diff --git a/compiler/rustc_borrowck/src/lib.rs b/compiler/rustc_borrowck/src/lib.rs
index 719862e67c873..753eed5306807 100644
--- a/compiler/rustc_borrowck/src/lib.rs
+++ b/compiler/rustc_borrowck/src/lib.rs
@@ -22,7 +22,7 @@ use rustc_errors::{Applicability, Diagnostic, DiagnosticBuilder, ErrorReported};
 use rustc_hir as hir;
 use rustc_hir::def_id::LocalDefId;
 use rustc_hir::Node;
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::ChunkedBitSet;
 use rustc_index::vec::IndexVec;
 use rustc_infer::infer::{InferCtxt, TyCtxtInferExt};
 use rustc_middle::mir::{
@@ -1667,7 +1667,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
         location: Location,
         desired_action: InitializationRequiringAction,
         place_span: (PlaceRef<'tcx>, Span),
-        maybe_uninits: &BitSet<MovePathIndex>,
+        maybe_uninits: &ChunkedBitSet<MovePathIndex>,
         from: u64,
         to: u64,
     ) {
diff --git a/compiler/rustc_data_structures/src/lib.rs b/compiler/rustc_data_structures/src/lib.rs
index ea02a73c42204..3e93fe9a11f72 100644
--- a/compiler/rustc_data_structures/src/lib.rs
+++ b/compiler/rustc_data_structures/src/lib.rs
@@ -37,6 +37,8 @@ extern crate cfg_if;
 #[macro_use]
 extern crate rustc_macros;
 
+pub use rustc_index::static_assert_size;
+
 #[inline(never)]
 #[cold]
 pub fn cold_path<F: FnOnce() -> R, R>(f: F) -> R {
diff --git a/compiler/rustc_data_structures/src/macros.rs b/compiler/rustc_data_structures/src/macros.rs
index 48dfbba7504ef..e05491f6ff6e4 100644
--- a/compiler/rustc_data_structures/src/macros.rs
+++ b/compiler/rustc_data_structures/src/macros.rs
@@ -1,11 +1,3 @@
-/// Type size assertion. The first argument is a type and the second argument is its expected size.
-#[macro_export]
-macro_rules! static_assert_size {
-    ($ty:ty, $size:expr) => {
-        const _: [(); $size] = [(); ::std::mem::size_of::<$ty>()];
-    };
-}
-
 #[macro_export]
 macro_rules! enum_from_u32 {
     ($(#[$attr:meta])* pub enum $name:ident {
diff --git a/compiler/rustc_index/src/bit_set.rs b/compiler/rustc_index/src/bit_set.rs
index 7f376c5fbe54b..12bde02949452 100644
--- a/compiler/rustc_index/src/bit_set.rs
+++ b/compiler/rustc_index/src/bit_set.rs
@@ -5,16 +5,37 @@ use std::iter;
 use std::marker::PhantomData;
 use std::mem;
 use std::ops::{BitAnd, BitAndAssign, BitOrAssign, Bound, Not, Range, RangeBounds, Shl};
+use std::rc::Rc;
 use std::slice;
 
 use rustc_macros::{Decodable, Encodable};
 
+use Chunk::*;
+
 #[cfg(test)]
 mod tests;
 
-pub type Word = u64;
-pub const WORD_BYTES: usize = mem::size_of::<Word>();
-pub const WORD_BITS: usize = WORD_BYTES * 8;
+type Word = u64;
+const WORD_BYTES: usize = mem::size_of::<Word>();
+const WORD_BITS: usize = WORD_BYTES * 8;
+
+// The choice of chunk size has some trade-offs.
+//
+// A big chunk size tends to favour cases where many large `ChunkedBitSet`s are
+// present, because they require fewer `Chunk`s, reducing the number of
+// allocations and reducing peak memory usage. Also, fewer chunk operations are
+// required, though more of them might be `Mixed`.
+//
+// A small chunk size tends to favour cases where many small `ChunkedBitSet`s
+// are present, because less space is wasted at the end of the final chunk (if
+// it's not full).
+const CHUNK_WORDS: usize = 32;
+const CHUNK_BITS: usize = CHUNK_WORDS * WORD_BITS; // 2048 bits
+
+/// ChunkSize is small to keep `Chunk` small. The static assertion ensures it's
+/// not too small.
+type ChunkSize = u16;
+const _: () = assert!(CHUNK_BITS <= ChunkSize::MAX as usize);
 
 pub trait BitRelations<Rhs> {
     fn union(&mut self, other: &Rhs) -> bool;
@@ -121,19 +142,12 @@ impl<T: Idx> BitSet<T> {
     /// Clear all elements.
     #[inline]
     pub fn clear(&mut self) {
-        for word in &mut self.words {
-            *word = 0;
-        }
+        self.words.fill(0);
     }
 
     /// Clear excess bits in the final word.
     fn clear_excess_bits(&mut self) {
-        let num_bits_in_final_word = self.domain_size % WORD_BITS;
-        if num_bits_in_final_word > 0 {
-            let mask = (1 << num_bits_in_final_word) - 1;
-            let final_word_idx = self.words.len() - 1;
-            self.words[final_word_idx] &= mask;
-        }
+        clear_excess_bits_in_final_word(self.domain_size, &mut self.words);
     }
 
     /// Count the number of set bits in the set.
@@ -203,9 +217,7 @@ impl<T: Idx> BitSet<T> {
 
     /// Sets all bits to true.
     pub fn insert_all(&mut self) {
-        for word in &mut self.words {
-            *word = !0;
-        }
+        self.words.fill(!0);
         self.clear_excess_bits();
     }
 
@@ -328,6 +340,407 @@ impl<T: Idx> BitRelations<BitSet<T>> for BitSet<T> {
     }
 }
 
+/// 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.
+///
+/// This type is especially efficient for sets that typically have a large
+/// `domain_size` with significant stretches of all zeros or all ones, and also
+/// some stretches with lots of 0s and 1s mixed in a way that causes trouble
+/// for `IntervalSet`.
+///
+/// `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(Debug, PartialEq, Eq)]
+pub struct ChunkedBitSet<T> {
+    domain_size: usize,
+
+    /// The chunks. Each one contains exactly CHUNK_BITS values, except the
+    /// last one which contains 1..=CHUNK_BITS values.
+    chunks: Box<[Chunk]>,
+
+    marker: PhantomData<T>,
+}
+
+// Note: the chunk domain size is duplicated in each variant. This is a bit
+// inconvenient, but it allows the type size to be smaller than if we had an
+// outer struct containing a chunk domain size plus the `Chunk`, because the
+// compiler can place the chunk domain size after the tag.
+#[derive(Clone, Debug, PartialEq, Eq)]
+enum Chunk {
+    /// A chunk that is all zeros; we don't represent the zeros explicitly.
+    Zeros(ChunkSize),
+
+    /// A chunk that is all ones; we don't represent the ones explicitly.
+    Ones(ChunkSize),
+
+    /// A chunk that has a mix of zeros and ones, which are represented
+    /// explicitly and densely. It never has all zeros or all ones.
+    ///
+    /// If this is the final chunk there may be excess, unused words. This
+    /// turns out to be both simpler and have better performance than
+    /// allocating the minimum number of words, largely because we avoid having
+    /// to store the length, which would make this type larger. These excess
+    /// words are always be zero, as are any excess bits in the final in-use
+    /// word.
+    ///
+    /// The second field is the count of 1s set in the chunk, and must satisfy
+    /// `0 < count < chunk_domain_size`.
+    ///
+    /// The words are within an `Rc` because it's surprisingly common to
+    /// duplicate an entire chunk, e.g. in `ChunkedBitSet::clone_from()`, or
+    /// when a `Mixed` chunk is union'd into a `Zeros` chunk. When we do need
+    /// to modify a chunk we use `Rc::make_mut`.
+    Mixed(ChunkSize, ChunkSize, Rc<[Word; CHUNK_WORDS]>),
+}
+
+// This type is used a lot. Make sure it doesn't unintentionally get bigger.
+#[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
+crate::static_assert_size!(Chunk, 16);
+
+impl<T> ChunkedBitSet<T> {
+    pub fn domain_size(&self) -> usize {
+        self.domain_size
+    }
+
+    #[cfg(test)]
+    fn assert_valid(&self) {
+        if self.domain_size == 0 {
+            assert!(self.chunks.is_empty());
+            return;
+        }
+
+        assert!((self.chunks.len() - 1) * CHUNK_BITS <= self.domain_size);
+        assert!(self.chunks.len() * CHUNK_BITS >= self.domain_size);
+        for chunk in self.chunks.iter() {
+            chunk.assert_valid();
+        }
+    }
+}
+
+impl<T: Idx> ChunkedBitSet<T> {
+    /// Creates a new bitset with a given `domain_size` and chunk kind.
+    fn new(domain_size: usize, is_empty: bool) -> Self {
+        let chunks = if domain_size == 0 {
+            Box::new([])
+        } else {
+            // All the chunks have a chunk_domain_size of `CHUNK_BITS` except
+            // the final one.
+            let final_chunk_domain_size = {
+                let n = domain_size % CHUNK_BITS;
+                if n == 0 { CHUNK_BITS } else { n }
+            };
+            let mut chunks =
+                vec![Chunk::new(CHUNK_BITS, is_empty); num_chunks(domain_size)].into_boxed_slice();
+            *chunks.last_mut().unwrap() = Chunk::new(final_chunk_domain_size, is_empty);
+            chunks
+        };
+        ChunkedBitSet { domain_size, chunks, marker: PhantomData }
+    }
+
+    /// Creates a new, empty bitset with a given `domain_size`.
+    #[inline]
+    pub fn new_empty(domain_size: usize) -> Self {
+        ChunkedBitSet::new(domain_size, /* is_empty */ true)
+    }
+
+    /// Creates a new, filled bitset with a given `domain_size`.
+    #[inline]
+    pub fn new_filled(domain_size: usize) -> Self {
+        ChunkedBitSet::new(domain_size, /* is_empty */ false)
+    }
+
+    #[cfg(test)]
+    fn chunks(&self) -> &[Chunk] {
+        &self.chunks
+    }
+
+    /// Count the number of bits in the set.
+    pub fn count(&self) -> usize {
+        self.chunks.iter().map(|chunk| chunk.count()).sum()
+    }
+
+    /// Returns `true` if `self` contains `elem`.
+    #[inline]
+    pub fn contains(&self, elem: T) -> bool {
+        assert!(elem.index() < self.domain_size);
+        let chunk = &self.chunks[chunk_index(elem)];
+        match &chunk {
+            Zeros(_) => false,
+            Ones(_) => true,
+            Mixed(_, _, words) => {
+                let (word_index, mask) = chunk_word_index_and_mask(elem);
+                (words[word_index] & mask) != 0
+            }
+        }
+    }
+
+    /// Insert `elem`. Returns whether the set has changed.
+    pub fn insert(&mut self, elem: T) -> bool {
+        assert!(elem.index() < self.domain_size);
+        let chunk_index = chunk_index(elem);
+        let chunk = &mut self.chunks[chunk_index];
+        match *chunk {
+            Zeros(chunk_domain_size) => {
+                if chunk_domain_size > 1 {
+                    // We take some effort to avoid copying the words.
+                    let words = Rc::<[Word; CHUNK_WORDS]>::new_zeroed();
+                    // SAFETY: `words` can safely be all zeroes.
+                    let mut words = unsafe { words.assume_init() };
+                    let words_ref = Rc::get_mut(&mut words).unwrap();
+
+                    let (word_index, mask) = chunk_word_index_and_mask(elem);
+                    words_ref[word_index] |= mask;
+                    *chunk = Mixed(chunk_domain_size, 1, words);
+                } else {
+                    *chunk = Ones(chunk_domain_size);
+                }
+                true
+            }
+            Ones(_) => false,
+            Mixed(chunk_domain_size, ref mut count, ref mut words) => {
+                // We skip all the work if the bit is already set.
+                let (word_index, mask) = chunk_word_index_and_mask(elem);
+                if (words[word_index] & mask) == 0 {
+                    *count += 1;
+                    if *count < chunk_domain_size {
+                        let words = Rc::make_mut(words);
+                        words[word_index] |= mask;
+                    } else {
+                        *chunk = Ones(chunk_domain_size);
+                    }
+                    true
+                } else {
+                    false
+                }
+            }
+        }
+    }
+
+    /// Sets all bits to true.
+    pub fn insert_all(&mut self) {
+        for chunk in self.chunks.iter_mut() {
+            *chunk = match *chunk {
+                Zeros(chunk_domain_size)
+                | Ones(chunk_domain_size)
+                | Mixed(chunk_domain_size, ..) => Ones(chunk_domain_size),
+            }
+        }
+    }
+
+    /// Returns `true` if the set has changed.
+    pub fn remove(&mut self, elem: T) -> bool {
+        assert!(elem.index() < self.domain_size);
+        let chunk_index = chunk_index(elem);
+        let chunk = &mut self.chunks[chunk_index];
+        match *chunk {
+            Zeros(_) => false,
+            Ones(chunk_domain_size) => {
+                if chunk_domain_size > 1 {
+                    // We take some effort to avoid copying the words.
+                    let words = Rc::<[Word; CHUNK_WORDS]>::new_zeroed();
+                    // SAFETY: `words` can safely be all zeroes.
+                    let mut words = unsafe { words.assume_init() };
+                    let words_ref = Rc::get_mut(&mut words).unwrap();
+
+                    // Set only the bits in use.
+                    let num_words = num_words(chunk_domain_size as usize);
+                    words_ref[..num_words].fill(!0);
+                    clear_excess_bits_in_final_word(
+                        chunk_domain_size as usize,
+                        &mut words_ref[..num_words],
+                    );
+                    let (word_index, mask) = chunk_word_index_and_mask(elem);
+                    words_ref[word_index] &= !mask;
+                    *chunk = Mixed(chunk_domain_size, chunk_domain_size - 1, words);
+                } else {
+                    *chunk = Zeros(chunk_domain_size);
+                }
+                true
+            }
+            Mixed(chunk_domain_size, ref mut count, ref mut words) => {
+                // We skip all the work if the bit is already clear.
+                let (word_index, mask) = chunk_word_index_and_mask(elem);
+                if (words[word_index] & mask) != 0 {
+                    *count -= 1;
+                    if *count > 0 {
+                        let words = Rc::make_mut(words);
+                        words[word_index] &= !mask;
+                    } else {
+                        *chunk = Zeros(chunk_domain_size);
+                    }
+                    true
+                } else {
+                    false
+                }
+            }
+        }
+    }
+
+    bit_relations_inherent_impls! {}
+}
+
+impl<T: Idx> BitRelations<ChunkedBitSet<T>> for ChunkedBitSet<T> {
+    fn union(&mut self, other: &ChunkedBitSet<T>) -> bool {
+        assert_eq!(self.domain_size, other.domain_size);
+        debug_assert_eq!(self.chunks.len(), other.chunks.len());
+
+        let mut changed = false;
+        for (mut self_chunk, other_chunk) in self.chunks.iter_mut().zip(other.chunks.iter()) {
+            match (&mut self_chunk, &other_chunk) {
+                (_, Zeros(_)) | (Ones(_), _) => {}
+                (Zeros(self_chunk_domain_size), Ones(other_chunk_domain_size))
+                | (Mixed(self_chunk_domain_size, ..), Ones(other_chunk_domain_size))
+                | (Zeros(self_chunk_domain_size), Mixed(other_chunk_domain_size, ..)) => {
+                    // `other_chunk` fully overwrites `self_chunk`
+                    debug_assert_eq!(self_chunk_domain_size, other_chunk_domain_size);
+                    *self_chunk = other_chunk.clone();
+                    changed = true;
+                }
+                (
+                    Mixed(
+                        self_chunk_domain_size,
+                        ref mut self_chunk_count,
+                        ref mut self_chunk_words,
+                    ),
+                    Mixed(_other_chunk_domain_size, _other_chunk_count, other_chunk_words),
+                ) => {
+                    // First check if the operation would change
+                    // `self_chunk.words`. If not, we can avoid allocating some
+                    // words, and this happens often enough that it's a
+                    // performance win. Also, we only need to operate on the
+                    // in-use words, hence the slicing.
+                    let op = |a, b| a | b;
+                    let num_words = num_words(*self_chunk_domain_size as usize);
+                    if bitwise_changes(
+                        &self_chunk_words[0..num_words],
+                        &other_chunk_words[0..num_words],
+                        op,
+                    ) {
+                        let self_chunk_words = Rc::make_mut(self_chunk_words);
+                        let has_changed = bitwise(
+                            &mut self_chunk_words[0..num_words],
+                            &other_chunk_words[0..num_words],
+                            op,
+                        );
+                        debug_assert!(has_changed);
+                        *self_chunk_count = self_chunk_words[0..num_words]
+                            .iter()
+                            .map(|w| w.count_ones() as ChunkSize)
+                            .sum();
+                        if *self_chunk_count == *self_chunk_domain_size {
+                            *self_chunk = Ones(*self_chunk_domain_size);
+                        }
+                        changed = true;
+                    }
+                }
+            }
+        }
+        changed
+    }
+
+    fn subtract(&mut self, _other: &ChunkedBitSet<T>) -> bool {
+        unimplemented!("implement if/when necessary");
+    }
+
+    fn intersect(&mut self, _other: &ChunkedBitSet<T>) -> bool {
+        unimplemented!("implement if/when necessary");
+    }
+}
+
+impl<T: Idx> BitRelations<HybridBitSet<T>> for ChunkedBitSet<T> {
+    fn union(&mut self, other: &HybridBitSet<T>) -> bool {
+        // FIXME: this is slow if `other` is dense, and could easily be
+        // improved, but it hasn't been a problem in practice so far.
+        assert_eq!(self.domain_size, other.domain_size());
+        sequential_update(|elem| self.insert(elem), other.iter())
+    }
+
+    fn subtract(&mut self, other: &HybridBitSet<T>) -> bool {
+        // FIXME: this is slow if `other` is dense, and could easily be
+        // improved, but it hasn't been a problem in practice so far.
+        assert_eq!(self.domain_size, other.domain_size());
+        sequential_update(|elem| self.remove(elem), other.iter())
+    }
+
+    fn intersect(&mut self, _other: &HybridBitSet<T>) -> bool {
+        unimplemented!("implement if/when necessary");
+    }
+}
+
+impl<T> Clone for ChunkedBitSet<T> {
+    fn clone(&self) -> Self {
+        ChunkedBitSet {
+            domain_size: self.domain_size,
+            chunks: self.chunks.clone(),
+            marker: PhantomData,
+        }
+    }
+
+    /// WARNING: this implementation of clone_from will 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) {
+        assert_eq!(self.domain_size, from.domain_size);
+        debug_assert_eq!(self.chunks.len(), from.chunks.len());
+
+        self.chunks.clone_from(&from.chunks)
+    }
+}
+
+impl Chunk {
+    #[cfg(test)]
+    fn assert_valid(&self) {
+        match *self {
+            Zeros(chunk_domain_size) | Ones(chunk_domain_size) => {
+                assert!(chunk_domain_size as usize <= CHUNK_BITS);
+            }
+            Mixed(chunk_domain_size, count, ref words) => {
+                assert!(chunk_domain_size as usize <= CHUNK_BITS);
+                assert!(0 < count && count < chunk_domain_size);
+
+                // Check the number of set bits matches `count`.
+                assert_eq!(
+                    words.iter().map(|w| w.count_ones() as ChunkSize).sum::<ChunkSize>(),
+                    count
+                );
+
+                // Check the not-in-use words are all zeroed.
+                let num_words = num_words(chunk_domain_size as usize);
+                if num_words < CHUNK_WORDS {
+                    assert_eq!(
+                        words[num_words..]
+                            .iter()
+                            .map(|w| w.count_ones() as ChunkSize)
+                            .sum::<ChunkSize>(),
+                        0
+                    );
+                }
+            }
+        }
+    }
+
+    fn new(chunk_domain_size: usize, is_empty: bool) -> Self {
+        debug_assert!(chunk_domain_size <= CHUNK_BITS);
+        let chunk_domain_size = chunk_domain_size as ChunkSize;
+        if is_empty { Zeros(chunk_domain_size) } else { Ones(chunk_domain_size) }
+    }
+
+    /// Count the number of 1s in the chunk.
+    fn count(&self) -> usize {
+        match *self {
+            Zeros(_) => 0,
+            Ones(chunk_domain_size) => chunk_domain_size as usize,
+            Mixed(_, count, _) => count as usize,
+        }
+    }
+}
+
 // Applies a function to mutate a bitset, and returns true if any
 // of the applications return true
 fn sequential_update<T: Idx>(
@@ -642,6 +1055,23 @@ where
     changed != 0
 }
 
+/// Does this bitwise operation change `out_vec`?
+#[inline]
+fn bitwise_changes<Op>(out_vec: &[Word], in_vec: &[Word], op: Op) -> bool
+where
+    Op: Fn(Word, Word) -> Word,
+{
+    assert_eq!(out_vec.len(), in_vec.len());
+    for (out_elem, in_elem) in iter::zip(out_vec, in_vec) {
+        let old_val = *out_elem;
+        let new_val = op(old_val, *in_elem);
+        if old_val != new_val {
+            return true;
+        }
+    }
+    false
+}
+
 const SPARSE_MAX: usize = 8;
 
 /// A fixed-size bitset type with a sparse representation and a maximum of
@@ -1136,18 +1566,7 @@ impl<R: Idx, C: Idx> BitMatrix<R, C> {
         for index in start..end {
             words[index] = !0;
         }
-        self.clear_excess_bits(row);
-    }
-
-    /// Clear excess bits in the final word of the row.
-    fn clear_excess_bits(&mut self, row: R) {
-        let num_bits_in_final_word = self.num_columns % WORD_BITS;
-        if num_bits_in_final_word > 0 {
-            let mask = (1 << num_bits_in_final_word) - 1;
-            let (_, end) = self.range(row);
-            let final_word_idx = end - 1;
-            self.words[final_word_idx] &= mask;
-        }
+        clear_excess_bits_in_final_word(self.num_columns, &mut self.words[..end]);
     }
 
     /// Gets a slice of the underlying words.
@@ -1339,6 +1758,12 @@ fn num_words<T: Idx>(domain_size: T) -> usize {
     (domain_size.index() + WORD_BITS - 1) / WORD_BITS
 }
 
+#[inline]
+fn num_chunks<T: Idx>(domain_size: T) -> usize {
+    assert!(domain_size.index() > 0);
+    (domain_size.index() + CHUNK_BITS - 1) / CHUNK_BITS
+}
+
 #[inline]
 fn word_index_and_mask<T: Idx>(elem: T) -> (usize, Word) {
     let elem = elem.index();
@@ -1347,6 +1772,25 @@ fn word_index_and_mask<T: Idx>(elem: T) -> (usize, Word) {
     (word_index, mask)
 }
 
+#[inline]
+fn chunk_index<T: Idx>(elem: T) -> usize {
+    elem.index() / CHUNK_BITS
+}
+
+#[inline]
+fn chunk_word_index_and_mask<T: Idx>(elem: T) -> (usize, Word) {
+    let chunk_elem = elem.index() % CHUNK_BITS;
+    word_index_and_mask(chunk_elem)
+}
+
+fn clear_excess_bits_in_final_word(domain_size: usize, words: &mut [Word]) {
+    let num_bits_in_final_word = domain_size % WORD_BITS;
+    if num_bits_in_final_word > 0 {
+        let mask = (1 << num_bits_in_final_word) - 1;
+        words[words.len() - 1] &= mask;
+    }
+}
+
 #[inline]
 fn max_bit(word: Word) -> usize {
     WORD_BITS - 1 - word.leading_zeros() as usize
diff --git a/compiler/rustc_index/src/bit_set/tests.rs b/compiler/rustc_index/src/bit_set/tests.rs
index e2b07305c9642..eec7dab5189a6 100644
--- a/compiler/rustc_index/src/bit_set/tests.rs
+++ b/compiler/rustc_index/src/bit_set/tests.rs
@@ -147,6 +147,201 @@ fn hybrid_bitset() {
     assert!(dense0.is_empty());
 }
 
+#[test]
+fn chunked_bitset() {
+    let mut b0 = ChunkedBitSet::<usize>::new_empty(0);
+    let b0b = b0.clone();
+    assert_eq!(b0, ChunkedBitSet { domain_size: 0, chunks: Box::new([]), marker: PhantomData });
+
+    // There are no valid insert/remove/contains operations on a 0-domain
+    // bitset, but we can test `union`.
+    b0.assert_valid();
+    assert!(!b0.union(&b0b));
+    assert_eq!(b0.chunks(), vec![]);
+    assert_eq!(b0.count(), 0);
+    b0.assert_valid();
+
+    //-----------------------------------------------------------------------
+
+    let mut b1 = ChunkedBitSet::<usize>::new_empty(1);
+    assert_eq!(
+        b1,
+        ChunkedBitSet { domain_size: 1, chunks: Box::new([Zeros(1)]), marker: PhantomData }
+    );
+
+    b1.assert_valid();
+    assert!(!b1.contains(0));
+    assert_eq!(b1.count(), 0);
+    assert!(b1.insert(0));
+    assert!(b1.contains(0));
+    assert_eq!(b1.count(), 1);
+    assert_eq!(b1.chunks(), [Ones(1)]);
+    assert!(!b1.insert(0));
+    assert!(b1.remove(0));
+    assert!(!b1.contains(0));
+    assert_eq!(b1.count(), 0);
+    assert_eq!(b1.chunks(), [Zeros(1)]);
+    b1.assert_valid();
+
+    //-----------------------------------------------------------------------
+
+    let mut b100 = ChunkedBitSet::<usize>::new_filled(100);
+    assert_eq!(
+        b100,
+        ChunkedBitSet { domain_size: 100, chunks: Box::new([Ones(100)]), marker: PhantomData }
+    );
+
+    b100.assert_valid();
+    for i in 0..100 {
+        assert!(b100.contains(i));
+    }
+    assert_eq!(b100.count(), 100);
+    assert!(b100.remove(3));
+    assert!(b100.insert(3));
+    assert_eq!(b100.chunks(), vec![Ones(100)]);
+    assert!(
+        b100.remove(20) && b100.remove(30) && b100.remove(40) && b100.remove(99) && b100.insert(30)
+    );
+    assert_eq!(b100.count(), 97);
+    assert!(!b100.contains(20) && b100.contains(30) && !b100.contains(99) && b100.contains(50));
+    assert_eq!(
+        b100.chunks(),
+        vec![Mixed(
+            100,
+            97,
+            #[rustfmt::skip]
+            Rc::new([
+                0b11111111_11111111_11111110_11111111_11111111_11101111_11111111_11111111,
+                0b00000000_00000000_00000000_00000111_11111111_11111111_11111111_11111111,
+                0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+                0, 0, 0, 0, 0,
+            ])
+        )],
+    );
+    b100.assert_valid();
+    let mut num_removed = 0;
+    for i in 0..100 {
+        if b100.remove(i) {
+            num_removed += 1;
+        }
+    }
+    assert_eq!(num_removed, 97);
+    assert_eq!(b100.chunks(), vec![Zeros(100)]);
+    b100.assert_valid();
+
+    //-----------------------------------------------------------------------
+
+    let mut b2548 = ChunkedBitSet::<usize>::new_empty(2548);
+    assert_eq!(
+        b2548,
+        ChunkedBitSet {
+            domain_size: 2548,
+            chunks: Box::new([Zeros(2048), Zeros(500)]),
+            marker: PhantomData,
+        }
+    );
+
+    b2548.assert_valid();
+    b2548.insert(14);
+    b2548.remove(14);
+    assert_eq!(b2548.chunks(), vec![Zeros(2048), Zeros(500)]);
+    b2548.insert_all();
+    for i in 0..2548 {
+        assert!(b2548.contains(i));
+    }
+    assert_eq!(b2548.count(), 2548);
+    assert_eq!(b2548.chunks(), vec![Ones(2048), Ones(500)]);
+    b2548.assert_valid();
+
+    //-----------------------------------------------------------------------
+
+    let mut b4096 = ChunkedBitSet::<usize>::new_empty(4096);
+    assert_eq!(
+        b4096,
+        ChunkedBitSet {
+            domain_size: 4096,
+            chunks: Box::new([Zeros(2048), Zeros(2048)]),
+            marker: PhantomData,
+        }
+    );
+
+    b4096.assert_valid();
+    for i in 0..4096 {
+        assert!(!b4096.contains(i));
+    }
+    assert!(b4096.insert(0) && b4096.insert(4095) && !b4096.insert(4095));
+    assert!(
+        b4096.contains(0) && !b4096.contains(2047) && !b4096.contains(2048) && b4096.contains(4095)
+    );
+    assert_eq!(
+        b4096.chunks(),
+        #[rustfmt::skip]
+        vec![
+            Mixed(2048, 1, Rc::new([
+                1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+                0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0
+            ])),
+            Mixed(2048, 1, Rc::new([
+                0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+                0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0x8000_0000_0000_0000
+            ])),
+        ],
+    );
+    assert_eq!(b4096.count(), 2);
+    b4096.assert_valid();
+
+    //-----------------------------------------------------------------------
+
+    let mut b10000 = ChunkedBitSet::<usize>::new_empty(10000);
+    assert_eq!(
+        b10000,
+        ChunkedBitSet {
+            domain_size: 10000,
+            chunks: Box::new([Zeros(2048), Zeros(2048), Zeros(2048), Zeros(2048), Zeros(1808),]),
+            marker: PhantomData,
+        }
+    );
+
+    b10000.assert_valid();
+    assert!(b10000.insert(3000) && b10000.insert(5000));
+    assert_eq!(
+        b10000.chunks(),
+        #[rustfmt::skip]
+        vec![
+            Zeros(2048),
+            Mixed(2048, 1, Rc::new([
+                0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0x0100_0000_0000_0000, 0,
+                0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+            ])),
+            Mixed(2048, 1, Rc::new([
+                0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0x0100, 0,
+                0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+            ])),
+            Zeros(2048),
+            Zeros(1808),
+        ],
+    );
+    let mut b10000b = ChunkedBitSet::<usize>::new_empty(10000);
+    b10000b.clone_from(&b10000);
+    assert_eq!(b10000, b10000b);
+    for i in 6000..7000 {
+        b10000b.insert(i);
+    }
+    assert_eq!(b10000b.count(), 1002);
+    b10000b.assert_valid();
+    b10000b.clone_from(&b10000);
+    assert_eq!(b10000b.count(), 2);
+    for i in 2000..8000 {
+        b10000b.insert(i);
+    }
+    b10000.union(&b10000b);
+    assert_eq!(b10000.count(), 6000);
+    b10000.union(&b10000b);
+    assert_eq!(b10000.count(), 6000);
+    b10000.assert_valid();
+    b10000b.assert_valid();
+}
+
 #[test]
 fn grow() {
     let mut set: GrowableBitSet<usize> = GrowableBitSet::with_capacity(65);
diff --git a/compiler/rustc_index/src/lib.rs b/compiler/rustc_index/src/lib.rs
index 7919e40925392..0cfb7bd110602 100644
--- a/compiler/rustc_index/src/lib.rs
+++ b/compiler/rustc_index/src/lib.rs
@@ -1,11 +1,21 @@
 #![feature(allow_internal_unstable)]
 #![feature(bench_black_box)]
 #![feature(extend_one)]
+#![feature(let_else)]
 #![feature(min_specialization)]
+#![feature(new_uninit)]
 #![feature(step_trait)]
+#![feature(stmt_expr_attributes)]
 #![feature(test)]
-#![feature(let_else)]
 
 pub mod bit_set;
 pub mod interval;
 pub mod vec;
+
+/// Type size assertion. The first argument is a type and the second argument is its expected size.
+#[macro_export]
+macro_rules! static_assert_size {
+    ($ty:ty, $size:expr) => {
+        const _: [(); $size] = [(); ::std::mem::size_of::<$ty>()];
+    };
+}
diff --git a/compiler/rustc_mir_dataflow/src/framework/cursor.rs b/compiler/rustc_mir_dataflow/src/framework/cursor.rs
index ba6b566a304a5..2de5a9d6991fe 100644
--- a/compiler/rustc_mir_dataflow/src/framework/cursor.rs
+++ b/compiler/rustc_mir_dataflow/src/framework/cursor.rs
@@ -1,10 +1,12 @@
 //! Random access inspection of the results of a dataflow analysis.
 
+use crate::framework::BitSetExt;
+
 use std::borrow::Borrow;
 use std::cmp::Ordering;
 
+#[cfg(debug_assertions)]
 use rustc_index::bit_set::BitSet;
-use rustc_index::vec::Idx;
 use rustc_middle::mir::{self, BasicBlock, Location};
 
 use super::{Analysis, Direction, Effect, EffectIndex, Results};
@@ -209,13 +211,13 @@ where
     }
 }
 
-impl<'mir, 'tcx, A, R, T> ResultsCursor<'mir, 'tcx, A, R>
+impl<'mir, 'tcx, A, R> ResultsCursor<'mir, 'tcx, A, R>
 where
-    A: Analysis<'tcx, Domain = BitSet<T>>,
-    T: Idx,
+    A: crate::GenKillAnalysis<'tcx>,
+    A::Domain: BitSetExt<A::Idx>,
     R: Borrow<Results<'tcx, A>>,
 {
-    pub fn contains(&self, elem: T) -> bool {
+    pub fn contains(&self, elem: A::Idx) -> bool {
         self.get().contains(elem)
     }
 }
diff --git a/compiler/rustc_mir_dataflow/src/framework/engine.rs b/compiler/rustc_mir_dataflow/src/framework/engine.rs
index e20358ba9896d..88ed0128a1fd3 100644
--- a/compiler/rustc_mir_dataflow/src/framework/engine.rs
+++ b/compiler/rustc_mir_dataflow/src/framework/engine.rs
@@ -1,6 +1,7 @@
 //! A solver for dataflow problems.
 
-use std::borrow::BorrowMut;
+use crate::framework::BitSetExt;
+
 use std::ffi::OsString;
 use std::path::PathBuf;
 
@@ -91,7 +92,7 @@ where
 impl<'a, 'tcx, A, D, T> Engine<'a, 'tcx, A>
 where
     A: GenKillAnalysis<'tcx, Idx = T, Domain = D>,
-    D: Clone + JoinSemiLattice + GenKill<T> + BorrowMut<BitSet<T>>,
+    D: Clone + JoinSemiLattice + GenKill<T> + BitSetExt<T>,
     T: Idx,
 {
     /// Creates a new `Engine` to solve a gen-kill dataflow problem.
@@ -106,7 +107,7 @@ where
 
         // Otherwise, compute and store the cumulative transfer function for each block.
 
-        let identity = GenKillSet::identity(analysis.bottom_value(body).borrow().domain_size());
+        let identity = GenKillSet::identity(analysis.bottom_value(body).domain_size());
         let mut trans_for_block = IndexVec::from_elem(identity, body.basic_blocks());
 
         for (block, block_data) in body.basic_blocks().iter_enumerated() {
@@ -115,7 +116,7 @@ where
         }
 
         let apply_trans = Box::new(move |bb: BasicBlock, state: &mut A::Domain| {
-            trans_for_block[bb].apply(state.borrow_mut());
+            trans_for_block[bb].apply(state);
         });
 
         Self::new(tcx, body, analysis, Some(apply_trans as Box<_>))
diff --git a/compiler/rustc_mir_dataflow/src/framework/fmt.rs b/compiler/rustc_mir_dataflow/src/framework/fmt.rs
index 1d1553bbbd984..99735673f4d58 100644
--- a/compiler/rustc_mir_dataflow/src/framework/fmt.rs
+++ b/compiler/rustc_mir_dataflow/src/framework/fmt.rs
@@ -1,7 +1,7 @@
 //! Custom formatting traits used when outputting Graphviz diagrams with the results of a dataflow
 //! analysis.
 
-use rustc_index::bit_set::{BitSet, HybridBitSet};
+use rustc_index::bit_set::{BitSet, ChunkedBitSet, HybridBitSet};
 use rustc_index::vec::Idx;
 use std::fmt;
 
@@ -133,6 +133,19 @@ where
     }
 }
 
+impl<T, C> DebugWithContext<C> for ChunkedBitSet<T>
+where
+    T: Idx + DebugWithContext<C>,
+{
+    fn fmt_with(&self, _ctxt: &C, _f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        unimplemented!("implement when/if needed");
+    }
+
+    fn fmt_diff_with(&self, _old: &Self, _ctxt: &C, _f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        unimplemented!("implement when/if needed");
+    }
+}
+
 impl<T, C> DebugWithContext<C> for &'_ T
 where
     T: DebugWithContext<C>,
diff --git a/compiler/rustc_mir_dataflow/src/framework/lattice.rs b/compiler/rustc_mir_dataflow/src/framework/lattice.rs
index f937b31f4cfc9..9a462f6e1a41a 100644
--- a/compiler/rustc_mir_dataflow/src/framework/lattice.rs
+++ b/compiler/rustc_mir_dataflow/src/framework/lattice.rs
@@ -38,7 +38,8 @@
 //! [Hasse diagram]: https://en.wikipedia.org/wiki/Hasse_diagram
 //! [poset]: https://en.wikipedia.org/wiki/Partially_ordered_set
 
-use rustc_index::bit_set::BitSet;
+use crate::framework::BitSetExt;
+use rustc_index::bit_set::{BitSet, ChunkedBitSet, HybridBitSet};
 use rustc_index::vec::{Idx, IndexVec};
 use std::iter;
 
@@ -145,6 +146,18 @@ impl<T: Idx> MeetSemiLattice for BitSet<T> {
     }
 }
 
+impl<T: Idx> JoinSemiLattice for ChunkedBitSet<T> {
+    fn join(&mut self, other: &Self) -> bool {
+        self.union(other)
+    }
+}
+
+impl<T: Idx> MeetSemiLattice for ChunkedBitSet<T> {
+    fn meet(&mut self, other: &Self) -> bool {
+        self.intersect(other)
+    }
+}
+
 /// The counterpart of a given semilattice `T` using the [inverse order].
 ///
 /// The dual of a join-semilattice is a meet-semilattice and vice versa. For example, the dual of a
@@ -155,15 +168,21 @@ impl<T: Idx> MeetSemiLattice for BitSet<T> {
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub struct Dual<T>(pub T);
 
-impl<T> std::borrow::Borrow<T> for Dual<T> {
-    fn borrow(&self) -> &T {
-        &self.0
+impl<T: Idx> BitSetExt<T> for Dual<BitSet<T>> {
+    fn domain_size(&self) -> usize {
+        self.0.domain_size()
+    }
+
+    fn contains(&self, elem: T) -> bool {
+        self.0.contains(elem)
+    }
+
+    fn union(&mut self, other: &HybridBitSet<T>) {
+        self.0.union(other);
     }
-}
 
-impl<T> std::borrow::BorrowMut<T> for Dual<T> {
-    fn borrow_mut(&mut self) -> &mut T {
-        &mut self.0
+    fn subtract(&mut self, other: &HybridBitSet<T>) {
+        self.0.subtract(other);
     }
 }
 
diff --git a/compiler/rustc_mir_dataflow/src/framework/mod.rs b/compiler/rustc_mir_dataflow/src/framework/mod.rs
index dac6720a6e543..c51dd06de251d 100644
--- a/compiler/rustc_mir_dataflow/src/framework/mod.rs
+++ b/compiler/rustc_mir_dataflow/src/framework/mod.rs
@@ -30,10 +30,9 @@
 //!
 //! [gen-kill]: https://en.wikipedia.org/wiki/Data-flow_analysis#Bit_vector_problems
 
-use std::borrow::BorrowMut;
 use std::cmp::Ordering;
 
-use rustc_index::bit_set::{BitSet, HybridBitSet};
+use rustc_index::bit_set::{BitSet, ChunkedBitSet, HybridBitSet};
 use rustc_index::vec::Idx;
 use rustc_middle::mir::{self, BasicBlock, Location};
 use rustc_middle::ty::TyCtxt;
@@ -52,6 +51,51 @@ pub use self::engine::{Engine, Results};
 pub use self::lattice::{JoinSemiLattice, MeetSemiLattice};
 pub use self::visitor::{visit_results, ResultsVisitable, ResultsVisitor};
 
+/// Analysis domains are all bitsets of various kinds. This trait holds
+/// operations needed by all of them.
+pub trait BitSetExt<T> {
+    fn domain_size(&self) -> usize;
+    fn contains(&self, elem: T) -> bool;
+    fn union(&mut self, other: &HybridBitSet<T>);
+    fn subtract(&mut self, other: &HybridBitSet<T>);
+}
+
+impl<T: Idx> BitSetExt<T> for BitSet<T> {
+    fn domain_size(&self) -> usize {
+        self.domain_size()
+    }
+
+    fn contains(&self, elem: T) -> bool {
+        self.contains(elem)
+    }
+
+    fn union(&mut self, other: &HybridBitSet<T>) {
+        self.union(other);
+    }
+
+    fn subtract(&mut self, other: &HybridBitSet<T>) {
+        self.subtract(other);
+    }
+}
+
+impl<T: Idx> BitSetExt<T> for ChunkedBitSet<T> {
+    fn domain_size(&self) -> usize {
+        self.domain_size()
+    }
+
+    fn contains(&self, elem: T) -> bool {
+        self.contains(elem)
+    }
+
+    fn union(&mut self, other: &HybridBitSet<T>) {
+        self.union(other);
+    }
+
+    fn subtract(&mut self, other: &HybridBitSet<T>) {
+        self.subtract(other);
+    }
+}
+
 /// Define the domain of a dataflow problem.
 ///
 /// This trait specifies the lattice on which this analysis operates (the domain) as well as its
@@ -303,7 +347,7 @@ pub trait GenKillAnalysis<'tcx>: Analysis<'tcx> {
 impl<'tcx, A> Analysis<'tcx> for A
 where
     A: GenKillAnalysis<'tcx>,
-    A::Domain: GenKill<A::Idx> + BorrowMut<BitSet<A::Idx>>,
+    A::Domain: GenKill<A::Idx> + BitSetExt<A::Idx>,
 {
     fn apply_statement_effect(
         &self,
@@ -435,7 +479,7 @@ impl<T: Idx> GenKillSet<T> {
         }
     }
 
-    pub fn apply(&self, state: &mut BitSet<T>) {
+    pub fn apply(&self, state: &mut impl BitSetExt<T>) {
         state.union(&self.gen);
         state.subtract(&self.kill);
     }
@@ -463,6 +507,16 @@ impl<T: Idx> GenKill<T> for BitSet<T> {
     }
 }
 
+impl<T: Idx> GenKill<T> for ChunkedBitSet<T> {
+    fn gen(&mut self, elem: T) {
+        self.insert(elem);
+    }
+
+    fn kill(&mut self, elem: T) {
+        self.remove(elem);
+    }
+}
+
 impl<T: Idx> GenKill<T> for lattice::Dual<BitSet<T>> {
     fn gen(&mut self, elem: T) {
         self.0.insert(elem);
diff --git a/compiler/rustc_mir_dataflow/src/impls/mod.rs b/compiler/rustc_mir_dataflow/src/impls/mod.rs
index 63d935db8ca13..9502c5d57d688 100644
--- a/compiler/rustc_mir_dataflow/src/impls/mod.rs
+++ b/compiler/rustc_mir_dataflow/src/impls/mod.rs
@@ -2,7 +2,7 @@
 //! bitvectors attached to each basic block, represented via a
 //! zero-sized structure.
 
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::{BitSet, ChunkedBitSet};
 use rustc_index::vec::Idx;
 use rustc_middle::mir::visit::{MirVisitable, Visitor};
 use rustc_middle::mir::{self, Body, Location};
@@ -286,12 +286,12 @@ impl<'a, 'tcx> DefinitelyInitializedPlaces<'a, 'tcx> {
 }
 
 impl<'tcx> AnalysisDomain<'tcx> for MaybeInitializedPlaces<'_, 'tcx> {
-    type Domain = BitSet<MovePathIndex>;
+    type Domain = ChunkedBitSet<MovePathIndex>;
     const NAME: &'static str = "maybe_init";
 
     fn bottom_value(&self, _: &mir::Body<'tcx>) -> Self::Domain {
         // bottom = uninitialized
-        BitSet::new_empty(self.move_data().move_paths.len())
+        ChunkedBitSet::new_empty(self.move_data().move_paths.len())
     }
 
     fn initialize_start_block(&self, _: &mir::Body<'tcx>, state: &mut Self::Domain) {
@@ -417,13 +417,13 @@ impl<'tcx> GenKillAnalysis<'tcx> for MaybeInitializedPlaces<'_, 'tcx> {
 }
 
 impl<'tcx> AnalysisDomain<'tcx> for MaybeUninitializedPlaces<'_, 'tcx> {
-    type Domain = BitSet<MovePathIndex>;
+    type Domain = ChunkedBitSet<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)
-        BitSet::new_empty(self.move_data().move_paths.len())
+        ChunkedBitSet::new_empty(self.move_data().move_paths.len())
     }
 
     // sets on_entry bits for Arg places
@@ -606,13 +606,13 @@ impl<'tcx> GenKillAnalysis<'tcx> for DefinitelyInitializedPlaces<'_, 'tcx> {
 }
 
 impl<'tcx> AnalysisDomain<'tcx> for EverInitializedPlaces<'_, 'tcx> {
-    type Domain = BitSet<InitIndex>;
+    type Domain = ChunkedBitSet<InitIndex>;
 
     const NAME: &'static str = "ever_init";
 
     fn bottom_value(&self, _: &mir::Body<'tcx>) -> Self::Domain {
         // bottom = no initialized variables by default
-        BitSet::new_empty(self.move_data().inits.len())
+        ChunkedBitSet::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_dataflow/src/rustc_peek.rs b/compiler/rustc_mir_dataflow/src/rustc_peek.rs
index 1746d5ee38b92..51ab5b43bff0a 100644
--- a/compiler/rustc_mir_dataflow/src/rustc_peek.rs
+++ b/compiler/rustc_mir_dataflow/src/rustc_peek.rs
@@ -1,5 +1,3 @@
-use std::borrow::Borrow;
-
 use rustc_ast::ast;
 use rustc_span::symbol::sym;
 use rustc_span::Span;
@@ -10,6 +8,7 @@ use rustc_middle::mir::MirPass;
 use rustc_middle::mir::{self, Body, Local, Location};
 use rustc_middle::ty::{self, Ty, TyCtxt};
 
+use crate::framework::BitSetExt;
 use crate::impls::{
     DefinitelyInitializedPlaces, MaybeInitializedPlaces, MaybeLiveLocals, MaybeUninitializedPlaces,
 };
@@ -248,7 +247,7 @@ pub trait RustcPeekAt<'tcx>: Analysis<'tcx> {
 impl<'tcx, A, D> RustcPeekAt<'tcx> for A
 where
     A: Analysis<'tcx, Domain = D> + HasMoveData<'tcx>,
-    D: JoinSemiLattice + Clone + Borrow<BitSet<MovePathIndex>>,
+    D: JoinSemiLattice + Clone + BitSetExt<MovePathIndex>,
 {
     fn peek_at(
         &self,
@@ -259,7 +258,7 @@ where
     ) {
         match self.move_data().rev_lookup.find(place.as_ref()) {
             LookupResult::Exact(peek_mpi) => {
-                let bit_state = flow_state.borrow().contains(peek_mpi);
+                let bit_state = flow_state.contains(peek_mpi);
                 debug!("rustc_peek({:?} = &{:?}) bit_state: {}", call.arg, place, bit_state);
                 if !bit_state {
                     tcx.sess.span_err(call.span, "rustc_peek: bit not set");
diff --git a/compiler/rustc_mir_transform/src/remove_uninit_drops.rs b/compiler/rustc_mir_transform/src/remove_uninit_drops.rs
index 6d082748939ac..4167f8df11cd8 100644
--- a/compiler/rustc_mir_transform/src/remove_uninit_drops.rs
+++ b/compiler/rustc_mir_transform/src/remove_uninit_drops.rs
@@ -1,4 +1,4 @@
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::ChunkedBitSet;
 use rustc_middle::mir::{Body, Field, Rvalue, Statement, StatementKind, TerminatorKind};
 use rustc_middle::ty::subst::SubstsRef;
 use rustc_middle::ty::{self, ParamEnv, Ty, TyCtxt, VariantDef};
@@ -89,7 +89,7 @@ impl<'tcx> MirPass<'tcx> for RemoveUninitDrops {
 fn is_needs_drop_and_init<'tcx>(
     tcx: TyCtxt<'tcx>,
     param_env: ParamEnv<'tcx>,
-    maybe_inits: &BitSet<MovePathIndex>,
+    maybe_inits: &ChunkedBitSet<MovePathIndex>,
     move_data: &MoveData<'tcx>,
     ty: Ty<'tcx>,
     mpi: MovePathIndex,