diff --git a/compiler/rustc_borrowck/src/lib.rs b/compiler/rustc_borrowck/src/lib.rs
index 9b7474c2208..d7db92da18f 100644
--- a/compiler/rustc_borrowck/src/lib.rs
+++ b/compiler/rustc_borrowck/src/lib.rs
@@ -26,7 +26,7 @@ use rustc_data_structures::graph::dominators::Dominators;
 use rustc_errors::Diag;
 use rustc_hir as hir;
 use rustc_hir::def_id::LocalDefId;
-use rustc_index::bit_set::{BitSet, ChunkedBitSet};
+use rustc_index::bit_set::{BitSet, MixedBitSet};
 use rustc_index::{IndexSlice, IndexVec};
 use rustc_infer::infer::{
     InferCtxt, NllRegionVariableOrigin, RegionVariableOrigin, TyCtxtInferExt,
@@ -1797,7 +1797,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, '_, 'tcx> {
         location: Location,
         desired_action: InitializationRequiringAction,
         place_span: (PlaceRef<'tcx>, Span),
-        maybe_uninits: &ChunkedBitSet<MovePathIndex>,
+        maybe_uninits: &MixedBitSet<MovePathIndex>,
         from: u64,
         to: u64,
     ) {
diff --git a/compiler/rustc_index/src/bit_set.rs b/compiler/rustc_index/src/bit_set.rs
index de6fa132ca0..aba1e938296 100644
--- a/compiler/rustc_index/src/bit_set.rs
+++ b/compiler/rustc_index/src/bit_set.rs
@@ -296,6 +296,111 @@ impl<T: Idx> From<GrowableBitSet<T>> for BitSet<T> {
     }
 }
 
+impl<T> Clone for BitSet<T> {
+    fn clone(&self) -> Self {
+        BitSet { domain_size: self.domain_size, words: self.words.clone(), marker: PhantomData }
+    }
+
+    fn clone_from(&mut self, from: &Self) {
+        self.domain_size = from.domain_size;
+        self.words.clone_from(&from.words);
+    }
+}
+
+impl<T: Idx> fmt::Debug for BitSet<T> {
+    fn fmt(&self, w: &mut fmt::Formatter<'_>) -> fmt::Result {
+        w.debug_list().entries(self.iter()).finish()
+    }
+}
+
+impl<T: Idx> ToString for BitSet<T> {
+    fn to_string(&self) -> String {
+        let mut result = String::new();
+        let mut sep = '[';
+
+        // Note: this is a little endian printout of bytes.
+
+        // i tracks how many bits we have printed so far.
+        let mut i = 0;
+        for word in &self.words {
+            let mut word = *word;
+            for _ in 0..WORD_BYTES {
+                // for each byte in `word`:
+                let remain = self.domain_size - i;
+                // If less than a byte remains, then mask just that many bits.
+                let mask = if remain <= 8 { (1 << remain) - 1 } else { 0xFF };
+                assert!(mask <= 0xFF);
+                let byte = word & mask;
+
+                result.push_str(&format!("{sep}{byte:02x}"));
+
+                if remain <= 8 {
+                    break;
+                }
+                word >>= 8;
+                i += 8;
+                sep = '-';
+            }
+            sep = '|';
+        }
+        result.push(']');
+
+        result
+    }
+}
+
+pub struct BitIter<'a, T: Idx> {
+    /// A copy of the current word, but with any already-visited bits cleared.
+    /// (This lets us use `trailing_zeros()` to find the next set bit.) When it
+    /// is reduced to 0, we move onto the next word.
+    word: Word,
+
+    /// The offset (measured in bits) of the current word.
+    offset: usize,
+
+    /// Underlying iterator over the words.
+    iter: slice::Iter<'a, Word>,
+
+    marker: PhantomData<T>,
+}
+
+impl<'a, T: Idx> BitIter<'a, T> {
+    #[inline]
+    fn new(words: &'a [Word]) -> BitIter<'a, T> {
+        // We initialize `word` and `offset` to degenerate values. On the first
+        // call to `next()` we will fall through to getting the first word from
+        // `iter`, which sets `word` to the first word (if there is one) and
+        // `offset` to 0. Doing it this way saves us from having to maintain
+        // additional state about whether we have started.
+        BitIter {
+            word: 0,
+            offset: usize::MAX - (WORD_BITS - 1),
+            iter: words.iter(),
+            marker: PhantomData,
+        }
+    }
+}
+
+impl<'a, T: Idx> Iterator for BitIter<'a, T> {
+    type Item = T;
+    fn next(&mut self) -> Option<T> {
+        loop {
+            if self.word != 0 {
+                // Get the position of the next set bit in the current word,
+                // then clear the bit.
+                let bit_pos = self.word.trailing_zeros() as usize;
+                self.word ^= 1 << bit_pos;
+                return Some(T::new(bit_pos + self.offset));
+            }
+
+            // Move onto the next word. `wrapping_add()` is needed to handle
+            // the degenerate initial value given to `offset` in `new()`.
+            self.word = *self.iter.next()?;
+            self.offset = self.offset.wrapping_add(WORD_BITS);
+        }
+    }
+}
+
 /// A fixed-size bitset type with a partially dense, partially sparse
 /// representation. The bitset is broken into chunks, and chunks that are all
 /// zeros or all ones are represented and handled very efficiently.
@@ -305,6 +410,9 @@ impl<T: Idx> From<GrowableBitSet<T>> for BitSet<T> {
 /// some stretches with lots of 0s and 1s mixed in a way that causes trouble
 /// for `IntervalSet`.
 ///
+/// Best used via `MixedBitSet`, rather than directly, because `MixedBitSet`
+/// has better performance for small bitsets.
+///
 /// `T` is an index type, typically a newtyped `usize` wrapper, but it can also
 /// just be `usize`.
 ///
@@ -958,117 +1066,12 @@ fn sequential_update<T: Idx>(
     it.fold(false, |changed, elem| self_update(elem) | changed)
 }
 
-impl<T> Clone for BitSet<T> {
-    fn clone(&self) -> Self {
-        BitSet { domain_size: self.domain_size, words: self.words.clone(), marker: PhantomData }
-    }
-
-    fn clone_from(&mut self, from: &Self) {
-        self.domain_size = from.domain_size;
-        self.words.clone_from(&from.words);
-    }
-}
-
-impl<T: Idx> fmt::Debug for BitSet<T> {
-    fn fmt(&self, w: &mut fmt::Formatter<'_>) -> fmt::Result {
-        w.debug_list().entries(self.iter()).finish()
-    }
-}
-
 impl<T: Idx> fmt::Debug for ChunkedBitSet<T> {
     fn fmt(&self, w: &mut fmt::Formatter<'_>) -> fmt::Result {
         w.debug_list().entries(self.iter()).finish()
     }
 }
 
-impl<T: Idx> ToString for BitSet<T> {
-    fn to_string(&self) -> String {
-        let mut result = String::new();
-        let mut sep = '[';
-
-        // Note: this is a little endian printout of bytes.
-
-        // i tracks how many bits we have printed so far.
-        let mut i = 0;
-        for word in &self.words {
-            let mut word = *word;
-            for _ in 0..WORD_BYTES {
-                // for each byte in `word`:
-                let remain = self.domain_size - i;
-                // If less than a byte remains, then mask just that many bits.
-                let mask = if remain <= 8 { (1 << remain) - 1 } else { 0xFF };
-                assert!(mask <= 0xFF);
-                let byte = word & mask;
-
-                result.push_str(&format!("{sep}{byte:02x}"));
-
-                if remain <= 8 {
-                    break;
-                }
-                word >>= 8;
-                i += 8;
-                sep = '-';
-            }
-            sep = '|';
-        }
-        result.push(']');
-
-        result
-    }
-}
-
-pub struct BitIter<'a, T: Idx> {
-    /// A copy of the current word, but with any already-visited bits cleared.
-    /// (This lets us use `trailing_zeros()` to find the next set bit.) When it
-    /// is reduced to 0, we move onto the next word.
-    word: Word,
-
-    /// The offset (measured in bits) of the current word.
-    offset: usize,
-
-    /// Underlying iterator over the words.
-    iter: slice::Iter<'a, Word>,
-
-    marker: PhantomData<T>,
-}
-
-impl<'a, T: Idx> BitIter<'a, T> {
-    #[inline]
-    fn new(words: &'a [Word]) -> BitIter<'a, T> {
-        // We initialize `word` and `offset` to degenerate values. On the first
-        // call to `next()` we will fall through to getting the first word from
-        // `iter`, which sets `word` to the first word (if there is one) and
-        // `offset` to 0. Doing it this way saves us from having to maintain
-        // additional state about whether we have started.
-        BitIter {
-            word: 0,
-            offset: usize::MAX - (WORD_BITS - 1),
-            iter: words.iter(),
-            marker: PhantomData,
-        }
-    }
-}
-
-impl<'a, T: Idx> Iterator for BitIter<'a, T> {
-    type Item = T;
-    fn next(&mut self) -> Option<T> {
-        loop {
-            if self.word != 0 {
-                // Get the position of the next set bit in the current word,
-                // then clear the bit.
-                let bit_pos = self.word.trailing_zeros() as usize;
-                self.word ^= 1 << bit_pos;
-                return Some(T::new(bit_pos + self.offset));
-            }
-
-            // Move onto the next word. `wrapping_add()` is needed to handle
-            // the degenerate initial value given to `offset` in `new()`.
-            self.word = *self.iter.next()?;
-            self.offset = self.offset.wrapping_add(WORD_BITS);
-        }
-    }
-}
-
 #[inline]
 fn bitwise<Op>(out_vec: &mut [Word], in_vec: &[Word], op: Op) -> bool
 where
@@ -1106,6 +1109,158 @@ 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+).
+///
+/// `T` is an index type, typically a newtyped `usize` wrapper, but it can also
+/// just be `usize`.
+///
+/// All operations that involve an element will panic if the element is equal
+/// to or greater than the domain size. All operations that involve two bitsets
+/// will panic if the bitsets have differing domain sizes.
+#[derive(PartialEq, Eq)]
+pub enum MixedBitSet<T> {
+    Small(BitSet<T>),
+    Large(ChunkedBitSet<T>),
+}
+
+impl<T> MixedBitSet<T> {
+    pub fn domain_size(&self) -> usize {
+        match self {
+            MixedBitSet::Small(set) => set.domain_size(),
+            MixedBitSet::Large(set) => set.domain_size(),
+        }
+    }
+}
+
+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))
+        } else {
+            MixedBitSet::Large(ChunkedBitSet::new_empty(domain_size))
+        }
+    }
+
+    #[inline]
+    pub fn is_empty(&self) -> bool {
+        match self {
+            MixedBitSet::Small(set) => set.is_empty(),
+            MixedBitSet::Large(set) => set.is_empty(),
+        }
+    }
+
+    #[inline]
+    pub fn contains(&self, elem: T) -> bool {
+        match self {
+            MixedBitSet::Small(set) => set.contains(elem),
+            MixedBitSet::Large(set) => set.contains(elem),
+        }
+    }
+
+    #[inline]
+    pub fn insert(&mut self, elem: T) -> bool {
+        match self {
+            MixedBitSet::Small(set) => set.insert(elem),
+            MixedBitSet::Large(set) => set.insert(elem),
+        }
+    }
+
+    pub fn insert_all(&mut self) {
+        match self {
+            MixedBitSet::Small(set) => set.insert_all(),
+            MixedBitSet::Large(set) => set.insert_all(),
+        }
+    }
+
+    #[inline]
+    pub fn remove(&mut self, elem: T) -> bool {
+        match self {
+            MixedBitSet::Small(set) => set.remove(elem),
+            MixedBitSet::Large(set) => set.remove(elem),
+        }
+    }
+
+    pub fn iter(&self) -> MixedBitIter<'_, T> {
+        match self {
+            MixedBitSet::Small(set) => MixedBitIter::Small(set.iter()),
+            MixedBitSet::Large(set) => MixedBitIter::Large(set.iter()),
+        }
+    }
+
+    bit_relations_inherent_impls! {}
+}
+
+impl<T> Clone for MixedBitSet<T> {
+    fn clone(&self) -> Self {
+        match self {
+            MixedBitSet::Small(set) => MixedBitSet::Small(set.clone()),
+            MixedBitSet::Large(set) => MixedBitSet::Large(set.clone()),
+        }
+    }
+
+    /// WARNING: this implementation of clone_from may panic if the two
+    /// bitsets have different domain sizes. This constraint is not inherent to
+    /// `clone_from`, but it works with the existing call sites and allows a
+    /// faster implementation, which is important because this function is hot.
+    fn clone_from(&mut self, from: &Self) {
+        match (self, from) {
+            (MixedBitSet::Small(set), MixedBitSet::Small(from)) => set.clone_from(from),
+            (MixedBitSet::Large(set), MixedBitSet::Large(from)) => set.clone_from(from),
+            _ => panic!("MixedBitSet size mismatch"),
+        }
+    }
+}
+
+impl<T: Idx> BitRelations<MixedBitSet<T>> for MixedBitSet<T> {
+    fn union(&mut self, other: &MixedBitSet<T>) -> bool {
+        match (self, other) {
+            (MixedBitSet::Small(set), MixedBitSet::Small(other)) => set.union(other),
+            (MixedBitSet::Large(set), MixedBitSet::Large(other)) => set.union(other),
+            _ => panic!("MixedBitSet size mismatch"),
+        }
+    }
+
+    fn subtract(&mut self, other: &MixedBitSet<T>) -> bool {
+        match (self, other) {
+            (MixedBitSet::Small(set), MixedBitSet::Small(other)) => set.subtract(other),
+            (MixedBitSet::Large(set), MixedBitSet::Large(other)) => set.subtract(other),
+            _ => panic!("MixedBitSet size mismatch"),
+        }
+    }
+
+    fn intersect(&mut self, _other: &MixedBitSet<T>) -> bool {
+        unimplemented!("implement if/when necessary");
+    }
+}
+
+impl<T: Idx> fmt::Debug for MixedBitSet<T> {
+    fn fmt(&self, w: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            MixedBitSet::Small(set) => set.fmt(w),
+            MixedBitSet::Large(set) => set.fmt(w),
+        }
+    }
+}
+
+pub enum MixedBitIter<'a, T: Idx> {
+    Small(BitIter<'a, T>),
+    Large(ChunkedBitIter<'a, T>),
+}
+
+impl<'a, T: Idx> Iterator for MixedBitIter<'a, T> {
+    type Item = T;
+    fn next(&mut self) -> Option<T> {
+        match self {
+            MixedBitIter::Small(iter) => iter.next(),
+            MixedBitIter::Large(iter) => iter.next(),
+        }
+    }
+}
+
 /// A resizable bitset type with a dense representation.
 ///
 /// `T` is an index type, typically a newtyped `usize` wrapper, but it can also
@@ -1374,7 +1529,7 @@ impl<R: Idx, C: Idx> fmt::Debug for BitMatrix<R, C> {
 /// sparse representation.
 ///
 /// Initially, every row has no explicit representation. If any bit within a
-/// row is set, the entire row is instantiated as `Some(<ChunkedBitSet>)`.
+/// row is set, the entire row is instantiated as `Some(<BitSet>)`.
 /// 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.
@@ -1388,7 +1543,7 @@ where
     C: Idx,
 {
     num_columns: usize,
-    rows: IndexVec<R, Option<ChunkedBitSet<C>>>,
+    rows: IndexVec<R, Option<BitSet<C>>>,
 }
 
 impl<R: Idx, C: Idx> SparseBitMatrix<R, C> {
@@ -1397,10 +1552,10 @@ impl<R: Idx, C: Idx> SparseBitMatrix<R, C> {
         Self { num_columns, rows: IndexVec::new() }
     }
 
-    fn ensure_row(&mut self, row: R) -> &mut ChunkedBitSet<C> {
-        // Instantiate any missing rows up to and including row `row` with an empty ChunkedBitSet.
-        // Then replace row `row` with a full ChunkedBitSet if necessary.
-        self.rows.get_or_insert_with(row, || ChunkedBitSet::new_empty(self.num_columns))
+    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))
     }
 
     /// Sets the cell at `(row, column)` to true. Put another way, insert
@@ -1474,7 +1629,7 @@ 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<&ChunkedBitSet<C>> {
+    pub fn row(&self, row: R) -> Option<&BitSet<C>> {
         self.rows.get(row)?.as_ref()
     }
 
@@ -1484,7 +1639,7 @@ impl<R: Idx, C: Idx> SparseBitMatrix<R, C> {
     /// Returns true if the row was changed.
     pub fn intersect_row<Set>(&mut self, row: R, set: &Set) -> bool
     where
-        ChunkedBitSet<C>: BitRelations<Set>,
+        BitSet<C>: BitRelations<Set>,
     {
         match self.rows.get_mut(row) {
             Some(Some(row)) => row.intersect(set),
@@ -1498,7 +1653,7 @@ impl<R: Idx, C: Idx> SparseBitMatrix<R, C> {
     /// Returns true if the row was changed.
     pub fn subtract_row<Set>(&mut self, row: R, set: &Set) -> bool
     where
-        ChunkedBitSet<C>: BitRelations<Set>,
+        BitSet<C>: BitRelations<Set>,
     {
         match self.rows.get_mut(row) {
             Some(Some(row)) => row.subtract(set),
@@ -1512,7 +1667,7 @@ impl<R: Idx, C: Idx> SparseBitMatrix<R, C> {
     /// Returns true if the row was changed.
     pub fn union_row<Set>(&mut self, row: R, set: &Set) -> bool
     where
-        ChunkedBitSet<C>: BitRelations<Set>,
+        BitSet<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 3f9198ce37f..f6142323979 100644
--- a/compiler/rustc_index/src/bit_set/tests.rs
+++ b/compiler/rustc_index/src/bit_set/tests.rs
@@ -503,15 +503,15 @@ fn sparse_matrix_operations() {
     matrix.insert(2, 99);
     matrix.insert(4, 0);
 
-    let mut disjoint: ChunkedBitSet<usize> = ChunkedBitSet::new_empty(100);
+    let mut disjoint: BitSet<usize> = BitSet::new_empty(100);
     disjoint.insert(33);
 
-    let mut superset = ChunkedBitSet::new_empty(100);
+    let mut superset = BitSet::new_empty(100);
     superset.insert(22);
     superset.insert(75);
     superset.insert(33);
 
-    let mut subset = ChunkedBitSet::new_empty(100);
+    let mut subset = BitSet::new_empty(100);
     subset.insert(22);
 
     // SparseBitMatrix::remove
diff --git a/compiler/rustc_mir_dataflow/src/framework/fmt.rs b/compiler/rustc_mir_dataflow/src/framework/fmt.rs
index dc176ba2d03..faf2c411dde 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};
+use rustc_index::bit_set::{BitSet, ChunkedBitSet, MixedBitSet};
 
 use super::lattice::MaybeReachable;
 
@@ -85,8 +85,8 @@ where
         let size = self.domain_size();
         assert_eq!(size, old.domain_size());
 
-        let mut set_in_self = ChunkedBitSet::new_empty(size);
-        let mut cleared_in_self = ChunkedBitSet::new_empty(size);
+        let mut set_in_self = MixedBitSet::new_empty(size);
+        let mut cleared_in_self = MixedBitSet::new_empty(size);
 
         for i in (0..size).map(T::new) {
             match (self.contains(i), old.contains(i)) {
@@ -112,8 +112,8 @@ where
         let size = self.domain_size();
         assert_eq!(size, old.domain_size());
 
-        let mut set_in_self = ChunkedBitSet::new_empty(size);
-        let mut cleared_in_self = ChunkedBitSet::new_empty(size);
+        let mut set_in_self = MixedBitSet::new_empty(size);
+        let mut cleared_in_self = MixedBitSet::new_empty(size);
 
         for i in (0..size).map(T::new) {
             match (self.contains(i), old.contains(i)) {
@@ -127,6 +127,26 @@ where
     }
 }
 
+impl<T, C> DebugWithContext<C> for MixedBitSet<T>
+where
+    T: Idx + DebugWithContext<C>,
+{
+    fn fmt_with(&self, ctxt: &C, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            MixedBitSet::Small(set) => set.fmt_with(ctxt, f),
+            MixedBitSet::Large(set) => set.fmt_with(ctxt, f),
+        }
+    }
+
+    fn fmt_diff_with(&self, old: &Self, ctxt: &C, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match (self, old) {
+            (MixedBitSet::Small(set), MixedBitSet::Small(old)) => set.fmt_diff_with(old, ctxt, f),
+            (MixedBitSet::Large(set), MixedBitSet::Large(old)) => set.fmt_diff_with(old, ctxt, f),
+            _ => panic!("MixedBitSet size mismatch"),
+        }
+    }
+}
+
 impl<S, C> DebugWithContext<C> for MaybeReachable<S>
 where
     S: DebugWithContext<C>,
@@ -159,8 +179,8 @@ where
 }
 
 fn fmt_diff<T, C>(
-    inserted: &ChunkedBitSet<T>,
-    removed: &ChunkedBitSet<T>,
+    inserted: &MixedBitSet<T>,
+    removed: &MixedBitSet<T>,
     ctxt: &C,
     f: &mut fmt::Formatter<'_>,
 ) -> fmt::Result
diff --git a/compiler/rustc_mir_dataflow/src/framework/lattice.rs b/compiler/rustc_mir_dataflow/src/framework/lattice.rs
index e2b56aedca3..e063eaf74bd 100644
--- a/compiler/rustc_mir_dataflow/src/framework/lattice.rs
+++ b/compiler/rustc_mir_dataflow/src/framework/lattice.rs
@@ -40,7 +40,7 @@
 
 use std::iter;
 
-use rustc_index::bit_set::{BitSet, ChunkedBitSet};
+use rustc_index::bit_set::{BitSet, MixedBitSet};
 use rustc_index::{Idx, IndexVec};
 
 use crate::framework::BitSetExt;
@@ -126,7 +126,7 @@ impl<T: Idx> JoinSemiLattice for BitSet<T> {
     }
 }
 
-impl<T: Idx> JoinSemiLattice for ChunkedBitSet<T> {
+impl<T: Idx> JoinSemiLattice for MixedBitSet<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 b9407882ec5..caff2a81ff3 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, ChunkedBitSet};
+use rustc_index::bit_set::{BitSet, MixedBitSet};
 use rustc_index::{Idx, IndexVec};
 use rustc_middle::bug;
 use rustc_middle::mir::{self, BasicBlock, CallReturnPlaces, Location, TerminatorEdges, traversal};
@@ -71,7 +71,7 @@ impl<T: Idx> BitSetExt<T> for BitSet<T> {
     }
 }
 
-impl<T: Idx> BitSetExt<T> for ChunkedBitSet<T> {
+impl<T: Idx> BitSetExt<T> for MixedBitSet<T> {
     fn contains(&self, elem: T) -> bool {
         self.contains(elem)
     }
@@ -327,7 +327,7 @@ impl<T: Idx> GenKill<T> for BitSet<T> {
     }
 }
 
-impl<T: Idx> GenKill<T> for ChunkedBitSet<T> {
+impl<T: Idx> GenKill<T> for MixedBitSet<T> {
     fn gen_(&mut self, elem: T) {
         self.insert(elem);
     }
diff --git a/compiler/rustc_mir_dataflow/src/impls/initialized.rs b/compiler/rustc_mir_dataflow/src/impls/initialized.rs
index 2c10d4b1cd3..91677657602 100644
--- a/compiler/rustc_mir_dataflow/src/impls/initialized.rs
+++ b/compiler/rustc_mir_dataflow/src/impls/initialized.rs
@@ -1,7 +1,7 @@
 use std::assert_matches::assert_matches;
 
 use rustc_index::Idx;
-use rustc_index::bit_set::{BitSet, ChunkedBitSet};
+use rustc_index::bit_set::{BitSet, MixedBitSet};
 use rustc_middle::bug;
 use rustc_middle::mir::{self, Body, CallReturnPlaces, Location, TerminatorEdges};
 use rustc_middle::ty::{self, TyCtxt};
@@ -70,7 +70,7 @@ impl<'a, 'tcx> MaybeInitializedPlaces<'a, 'tcx> {
     pub fn is_unwind_dead(
         &self,
         place: mir::Place<'tcx>,
-        state: &MaybeReachable<ChunkedBitSet<MovePathIndex>>,
+        state: &MaybeReachable<MixedBitSet<MovePathIndex>>,
     ) -> bool {
         if let LookupResult::Exact(path) = self.move_data().rev_lookup.find(place.as_ref()) {
             let mut maybe_live = false;
@@ -244,8 +244,8 @@ impl<'tcx> MaybeUninitializedPlaces<'_, 'tcx> {
 
 impl<'tcx> Analysis<'tcx> for MaybeInitializedPlaces<'_, 'tcx> {
     /// There can be many more `MovePathIndex` than there are locals in a MIR body.
-    /// We use a chunked bitset to avoid paying too high a memory footprint.
-    type Domain = MaybeReachable<ChunkedBitSet<MovePathIndex>>;
+    /// We use a mixed bitset to avoid paying too high a memory footprint.
+    type Domain = MaybeReachable<MixedBitSet<MovePathIndex>>;
 
     const NAME: &'static str = "maybe_init";
 
@@ -256,7 +256,7 @@ impl<'tcx> Analysis<'tcx> for MaybeInitializedPlaces<'_, 'tcx> {
 
     fn initialize_start_block(&self, _: &mir::Body<'tcx>, state: &mut Self::Domain) {
         *state =
-            MaybeReachable::Reachable(ChunkedBitSet::new_empty(self.move_data().move_paths.len()));
+            MaybeReachable::Reachable(MixedBitSet::new_empty(self.move_data().move_paths.len()));
         drop_flag_effects_for_function_entry(self.body, self.move_data, |path, s| {
             assert!(s == DropFlagState::Present);
             state.gen_(path);
@@ -371,14 +371,14 @@ impl<'tcx> Analysis<'tcx> for MaybeInitializedPlaces<'_, 'tcx> {
 
 impl<'tcx> Analysis<'tcx> for MaybeUninitializedPlaces<'_, 'tcx> {
     /// There can be many more `MovePathIndex` than there are locals in a MIR body.
-    /// We use a chunked bitset to avoid paying too high a memory footprint.
-    type Domain = ChunkedBitSet<MovePathIndex>;
+    /// We use a mixed bitset to avoid paying too high a memory footprint.
+    type Domain = MixedBitSet<MovePathIndex>;
 
     const NAME: &'static str = "maybe_uninit";
 
     fn bottom_value(&self, _: &mir::Body<'tcx>) -> Self::Domain {
         // bottom = initialized (start_block_effect counters this at outset)
-        ChunkedBitSet::new_empty(self.move_data().move_paths.len())
+        MixedBitSet::new_empty(self.move_data().move_paths.len())
     }
 
     // sets on_entry bits for Arg places
@@ -492,14 +492,14 @@ impl<'tcx> Analysis<'tcx> for MaybeUninitializedPlaces<'_, 'tcx> {
 
 impl<'tcx> Analysis<'tcx> for EverInitializedPlaces<'_, 'tcx> {
     /// There can be many more `InitIndex` than there are locals in a MIR body.
-    /// We use a chunked bitset to avoid paying too high a memory footprint.
-    type Domain = ChunkedBitSet<InitIndex>;
+    /// We use a mixed bitset to avoid paying too high a memory footprint.
+    type Domain = MixedBitSet<InitIndex>;
 
     const NAME: &'static str = "ever_init";
 
     fn bottom_value(&self, _: &mir::Body<'tcx>) -> Self::Domain {
         // bottom = no initialized variables by default
-        ChunkedBitSet::new_empty(self.move_data().inits.len())
+        MixedBitSet::new_empty(self.move_data().inits.len())
     }
 
     fn initialize_start_block(&self, body: &mir::Body<'tcx>, state: &mut Self::Domain) {
diff --git a/compiler/rustc_mir_transform/src/lint_tail_expr_drop_order.rs b/compiler/rustc_mir_transform/src/lint_tail_expr_drop_order.rs
index 732a9cd9890..7fb421dea0c 100644
--- a/compiler/rustc_mir_transform/src/lint_tail_expr_drop_order.rs
+++ b/compiler/rustc_mir_transform/src/lint_tail_expr_drop_order.rs
@@ -7,7 +7,7 @@ use rustc_data_structures::unord::{UnordMap, UnordSet};
 use rustc_errors::Subdiagnostic;
 use rustc_hir::CRATE_HIR_ID;
 use rustc_hir::def_id::{DefId, LocalDefId};
-use rustc_index::bit_set::ChunkedBitSet;
+use rustc_index::bit_set::MixedBitSet;
 use rustc_index::{IndexSlice, IndexVec};
 use rustc_macros::{LintDiagnostic, Subdiagnostic};
 use rustc_middle::bug;
@@ -49,24 +49,24 @@ struct DropsReachable<'a, 'mir, 'tcx> {
     move_data: &'a MoveData<'tcx>,
     maybe_init: &'a mut ResultsCursor<'mir, 'tcx, MaybeInitializedPlaces<'mir, 'tcx>>,
     block_drop_value_info: &'a mut IndexSlice<BasicBlock, MovePathIndexAtBlock>,
-    collected_drops: &'a mut ChunkedBitSet<MovePathIndex>,
-    visited: FxHashMap<BasicBlock, Rc<RefCell<ChunkedBitSet<MovePathIndex>>>>,
+    collected_drops: &'a mut MixedBitSet<MovePathIndex>,
+    visited: FxHashMap<BasicBlock, Rc<RefCell<MixedBitSet<MovePathIndex>>>>,
 }
 
 impl<'a, 'mir, 'tcx> DropsReachable<'a, 'mir, 'tcx> {
     fn visit(&mut self, block: BasicBlock) {
         let move_set_size = self.move_data.move_paths.len();
-        let make_new_path_set = || Rc::new(RefCell::new(ChunkedBitSet::new_empty(move_set_size)));
+        let make_new_path_set = || Rc::new(RefCell::new(MixedBitSet::new_empty(move_set_size)));
 
         let data = &self.body.basic_blocks[block];
         let Some(terminator) = &data.terminator else { return };
-        // Given that we observe these dropped locals here at `block` so far,
-        // we will try to update the successor blocks.
-        // An occupied entry at `block` in `self.visited` signals that we have visited `block` before.
+        // Given that we observe these dropped locals here at `block` so far, we will try to update
+        // the successor blocks. An occupied entry at `block` in `self.visited` signals that we
+        // have visited `block` before.
         let dropped_local_here =
             Rc::clone(self.visited.entry(block).or_insert_with(make_new_path_set));
-        // We could have invoked reverse lookup for a `MovePathIndex` every time, but unfortunately it is expensive.
-        // Let's cache them in `self.block_drop_value_info`.
+        // We could have invoked reverse lookup for a `MovePathIndex` every time, but unfortunately
+        // it is expensive. Let's cache them in `self.block_drop_value_info`.
         match self.block_drop_value_info[block] {
             MovePathIndexAtBlock::Some(dropped) => {
                 dropped_local_here.borrow_mut().insert(dropped);
@@ -76,23 +76,24 @@ impl<'a, 'mir, 'tcx> DropsReachable<'a, 'mir, 'tcx> {
                     && let LookupResult::Exact(idx) | LookupResult::Parent(Some(idx)) =
                         self.move_data.rev_lookup.find(place.as_ref())
                 {
-                    // Since we are working with MIRs at a very early stage,
-                    // observing a `drop` terminator is not indicative enough that
-                    // the drop will definitely happen.
-                    // That is decided in the drop elaboration pass instead.
-                    // Therefore, we need to consult with the maybe-initialization information.
+                    // Since we are working with MIRs at a very early stage, observing a `drop`
+                    // terminator is not indicative enough that the drop will definitely happen.
+                    // That is decided in the drop elaboration pass instead. Therefore, we need to
+                    // consult with the maybe-initialization information.
                     self.maybe_init.seek_before_primary_effect(Location {
                         block,
                         statement_index: data.statements.len(),
                     });
 
-                    // Check if the drop of `place` under inspection is really in effect.
-                    // This is true only when `place` may have been initialized along a control flow path from a BID to the drop program point today.
-                    // In other words, this is where the drop of `place` will happen in the future instead.
+                    // Check if the drop of `place` under inspection is really in effect. This is
+                    // true only when `place` may have been initialized along a control flow path
+                    // from a BID to the drop program point today. In other words, this is where
+                    // the drop of `place` will happen in the future instead.
                     if let MaybeReachable::Reachable(maybe_init) = self.maybe_init.get()
                         && maybe_init.contains(idx)
                     {
-                        // We also cache the drop information, so that we do not need to check on data-flow cursor again
+                        // We also cache the drop information, so that we do not need to check on
+                        // data-flow cursor again.
                         self.block_drop_value_info[block] = MovePathIndexAtBlock::Some(idx);
                         dropped_local_here.borrow_mut().insert(idx);
                     } else {
@@ -139,8 +140,9 @@ impl<'a, 'mir, 'tcx> DropsReachable<'a, 'mir, 'tcx> {
                 // Let's check the observed dropped places in.
                 self.collected_drops.union(&*dropped_local_there.borrow());
                 if self.drop_span.is_none() {
-                    // FIXME(@dingxiangfei2009): it turns out that `self.body.source_scopes` are still a bit wonky.
-                    // There is a high chance that this span still points to a block rather than a statement semicolon.
+                    // FIXME(@dingxiangfei2009): it turns out that `self.body.source_scopes` are
+                    // still a bit wonky. There is a high chance that this span still points to a
+                    // block rather than a statement semicolon.
                     *self.drop_span = Some(terminator.source_info.span);
                 }
                 // Now we have discovered a simple control flow path from a future drop point
@@ -394,10 +396,10 @@ pub(crate) fn run_lint<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId, body: &Body<
     for (&block, candidates) in &bid_per_block {
         // We will collect drops on locals on paths between BID points to their actual drop locations
         // into `all_locals_dropped`.
-        let mut all_locals_dropped = ChunkedBitSet::new_empty(move_data.move_paths.len());
+        let mut all_locals_dropped = MixedBitSet::new_empty(move_data.move_paths.len());
         let mut drop_span = None;
         for &(_, place) in candidates.iter() {
-            let mut collected_drops = ChunkedBitSet::new_empty(move_data.move_paths.len());
+            let mut collected_drops = MixedBitSet::new_empty(move_data.move_paths.len());
             // ## On detecting change in relative drop order ##
             // Iterate through each BID-containing block `block`.
             // If the place `P` targeted by the BID is "maybe initialized",
@@ -425,8 +427,9 @@ pub(crate) fn run_lint<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId, body: &Body<
 
         // We shall now exclude some local bindings for the following cases.
         {
-            let mut to_exclude = ChunkedBitSet::new_empty(all_locals_dropped.domain_size());
-            // We will now do subtraction from the candidate dropped locals, because of the following reasons.
+            let mut to_exclude = MixedBitSet::new_empty(all_locals_dropped.domain_size());
+            // We will now do subtraction from the candidate dropped locals, because of the
+            // following reasons.
             for path_idx in all_locals_dropped.iter() {
                 let move_path = &move_data.move_paths[path_idx];
                 let dropped_local = move_path.place.local;
diff --git a/compiler/rustc_mir_transform/src/remove_uninit_drops.rs b/compiler/rustc_mir_transform/src/remove_uninit_drops.rs
index f786c676e9e..e955d8277a4 100644
--- a/compiler/rustc_mir_transform/src/remove_uninit_drops.rs
+++ b/compiler/rustc_mir_transform/src/remove_uninit_drops.rs
@@ -1,5 +1,5 @@
 use rustc_abi::FieldIdx;
-use rustc_index::bit_set::ChunkedBitSet;
+use rustc_index::bit_set::MixedBitSet;
 use rustc_middle::mir::{Body, TerminatorKind};
 use rustc_middle::ty::{self, GenericArgsRef, Ty, TyCtxt, VariantDef};
 use rustc_mir_dataflow::impls::MaybeInitializedPlaces;
@@ -67,7 +67,7 @@ impl<'tcx> crate::MirPass<'tcx> for RemoveUninitDrops {
 fn is_needs_drop_and_init<'tcx>(
     tcx: TyCtxt<'tcx>,
     typing_env: ty::TypingEnv<'tcx>,
-    maybe_inits: &ChunkedBitSet<MovePathIndex>,
+    maybe_inits: &MixedBitSet<MovePathIndex>,
     move_data: &MoveData<'tcx>,
     ty: Ty<'tcx>,
     mpi: MovePathIndex,