionId, Op};
 use tracing::{debug, debug_span, instrument};
 
@@ -77,7 +77,7 @@ impl CoverageCounters {
     /// counters or counter expressions for nodes and edges as required.
     pub(super) fn make_bcb_counters(
         graph: &CoverageGraph,
-        bcb_needs_counter: &BitSet<BasicCoverageBlock>,
+        bcb_needs_counter: &DenseBitSet<BasicCoverageBlock>,
     ) -> Self {
         let mut builder = CountersBuilder::new(graph, bcb_needs_counter);
         builder.make_bcb_counters();
@@ -220,13 +220,16 @@ fn sibling_out_edge_targets(
 /// the set of nodes that need counters.
 struct CountersBuilder<'a> {
     graph: &'a CoverageGraph,
-    bcb_needs_counter: &'a BitSet<BasicCoverageBlock>,
+    bcb_needs_counter: &'a DenseBitSet<BasicCoverageBlock>,
 
     site_counters: FxHashMap<Site, SiteCounter>,
 }
 
 impl<'a> CountersBuilder<'a> {
-    fn new(graph: &'a CoverageGraph, bcb_needs_counter: &'a BitSet<BasicCoverageBlock>) -> Self {
+    fn new(
+        graph: &'a CoverageGraph,
+        bcb_needs_counter: &'a DenseBitSet<BasicCoverageBlock>,
+    ) -> Self {
         assert_eq!(graph.num_nodes(), bcb_needs_counter.domain_size());
         Self { graph, bcb_needs_counter, site_counters: FxHashMap::default() }
     }
diff --git a/compiler/rustc_mir_transform/src/coverage/graph.rs b/compiler/rustc_mir_transform/src/coverage/graph.rs
index ad6774fccd6..3fa8b063fa7 100644
--- a/compiler/rustc_mir_transform/src/coverage/graph.rs
+++ b/compiler/rustc_mir_transform/src/coverage/graph.rs
@@ -8,7 +8,7 @@ use rustc_data_structures::fx::FxHashSet;
 use rustc_data_structures::graph::dominators::Dominators;
 use rustc_data_structures::graph::{self, DirectedGraph, StartNode};
 use rustc_index::IndexVec;
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::mir::{self, BasicBlock, Terminator, TerminatorKind};
 use tracing::debug;
 
@@ -27,7 +27,7 @@ pub(crate) struct CoverageGraph {
     /// their relative order is consistent but arbitrary.
     dominator_order_rank: IndexVec<BasicCoverageBlock, u32>,
     /// A loop header is a node that dominates one or more of its predecessors.
-    is_loop_header: BitSet<BasicCoverageBlock>,
+    is_loop_header: DenseBitSet<BasicCoverageBlock>,
     /// For each node, the loop header node of its nearest enclosing loop.
     /// This forms a linked list that can be traversed to find all enclosing loops.
     enclosing_loop_header: IndexVec<BasicCoverageBlock, Option<BasicCoverageBlock>>,
@@ -72,7 +72,7 @@ impl CoverageGraph {
             predecessors,
             dominators: None,
             dominator_order_rank: IndexVec::from_elem_n(0, num_nodes),
-            is_loop_header: BitSet::new_empty(num_nodes),
+            is_loop_header: DenseBitSet::new_empty(num_nodes),
             enclosing_loop_header: IndexVec::from_elem_n(None, num_nodes),
         };
         assert_eq!(num_nodes, this.num_nodes());
diff --git a/compiler/rustc_mir_transform/src/coverage/mappings.rs b/compiler/rustc_mir_transform/src/coverage/mappings.rs
index 5bd20e00eb6..8d0d92dc367 100644
--- a/compiler/rustc_mir_transform/src/coverage/mappings.rs
+++ b/compiler/rustc_mir_transform/src/coverage/mappings.rs
@@ -3,7 +3,7 @@ use std::collections::BTreeSet;
 use rustc_data_structures::fx::FxIndexMap;
 use rustc_data_structures::graph::DirectedGraph;
 use rustc_index::IndexVec;
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::mir::coverage::{
     BlockMarkerId, BranchSpan, ConditionId, ConditionInfo, CoverageInfoHi, CoverageKind,
 };
@@ -128,7 +128,7 @@ pub(super) fn extract_all_mapping_info_from_mir<'tcx>(
 }
 
 impl ExtractedMappings {
-    pub(super) fn all_bcbs_with_counter_mappings(&self) -> BitSet<BasicCoverageBlock> {
+    pub(super) fn all_bcbs_with_counter_mappings(&self) -> DenseBitSet<BasicCoverageBlock> {
         // Fully destructure self to make sure we don't miss any fields that have mappings.
         let Self {
             num_bcbs,
@@ -140,7 +140,7 @@ impl ExtractedMappings {
         } = self;
 
         // Identify which BCBs have one or more mappings.
-        let mut bcbs_with_counter_mappings = BitSet::new_empty(*num_bcbs);
+        let mut bcbs_with_counter_mappings = DenseBitSet::new_empty(*num_bcbs);
         let mut insert = |bcb| {
             bcbs_with_counter_mappings.insert(bcb);
         };
@@ -172,8 +172,8 @@ impl ExtractedMappings {
     }
 
     /// Returns the set of BCBs that have one or more `Code` mappings.
-    pub(super) fn bcbs_with_ordinary_code_mappings(&self) -> BitSet<BasicCoverageBlock> {
-        let mut bcbs = BitSet::new_empty(self.num_bcbs);
+    pub(super) fn bcbs_with_ordinary_code_mappings(&self) -> DenseBitSet<BasicCoverageBlock> {
+        let mut bcbs = DenseBitSet::new_empty(self.num_bcbs);
         for &CodeMapping { span: _, bcb } in &self.code_mappings {
             bcbs.insert(bcb);
         }
diff --git a/compiler/rustc_mir_transform/src/coverage/query.rs b/compiler/rustc_mir_transform/src/coverage/query.rs
index edaec3c7965..3e7cf8541c2 100644
--- a/compiler/rustc_mir_transform/src/coverage/query.rs
+++ b/compiler/rustc_mir_transform/src/coverage/query.rs
@@ -1,5 +1,5 @@
 use rustc_data_structures::captures::Captures;
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags;
 use rustc_middle::mir::coverage::{
     CounterId, CovTerm, CoverageIdsInfo, CoverageKind, Expression, ExpressionId,
@@ -92,13 +92,13 @@ fn coverage_ids_info<'tcx>(
 
     let Some(fn_cov_info) = mir_body.function_coverage_info.as_deref() else {
         return CoverageIdsInfo {
-            counters_seen: BitSet::new_empty(0),
-            zero_expressions: BitSet::new_empty(0),
+            counters_seen: DenseBitSet::new_empty(0),
+            zero_expressions: DenseBitSet::new_empty(0),
         };
     };
 
-    let mut counters_seen = BitSet::new_empty(fn_cov_info.num_counters);
-    let mut expressions_seen = BitSet::new_filled(fn_cov_info.expressions.len());
+    let mut counters_seen = DenseBitSet::new_empty(fn_cov_info.num_counters);
+    let mut expressions_seen = DenseBitSet::new_filled(fn_cov_info.expressions.len());
 
     // For each expression ID that is directly used by one or more mappings,
     // mark it as not-yet-seen. This indicates that we expect to see a
@@ -148,23 +148,23 @@ fn is_inlined(body: &Body<'_>, statement: &Statement<'_>) -> bool {
     scope_data.inlined.is_some() || scope_data.inlined_parent_scope.is_some()
 }
 
-/// Identify expressions that will always have a value of zero, and note
-/// their IDs in a `BitSet`. Mappings that refer to a zero expression
-/// can instead become mappings to a constant zero value.
+/// Identify expressions that will always have a value of zero, and note their
+/// IDs in a `DenseBitSet`. Mappings that refer to a zero expression can instead
+/// become mappings to a constant zero value.
 ///
 /// This function mainly exists to preserve the simplifications that were
 /// already being performed by the Rust-side expression renumbering, so that
 /// the resulting coverage mappings don't get worse.
 fn identify_zero_expressions(
     fn_cov_info: &FunctionCoverageInfo,
-    counters_seen: &BitSet<CounterId>,
-    expressions_seen: &BitSet<ExpressionId>,
-) -> BitSet<ExpressionId> {
+    counters_seen: &DenseBitSet<CounterId>,
+    expressions_seen: &DenseBitSet<ExpressionId>,
+) -> DenseBitSet<ExpressionId> {
     // The set of expressions that either were optimized out entirely, or
     // have zero as both of their operands, and will therefore always have
     // a value of zero. Other expressions that refer to these as operands
     // can have those operands replaced with `CovTerm::Zero`.
-    let mut zero_expressions = BitSet::new_empty(fn_cov_info.expressions.len());
+    let mut zero_expressions = DenseBitSet::new_empty(fn_cov_info.expressions.len());
 
     // Simplify a copy of each expression based on lower-numbered expressions,
     // and then update the set of always-zero expressions if necessary.
@@ -228,8 +228,8 @@ fn identify_zero_expressions(
 /// into account knowledge of which counters are unused and which expressions
 /// are always zero.
 fn is_zero_term(
-    counters_seen: &BitSet<CounterId>,
-    zero_expressions: &BitSet<ExpressionId>,
+    counters_seen: &DenseBitSet<CounterId>,
+    zero_expressions: &DenseBitSet<ExpressionId>,
     term: CovTerm,
 ) -> bool {
     match term {
diff --git a/compiler/rustc_mir_transform/src/dead_store_elimination.rs b/compiler/rustc_mir_transform/src/dead_store_elimination.rs
index 0c75cdadc92..434e921d439 100644
--- a/compiler/rustc_mir_transform/src/dead_store_elimination.rs
+++ b/compiler/rustc_mir_transform/src/dead_store_elimination.rs
@@ -26,8 +26,8 @@ use crate::util::is_within_packed;
 
 /// Performs the optimization on the body
 ///
-/// The `borrowed` set must be a `BitSet` of all the locals that are ever borrowed in this body. It
-/// can be generated via the [`borrowed_locals`] function.
+/// The `borrowed` set must be a `DenseBitSet` of all the locals that are ever borrowed in this
+/// body. It can be generated via the [`borrowed_locals`] function.
 fn eliminate<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
     let borrowed_locals = borrowed_locals(body);
 
diff --git a/compiler/rustc_mir_transform/src/deduce_param_attrs.rs b/compiler/rustc_mir_transform/src/deduce_param_attrs.rs
index 67b215c7c9d..049f13ce96d 100644
--- a/compiler/rustc_mir_transform/src/deduce_param_attrs.rs
+++ b/compiler/rustc_mir_transform/src/deduce_param_attrs.rs
@@ -6,7 +6,7 @@
 //! dependent crates can use them.
 
 use rustc_hir::def_id::LocalDefId;
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::mir::visit::{NonMutatingUseContext, PlaceContext, Visitor};
 use rustc_middle::mir::{Body, Location, Operand, Place, RETURN_PLACE, Terminator, TerminatorKind};
 use rustc_middle::ty::{self, DeducedParamAttrs, Ty, TyCtxt};
@@ -18,13 +18,13 @@ struct DeduceReadOnly {
     /// Each bit is indexed by argument number, starting at zero (so 0 corresponds to local decl
     /// 1). The bit is true if the argument may have been mutated or false if we know it hasn't
     /// been up to the point we're at.
-    mutable_args: BitSet<usize>,
+    mutable_args: DenseBitSet<usize>,
 }
 
 impl DeduceReadOnly {
     /// Returns a new DeduceReadOnly instance.
     fn new(arg_count: usize) -> Self {
-        Self { mutable_args: BitSet::new_empty(arg_count) }
+        Self { mutable_args: DenseBitSet::new_empty(arg_count) }
     }
 }
 
diff --git a/compiler/rustc_mir_transform/src/dest_prop.rs b/compiler/rustc_mir_transform/src/dest_prop.rs
index e99bee6a01f..1ac4d835946 100644
--- a/compiler/rustc_mir_transform/src/dest_prop.rs
+++ b/compiler/rustc_mir_transform/src/dest_prop.rs
@@ -132,7 +132,7 @@
 //! [attempt 3]: https://github.com/rust-lang/rust/pull/72632
 
 use rustc_data_structures::fx::{FxIndexMap, IndexEntry, IndexOccupiedEntry};
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_index::interval::SparseIntervalMatrix;
 use rustc_middle::bug;
 use rustc_middle::mir::visit::{MutVisitor, PlaceContext, Visitor};
@@ -204,7 +204,8 @@ impl<'tcx> crate::MirPass<'tcx> for DestinationPropagation {
             // Because we only filter once per round, it is unsound to use a local for more than
             // one merge operation within a single round of optimizations. We store here which ones
             // we have already used.
-            let mut merged_locals: BitSet<Local> = BitSet::new_empty(body.local_decls.len());
+            let mut merged_locals: DenseBitSet<Local> =
+                DenseBitSet::new_empty(body.local_decls.len());
 
             // This is the set of merges we will apply this round. It is a subset of the candidates.
             let mut merges = FxIndexMap::default();
@@ -274,7 +275,7 @@ fn apply_merges<'tcx>(
     body: &mut Body<'tcx>,
     tcx: TyCtxt<'tcx>,
     merges: FxIndexMap<Local, Local>,
-    merged_locals: BitSet<Local>,
+    merged_locals: DenseBitSet<Local>,
 ) {
     let mut merger = Merger { tcx, merges, merged_locals };
     merger.visit_body_preserves_cfg(body);
@@ -283,7 +284,7 @@ fn apply_merges<'tcx>(
 struct Merger<'tcx> {
     tcx: TyCtxt<'tcx>,
     merges: FxIndexMap<Local, Local>,
-    merged_locals: BitSet<Local>,
+    merged_locals: DenseBitSet<Local>,
 }
 
 impl<'tcx> MutVisitor<'tcx> for Merger<'tcx> {
@@ -351,7 +352,7 @@ impl Candidates {
     /// Collects the candidates for merging.
     ///
     /// This is responsible for enforcing the first and third bullet point.
-    fn reset_and_find<'tcx>(&mut self, body: &Body<'tcx>, borrowed: &BitSet<Local>) {
+    fn reset_and_find<'tcx>(&mut self, body: &Body<'tcx>, borrowed: &DenseBitSet<Local>) {
         self.c.clear();
         self.reverse.clear();
         let mut visitor = FindAssignments { body, candidates: &mut self.c, borrowed };
@@ -735,7 +736,7 @@ fn places_to_candidate_pair<'tcx>(
 struct FindAssignments<'a, 'tcx> {
     body: &'a Body<'tcx>,
     candidates: &'a mut FxIndexMap<Local, Vec<Local>>,
-    borrowed: &'a BitSet<Local>,
+    borrowed: &'a DenseBitSet<Local>,
 }
 
 impl<'tcx> Visitor<'tcx> for FindAssignments<'_, 'tcx> {
diff --git a/compiler/rustc_mir_transform/src/elaborate_drops.rs b/compiler/rustc_mir_transform/src/elaborate_drops.rs
index 3ebc9113725..988f1a25561 100644
--- a/compiler/rustc_mir_transform/src/elaborate_drops.rs
+++ b/compiler/rustc_mir_transform/src/elaborate_drops.rs
@@ -2,7 +2,7 @@ use std::fmt;
 
 use rustc_abi::{FieldIdx, VariantIdx};
 use rustc_index::IndexVec;
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::mir::patch::MirPatch;
 use rustc_middle::mir::*;
 use rustc_middle::ty::{self, TyCtxt};
@@ -96,10 +96,10 @@ impl<'tcx> crate::MirPass<'tcx> for ElaborateDrops {
 fn compute_dead_unwinds<'a, 'tcx>(
     body: &'a Body<'tcx>,
     flow_inits: &mut ResultsCursor<'a, 'tcx, MaybeInitializedPlaces<'a, 'tcx>>,
-) -> BitSet<BasicBlock> {
+) -> DenseBitSet<BasicBlock> {
     // We only need to do this pass once, because unwind edges can only
     // reach cleanup blocks, which can't have unwind edges themselves.
-    let mut dead_unwinds = BitSet::new_empty(body.basic_blocks.len());
+    let mut dead_unwinds = DenseBitSet::new_empty(body.basic_blocks.len());
     for (bb, bb_data) in body.basic_blocks.iter_enumerated() {
         let TerminatorKind::Drop { place, unwind: UnwindAction::Cleanup(_), .. } =
             bb_data.terminator().kind
diff --git a/compiler/rustc_mir_transform/src/gvn.rs b/compiler/rustc_mir_transform/src/gvn.rs
index 71bec38c405..affc4cf0afc 100644
--- a/compiler/rustc_mir_transform/src/gvn.rs
+++ b/compiler/rustc_mir_transform/src/gvn.rs
@@ -94,7 +94,7 @@ use rustc_const_eval::interpret::{
 use rustc_data_structures::fx::FxIndexSet;
 use rustc_data_structures::graph::dominators::Dominators;
 use rustc_hir::def::DefKind;
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_index::{IndexVec, newtype_index};
 use rustc_middle::bug;
 use rustc_middle::mir::interpret::GlobalAlloc;
@@ -256,7 +256,7 @@ struct VnState<'body, 'tcx> {
     feature_unsized_locals: bool,
     ssa: &'body SsaLocals,
     dominators: Dominators<BasicBlock>,
-    reused_locals: BitSet<Local>,
+    reused_locals: DenseBitSet<Local>,
 }
 
 impl<'body, 'tcx> VnState<'body, 'tcx> {
@@ -287,7 +287,7 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
             feature_unsized_locals: tcx.features().unsized_locals(),
             ssa,
             dominators,
-            reused_locals: BitSet::new_empty(local_decls.len()),
+            reused_locals: DenseBitSet::new_empty(local_decls.len()),
         }
     }
 
@@ -1714,7 +1714,7 @@ impl<'tcx> MutVisitor<'tcx> for VnState<'_, 'tcx> {
 
 struct StorageRemover<'tcx> {
     tcx: TyCtxt<'tcx>,
-    reused_locals: BitSet<Local>,
+    reused_locals: DenseBitSet<Local>,
 }
 
 impl<'tcx> MutVisitor<'tcx> for StorageRemover<'tcx> {
diff --git a/compiler/rustc_mir_transform/src/inline.rs b/compiler/rustc_mir_transform/src/inline.rs
index e4daa2b9757..470393c9ae1 100644
--- a/compiler/rustc_mir_transform/src/inline.rs
+++ b/compiler/rustc_mir_transform/src/inline.rs
@@ -8,7 +8,7 @@ use rustc_attr_parsing::InlineAttr;
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::DefId;
 use rustc_index::Idx;
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::bug;
 use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrs;
 use rustc_middle::mir::visit::*;
@@ -369,7 +369,7 @@ impl<'tcx> Inliner<'tcx> for NormalInliner<'tcx> {
 
         // Traverse the MIR manually so we can account for the effects of inlining on the CFG.
         let mut work_list = vec![START_BLOCK];
-        let mut visited = BitSet::new_empty(callee_body.basic_blocks.len());
+        let mut visited = DenseBitSet::new_empty(callee_body.basic_blocks.len());
         while let Some(bb) = work_list.pop() {
             if !visited.insert(bb.index()) {
                 continue;
@@ -885,7 +885,7 @@ fn inline_call<'tcx, I: Inliner<'tcx>>(
         in_cleanup_block: false,
         return_block,
         tcx,
-        always_live_locals: BitSet::new_filled(callee_body.local_decls.len()),
+        always_live_locals: DenseBitSet::new_filled(callee_body.local_decls.len()),
     };
 
     // Map all `Local`s, `SourceScope`s and `BasicBlock`s to new ones
@@ -1127,7 +1127,7 @@ struct Integrator<'a, 'tcx> {
     in_cleanup_block: bool,
     return_block: Option<BasicBlock>,
     tcx: TyCtxt<'tcx>,
-    always_live_locals: BitSet<Local>,
+    always_live_locals: DenseBitSet<Local>,
 }
 
 impl Integrator<'_, '_> {
diff --git a/compiler/rustc_mir_transform/src/jump_threading.rs b/compiler/rustc_mir_transform/src/jump_threading.rs
index 8feb90ff7a0..c73a03489c5 100644
--- a/compiler/rustc_mir_transform/src/jump_threading.rs
+++ b/compiler/rustc_mir_transform/src/jump_threading.rs
@@ -40,7 +40,7 @@ use rustc_const_eval::const_eval::DummyMachine;
 use rustc_const_eval::interpret::{ImmTy, Immediate, InterpCx, OpTy, Projectable};
 use rustc_data_structures::fx::FxHashSet;
 use rustc_index::IndexVec;
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::bug;
 use rustc_middle::mir::interpret::Scalar;
 use rustc_middle::mir::visit::Visitor;
@@ -121,7 +121,7 @@ struct TOFinder<'a, 'tcx> {
     ecx: InterpCx<'tcx, DummyMachine>,
     body: &'a Body<'tcx>,
     map: Map<'tcx>,
-    loop_headers: BitSet<BasicBlock>,
+    loop_headers: DenseBitSet<BasicBlock>,
     /// We use an arena to avoid cloning the slices when cloning `state`.
     arena: &'a DroplessArena,
     opportunities: Vec<ThreadingOpportunity>,
@@ -832,8 +832,8 @@ enum Update {
 /// at least a predecessor which it dominates. This definition is only correct for reducible CFGs.
 /// But if the CFG is already irreducible, there is no point in trying much harder.
 /// is already irreducible.
-fn loop_headers(body: &Body<'_>) -> BitSet<BasicBlock> {
-    let mut loop_headers = BitSet::new_empty(body.basic_blocks.len());
+fn loop_headers(body: &Body<'_>) -> DenseBitSet<BasicBlock> {
+    let mut loop_headers = DenseBitSet::new_empty(body.basic_blocks.len());
     let dominators = body.basic_blocks.dominators();
     // Only visit reachable blocks.
     for (bb, bbdata) in traversal::preorder(body) {
diff --git a/compiler/rustc_mir_transform/src/known_panics_lint.rs b/compiler/rustc_mir_transform/src/known_panics_lint.rs
index f1705d0c831..9b3a0e67295 100644
--- a/compiler/rustc_mir_transform/src/known_panics_lint.rs
+++ b/compiler/rustc_mir_transform/src/known_panics_lint.rs
@@ -13,7 +13,7 @@ use rustc_data_structures::fx::FxHashSet;
 use rustc_hir::HirId;
 use rustc_hir::def::DefKind;
 use rustc_index::IndexVec;
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::bug;
 use rustc_middle::mir::visit::{MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor};
 use rustc_middle::mir::*;
@@ -67,7 +67,7 @@ struct ConstPropagator<'mir, 'tcx> {
     tcx: TyCtxt<'tcx>,
     typing_env: ty::TypingEnv<'tcx>,
     worklist: Vec<BasicBlock>,
-    visited_blocks: BitSet<BasicBlock>,
+    visited_blocks: DenseBitSet<BasicBlock>,
     locals: IndexVec<Local, Value<'tcx>>,
     body: &'mir Body<'tcx>,
     written_only_inside_own_block_locals: FxHashSet<Local>,
@@ -190,7 +190,7 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
             tcx,
             typing_env,
             worklist: vec![START_BLOCK],
-            visited_blocks: BitSet::new_empty(body.basic_blocks.len()),
+            visited_blocks: DenseBitSet::new_empty(body.basic_blocks.len()),
             locals: IndexVec::from_elem_n(Value::Uninit, body.local_decls.len()),
             body,
             can_const_prop,
@@ -852,7 +852,7 @@ enum ConstPropMode {
 struct CanConstProp {
     can_const_prop: IndexVec<Local, ConstPropMode>,
     // False at the beginning. Once set, no more assignments are allowed to that local.
-    found_assignment: BitSet<Local>,
+    found_assignment: DenseBitSet<Local>,
 }
 
 impl CanConstProp {
@@ -864,7 +864,7 @@ impl CanConstProp {
     ) -> IndexVec<Local, ConstPropMode> {
         let mut cpv = CanConstProp {
             can_const_prop: IndexVec::from_elem(ConstPropMode::FullConstProp, &body.local_decls),
-            found_assignment: BitSet::new_empty(body.local_decls.len()),
+            found_assignment: DenseBitSet::new_empty(body.local_decls.len()),
         };
         for (local, val) in cpv.can_const_prop.iter_enumerated_mut() {
             let ty = body.local_decls[local].ty;
diff --git a/compiler/rustc_mir_transform/src/lint.rs b/compiler/rustc_mir_transform/src/lint.rs
index 29e762af8de..f472c7cb493 100644
--- a/compiler/rustc_mir_transform/src/lint.rs
+++ b/compiler/rustc_mir_transform/src/lint.rs
@@ -5,7 +5,7 @@
 use std::borrow::Cow;
 
 use rustc_data_structures::fx::FxHashSet;
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::mir::visit::{PlaceContext, Visitor};
 use rustc_middle::mir::*;
 use rustc_middle::ty::TyCtxt;
@@ -43,7 +43,7 @@ struct Lint<'a, 'tcx> {
     when: String,
     body: &'a Body<'tcx>,
     is_fn_like: bool,
-    always_live_locals: &'a BitSet<Local>,
+    always_live_locals: &'a DenseBitSet<Local>,
     maybe_storage_live: ResultsCursor<'a, 'tcx, MaybeStorageLive<'a>>,
     maybe_storage_dead: ResultsCursor<'a, 'tcx, MaybeStorageDead<'a>>,
     places: FxHashSet<PlaceRef<'tcx>>,
diff --git a/compiler/rustc_mir_transform/src/multiple_return_terminators.rs b/compiler/rustc_mir_transform/src/multiple_return_terminators.rs
index a9227524ce5..6dfa14d6b52 100644
--- a/compiler/rustc_mir_transform/src/multiple_return_terminators.rs
+++ b/compiler/rustc_mir_transform/src/multiple_return_terminators.rs
@@ -1,7 +1,7 @@
 //! This pass removes jumps to basic blocks containing only a return, and replaces them with a
 //! return instead.
 
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::mir::*;
 use rustc_middle::ty::TyCtxt;
 
@@ -16,7 +16,7 @@ impl<'tcx> crate::MirPass<'tcx> for MultipleReturnTerminators {
 
     fn run_pass(&self, _: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
         // find basic blocks with no statement and a return terminator
-        let mut bbs_simple_returns = BitSet::new_empty(body.basic_blocks.len());
+        let mut bbs_simple_returns = DenseBitSet::new_empty(body.basic_blocks.len());
         let bbs = body.basic_blocks_mut();
         for idx in bbs.indices() {
             if bbs[idx].statements.is_empty()
diff --git a/compiler/rustc_mir_transform/src/nrvo.rs b/compiler/rustc_mir_transform/src/nrvo.rs
index cd026ed6806..35872de3852 100644
--- a/compiler/rustc_mir_transform/src/nrvo.rs
+++ b/compiler/rustc_mir_transform/src/nrvo.rs
@@ -1,7 +1,7 @@
 //! See the docs for [`RenameReturnPlace`].
 
 use rustc_hir::Mutability;
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::bug;
 use rustc_middle::mir::visit::{MutVisitor, NonUseContext, PlaceContext, Visitor};
 use rustc_middle::mir::{self, BasicBlock, Local, Location};
@@ -116,7 +116,7 @@ fn local_eligible_for_nrvo(body: &mir::Body<'_>) -> Option<Local> {
 
 fn find_local_assigned_to_return_place(start: BasicBlock, body: &mir::Body<'_>) -> Option<Local> {
     let mut block = start;
-    let mut seen = BitSet::new_empty(body.basic_blocks.len());
+    let mut seen = DenseBitSet::new_empty(body.basic_blocks.len());
 
     // Iterate as long as `block` has exactly one predecessor that we have not yet visited.
     while seen.insert(block) {
diff --git a/compiler/rustc_mir_transform/src/prettify.rs b/compiler/rustc_mir_transform/src/prettify.rs
index 937c207776b..51abd4da86e 100644
--- a/compiler/rustc_mir_transform/src/prettify.rs
+++ b/compiler/rustc_mir_transform/src/prettify.rs
@@ -4,7 +4,7 @@
 //! (`-Zmir-enable-passes=+ReorderBasicBlocks,+ReorderLocals`)
 //! to make the MIR easier to read for humans.
 
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_index::{IndexSlice, IndexVec};
 use rustc_middle::mir::visit::{MutVisitor, PlaceContext, Visitor};
 use rustc_middle::mir::*;
@@ -51,8 +51,10 @@ impl<'tcx> crate::MirPass<'tcx> for ReorderLocals {
     }
 
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
-        let mut finder =
-            LocalFinder { map: IndexVec::new(), seen: BitSet::new_empty(body.local_decls.len()) };
+        let mut finder = LocalFinder {
+            map: IndexVec::new(),
+            seen: DenseBitSet::new_empty(body.local_decls.len()),
+        };
 
         // We can't reorder the return place or the arguments
         for local in (0..=body.arg_count).map(Local::from_usize) {
@@ -113,7 +115,7 @@ impl<'tcx> MutVisitor<'tcx> for BasicBlockUpdater<'tcx> {
 
 struct LocalFinder {
     map: IndexVec<Local, Local>,
-    seen: BitSet<Local>,
+    seen: DenseBitSet<Local>,
 }
 
 impl LocalFinder {
diff --git a/compiler/rustc_mir_transform/src/ref_prop.rs b/compiler/rustc_mir_transform/src/ref_prop.rs
index 96bcdfa6fac..95b05f94270 100644
--- a/compiler/rustc_mir_transform/src/ref_prop.rs
+++ b/compiler/rustc_mir_transform/src/ref_prop.rs
@@ -2,7 +2,7 @@ use std::borrow::Cow;
 
 use rustc_data_structures::fx::FxHashSet;
 use rustc_index::IndexVec;
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::bug;
 use rustc_middle::mir::visit::*;
 use rustc_middle::mir::*;
@@ -132,7 +132,7 @@ fn compute_replacement<'tcx>(
     let mut targets = IndexVec::from_elem(Value::Unknown, &body.local_decls);
     // Set of locals for which we will remove their storage statement. This is useful for
     // reborrowed references.
-    let mut storage_to_remove = BitSet::new_empty(body.local_decls.len());
+    let mut storage_to_remove = DenseBitSet::new_empty(body.local_decls.len());
 
     let fully_replacable_locals = fully_replacable_locals(ssa);
 
@@ -324,8 +324,8 @@ fn compute_replacement<'tcx>(
 ///
 /// We consider a local to be replacable iff it's only used in a `Deref` projection `*_local` or
 /// non-use position (like storage statements and debuginfo).
-fn fully_replacable_locals(ssa: &SsaLocals) -> BitSet<Local> {
-    let mut replacable = BitSet::new_empty(ssa.num_locals());
+fn fully_replacable_locals(ssa: &SsaLocals) -> DenseBitSet<Local> {
+    let mut replacable = DenseBitSet::new_empty(ssa.num_locals());
 
     // First pass: for each local, whether its uses can be fully replaced.
     for local in ssa.locals() {
@@ -344,7 +344,7 @@ fn fully_replacable_locals(ssa: &SsaLocals) -> BitSet<Local> {
 struct Replacer<'tcx> {
     tcx: TyCtxt<'tcx>,
     targets: IndexVec<Local, Value<'tcx>>,
-    storage_to_remove: BitSet<Local>,
+    storage_to_remove: DenseBitSet<Local>,
     allowed_replacements: FxHashSet<(Local, Location)>,
     any_replacement: bool,
 }
diff --git a/compiler/rustc_mir_transform/src/remove_noop_landing_pads.rs b/compiler/rustc_mir_transform/src/remove_noop_landing_pads.rs
index fd49e956f43..76a3edfe0be 100644
--- a/compiler/rustc_mir_transform/src/remove_noop_landing_pads.rs
+++ b/compiler/rustc_mir_transform/src/remove_noop_landing_pads.rs
@@ -1,4 +1,4 @@
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::mir::patch::MirPatch;
 use rustc_middle::mir::*;
 use rustc_middle::ty::TyCtxt;
@@ -40,7 +40,7 @@ impl<'tcx> crate::MirPass<'tcx> for RemoveNoopLandingPads {
 
         let mut jumps_folded = 0;
         let mut landing_pads_removed = 0;
-        let mut nop_landing_pads = BitSet::new_empty(body.basic_blocks.len());
+        let mut nop_landing_pads = DenseBitSet::new_empty(body.basic_blocks.len());
 
         // This is a post-order traversal, so that if A post-dominates B
         // then A will be visited before B.
@@ -81,7 +81,7 @@ impl RemoveNoopLandingPads {
         &self,
         bb: BasicBlock,
         body: &Body<'_>,
-        nop_landing_pads: &BitSet<BasicBlock>,
+        nop_landing_pads: &DenseBitSet<BasicBlock>,
     ) -> bool {
         for stmt in &body[bb].statements {
             match &stmt.kind {
diff --git a/compiler/rustc_mir_transform/src/single_use_consts.rs b/compiler/rustc_mir_transform/src/single_use_consts.rs
index 277a33c0311..10b3c0ae94f 100644
--- a/compiler/rustc_mir_transform/src/single_use_consts.rs
+++ b/compiler/rustc_mir_transform/src/single_use_consts.rs
@@ -1,5 +1,5 @@
 use rustc_index::IndexVec;
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::bug;
 use rustc_middle::mir::visit::{MutVisitor, PlaceContext, Visitor};
 use rustc_middle::mir::*;
@@ -28,9 +28,9 @@ impl<'tcx> crate::MirPass<'tcx> for SingleUseConsts {
 
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
         let mut finder = SingleUseConstsFinder {
-            ineligible_locals: BitSet::new_empty(body.local_decls.len()),
+            ineligible_locals: DenseBitSet::new_empty(body.local_decls.len()),
             locations: IndexVec::from_elem(LocationPair::new(), &body.local_decls),
-            locals_in_debug_info: BitSet::new_empty(body.local_decls.len()),
+            locals_in_debug_info: DenseBitSet::new_empty(body.local_decls.len()),
         };
 
         finder.ineligible_locals.insert_range(..=Local::from_usize(body.arg_count));
@@ -96,9 +96,9 @@ impl LocationPair {
 }
 
 struct SingleUseConstsFinder {
-    ineligible_locals: BitSet<Local>,
+    ineligible_locals: DenseBitSet<Local>,
     locations: IndexVec<Local, LocationPair>,
-    locals_in_debug_info: BitSet<Local>,
+    locals_in_debug_info: DenseBitSet<Local>,
 }
 
 impl<'tcx> Visitor<'tcx> for SingleUseConstsFinder {
diff --git a/compiler/rustc_mir_transform/src/sroa.rs b/compiler/rustc_mir_transform/src/sroa.rs
index 52b9ec1e0a3..d54ea3feab6 100644
--- a/compiler/rustc_mir_transform/src/sroa.rs
+++ b/compiler/rustc_mir_transform/src/sroa.rs
@@ -2,7 +2,7 @@ use rustc_abi::{FIRST_VARIANT, FieldIdx};
 use rustc_data_structures::flat_map_in_place::FlatMapInPlace;
 use rustc_hir::LangItem;
 use rustc_index::IndexVec;
-use rustc_index::bit_set::{BitSet, GrowableBitSet};
+use rustc_index::bit_set::{DenseBitSet, GrowableBitSet};
 use rustc_middle::bug;
 use rustc_middle::mir::patch::MirPatch;
 use rustc_middle::mir::visit::*;
@@ -60,9 +60,9 @@ impl<'tcx> crate::MirPass<'tcx> for ScalarReplacementOfAggregates {
 fn escaping_locals<'tcx>(
     tcx: TyCtxt<'tcx>,
     typing_env: ty::TypingEnv<'tcx>,
-    excluded: &BitSet<Local>,
+    excluded: &DenseBitSet<Local>,
     body: &Body<'tcx>,
-) -> BitSet<Local> {
+) -> DenseBitSet<Local> {
     let is_excluded_ty = |ty: Ty<'tcx>| {
         if ty.is_union() || ty.is_enum() {
             return true;
@@ -97,7 +97,7 @@ fn escaping_locals<'tcx>(
         false
     };
 
-    let mut set = BitSet::new_empty(body.local_decls.len());
+    let mut set = DenseBitSet::new_empty(body.local_decls.len());
     set.insert_range(RETURN_PLACE..=Local::from_usize(body.arg_count));
     for (local, decl) in body.local_decls().iter_enumerated() {
         if excluded.contains(local) || is_excluded_ty(decl.ty) {
@@ -109,7 +109,7 @@ fn escaping_locals<'tcx>(
     return visitor.set;
 
     struct EscapeVisitor {
-        set: BitSet<Local>,
+        set: DenseBitSet<Local>,
     }
 
     impl<'tcx> Visitor<'tcx> for EscapeVisitor {
@@ -198,7 +198,7 @@ fn compute_flattening<'tcx>(
     tcx: TyCtxt<'tcx>,
     typing_env: ty::TypingEnv<'tcx>,
     body: &mut Body<'tcx>,
-    escaping: BitSet<Local>,
+    escaping: DenseBitSet<Local>,
 ) -> ReplacementMap<'tcx> {
     let mut fragments = IndexVec::from_elem(None, &body.local_decls);
 
@@ -226,8 +226,8 @@ fn replace_flattened_locals<'tcx>(
     tcx: TyCtxt<'tcx>,
     body: &mut Body<'tcx>,
     replacements: ReplacementMap<'tcx>,
-) -> BitSet<Local> {
-    let mut all_dead_locals = BitSet::new_empty(replacements.fragments.len());
+) -> DenseBitSet<Local> {
+    let mut all_dead_locals = DenseBitSet::new_empty(replacements.fragments.len());
     for (local, replacements) in replacements.fragments.iter_enumerated() {
         if replacements.is_some() {
             all_dead_locals.insert(local);
@@ -267,7 +267,7 @@ struct ReplacementVisitor<'tcx, 'll> {
     /// Work to do.
     replacements: &'ll ReplacementMap<'tcx>,
     /// This is used to check that we are not leaving references to replaced locals behind.
-    all_dead_locals: BitSet<Local>,
+    all_dead_locals: DenseBitSet<Local>,
     patch: MirPatch<'tcx>,
 }
 
diff --git a/compiler/rustc_mir_transform/src/ssa.rs b/compiler/rustc_mir_transform/src/ssa.rs
index 5653aef0aae..a24b3b2e80f 100644
--- a/compiler/rustc_mir_transform/src/ssa.rs
+++ b/compiler/rustc_mir_transform/src/ssa.rs
@@ -7,7 +7,7 @@
 //! of a `Freeze` local. Those can still be considered to be SSA.
 
 use rustc_data_structures::graph::dominators::Dominators;
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_index::{IndexSlice, IndexVec};
 use rustc_middle::bug;
 use rustc_middle::middle::resolve_bound_vars::Set1;
@@ -29,7 +29,7 @@ pub(super) struct SsaLocals {
     /// We ignore non-uses (Storage statements, debuginfo).
     direct_uses: IndexVec<Local, u32>,
     /// Set of SSA locals that are immutably borrowed.
-    borrowed_locals: BitSet<Local>,
+    borrowed_locals: DenseBitSet<Local>,
 }
 
 pub(super) enum AssignedValue<'a, 'tcx> {
@@ -50,7 +50,7 @@ impl SsaLocals {
         let dominators = body.basic_blocks.dominators();
 
         let direct_uses = IndexVec::from_elem(0, &body.local_decls);
-        let borrowed_locals = BitSet::new_empty(body.local_decls.len());
+        let borrowed_locals = DenseBitSet::new_empty(body.local_decls.len());
         let mut visitor = SsaVisitor {
             body,
             assignments,
@@ -202,12 +202,12 @@ impl SsaLocals {
     }
 
     /// Set of SSA locals that are immutably borrowed.
-    pub(super) fn borrowed_locals(&self) -> &BitSet<Local> {
+    pub(super) fn borrowed_locals(&self) -> &DenseBitSet<Local> {
         &self.borrowed_locals
     }
 
     /// Make a property uniform on a copy equivalence class by removing elements.
-    pub(super) fn meet_copy_equivalence(&self, property: &mut BitSet<Local>) {
+    pub(super) fn meet_copy_equivalence(&self, property: &mut DenseBitSet<Local>) {
         // Consolidate to have a local iff all its copies are.
         //
         // `copy_classes` defines equivalence classes between locals. The `local`s that recursively
@@ -241,7 +241,7 @@ struct SsaVisitor<'a, 'tcx> {
     assignment_order: Vec<Local>,
     direct_uses: IndexVec<Local, u32>,
     // Track locals that are immutably borrowed, so we can check their type is `Freeze` later.
-    borrowed_locals: BitSet<Local>,
+    borrowed_locals: DenseBitSet<Local>,
 }
 
 impl SsaVisitor<'_, '_> {
@@ -396,7 +396,7 @@ pub(crate) struct StorageLiveLocals {
 impl StorageLiveLocals {
     pub(crate) fn new(
         body: &Body<'_>,
-        always_storage_live_locals: &BitSet<Local>,
+        always_storage_live_locals: &DenseBitSet<Local>,
     ) -> StorageLiveLocals {
         let mut storage_live = IndexVec::from_elem(Set1::Empty, &body.local_decls);
         for local in always_storage_live_locals.iter() {
diff --git a/compiler/rustc_mir_transform/src/validate.rs b/compiler/rustc_mir_transform/src/validate.rs
index 035670d4903..414477d9004 100644
--- a/compiler/rustc_mir_transform/src/validate.rs
+++ b/compiler/rustc_mir_transform/src/validate.rs
@@ -5,7 +5,7 @@ use rustc_attr_parsing::InlineAttr;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_hir::LangItem;
 use rustc_index::IndexVec;
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_infer::infer::TyCtxtInferExt;
 use rustc_infer::traits::{Obligation, ObligationCause};
 use rustc_middle::mir::coverage::CoverageKind;
@@ -98,7 +98,7 @@ struct CfgChecker<'a, 'tcx> {
     body: &'a Body<'tcx>,
     tcx: TyCtxt<'tcx>,
     unwind_edge_count: usize,
-    reachable_blocks: BitSet<BasicBlock>,
+    reachable_blocks: DenseBitSet<BasicBlock>,
     value_cache: FxHashSet<u128>,
     // If `false`, then the MIR must not contain `UnwindAction::Continue` or
     // `TerminatorKind::Resume`.
diff --git a/compiler/rustc_pattern_analysis/src/constructor.rs b/compiler/rustc_pattern_analysis/src/constructor.rs
index 8fce4266345..4ce868f014f 100644
--- a/compiler/rustc_pattern_analysis/src/constructor.rs
+++ b/compiler/rustc_pattern_analysis/src/constructor.rs
@@ -182,7 +182,7 @@ use std::iter::once;
 
 use rustc_apfloat::ieee::{DoubleS, HalfS, IeeeFloat, QuadS, SingleS};
 use rustc_index::IndexVec;
-use rustc_index::bit_set::{BitSet, GrowableBitSet};
+use rustc_index::bit_set::{DenseBitSet, GrowableBitSet};
 use smallvec::SmallVec;
 
 use self::Constructor::*;
@@ -1072,7 +1072,7 @@ impl<Cx: PatCx> ConstructorSet<Cx> {
                 }
             }
             ConstructorSet::Variants { variants, non_exhaustive } => {
-                let mut seen_set = BitSet::new_empty(variants.len());
+                let mut seen_set = DenseBitSet::new_empty(variants.len());
                 for idx in seen.iter().filter_map(|c| c.as_variant()) {
                     seen_set.insert(idx);
                 }
diff --git a/compiler/rustc_pattern_analysis/src/usefulness.rs b/compiler/rustc_pattern_analysis/src/usefulness.rs
index 99261eaa95c..cc09cd491af 100644
--- a/compiler/rustc_pattern_analysis/src/usefulness.rs
+++ b/compiler/rustc_pattern_analysis/src/usefulness.rs
@@ -712,7 +712,7 @@ use std::fmt;
 #[cfg(feature = "rustc")]
 use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_hash::{FxHashMap, FxHashSet};
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use smallvec::{SmallVec, smallvec};
 use tracing::{debug, instrument};
 
@@ -1129,7 +1129,7 @@ struct MatrixRow<'p, Cx: PatCx> {
     /// ```
     /// Here the `(true, true)` case is irrelevant. Since we skip it, we will not detect that row 0
     /// intersects rows 1 and 2.
-    intersects_at_least: BitSet<usize>,
+    intersects_at_least: DenseBitSet<usize>,
     /// Whether the head pattern is a branch (see definition of "branch pattern" at
     /// [`BranchPatUsefulness`])
     head_is_branch: bool,
@@ -1142,7 +1142,7 @@ impl<'p, Cx: PatCx> MatrixRow<'p, Cx> {
             parent_row: arm_id,
             is_under_guard: arm.has_guard,
             useful: false,
-            intersects_at_least: BitSet::new_empty(0), // Initialized in `Matrix::push`.
+            intersects_at_least: DenseBitSet::new_empty(0), // Initialized in `Matrix::push`.
             // This pattern is a branch because it comes from a match arm.
             head_is_branch: true,
         }
@@ -1171,7 +1171,7 @@ impl<'p, Cx: PatCx> MatrixRow<'p, Cx> {
             parent_row,
             is_under_guard: self.is_under_guard,
             useful: false,
-            intersects_at_least: BitSet::new_empty(0), // Initialized in `Matrix::push`.
+            intersects_at_least: DenseBitSet::new_empty(0), // Initialized in `Matrix::push`.
             head_is_branch: is_or_pat,
         })
     }
@@ -1191,7 +1191,7 @@ impl<'p, Cx: PatCx> MatrixRow<'p, Cx> {
             parent_row,
             is_under_guard: self.is_under_guard,
             useful: false,
-            intersects_at_least: BitSet::new_empty(0), // Initialized in `Matrix::push`.
+            intersects_at_least: DenseBitSet::new_empty(0), // Initialized in `Matrix::push`.
             head_is_branch: false,
         })
     }
@@ -1230,7 +1230,7 @@ struct Matrix<'p, Cx: PatCx> {
 impl<'p, Cx: PatCx> Matrix<'p, Cx> {
     /// Pushes a new row to the matrix. Internal method, prefer [`Matrix::new`].
     fn push(&mut self, mut row: MatrixRow<'p, Cx>) {
-        row.intersects_at_least = BitSet::new_empty(self.rows.len());
+        row.intersects_at_least = DenseBitSet::new_empty(self.rows.len());
         self.rows.push(row);
     }
 
@@ -1824,7 +1824,7 @@ pub struct UsefulnessReport<'p, Cx: PatCx> {
     pub non_exhaustiveness_witnesses: Vec<WitnessPat<Cx>>,
     /// For each arm, a set of indices of arms above it that have non-empty intersection, i.e. there
     /// is a value matched by both arms. This may miss real intersections.
-    pub arm_intersections: Vec<BitSet<usize>>,
+    pub arm_intersections: Vec<DenseBitSet<usize>>,
 }
 
 /// Computes whether a match is exhaustive and which of its arms are useful.
diff --git a/compiler/rustc_ty_utils/src/layout.rs b/compiler/rustc_ty_utils/src/layout.rs
index fae787f9a40..ab606478c51 100644
--- a/compiler/rustc_ty_utils/src/layout.rs
+++ b/compiler/rustc_ty_utils/src/layout.rs
@@ -9,7 +9,7 @@ use rustc_abi::{
     HasDataLayout, Layout, LayoutCalculatorError, LayoutData, Niche, ReprOptions, Scalar, Size,
     StructKind, TagEncoding, VariantIdx, Variants, WrappingRange,
 };
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_index::{IndexSlice, IndexVec};
 use rustc_middle::bug;
 use rustc_middle::mir::{CoroutineLayout, CoroutineSavedLocal};
@@ -724,7 +724,7 @@ enum SavedLocalEligibility {
 /// Compute the eligibility and assignment of each local.
 fn coroutine_saved_local_eligibility(
     info: &CoroutineLayout<'_>,
-) -> (BitSet<CoroutineSavedLocal>, IndexVec<CoroutineSavedLocal, SavedLocalEligibility>) {
+) -> (DenseBitSet<CoroutineSavedLocal>, IndexVec<CoroutineSavedLocal, SavedLocalEligibility>) {
     use SavedLocalEligibility::*;
 
     let mut assignments: IndexVec<CoroutineSavedLocal, SavedLocalEligibility> =
@@ -732,7 +732,7 @@ fn coroutine_saved_local_eligibility(
 
     // The saved locals not eligible for overlap. These will get
     // "promoted" to the prefix of our coroutine.
-    let mut ineligible_locals = BitSet::new_empty(info.field_tys.len());
+    let mut ineligible_locals = DenseBitSet::new_empty(info.field_tys.len());
 
     // Figure out which of our saved locals are fields in only
     // one variant. The rest are deemed ineligible for overlap.
@@ -792,7 +792,7 @@ fn coroutine_saved_local_eligibility(
     // lay them out with the other locals in the prefix and eliminate
     // unnecessary padding bytes.
     {
-        let mut used_variants = BitSet::new_empty(info.variant_fields.len());
+        let mut used_variants = DenseBitSet::new_empty(info.variant_fields.len());
         for assignment in &assignments {
             if let Assigned(idx) = assignment {
                 used_variants.insert(*idx);
diff --git a/compiler/rustc_ty_utils/src/representability.rs b/compiler/rustc_ty_utils/src/representability.rs
index 0ffb7f62496..98b1550e1a3 100644
--- a/compiler/rustc_ty_utils/src/representability.rs
+++ b/compiler/rustc_ty_utils/src/representability.rs
@@ -1,5 +1,5 @@
 use rustc_hir::def::DefKind;
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::bug;
 use rustc_middle::query::Providers;
 use rustc_middle::ty::{self, Representability, Ty, TyCtxt};
@@ -83,10 +83,10 @@ fn representability_adt_ty<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> Representab
     Representability::Representable
 }
 
-fn params_in_repr(tcx: TyCtxt<'_>, def_id: LocalDefId) -> BitSet<u32> {
+fn params_in_repr(tcx: TyCtxt<'_>, def_id: LocalDefId) -> DenseBitSet<u32> {
     let adt_def = tcx.adt_def(def_id);
     let generics = tcx.generics_of(def_id);
-    let mut params_in_repr = BitSet::new_empty(generics.own_params.len());
+    let mut params_in_repr = DenseBitSet::new_empty(generics.own_params.len());
     for variant in adt_def.variants() {
         for field in variant.fields.iter() {
             params_in_repr_ty(
@@ -99,7 +99,7 @@ fn params_in_repr(tcx: TyCtxt<'_>, def_id: LocalDefId) -> BitSet<u32> {
     params_in_repr
 }
 
-fn params_in_repr_ty<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>, params_in_repr: &mut BitSet<u32>) {
+fn params_in_repr_ty<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>, params_in_repr: &mut DenseBitSet<u32>) {
     match *ty.kind() {
         ty::Adt(adt, args) => {
             let inner_params_in_repr = tcx.params_in_repr(adt.did());
diff --git a/compiler/rustc_ty_utils/src/ty.rs b/compiler/rustc_ty_utils/src/ty.rs
index 7eed32e3a33..8ed45b4e541 100644
--- a/compiler/rustc_ty_utils/src/ty.rs
+++ b/compiler/rustc_ty_utils/src/ty.rs
@@ -2,7 +2,7 @@ use rustc_data_structures::fx::FxHashSet;
 use rustc_hir as hir;
 use rustc_hir::LangItem;
 use rustc_hir::def::DefKind;
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::bug;
 use rustc_middle::query::Providers;
 use rustc_middle::ty::fold::fold_regions;
@@ -317,7 +317,7 @@ fn asyncness(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Asyncness {
     })
 }
 
-fn unsizing_params_for_adt<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId) -> BitSet<u32> {
+fn unsizing_params_for_adt<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId) -> DenseBitSet<u32> {
     let def = tcx.adt_def(def_id);
     let num_params = tcx.generics_of(def_id).count();
 
@@ -338,10 +338,10 @@ fn unsizing_params_for_adt<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId) -> BitSet<u32
 
     // The last field of the structure has to exist and contain type/const parameters.
     let Some((tail_field, prefix_fields)) = def.non_enum_variant().fields.raw.split_last() else {
-        return BitSet::new_empty(num_params);
+        return DenseBitSet::new_empty(num_params);
     };
 
-    let mut unsizing_params = BitSet::new_empty(num_params);
+    let mut unsizing_params = DenseBitSet::new_empty(num_params);
     for arg in tcx.type_of(tail_field.did).instantiate_identity().walk() {
         if let Some(i) = maybe_unsizing_param_idx(arg) {
             unsizing_params.insert(i);
diff --git a/compiler/rustc_type_ir/src/interner.rs b/compiler/rustc_type_ir/src/interner.rs
index 025ec7ae896..4fec606a831 100644
--- a/compiler/rustc_type_ir/src/interner.rs
+++ b/compiler/rustc_type_ir/src/interner.rs
@@ -3,7 +3,7 @@ use std::hash::Hash;
 use std::ops::Deref;
 
 use rustc_ast_ir::Movability;
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use smallvec::SmallVec;
 
 use crate::fold::TypeFoldable;
@@ -282,7 +282,7 @@ pub trait Interner:
     fn coroutine_is_gen(self, coroutine_def_id: Self::DefId) -> bool;
     fn coroutine_is_async_gen(self, coroutine_def_id: Self::DefId) -> bool;
 
-    type UnsizingParams: Deref<Target = BitSet<u32>>;
+    type UnsizingParams: Deref<Target = DenseBitSet<u32>>;
     fn unsizing_params_for_adt(self, adt_def_id: Self::DefId) -> Self::UnsizingParams;
 
     fn find_const_ty_from_env(