Auto merge of #74862 - mark-i-m:mv-compiler, r=petrochenkov

Move almost all compiler crates to compiler/

This PR implements https://github.com/rust-lang/compiler-team/issues/336 and moves all `rustc_*` crates from `src` to the new `compiler` directory.

`librustc_foo` directories are renamed to `rustc_foo`.
`src` directories are introduced inside `rustc_*` directories to mirror the scheme already use for `library` crates.

5 files changed, 2438 insertions, 0 deletions

diff --git a/compiler/rustc_index/src/bit_set.rs b/compiler/rustc_index/src/bit_set.rs
new file mode 100644
index 00000000000..c43d1a6830d
--- /dev/null
+++ b/compiler/rustc_index/src/bit_set.rs
@@ -0,0 +1,1164 @@
+use crate::vec::{Idx, IndexVec};
+use arrayvec::ArrayVec;
+use std::fmt;
+use std::iter;
+use std::marker::PhantomData;
+use std::mem;
+use std::ops::{BitAnd, BitAndAssign, BitOrAssign, Not, Range, Shl};
+use std::slice;
+
+use rustc_macros::{Decodable, Encodable};
+
+#[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;
+
+/// A fixed-size bitset type with a dense representation.
+///
+/// NOTE: Use [`GrowableBitSet`] if you need support for resizing after creation.
+///
+/// `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.
+///
+/// [`GrowableBitSet`]: struct.GrowableBitSet.html
+#[derive(Clone, Eq, PartialEq, Decodable, Encodable)]
+pub struct BitSet<T: Idx> {
+    domain_size: usize,
+    words: Vec<Word>,
+    marker: PhantomData<T>,
+}
+
+impl<T: Idx> BitSet<T> {
+    /// Creates a new, empty bitset with a given `domain_size`.
+    #[inline]
+    pub fn new_empty(domain_size: usize) -> BitSet<T> {
+        let num_words = num_words(domain_size);
+        BitSet { domain_size, words: vec![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> {
+        let num_words = num_words(domain_size);
+        let mut result = BitSet { domain_size, words: vec![!0; num_words], marker: PhantomData };
+        result.clear_excess_bits();
+        result
+    }
+
+    /// Gets the domain size.
+    pub fn domain_size(&self) -> usize {
+        self.domain_size
+    }
+
+    /// Clear all elements.
+    #[inline]
+    pub fn clear(&mut self) {
+        for word in &mut self.words {
+            *word = 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;
+        }
+    }
+
+    /// Efficiently overwrite `self` with `other`.
+    pub fn overwrite(&mut self, other: &BitSet<T>) {
+        assert!(self.domain_size == other.domain_size);
+        self.words.clone_from_slice(&other.words);
+    }
+
+    /// Count the number of set bits in the set.
+    pub fn count(&self) -> usize {
+        self.words.iter().map(|e| e.count_ones() as usize).sum()
+    }
+
+    /// Returns `true` if `self` contains `elem`.
+    #[inline]
+    pub fn contains(&self, elem: T) -> bool {
+        assert!(elem.index() < self.domain_size);
+        let (word_index, mask) = word_index_and_mask(elem);
+        (self.words[word_index] & mask) != 0
+    }
+
+    /// Is `self` is a (non-strict) superset of `other`?
+    #[inline]
+    pub fn superset(&self, other: &BitSet<T>) -> bool {
+        assert_eq!(self.domain_size, other.domain_size);
+        self.words.iter().zip(&other.words).all(|(a, b)| (a & b) == *b)
+    }
+
+    /// Is the set empty?
+    #[inline]
+    pub fn is_empty(&self) -> bool {
+        self.words.iter().all(|a| *a == 0)
+    }
+
+    /// Insert `elem`. Returns whether the set has changed.
+    #[inline]
+    pub fn insert(&mut self, elem: T) -> bool {
+        assert!(elem.index() < self.domain_size);
+        let (word_index, mask) = word_index_and_mask(elem);
+        let word_ref = &mut self.words[word_index];
+        let word = *word_ref;
+        let new_word = word | mask;
+        *word_ref = new_word;
+        new_word != word
+    }
+
+    /// Sets all bits to true.
+    pub fn insert_all(&mut self) {
+        for word in &mut self.words {
+            *word = !0;
+        }
+        self.clear_excess_bits();
+    }
+
+    /// Returns `true` if the set has changed.
+    #[inline]
+    pub fn remove(&mut self, elem: T) -> bool {
+        assert!(elem.index() < self.domain_size);
+        let (word_index, mask) = word_index_and_mask(elem);
+        let word_ref = &mut self.words[word_index];
+        let word = *word_ref;
+        let new_word = word & !mask;
+        *word_ref = new_word;
+        new_word != word
+    }
+
+    /// Sets `self = self | other` and returns `true` if `self` changed
+    /// (i.e., if new bits were added).
+    pub fn union(&mut self, other: &impl UnionIntoBitSet<T>) -> bool {
+        other.union_into(self)
+    }
+
+    /// Sets `self = self - other` and returns `true` if `self` changed.
+    /// (i.e., if any bits were removed).
+    pub fn subtract(&mut self, other: &impl SubtractFromBitSet<T>) -> bool {
+        other.subtract_from(self)
+    }
+
+    /// Sets `self = self & other` and return `true` if `self` changed.
+    /// (i.e., if any bits were removed).
+    pub fn intersect(&mut self, other: &BitSet<T>) -> bool {
+        assert_eq!(self.domain_size, other.domain_size);
+        bitwise(&mut self.words, &other.words, |a, b| a & b)
+    }
+
+    /// Gets a slice of the underlying words.
+    pub fn words(&self) -> &[Word] {
+        &self.words
+    }
+
+    /// Iterates over the indices of set bits in a sorted order.
+    #[inline]
+    pub fn iter(&self) -> BitIter<'_, T> {
+        BitIter::new(&self.words)
+    }
+
+    /// Duplicates the set as a hybrid set.
+    pub fn to_hybrid(&self) -> HybridBitSet<T> {
+        // Note: we currently don't bother trying to make a Sparse set.
+        HybridBitSet::Dense(self.to_owned())
+    }
+
+    /// Set `self = self | other`. In contrast to `union` returns `true` if the set contains at
+    /// least one bit that is not in `other` (i.e. `other` is not a superset of `self`).
+    ///
+    /// This is an optimization for union of a hybrid bitset.
+    fn reverse_union_sparse(&mut self, sparse: &SparseBitSet<T>) -> bool {
+        assert!(sparse.domain_size == self.domain_size);
+        self.clear_excess_bits();
+
+        let mut not_already = false;
+        // Index of the current word not yet merged.
+        let mut current_index = 0;
+        // Mask of bits that came from the sparse set in the current word.
+        let mut new_bit_mask = 0;
+        for (word_index, mask) in sparse.iter().map(|x| word_index_and_mask(*x)) {
+            // Next bit is in a word not inspected yet.
+            if word_index > current_index {
+                self.words[current_index] |= new_bit_mask;
+                // Were there any bits in the old word that did not occur in the sparse set?
+                not_already |= (self.words[current_index] ^ new_bit_mask) != 0;
+                // Check all words we skipped for any set bit.
+                not_already |= self.words[current_index + 1..word_index].iter().any(|&x| x != 0);
+                // Update next word.
+                current_index = word_index;
+                // Reset bit mask, no bits have been merged yet.
+                new_bit_mask = 0;
+            }
+            // Add bit and mark it as coming from the sparse set.
+            // self.words[word_index] |= mask;
+            new_bit_mask |= mask;
+        }
+        self.words[current_index] |= new_bit_mask;
+        // Any bits in the last inspected word that were not in the sparse set?
+        not_already |= (self.words[current_index] ^ new_bit_mask) != 0;
+        // Any bits in the tail? Note `clear_excess_bits` before.
+        not_already |= self.words[current_index + 1..].iter().any(|&x| x != 0);
+
+        not_already
+    }
+}
+
+/// This is implemented by all the bitsets so that BitSet::union() can be
+/// passed any type of bitset.
+pub trait UnionIntoBitSet<T: Idx> {
+    // Performs `other = other | self`.
+    fn union_into(&self, other: &mut BitSet<T>) -> bool;
+}
+
+/// This is implemented by all the bitsets so that BitSet::subtract() can be
+/// passed any type of bitset.
+pub trait SubtractFromBitSet<T: Idx> {
+    // Performs `other = other - self`.
+    fn subtract_from(&self, other: &mut BitSet<T>) -> bool;
+}
+
+impl<T: Idx> UnionIntoBitSet<T> for BitSet<T> {
+    fn union_into(&self, other: &mut BitSet<T>) -> bool {
+        assert_eq!(self.domain_size, other.domain_size);
+        bitwise(&mut other.words, &self.words, |a, b| a | b)
+    }
+}
+
+impl<T: Idx> SubtractFromBitSet<T> for BitSet<T> {
+    fn subtract_from(&self, other: &mut BitSet<T>) -> bool {
+        assert_eq!(self.domain_size, other.domain_size);
+        bitwise(&mut other.words, &self.words, |a, b| a & !b)
+    }
+}
+
+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!("{}{:02x}", sep, byte));
+
+                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;
+                let bit = 1 << bit_pos;
+                self.word ^= bit;
+                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()`.
+            let word = self.iter.next()?;
+            self.word = *word;
+            self.offset = self.offset.wrapping_add(WORD_BITS);
+        }
+    }
+}
+
+#[inline]
+fn bitwise<Op>(out_vec: &mut [Word], in_vec: &[Word], op: Op) -> bool
+where
+    Op: Fn(Word, Word) -> Word,
+{
+    assert_eq!(out_vec.len(), in_vec.len());
+    let mut changed = false;
+    for (out_elem, in_elem) in out_vec.iter_mut().zip(in_vec.iter()) {
+        let old_val = *out_elem;
+        let new_val = op(old_val, *in_elem);
+        *out_elem = new_val;
+        changed |= old_val != new_val;
+    }
+    changed
+}
+
+const SPARSE_MAX: usize = 8;
+
+/// A fixed-size bitset type with a sparse representation and a maximum of
+/// `SPARSE_MAX` elements. The elements are stored as a sorted `ArrayVec` with
+/// no duplicates.
+///
+/// This type is used by `HybridBitSet`; do not use directly.
+#[derive(Clone, Debug)]
+pub struct SparseBitSet<T: Idx> {
+    domain_size: usize,
+    elems: ArrayVec<[T; SPARSE_MAX]>,
+}
+
+impl<T: Idx> SparseBitSet<T> {
+    fn new_empty(domain_size: usize) -> Self {
+        SparseBitSet { domain_size, elems: ArrayVec::new() }
+    }
+
+    fn len(&self) -> usize {
+        self.elems.len()
+    }
+
+    fn is_empty(&self) -> bool {
+        self.elems.len() == 0
+    }
+
+    fn contains(&self, elem: T) -> bool {
+        assert!(elem.index() < self.domain_size);
+        self.elems.contains(&elem)
+    }
+
+    fn insert(&mut self, elem: T) -> bool {
+        assert!(elem.index() < self.domain_size);
+        let changed = if let Some(i) = self.elems.iter().position(|&e| e >= elem) {
+            if self.elems[i] == elem {
+                // `elem` is already in the set.
+                false
+            } else {
+                // `elem` is smaller than one or more existing elements.
+                self.elems.insert(i, elem);
+                true
+            }
+        } else {
+            // `elem` is larger than all existing elements.
+            self.elems.push(elem);
+            true
+        };
+        assert!(self.len() <= SPARSE_MAX);
+        changed
+    }
+
+    fn remove(&mut self, elem: T) -> bool {
+        assert!(elem.index() < self.domain_size);
+        if let Some(i) = self.elems.iter().position(|&e| e == elem) {
+            self.elems.remove(i);
+            true
+        } else {
+            false
+        }
+    }
+
+    fn to_dense(&self) -> BitSet<T> {
+        let mut dense = BitSet::new_empty(self.domain_size);
+        for elem in self.elems.iter() {
+            dense.insert(*elem);
+        }
+        dense
+    }
+
+    fn iter(&self) -> slice::Iter<'_, T> {
+        self.elems.iter()
+    }
+}
+
+impl<T: Idx> UnionIntoBitSet<T> for SparseBitSet<T> {
+    fn union_into(&self, other: &mut BitSet<T>) -> bool {
+        assert_eq!(self.domain_size, other.domain_size);
+        let mut changed = false;
+        for elem in self.iter() {
+            changed |= other.insert(*elem);
+        }
+        changed
+    }
+}
+
+impl<T: Idx> SubtractFromBitSet<T> for SparseBitSet<T> {
+    fn subtract_from(&self, other: &mut BitSet<T>) -> bool {
+        assert_eq!(self.domain_size, other.domain_size);
+        let mut changed = false;
+        for elem in self.iter() {
+            changed |= other.remove(*elem);
+        }
+        changed
+    }
+}
+
+/// A fixed-size bitset type with a hybrid representation: sparse when there
+/// are up to a `SPARSE_MAX` elements in the set, but dense when there are more
+/// than `SPARSE_MAX`.
+///
+/// This type is especially efficient for sets that typically have a small
+/// number of elements, but a large `domain_size`, and are cleared frequently.
+///
+/// `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(Clone, Debug)]
+pub enum HybridBitSet<T: Idx> {
+    Sparse(SparseBitSet<T>),
+    Dense(BitSet<T>),
+}
+
+impl<T: Idx> HybridBitSet<T> {
+    pub fn new_empty(domain_size: usize) -> Self {
+        HybridBitSet::Sparse(SparseBitSet::new_empty(domain_size))
+    }
+
+    fn domain_size(&self) -> usize {
+        match self {
+            HybridBitSet::Sparse(sparse) => sparse.domain_size,
+            HybridBitSet::Dense(dense) => dense.domain_size,
+        }
+    }
+
+    pub fn clear(&mut self) {
+        let domain_size = self.domain_size();
+        *self = HybridBitSet::new_empty(domain_size);
+    }
+
+    pub fn contains(&self, elem: T) -> bool {
+        match self {
+            HybridBitSet::Sparse(sparse) => sparse.contains(elem),
+            HybridBitSet::Dense(dense) => dense.contains(elem),
+        }
+    }
+
+    pub fn superset(&self, other: &HybridBitSet<T>) -> bool {
+        match (self, other) {
+            (HybridBitSet::Dense(self_dense), HybridBitSet::Dense(other_dense)) => {
+                self_dense.superset(other_dense)
+            }
+            _ => {
+                assert!(self.domain_size() == other.domain_size());
+                other.iter().all(|elem| self.contains(elem))
+            }
+        }
+    }
+
+    pub fn is_empty(&self) -> bool {
+        match self {
+            HybridBitSet::Sparse(sparse) => sparse.is_empty(),
+            HybridBitSet::Dense(dense) => dense.is_empty(),
+        }
+    }
+
+    pub fn insert(&mut self, elem: T) -> bool {
+        // No need to check `elem` against `self.domain_size` here because all
+        // the match cases check it, one way or another.
+        match self {
+            HybridBitSet::Sparse(sparse) if sparse.len() < SPARSE_MAX => {
+                // The set is sparse and has space for `elem`.
+                sparse.insert(elem)
+            }
+            HybridBitSet::Sparse(sparse) if sparse.contains(elem) => {
+                // The set is sparse and does not have space for `elem`, but
+                // that doesn't matter because `elem` is already present.
+                false
+            }
+            HybridBitSet::Sparse(sparse) => {
+                // The set is sparse and full. Convert to a dense set.
+                let mut dense = sparse.to_dense();
+                let changed = dense.insert(elem);
+                assert!(changed);
+                *self = HybridBitSet::Dense(dense);
+                changed
+            }
+            HybridBitSet::Dense(dense) => dense.insert(elem),
+        }
+    }
+
+    pub fn insert_all(&mut self) {
+        let domain_size = self.domain_size();
+        match self {
+            HybridBitSet::Sparse(_) => {
+                *self = HybridBitSet::Dense(BitSet::new_filled(domain_size));
+            }
+            HybridBitSet::Dense(dense) => dense.insert_all(),
+        }
+    }
+
+    pub fn remove(&mut self, elem: T) -> bool {
+        // Note: we currently don't bother going from Dense back to Sparse.
+        match self {
+            HybridBitSet::Sparse(sparse) => sparse.remove(elem),
+            HybridBitSet::Dense(dense) => dense.remove(elem),
+        }
+    }
+
+    pub fn union(&mut self, other: &HybridBitSet<T>) -> bool {
+        match self {
+            HybridBitSet::Sparse(self_sparse) => {
+                match other {
+                    HybridBitSet::Sparse(other_sparse) => {
+                        // Both sets are sparse. Add the elements in
+                        // `other_sparse` to `self` one at a time. This
+                        // may or may not cause `self` to be densified.
+                        assert_eq!(self.domain_size(), other.domain_size());
+                        let mut changed = false;
+                        for elem in other_sparse.iter() {
+                            changed |= self.insert(*elem);
+                        }
+                        changed
+                    }
+                    HybridBitSet::Dense(other_dense) => {
+                        // `self` is sparse and `other` is dense. To
+                        // merge them, we have two available strategies:
+                        // * Densify `self` then merge other
+                        // * Clone other then integrate bits from `self`
+                        // The second strategy requires dedicated method
+                        // since the usual `union` returns the wrong
+                        // result. In the dedicated case the computation
+                        // is slightly faster if the bits of the sparse
+                        // bitset map to only few words of the dense
+                        // representation, i.e. indices are near each
+                        // other.
+                        //
+                        // Benchmarking seems to suggest that the second
+                        // option is worth it.
+                        let mut new_dense = other_dense.clone();
+                        let changed = new_dense.reverse_union_sparse(self_sparse);
+                        *self = HybridBitSet::Dense(new_dense);
+                        changed
+                    }
+                }
+            }
+
+            HybridBitSet::Dense(self_dense) => self_dense.union(other),
+        }
+    }
+
+    /// Converts to a dense set, consuming itself in the process.
+    pub fn to_dense(self) -> BitSet<T> {
+        match self {
+            HybridBitSet::Sparse(sparse) => sparse.to_dense(),
+            HybridBitSet::Dense(dense) => dense,
+        }
+    }
+
+    pub fn iter(&self) -> HybridIter<'_, T> {
+        match self {
+            HybridBitSet::Sparse(sparse) => HybridIter::Sparse(sparse.iter()),
+            HybridBitSet::Dense(dense) => HybridIter::Dense(dense.iter()),
+        }
+    }
+}
+
+impl<T: Idx> UnionIntoBitSet<T> for HybridBitSet<T> {
+    fn union_into(&self, other: &mut BitSet<T>) -> bool {
+        match self {
+            HybridBitSet::Sparse(sparse) => sparse.union_into(other),
+            HybridBitSet::Dense(dense) => dense.union_into(other),
+        }
+    }
+}
+
+impl<T: Idx> SubtractFromBitSet<T> for HybridBitSet<T> {
+    fn subtract_from(&self, other: &mut BitSet<T>) -> bool {
+        match self {
+            HybridBitSet::Sparse(sparse) => sparse.subtract_from(other),
+            HybridBitSet::Dense(dense) => dense.subtract_from(other),
+        }
+    }
+}
+
+pub enum HybridIter<'a, T: Idx> {
+    Sparse(slice::Iter<'a, T>),
+    Dense(BitIter<'a, T>),
+}
+
+impl<'a, T: Idx> Iterator for HybridIter<'a, T> {
+    type Item = T;
+
+    fn next(&mut self) -> Option<T> {
+        match self {
+            HybridIter::Sparse(sparse) => sparse.next().copied(),
+            HybridIter::Dense(dense) => dense.next(),
+        }
+    }
+}
+
+/// A resizable bitset type with a dense representation.
+///
+/// `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.
+#[derive(Clone, Debug, PartialEq)]
+pub struct GrowableBitSet<T: Idx> {
+    bit_set: BitSet<T>,
+}
+
+impl<T: Idx> GrowableBitSet<T> {
+    /// Ensure that the set can hold at least `min_domain_size` elements.
+    pub fn ensure(&mut self, min_domain_size: usize) {
+        if self.bit_set.domain_size < min_domain_size {
+            self.bit_set.domain_size = min_domain_size;
+        }
+
+        let min_num_words = num_words(min_domain_size);
+        if self.bit_set.words.len() < min_num_words {
+            self.bit_set.words.resize(min_num_words, 0)
+        }
+    }
+
+    pub fn new_empty() -> GrowableBitSet<T> {
+        GrowableBitSet { bit_set: BitSet::new_empty(0) }
+    }
+
+    pub fn with_capacity(capacity: usize) -> GrowableBitSet<T> {
+        GrowableBitSet { bit_set: BitSet::new_empty(capacity) }
+    }
+
+    /// Returns `true` if the set has changed.
+    #[inline]
+    pub fn insert(&mut self, elem: T) -> bool {
+        self.ensure(elem.index() + 1);
+        self.bit_set.insert(elem)
+    }
+
+    #[inline]
+    pub fn contains(&self, elem: T) -> bool {
+        let (word_index, mask) = word_index_and_mask(elem);
+        if let Some(word) = self.bit_set.words.get(word_index) { (word & mask) != 0 } else { false }
+    }
+}
+
+/// A fixed-size 2D bit matrix type with a dense representation.
+///
+/// `R` and `C` are index types used to identify rows and columns respectively;
+/// typically newtyped `usize` wrappers, but they can also just be `usize`.
+///
+/// All operations that involve a row and/or column index will panic if the
+/// index exceeds the relevant bound.
+#[derive(Clone, Eq, PartialEq, Decodable, Encodable)]
+pub struct BitMatrix<R: Idx, C: Idx> {
+    num_rows: usize,
+    num_columns: usize,
+    words: Vec<Word>,
+    marker: PhantomData<(R, C)>,
+}
+
+impl<R: Idx, C: Idx> BitMatrix<R, C> {
+    /// Creates a new `rows x columns` matrix, initially empty.
+    pub fn new(num_rows: usize, num_columns: usize) -> BitMatrix<R, C> {
+        // For every element, we need one bit for every other
+        // element. Round up to an even number of words.
+        let words_per_row = num_words(num_columns);
+        BitMatrix {
+            num_rows,
+            num_columns,
+            words: vec![0; num_rows * words_per_row],
+            marker: PhantomData,
+        }
+    }
+
+    /// 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> {
+        let num_columns = row.domain_size();
+        let words_per_row = num_words(num_columns);
+        assert_eq!(words_per_row, row.words().len());
+        BitMatrix {
+            num_rows,
+            num_columns,
+            words: iter::repeat(row.words()).take(num_rows).flatten().cloned().collect(),
+            marker: PhantomData,
+        }
+    }
+
+    pub fn rows(&self) -> impl Iterator<Item = R> {
+        (0..self.num_rows).map(R::new)
+    }
+
+    /// The range of bits for a given row.
+    fn range(&self, row: R) -> (usize, usize) {
+        let words_per_row = num_words(self.num_columns);
+        let start = row.index() * words_per_row;
+        (start, start + words_per_row)
+    }
+
+    /// Sets the cell at `(row, column)` to true. Put another way, insert
+    /// `column` to the bitset for `row`.
+    ///
+    /// Returns `true` if this changed the matrix.
+    pub fn insert(&mut self, row: R, column: C) -> bool {
+        assert!(row.index() < self.num_rows && column.index() < self.num_columns);
+        let (start, _) = self.range(row);
+        let (word_index, mask) = word_index_and_mask(column);
+        let words = &mut self.words[..];
+        let word = words[start + word_index];
+        let new_word = word | mask;
+        words[start + word_index] = new_word;
+        word != new_word
+    }
+
+    /// Do the bits from `row` contain `column`? Put another way, is
+    /// the matrix cell at `(row, column)` true?  Put yet another way,
+    /// if the matrix represents (transitive) reachability, can
+    /// `row` reach `column`?
+    pub fn contains(&self, row: R, column: C) -> bool {
+        assert!(row.index() < self.num_rows && column.index() < self.num_columns);
+        let (start, _) = self.range(row);
+        let (word_index, mask) = word_index_and_mask(column);
+        (self.words[start + word_index] & mask) != 0
+    }
+
+    /// Returns those indices that are true in rows `a` and `b`. This
+    /// is an *O*(*n*) operation where *n* is the number of elements
+    /// (somewhat independent from the actual size of the
+    /// intersection, in particular).
+    pub fn intersect_rows(&self, row1: R, row2: R) -> Vec<C> {
+        assert!(row1.index() < self.num_rows && row2.index() < self.num_rows);
+        let (row1_start, row1_end) = self.range(row1);
+        let (row2_start, row2_end) = self.range(row2);
+        let mut result = Vec::with_capacity(self.num_columns);
+        for (base, (i, j)) in (row1_start..row1_end).zip(row2_start..row2_end).enumerate() {
+            let mut v = self.words[i] & self.words[j];
+            for bit in 0..WORD_BITS {
+                if v == 0 {
+                    break;
+                }
+                if v & 0x1 != 0 {
+                    result.push(C::new(base * WORD_BITS + bit));
+                }
+                v >>= 1;
+            }
+        }
+        result
+    }
+
+    /// Adds the bits from row `read` to the bits from row `write`, and
+    /// returns `true` if anything changed.
+    ///
+    /// This is used when computing transitive reachability because if
+    /// you have an edge `write -> read`, because in that case
+    /// `write` can reach everything that `read` can (and
+    /// potentially more).
+    pub fn union_rows(&mut self, read: R, write: R) -> bool {
+        assert!(read.index() < self.num_rows && write.index() < self.num_rows);
+        let (read_start, read_end) = self.range(read);
+        let (write_start, write_end) = self.range(write);
+        let words = &mut self.words[..];
+        let mut changed = false;
+        for (read_index, write_index) in (read_start..read_end).zip(write_start..write_end) {
+            let word = words[write_index];
+            let new_word = word | words[read_index];
+            words[write_index] = new_word;
+            changed |= word != new_word;
+        }
+        changed
+    }
+
+    /// 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 {
+        assert!(write.index() < self.num_rows);
+        assert_eq!(with.domain_size(), self.num_columns);
+        let (write_start, write_end) = self.range(write);
+        let mut changed = false;
+        for (read_index, write_index) in (0..with.words().len()).zip(write_start..write_end) {
+            let word = self.words[write_index];
+            let new_word = word | with.words()[read_index];
+            self.words[write_index] = new_word;
+            changed |= word != new_word;
+        }
+        changed
+    }
+
+    /// Sets every cell in `row` to true.
+    pub fn insert_all_into_row(&mut self, row: R) {
+        assert!(row.index() < self.num_rows);
+        let (start, end) = self.range(row);
+        let words = &mut self.words[..];
+        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;
+        }
+    }
+
+    /// Gets a slice of the underlying words.
+    pub fn words(&self) -> &[Word] {
+        &self.words
+    }
+
+    /// Iterates through all the columns set to true in a given row of
+    /// the matrix.
+    pub fn iter(&self, row: R) -> BitIter<'_, C> {
+        assert!(row.index() < self.num_rows);
+        let (start, end) = self.range(row);
+        BitIter::new(&self.words[start..end])
+    }
+
+    /// Returns the number of elements in `row`.
+    pub fn count(&self, row: R) -> usize {
+        let (start, end) = self.range(row);
+        self.words[start..end].iter().map(|e| e.count_ones() as usize).sum()
+    }
+}
+
+impl<R: Idx, C: Idx> fmt::Debug for BitMatrix<R, C> {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        /// Forces its contents to print in regular mode instead of alternate mode.
+        struct OneLinePrinter<T>(T);
+        impl<T: fmt::Debug> fmt::Debug for OneLinePrinter<T> {
+            fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+                write!(fmt, "{:?}", self.0)
+            }
+        }
+
+        write!(fmt, "BitMatrix({}x{}) ", self.num_rows, self.num_columns)?;
+        let items = self.rows().flat_map(|r| self.iter(r).map(move |c| (r, c)));
+        fmt.debug_set().entries(items.map(OneLinePrinter)).finish()
+    }
+}
+
+/// 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(<HybridBitSet>)`.
+/// 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.
+///
+/// `R` and `C` are index types used to identify rows and columns respectively;
+/// typically newtyped `usize` wrappers, but they can also just be `usize`.
+#[derive(Clone, Debug)]
+pub struct SparseBitMatrix<R, C>
+where
+    R: Idx,
+    C: Idx,
+{
+    num_columns: usize,
+    rows: IndexVec<R, Option<HybridBitSet<C>>>,
+}
+
+impl<R: Idx, C: Idx> SparseBitMatrix<R, C> {
+    /// Creates a new empty sparse bit matrix with no rows or columns.
+    pub fn new(num_columns: usize) -> Self {
+        Self { num_columns, rows: IndexVec::new() }
+    }
+
+    fn ensure_row(&mut self, row: R) -> &mut HybridBitSet<C> {
+        // Instantiate any missing rows up to and including row `row` with an
+        // empty HybridBitSet.
+        self.rows.ensure_contains_elem(row, || None);
+
+        // Then replace row `row` with a full HybridBitSet if necessary.
+        let num_columns = self.num_columns;
+        self.rows[row].get_or_insert_with(|| HybridBitSet::new_empty(num_columns))
+    }
+
+    /// Sets the cell at `(row, column)` to true. Put another way, insert
+    /// `column` to the bitset for `row`.
+    ///
+    /// Returns `true` if this changed the matrix.
+    pub fn insert(&mut self, row: R, column: C) -> bool {
+        self.ensure_row(row).insert(column)
+    }
+
+    /// Do the bits from `row` contain `column`? Put another way, is
+    /// the matrix cell at `(row, column)` true?  Put yet another way,
+    /// if the matrix represents (transitive) reachability, can
+    /// `row` reach `column`?
+    pub fn contains(&self, row: R, column: C) -> bool {
+        self.row(row).map_or(false, |r| r.contains(column))
+    }
+
+    /// Adds the bits from row `read` to the bits from row `write`, and
+    /// returns `true` if anything changed.
+    ///
+    /// This is used when computing transitive reachability because if
+    /// you have an edge `write -> read`, because in that case
+    /// `write` can reach everything that `read` can (and
+    /// potentially more).
+    pub fn union_rows(&mut self, read: R, write: R) -> bool {
+        if read == write || self.row(read).is_none() {
+            return false;
+        }
+
+        self.ensure_row(write);
+        if let (Some(read_row), Some(write_row)) = self.rows.pick2_mut(read, write) {
+            write_row.union(read_row)
+        } else {
+            unreachable!()
+        }
+    }
+
+    /// Union a row, `from`, into the `into` row.
+    pub fn union_into_row(&mut self, into: R, from: &HybridBitSet<C>) -> bool {
+        self.ensure_row(into).union(from)
+    }
+
+    /// Insert all bits in the given row.
+    pub fn insert_all_into_row(&mut self, row: R) {
+        self.ensure_row(row).insert_all();
+    }
+
+    pub fn rows(&self) -> impl Iterator<Item = R> {
+        self.rows.indices()
+    }
+
+    /// Iterates through all the columns set to true in a given row of
+    /// the matrix.
+    pub fn iter<'a>(&'a self, row: R) -> impl Iterator<Item = C> + 'a {
+        self.row(row).into_iter().flat_map(|r| r.iter())
+    }
+
+    pub fn row(&self, row: R) -> Option<&HybridBitSet<C>> {
+        if let Some(Some(row)) = self.rows.get(row) { Some(row) } else { None }
+    }
+}
+
+#[inline]
+fn num_words<T: Idx>(domain_size: T) -> usize {
+    (domain_size.index() + WORD_BITS - 1) / WORD_BITS
+}
+
+#[inline]
+fn word_index_and_mask<T: Idx>(elem: T) -> (usize, Word) {
+    let elem = elem.index();
+    let word_index = elem / WORD_BITS;
+    let mask = 1 << (elem % WORD_BITS);
+    (word_index, mask)
+}
+
+/// Integral type used to represent the bit set.
+pub trait FiniteBitSetTy:
+    BitAnd<Output = Self>
+    + BitAndAssign
+    + BitOrAssign
+    + Clone
+    + Copy
+    + Shl
+    + Not<Output = Self>
+    + PartialEq
+    + Sized
+{
+    /// Size of the domain representable by this type, e.g. 64 for `u64`.
+    const DOMAIN_SIZE: u32;
+
+    /// Value which represents the `FiniteBitSet` having every bit set.
+    const FILLED: Self;
+    /// Value which represents the `FiniteBitSet` having no bits set.
+    const EMPTY: Self;
+
+    /// Value for one as the integral type.
+    const ONE: Self;
+    /// Value for zero as the integral type.
+    const ZERO: Self;
+
+    /// Perform a checked left shift on the integral type.
+    fn checked_shl(self, rhs: u32) -> Option<Self>;
+    /// Perform a checked right shift on the integral type.
+    fn checked_shr(self, rhs: u32) -> Option<Self>;
+}
+
+impl FiniteBitSetTy for u32 {
+    const DOMAIN_SIZE: u32 = 32;
+
+    const FILLED: Self = Self::MAX;
+    const EMPTY: Self = Self::MIN;
+
+    const ONE: Self = 1u32;
+    const ZERO: Self = 0u32;
+
+    fn checked_shl(self, rhs: u32) -> Option<Self> {
+        self.checked_shl(rhs)
+    }
+
+    fn checked_shr(self, rhs: u32) -> Option<Self> {
+        self.checked_shr(rhs)
+    }
+}
+
+impl std::fmt::Debug for FiniteBitSet<u32> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{:032b}", self.0)
+    }
+}
+
+impl FiniteBitSetTy for u64 {
+    const DOMAIN_SIZE: u32 = 64;
+
+    const FILLED: Self = Self::MAX;
+    const EMPTY: Self = Self::MIN;
+
+    const ONE: Self = 1u64;
+    const ZERO: Self = 0u64;
+
+    fn checked_shl(self, rhs: u32) -> Option<Self> {
+        self.checked_shl(rhs)
+    }
+
+    fn checked_shr(self, rhs: u32) -> Option<Self> {
+        self.checked_shr(rhs)
+    }
+}
+
+impl std::fmt::Debug for FiniteBitSet<u64> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{:064b}", self.0)
+    }
+}
+
+impl FiniteBitSetTy for u128 {
+    const DOMAIN_SIZE: u32 = 128;
+
+    const FILLED: Self = Self::MAX;
+    const EMPTY: Self = Self::MIN;
+
+    const ONE: Self = 1u128;
+    const ZERO: Self = 0u128;
+
+    fn checked_shl(self, rhs: u32) -> Option<Self> {
+        self.checked_shl(rhs)
+    }
+
+    fn checked_shr(self, rhs: u32) -> Option<Self> {
+        self.checked_shr(rhs)
+    }
+}
+
+impl std::fmt::Debug for FiniteBitSet<u128> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{:0128b}", self.0)
+    }
+}
+
+/// A fixed-sized bitset type represented by an integer type. Indices outwith than the range
+/// representable by `T` are considered set.
+#[derive(Copy, Clone, Eq, PartialEq, Decodable, Encodable)]
+pub struct FiniteBitSet<T: FiniteBitSetTy>(pub T);
+
+impl<T: FiniteBitSetTy> FiniteBitSet<T> {
+    /// Creates a new, empty bitset.
+    pub fn new_empty() -> Self {
+        Self(T::EMPTY)
+    }
+
+    /// Sets the `index`th bit.
+    pub fn set(&mut self, index: u32) {
+        self.0 |= T::ONE.checked_shl(index).unwrap_or(T::ZERO);
+    }
+
+    /// Unsets the `index`th bit.
+    pub fn clear(&mut self, index: u32) {
+        self.0 &= !T::ONE.checked_shl(index).unwrap_or(T::ZERO);
+    }
+
+    /// Sets the `i`th to `j`th bits.
+    pub fn set_range(&mut self, range: Range<u32>) {
+        let bits = T::FILLED
+            .checked_shl(range.end - range.start)
+            .unwrap_or(T::ZERO)
+            .not()
+            .checked_shl(range.start)
+            .unwrap_or(T::ZERO);
+        self.0 |= bits;
+    }
+
+    /// Is the set empty?
+    pub fn is_empty(&self) -> bool {
+        self.0 == T::EMPTY
+    }
+
+    /// Returns the domain size of the bitset.
+    pub fn within_domain(&self, index: u32) -> bool {
+        index < T::DOMAIN_SIZE
+    }
+
+    /// Returns if the `index`th bit is set.
+    pub fn contains(&self, index: u32) -> Option<bool> {
+        self.within_domain(index)
+            .then(|| ((self.0.checked_shr(index).unwrap_or(T::ONE)) & T::ONE) == T::ONE)
+    }
+}
+
+impl<T: FiniteBitSetTy> Default for FiniteBitSet<T> {
+    fn default() -> Self {
+        Self::new_empty()
+    }
+}
diff --git a/compiler/rustc_index/src/bit_set/tests.rs b/compiler/rustc_index/src/bit_set/tests.rs
new file mode 100644
index 00000000000..6cc3e9427d1
--- /dev/null
+++ b/compiler/rustc_index/src/bit_set/tests.rs
@@ -0,0 +1,366 @@
+use super::*;
+
+extern crate test;
+use test::Bencher;
+
+#[test]
+fn test_new_filled() {
+    for i in 0..128 {
+        let idx_buf = BitSet::new_filled(i);
+        let elems: Vec<usize> = idx_buf.iter().collect();
+        let expected: Vec<usize> = (0..i).collect();
+        assert_eq!(elems, expected);
+    }
+}
+
+#[test]
+fn bitset_iter_works() {
+    let mut bitset: BitSet<usize> = BitSet::new_empty(100);
+    bitset.insert(1);
+    bitset.insert(10);
+    bitset.insert(19);
+    bitset.insert(62);
+    bitset.insert(63);
+    bitset.insert(64);
+    bitset.insert(65);
+    bitset.insert(66);
+    bitset.insert(99);
+    assert_eq!(bitset.iter().collect::<Vec<_>>(), [1, 10, 19, 62, 63, 64, 65, 66, 99]);
+}
+
+#[test]
+fn bitset_iter_works_2() {
+    let mut bitset: BitSet<usize> = BitSet::new_empty(320);
+    bitset.insert(0);
+    bitset.insert(127);
+    bitset.insert(191);
+    bitset.insert(255);
+    bitset.insert(319);
+    assert_eq!(bitset.iter().collect::<Vec<_>>(), [0, 127, 191, 255, 319]);
+}
+
+#[test]
+fn union_two_sets() {
+    let mut set1: BitSet<usize> = BitSet::new_empty(65);
+    let mut set2: BitSet<usize> = BitSet::new_empty(65);
+    assert!(set1.insert(3));
+    assert!(!set1.insert(3));
+    assert!(set2.insert(5));
+    assert!(set2.insert(64));
+    assert!(set1.union(&set2));
+    assert!(!set1.union(&set2));
+    assert!(set1.contains(3));
+    assert!(!set1.contains(4));
+    assert!(set1.contains(5));
+    assert!(!set1.contains(63));
+    assert!(set1.contains(64));
+}
+
+#[test]
+fn hybrid_bitset() {
+    let mut sparse038: HybridBitSet<usize> = HybridBitSet::new_empty(256);
+    assert!(sparse038.is_empty());
+    assert!(sparse038.insert(0));
+    assert!(sparse038.insert(1));
+    assert!(sparse038.insert(8));
+    assert!(sparse038.insert(3));
+    assert!(!sparse038.insert(3));
+    assert!(sparse038.remove(1));
+    assert!(!sparse038.is_empty());
+    assert_eq!(sparse038.iter().collect::<Vec<_>>(), [0, 3, 8]);
+
+    for i in 0..256 {
+        if i == 0 || i == 3 || i == 8 {
+            assert!(sparse038.contains(i));
+        } else {
+            assert!(!sparse038.contains(i));
+        }
+    }
+
+    let mut sparse01358 = sparse038.clone();
+    assert!(sparse01358.insert(1));
+    assert!(sparse01358.insert(5));
+    assert_eq!(sparse01358.iter().collect::<Vec<_>>(), [0, 1, 3, 5, 8]);
+
+    let mut dense10 = HybridBitSet::new_empty(256);
+    for i in 0..10 {
+        assert!(dense10.insert(i));
+    }
+    assert!(!dense10.is_empty());
+    assert_eq!(dense10.iter().collect::<Vec<_>>(), [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);
+
+    let mut dense256 = HybridBitSet::new_empty(256);
+    assert!(dense256.is_empty());
+    dense256.insert_all();
+    assert!(!dense256.is_empty());
+    for i in 0..256 {
+        assert!(dense256.contains(i));
+    }
+
+    assert!(sparse038.superset(&sparse038)); // sparse + sparse (self)
+    assert!(sparse01358.superset(&sparse038)); // sparse + sparse
+    assert!(dense10.superset(&sparse038)); // dense + sparse
+    assert!(dense10.superset(&dense10)); // dense + dense (self)
+    assert!(dense256.superset(&dense10)); // dense + dense
+
+    let mut hybrid = sparse038;
+    assert!(!sparse01358.union(&hybrid)); // no change
+    assert!(hybrid.union(&sparse01358));
+    assert!(hybrid.superset(&sparse01358) && sparse01358.superset(&hybrid));
+    assert!(!dense10.union(&sparse01358));
+    assert!(!dense256.union(&dense10));
+    let mut dense = dense10;
+    assert!(dense.union(&dense256));
+    assert!(dense.superset(&dense256) && dense256.superset(&dense));
+    assert!(hybrid.union(&dense256));
+    assert!(hybrid.superset(&dense256) && dense256.superset(&hybrid));
+
+    assert_eq!(dense256.iter().count(), 256);
+    let mut dense0 = dense256;
+    for i in 0..256 {
+        assert!(dense0.remove(i));
+    }
+    assert!(!dense0.remove(0));
+    assert!(dense0.is_empty());
+}
+
+#[test]
+fn grow() {
+    let mut set: GrowableBitSet<usize> = GrowableBitSet::with_capacity(65);
+    for index in 0..65 {
+        assert!(set.insert(index));
+        assert!(!set.insert(index));
+    }
+    set.ensure(128);
+
+    // Check if the bits set before growing are still set
+    for index in 0..65 {
+        assert!(set.contains(index));
+    }
+
+    // Check if the new bits are all un-set
+    for index in 65..128 {
+        assert!(!set.contains(index));
+    }
+
+    // Check that we can set all new bits without running out of bounds
+    for index in 65..128 {
+        assert!(set.insert(index));
+        assert!(!set.insert(index));
+    }
+}
+
+#[test]
+fn matrix_intersection() {
+    let mut matrix: BitMatrix<usize, usize> = BitMatrix::new(200, 200);
+
+    // (*) Elements reachable from both 2 and 65.
+
+    matrix.insert(2, 3);
+    matrix.insert(2, 6);
+    matrix.insert(2, 10); // (*)
+    matrix.insert(2, 64); // (*)
+    matrix.insert(2, 65);
+    matrix.insert(2, 130);
+    matrix.insert(2, 160); // (*)
+
+    matrix.insert(64, 133);
+
+    matrix.insert(65, 2);
+    matrix.insert(65, 8);
+    matrix.insert(65, 10); // (*)
+    matrix.insert(65, 64); // (*)
+    matrix.insert(65, 68);
+    matrix.insert(65, 133);
+    matrix.insert(65, 160); // (*)
+
+    let intersection = matrix.intersect_rows(2, 64);
+    assert!(intersection.is_empty());
+
+    let intersection = matrix.intersect_rows(2, 65);
+    assert_eq!(intersection, &[10, 64, 160]);
+}
+
+#[test]
+fn matrix_iter() {
+    let mut matrix: BitMatrix<usize, usize> = BitMatrix::new(64, 100);
+    matrix.insert(3, 22);
+    matrix.insert(3, 75);
+    matrix.insert(2, 99);
+    matrix.insert(4, 0);
+    matrix.union_rows(3, 5);
+    matrix.insert_all_into_row(6);
+
+    let expected = [99];
+    let mut iter = expected.iter();
+    for i in matrix.iter(2) {
+        let j = *iter.next().unwrap();
+        assert_eq!(i, j);
+    }
+    assert!(iter.next().is_none());
+
+    let expected = [22, 75];
+    let mut iter = expected.iter();
+    assert_eq!(matrix.count(3), expected.len());
+    for i in matrix.iter(3) {
+        let j = *iter.next().unwrap();
+        assert_eq!(i, j);
+    }
+    assert!(iter.next().is_none());
+
+    let expected = [0];
+    let mut iter = expected.iter();
+    assert_eq!(matrix.count(4), expected.len());
+    for i in matrix.iter(4) {
+        let j = *iter.next().unwrap();
+        assert_eq!(i, j);
+    }
+    assert!(iter.next().is_none());
+
+    let expected = [22, 75];
+    let mut iter = expected.iter();
+    assert_eq!(matrix.count(5), expected.len());
+    for i in matrix.iter(5) {
+        let j = *iter.next().unwrap();
+        assert_eq!(i, j);
+    }
+    assert!(iter.next().is_none());
+
+    assert_eq!(matrix.count(6), 100);
+    let mut count = 0;
+    for (idx, i) in matrix.iter(6).enumerate() {
+        assert_eq!(idx, i);
+        count += 1;
+    }
+    assert_eq!(count, 100);
+
+    if let Some(i) = matrix.iter(7).next() {
+        panic!("expected no elements in row, but contains element {:?}", i);
+    }
+}
+
+#[test]
+fn sparse_matrix_iter() {
+    let mut matrix: SparseBitMatrix<usize, usize> = SparseBitMatrix::new(100);
+    matrix.insert(3, 22);
+    matrix.insert(3, 75);
+    matrix.insert(2, 99);
+    matrix.insert(4, 0);
+    matrix.union_rows(3, 5);
+
+    let expected = [99];
+    let mut iter = expected.iter();
+    for i in matrix.iter(2) {
+        let j = *iter.next().unwrap();
+        assert_eq!(i, j);
+    }
+    assert!(iter.next().is_none());
+
+    let expected = [22, 75];
+    let mut iter = expected.iter();
+    for i in matrix.iter(3) {
+        let j = *iter.next().unwrap();
+        assert_eq!(i, j);
+    }
+    assert!(iter.next().is_none());
+
+    let expected = [0];
+    let mut iter = expected.iter();
+    for i in matrix.iter(4) {
+        let j = *iter.next().unwrap();
+        assert_eq!(i, j);
+    }
+    assert!(iter.next().is_none());
+
+    let expected = [22, 75];
+    let mut iter = expected.iter();
+    for i in matrix.iter(5) {
+        let j = *iter.next().unwrap();
+        assert_eq!(i, j);
+    }
+    assert!(iter.next().is_none());
+}
+
+/// Merge dense hybrid set into empty sparse hybrid set.
+#[bench]
+fn union_hybrid_sparse_empty_to_dense(b: &mut Bencher) {
+    let mut pre_dense: HybridBitSet<usize> = HybridBitSet::new_empty(256);
+    for i in 0..10 {
+        assert!(pre_dense.insert(i));
+    }
+    let pre_sparse: HybridBitSet<usize> = HybridBitSet::new_empty(256);
+    b.iter(|| {
+        let dense = pre_dense.clone();
+        let mut sparse = pre_sparse.clone();
+        sparse.union(&dense);
+    })
+}
+
+/// Merge dense hybrid set into full hybrid set with same indices.
+#[bench]
+fn union_hybrid_sparse_full_to_dense(b: &mut Bencher) {
+    let mut pre_dense: HybridBitSet<usize> = HybridBitSet::new_empty(256);
+    for i in 0..10 {
+        assert!(pre_dense.insert(i));
+    }
+    let mut pre_sparse: HybridBitSet<usize> = HybridBitSet::new_empty(256);
+    for i in 0..SPARSE_MAX {
+        assert!(pre_sparse.insert(i));
+    }
+    b.iter(|| {
+        let dense = pre_dense.clone();
+        let mut sparse = pre_sparse.clone();
+        sparse.union(&dense);
+    })
+}
+
+/// Merge dense hybrid set into full hybrid set with indices over the whole domain.
+#[bench]
+fn union_hybrid_sparse_domain_to_dense(b: &mut Bencher) {
+    let mut pre_dense: HybridBitSet<usize> = HybridBitSet::new_empty(SPARSE_MAX * 64);
+    for i in 0..10 {
+        assert!(pre_dense.insert(i));
+    }
+    let mut pre_sparse: HybridBitSet<usize> = HybridBitSet::new_empty(SPARSE_MAX * 64);
+    for i in 0..SPARSE_MAX {
+        assert!(pre_sparse.insert(i * 64));
+    }
+    b.iter(|| {
+        let dense = pre_dense.clone();
+        let mut sparse = pre_sparse.clone();
+        sparse.union(&dense);
+    })
+}
+
+/// Merge dense hybrid set into empty hybrid set where the domain is very small.
+#[bench]
+fn union_hybrid_sparse_empty_small_domain(b: &mut Bencher) {
+    let mut pre_dense: HybridBitSet<usize> = HybridBitSet::new_empty(SPARSE_MAX);
+    for i in 0..SPARSE_MAX {
+        assert!(pre_dense.insert(i));
+    }
+    let pre_sparse: HybridBitSet<usize> = HybridBitSet::new_empty(SPARSE_MAX);
+    b.iter(|| {
+        let dense = pre_dense.clone();
+        let mut sparse = pre_sparse.clone();
+        sparse.union(&dense);
+    })
+}
+
+/// Merge dense hybrid set into full hybrid set where the domain is very small.
+#[bench]
+fn union_hybrid_sparse_full_small_domain(b: &mut Bencher) {
+    let mut pre_dense: HybridBitSet<usize> = HybridBitSet::new_empty(SPARSE_MAX);
+    for i in 0..SPARSE_MAX {
+        assert!(pre_dense.insert(i));
+    }
+    let mut pre_sparse: HybridBitSet<usize> = HybridBitSet::new_empty(SPARSE_MAX);
+    for i in 0..SPARSE_MAX {
+        assert!(pre_sparse.insert(i));
+    }
+    b.iter(|| {
+        let dense = pre_dense.clone();
+        let mut sparse = pre_sparse.clone();
+        sparse.union(&dense);
+    })
+}
diff --git a/compiler/rustc_index/src/lib.rs b/compiler/rustc_index/src/lib.rs
new file mode 100644
index 00000000000..7ee881b0639
--- /dev/null
+++ b/compiler/rustc_index/src/lib.rs
@@ -0,0 +1,11 @@
+#![feature(allow_internal_unstable)]
+#![feature(bool_to_option)]
+#![feature(const_fn)]
+#![feature(const_panic)]
+#![feature(extend_one)]
+#![feature(unboxed_closures)]
+#![feature(test)]
+#![feature(fn_traits)]
+
+pub mod bit_set;
+pub mod vec;
diff --git a/compiler/rustc_index/src/vec.rs b/compiler/rustc_index/src/vec.rs
new file mode 100644
index 00000000000..2420f82c041
--- /dev/null
+++ b/compiler/rustc_index/src/vec.rs
@@ -0,0 +1,846 @@
+use rustc_serialize::{Decodable, Decoder, Encodable, Encoder};
+
+use std::fmt;
+use std::fmt::Debug;
+use std::hash::Hash;
+use std::iter::{self, FromIterator};
+use std::marker::PhantomData;
+use std::ops::{Index, IndexMut, Range, RangeBounds};
+use std::slice;
+use std::vec;
+
+/// Represents some newtyped `usize` wrapper.
+///
+/// Purpose: avoid mixing indexes for different bitvector domains.
+pub trait Idx: Copy + 'static + Ord + Debug + Hash {
+    fn new(idx: usize) -> Self;
+
+    fn index(self) -> usize;
+
+    fn increment_by(&mut self, amount: usize) {
+        *self = self.plus(amount);
+    }
+
+    fn plus(self, amount: usize) -> Self {
+        Self::new(self.index() + amount)
+    }
+}
+
+impl Idx for usize {
+    #[inline]
+    fn new(idx: usize) -> Self {
+        idx
+    }
+    #[inline]
+    fn index(self) -> usize {
+        self
+    }
+}
+
+impl Idx for u32 {
+    #[inline]
+    fn new(idx: usize) -> Self {
+        assert!(idx <= u32::MAX as usize);
+        idx as u32
+    }
+    #[inline]
+    fn index(self) -> usize {
+        self as usize
+    }
+}
+
+/// Creates a struct type `S` that can be used as an index with
+/// `IndexVec` and so on.
+///
+/// There are two ways of interacting with these indices:
+///
+/// - The `From` impls are the preferred way. So you can do
+///   `S::from(v)` with a `usize` or `u32`. And you can convert back
+///   to an integer with `u32::from(s)`.
+///
+/// - Alternatively, you can use the methods `S::new(v)` and `s.index()`
+///   to create/return a value.
+///
+/// Internally, the index uses a u32, so the index must not exceed
+/// `u32::MAX`. You can also customize things like the `Debug` impl,
+/// what traits are derived, and so forth via the macro.
+#[macro_export]
+#[allow_internal_unstable(step_trait, step_trait_ext, rustc_attrs)]
+macro_rules! newtype_index {
+    // ---- public rules ----
+
+    // Use default constants
+    ($(#[$attrs:meta])* $v:vis struct $name:ident { .. }) => (
+        $crate::newtype_index!(
+            // Leave out derives marker so we can use its absence to ensure it comes first
+            @attrs        [$(#[$attrs])*]
+            @type         [$name]
+            // shave off 256 indices at the end to allow space for packing these indices into enums
+            @max          [0xFFFF_FF00]
+            @vis          [$v]
+            @debug_format ["{}"]);
+    );
+
+    // Define any constants
+    ($(#[$attrs:meta])* $v:vis struct $name:ident { $($tokens:tt)+ }) => (
+        $crate::newtype_index!(
+            // Leave out derives marker so we can use its absence to ensure it comes first
+            @attrs        [$(#[$attrs])*]
+            @type         [$name]
+            // shave off 256 indices at the end to allow space for packing these indices into enums
+            @max          [0xFFFF_FF00]
+            @vis          [$v]
+            @debug_format ["{}"]
+                          $($tokens)+);
+    );
+
+    // ---- private rules ----
+
+    // Base case, user-defined constants (if any) have already been defined
+    (@derives      [$($derives:ident,)*]
+     @attrs        [$(#[$attrs:meta])*]
+     @type         [$type:ident]
+     @max          [$max:expr]
+     @vis          [$v:vis]
+     @debug_format [$debug_format:tt]) => (
+        $(#[$attrs])*
+        #[derive(Copy, PartialEq, Eq, Hash, PartialOrd, Ord, $($derives),*)]
+        #[rustc_layout_scalar_valid_range_end($max)]
+        $v struct $type {
+            private: u32
+        }
+
+        impl Clone for $type {
+            fn clone(&self) -> Self {
+                *self
+            }
+        }
+
+        impl $type {
+            $v const MAX_AS_U32: u32 = $max;
+
+            $v const MAX: Self = Self::from_u32($max);
+
+            #[inline]
+            $v const fn from_usize(value: usize) -> Self {
+                assert!(value <= ($max as usize));
+                unsafe {
+                    Self::from_u32_unchecked(value as u32)
+                }
+            }
+
+            #[inline]
+            $v const fn from_u32(value: u32) -> Self {
+                assert!(value <= $max);
+                unsafe {
+                    Self::from_u32_unchecked(value)
+                }
+            }
+
+            #[inline]
+            $v const unsafe fn from_u32_unchecked(value: u32) -> Self {
+                Self { private: value }
+            }
+
+            /// Extracts the value of this index as an integer.
+            #[inline]
+            $v const fn index(self) -> usize {
+                self.as_usize()
+            }
+
+            /// Extracts the value of this index as a `u32`.
+            #[inline]
+            $v const fn as_u32(self) -> u32 {
+                self.private
+            }
+
+            /// Extracts the value of this index as a `usize`.
+            #[inline]
+            $v const fn as_usize(self) -> usize {
+                self.as_u32() as usize
+            }
+        }
+
+        impl std::ops::Add<usize> for $type {
+            type Output = Self;
+
+            fn add(self, other: usize) -> Self {
+                Self::from_usize(self.index() + other)
+            }
+        }
+
+        impl $crate::vec::Idx for $type {
+            #[inline]
+            fn new(value: usize) -> Self {
+                Self::from_usize(value)
+            }
+
+            #[inline]
+            fn index(self) -> usize {
+                self.as_usize()
+            }
+        }
+
+        unsafe impl ::std::iter::Step for $type {
+            #[inline]
+            fn steps_between(start: &Self, end: &Self) -> Option<usize> {
+                <usize as ::std::iter::Step>::steps_between(
+                    &Self::index(*start),
+                    &Self::index(*end),
+                )
+            }
+
+            #[inline]
+            fn forward_checked(start: Self, u: usize) -> Option<Self> {
+                Self::index(start).checked_add(u).map(Self::from_usize)
+            }
+
+            #[inline]
+            fn backward_checked(start: Self, u: usize) -> Option<Self> {
+                Self::index(start).checked_sub(u).map(Self::from_usize)
+            }
+        }
+
+        impl From<$type> for u32 {
+            #[inline]
+            fn from(v: $type) -> u32 {
+                v.as_u32()
+            }
+        }
+
+        impl From<$type> for usize {
+            #[inline]
+            fn from(v: $type) -> usize {
+                v.as_usize()
+            }
+        }
+
+        impl From<usize> for $type {
+            #[inline]
+            fn from(value: usize) -> Self {
+                Self::from_usize(value)
+            }
+        }
+
+        impl From<u32> for $type {
+            #[inline]
+            fn from(value: u32) -> Self {
+                Self::from_u32(value)
+            }
+        }
+
+        $crate::newtype_index!(
+            @handle_debug
+            @derives      [$($derives,)*]
+            @type         [$type]
+            @debug_format [$debug_format]);
+    );
+
+    // base case for handle_debug where format is custom. No Debug implementation is emitted.
+    (@handle_debug
+     @derives      [$($_derives:ident,)*]
+     @type         [$type:ident]
+     @debug_format [custom]) => ();
+
+    // base case for handle_debug, no debug overrides found, so use default
+    (@handle_debug
+     @derives      []
+     @type         [$type:ident]
+     @debug_format [$debug_format:tt]) => (
+        impl ::std::fmt::Debug for $type {
+            fn fmt(&self, fmt: &mut ::std::fmt::Formatter<'_>) -> ::std::fmt::Result {
+                write!(fmt, $debug_format, self.as_u32())
+            }
+        }
+    );
+
+    // Debug is requested for derive, don't generate any Debug implementation.
+    (@handle_debug
+     @derives      [Debug, $($derives:ident,)*]
+     @type         [$type:ident]
+     @debug_format [$debug_format:tt]) => ();
+
+    // It's not Debug, so just pop it off the front of the derives stack and check the rest.
+    (@handle_debug
+     @derives      [$_derive:ident, $($derives:ident,)*]
+     @type         [$type:ident]
+     @debug_format [$debug_format:tt]) => (
+        $crate::newtype_index!(
+            @handle_debug
+            @derives      [$($derives,)*]
+            @type         [$type]
+            @debug_format [$debug_format]);
+    );
+
+    // Append comma to end of derives list if it's missing
+    (@attrs        [$(#[$attrs:meta])*]
+     @type         [$type:ident]
+     @max          [$max:expr]
+     @vis          [$v:vis]
+     @debug_format [$debug_format:tt]
+                   derive [$($derives:ident),*]
+                   $($tokens:tt)*) => (
+        $crate::newtype_index!(
+            @attrs        [$(#[$attrs])*]
+            @type         [$type]
+            @max          [$max]
+            @vis          [$v]
+            @debug_format [$debug_format]
+                          derive [$($derives,)*]
+                          $($tokens)*);
+    );
+
+    // By not including the @derives marker in this list nor in the default args, we can force it
+    // to come first if it exists. When encodable is custom, just use the derives list as-is.
+    (@attrs        [$(#[$attrs:meta])*]
+     @type         [$type:ident]
+     @max          [$max:expr]
+     @vis          [$v:vis]
+     @debug_format [$debug_format:tt]
+                   derive [$($derives:ident,)+]
+                   ENCODABLE = custom
+                   $($tokens:tt)*) => (
+        $crate::newtype_index!(
+            @attrs        [$(#[$attrs])*]
+            @derives      [$($derives,)+]
+            @type         [$type]
+            @max          [$max]
+            @vis          [$v]
+            @debug_format [$debug_format]
+                          $($tokens)*);
+    );
+
+    // By not including the @derives marker in this list nor in the default args, we can force it
+    // to come first if it exists. When encodable isn't custom, add serialization traits by default.
+    (@attrs        [$(#[$attrs:meta])*]
+     @type         [$type:ident]
+     @max          [$max:expr]
+     @vis          [$v:vis]
+     @debug_format [$debug_format:tt]
+                   derive [$($derives:ident,)+]
+                   $($tokens:tt)*) => (
+        $crate::newtype_index!(
+            @derives      [$($derives,)+]
+            @attrs        [$(#[$attrs])*]
+            @type         [$type]
+            @max          [$max]
+            @vis          [$v]
+            @debug_format [$debug_format]
+                          $($tokens)*);
+        $crate::newtype_index!(@serializable $type);
+    );
+
+    // The case where no derives are added, but encodable is overridden. Don't
+    // derive serialization traits
+    (@attrs        [$(#[$attrs:meta])*]
+     @type         [$type:ident]
+     @max          [$max:expr]
+     @vis          [$v:vis]
+     @debug_format [$debug_format:tt]
+                   ENCODABLE = custom
+                   $($tokens:tt)*) => (
+        $crate::newtype_index!(
+            @derives      []
+            @attrs        [$(#[$attrs])*]
+            @type         [$type]
+            @max          [$max]
+            @vis          [$v]
+            @debug_format [$debug_format]
+                          $($tokens)*);
+    );
+
+    // The case where no derives are added, add serialization derives by default
+    (@attrs        [$(#[$attrs:meta])*]
+     @type         [$type:ident]
+     @max          [$max:expr]
+     @vis          [$v:vis]
+     @debug_format [$debug_format:tt]
+                   $($tokens:tt)*) => (
+        $crate::newtype_index!(
+            @derives      []
+            @attrs        [$(#[$attrs])*]
+            @type         [$type]
+            @max          [$max]
+            @vis          [$v]
+            @debug_format [$debug_format]
+                          $($tokens)*);
+        $crate::newtype_index!(@serializable $type);
+    );
+
+    (@serializable $type:ident) => (
+        impl<D: ::rustc_serialize::Decoder> ::rustc_serialize::Decodable<D> for $type {
+            fn decode(d: &mut D) -> Result<Self, D::Error> {
+                d.read_u32().map(Self::from_u32)
+            }
+        }
+        impl<E: ::rustc_serialize::Encoder> ::rustc_serialize::Encodable<E> for $type {
+            fn encode(&self, e: &mut E) -> Result<(), E::Error> {
+                e.emit_u32(self.private)
+            }
+        }
+    );
+
+    // Rewrite final without comma to one that includes comma
+    (@derives      [$($derives:ident,)*]
+     @attrs        [$(#[$attrs:meta])*]
+     @type         [$type:ident]
+     @max          [$max:expr]
+     @vis          [$v:vis]
+     @debug_format [$debug_format:tt]
+                   $name:ident = $constant:expr) => (
+        $crate::newtype_index!(
+            @derives      [$($derives,)*]
+            @attrs        [$(#[$attrs])*]
+            @type         [$type]
+            @max          [$max]
+            @vis          [$v]
+            @debug_format [$debug_format]
+                          $name = $constant,);
+    );
+
+    // Rewrite final const without comma to one that includes comma
+    (@derives      [$($derives:ident,)*]
+     @attrs        [$(#[$attrs:meta])*]
+     @type         [$type:ident]
+     @max          [$max:expr]
+     @vis          [$v:vis]
+     @debug_format [$debug_format:tt]
+                   $(#[doc = $doc:expr])*
+                   const $name:ident = $constant:expr) => (
+        $crate::newtype_index!(
+            @derives      [$($derives,)*]
+            @attrs        [$(#[$attrs])*]
+            @type         [$type]
+            @max          [$max]
+            @vis          [$v]
+            @debug_format [$debug_format]
+                          $(#[doc = $doc])* const $name = $constant,);
+    );
+
+    // Replace existing default for max
+    (@derives      [$($derives:ident,)*]
+     @attrs        [$(#[$attrs:meta])*]
+     @type         [$type:ident]
+     @max          [$_max:expr]
+     @vis          [$v:vis]
+     @debug_format [$debug_format:tt]
+                   MAX = $max:expr,
+                   $($tokens:tt)*) => (
+        $crate::newtype_index!(
+            @derives      [$($derives,)*]
+            @attrs        [$(#[$attrs])*]
+            @type         [$type]
+            @max          [$max]
+            @vis          [$v]
+            @debug_format [$debug_format]
+                          $($tokens)*);
+    );
+
+    // Replace existing default for debug_format
+    (@derives      [$($derives:ident,)*]
+     @attrs        [$(#[$attrs:meta])*]
+     @type         [$type:ident]
+     @max          [$max:expr]
+     @vis          [$v:vis]
+     @debug_format [$_debug_format:tt]
+                   DEBUG_FORMAT = $debug_format:tt,
+                   $($tokens:tt)*) => (
+        $crate::newtype_index!(
+            @derives      [$($derives,)*]
+            @attrs        [$(#[$attrs])*]
+            @type         [$type]
+            @max          [$max]
+            @vis          [$v]
+            @debug_format [$debug_format]
+                          $($tokens)*);
+    );
+
+    // Assign a user-defined constant
+    (@derives      [$($derives:ident,)*]
+     @attrs        [$(#[$attrs:meta])*]
+     @type         [$type:ident]
+     @max          [$max:expr]
+     @vis          [$v:vis]
+     @debug_format [$debug_format:tt]
+                   $(#[doc = $doc:expr])*
+                   const $name:ident = $constant:expr,
+                   $($tokens:tt)*) => (
+        $(#[doc = $doc])*
+        $v const $name: $type = $type::from_u32($constant);
+        $crate::newtype_index!(
+            @derives      [$($derives,)*]
+            @attrs        [$(#[$attrs])*]
+            @type         [$type]
+            @max          [$max]
+            @vis          [$v]
+            @debug_format [$debug_format]
+                          $($tokens)*);
+    );
+}
+
+#[derive(Clone, PartialEq, Eq, Hash)]
+pub struct IndexVec<I: Idx, T> {
+    pub raw: Vec<T>,
+    _marker: PhantomData<fn(&I)>,
+}
+
+// Whether `IndexVec` is `Send` depends only on the data,
+// not the phantom data.
+unsafe impl<I: Idx, T> Send for IndexVec<I, T> where T: Send {}
+
+impl<S: Encoder, I: Idx, T: Encodable<S>> Encodable<S> for IndexVec<I, T> {
+    fn encode(&self, s: &mut S) -> Result<(), S::Error> {
+        Encodable::encode(&self.raw, s)
+    }
+}
+
+impl<S: Encoder, I: Idx, T: Encodable<S>> Encodable<S> for &IndexVec<I, T> {
+    fn encode(&self, s: &mut S) -> Result<(), S::Error> {
+        Encodable::encode(&self.raw, s)
+    }
+}
+
+impl<D: Decoder, I: Idx, T: Decodable<D>> Decodable<D> for IndexVec<I, T> {
+    fn decode(d: &mut D) -> Result<Self, D::Error> {
+        Decodable::decode(d).map(|v| IndexVec { raw: v, _marker: PhantomData })
+    }
+}
+
+impl<I: Idx, T: fmt::Debug> fmt::Debug for IndexVec<I, T> {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Debug::fmt(&self.raw, fmt)
+    }
+}
+
+pub type Enumerated<I, J> = iter::Map<iter::Enumerate<J>, IntoIdx<I>>;
+
+impl<I: Idx, T> IndexVec<I, T> {
+    #[inline]
+    pub fn new() -> Self {
+        IndexVec { raw: Vec::new(), _marker: PhantomData }
+    }
+
+    #[inline]
+    pub fn from_raw(raw: Vec<T>) -> Self {
+        IndexVec { raw, _marker: PhantomData }
+    }
+
+    #[inline]
+    pub fn with_capacity(capacity: usize) -> Self {
+        IndexVec { raw: Vec::with_capacity(capacity), _marker: PhantomData }
+    }
+
+    #[inline]
+    pub fn from_elem<S>(elem: T, universe: &IndexVec<I, S>) -> Self
+    where
+        T: Clone,
+    {
+        IndexVec { raw: vec![elem; universe.len()], _marker: PhantomData }
+    }
+
+    #[inline]
+    pub fn from_elem_n(elem: T, n: usize) -> Self
+    where
+        T: Clone,
+    {
+        IndexVec { raw: vec![elem; n], _marker: PhantomData }
+    }
+
+    /// Create an `IndexVec` with `n` elements, where the value of each
+    /// element is the result of `func(i)`. (The underlying vector will
+    /// be allocated only once, with a capacity of at least `n`.)
+    #[inline]
+    pub fn from_fn_n(func: impl FnMut(I) -> T, n: usize) -> Self {
+        let indices = (0..n).map(I::new);
+        Self::from_raw(indices.map(func).collect())
+    }
+
+    #[inline]
+    pub fn push(&mut self, d: T) -> I {
+        let idx = I::new(self.len());
+        self.raw.push(d);
+        idx
+    }
+
+    #[inline]
+    pub fn pop(&mut self) -> Option<T> {
+        self.raw.pop()
+    }
+
+    #[inline]
+    pub fn len(&self) -> usize {
+        self.raw.len()
+    }
+
+    /// Gives the next index that will be assigned when `push` is
+    /// called.
+    #[inline]
+    pub fn next_index(&self) -> I {
+        I::new(self.len())
+    }
+
+    #[inline]
+    pub fn is_empty(&self) -> bool {
+        self.raw.is_empty()
+    }
+
+    #[inline]
+    pub fn into_iter(self) -> vec::IntoIter<T> {
+        self.raw.into_iter()
+    }
+
+    #[inline]
+    pub fn into_iter_enumerated(self) -> Enumerated<I, vec::IntoIter<T>> {
+        self.raw.into_iter().enumerate().map(IntoIdx { _marker: PhantomData })
+    }
+
+    #[inline]
+    pub fn iter(&self) -> slice::Iter<'_, T> {
+        self.raw.iter()
+    }
+
+    #[inline]
+    pub fn iter_enumerated(&self) -> Enumerated<I, slice::Iter<'_, T>> {
+        self.raw.iter().enumerate().map(IntoIdx { _marker: PhantomData })
+    }
+
+    #[inline]
+    pub fn indices(&self) -> iter::Map<Range<usize>, IntoIdx<I>> {
+        (0..self.len()).map(IntoIdx { _marker: PhantomData })
+    }
+
+    #[inline]
+    pub fn iter_mut(&mut self) -> slice::IterMut<'_, T> {
+        self.raw.iter_mut()
+    }
+
+    #[inline]
+    pub fn iter_enumerated_mut(&mut self) -> Enumerated<I, slice::IterMut<'_, T>> {
+        self.raw.iter_mut().enumerate().map(IntoIdx { _marker: PhantomData })
+    }
+
+    #[inline]
+    pub fn drain<'a, R: RangeBounds<usize>>(
+        &'a mut self,
+        range: R,
+    ) -> impl Iterator<Item = T> + 'a {
+        self.raw.drain(range)
+    }
+
+    #[inline]
+    pub fn drain_enumerated<'a, R: RangeBounds<usize>>(
+        &'a mut self,
+        range: R,
+    ) -> impl Iterator<Item = (I, T)> + 'a {
+        self.raw.drain(range).enumerate().map(IntoIdx { _marker: PhantomData })
+    }
+
+    #[inline]
+    pub fn last(&self) -> Option<I> {
+        self.len().checked_sub(1).map(I::new)
+    }
+
+    #[inline]
+    pub fn shrink_to_fit(&mut self) {
+        self.raw.shrink_to_fit()
+    }
+
+    #[inline]
+    pub fn swap(&mut self, a: I, b: I) {
+        self.raw.swap(a.index(), b.index())
+    }
+
+    #[inline]
+    pub fn truncate(&mut self, a: usize) {
+        self.raw.truncate(a)
+    }
+
+    #[inline]
+    pub fn get(&self, index: I) -> Option<&T> {
+        self.raw.get(index.index())
+    }
+
+    #[inline]
+    pub fn get_mut(&mut self, index: I) -> Option<&mut T> {
+        self.raw.get_mut(index.index())
+    }
+
+    /// Returns mutable references to two distinct elements, a and b. Panics if a == b.
+    #[inline]
+    pub fn pick2_mut(&mut self, a: I, b: I) -> (&mut T, &mut T) {
+        let (ai, bi) = (a.index(), b.index());
+        assert!(ai != bi);
+
+        if ai < bi {
+            let (c1, c2) = self.raw.split_at_mut(bi);
+            (&mut c1[ai], &mut c2[0])
+        } else {
+            let (c2, c1) = self.pick2_mut(b, a);
+            (c1, c2)
+        }
+    }
+
+    /// Returns mutable references to three distinct elements or panics otherwise.
+    #[inline]
+    pub fn pick3_mut(&mut self, a: I, b: I, c: I) -> (&mut T, &mut T, &mut T) {
+        let (ai, bi, ci) = (a.index(), b.index(), c.index());
+        assert!(ai != bi && bi != ci && ci != ai);
+        let len = self.raw.len();
+        assert!(ai < len && bi < len && ci < len);
+        let ptr = self.raw.as_mut_ptr();
+        unsafe { (&mut *ptr.add(ai), &mut *ptr.add(bi), &mut *ptr.add(ci)) }
+    }
+
+    pub fn convert_index_type<Ix: Idx>(self) -> IndexVec<Ix, T> {
+        IndexVec { raw: self.raw, _marker: PhantomData }
+    }
+}
+
+impl<I: Idx, T: Clone> IndexVec<I, T> {
+    /// Grows the index vector so that it contains an entry for
+    /// `elem`; if that is already true, then has no
+    /// effect. Otherwise, inserts new values as needed by invoking
+    /// `fill_value`.
+    #[inline]
+    pub fn ensure_contains_elem(&mut self, elem: I, fill_value: impl FnMut() -> T) {
+        let min_new_len = elem.index() + 1;
+        if self.len() < min_new_len {
+            self.raw.resize_with(min_new_len, fill_value);
+        }
+    }
+
+    #[inline]
+    pub fn resize(&mut self, new_len: usize, value: T) {
+        self.raw.resize(new_len, value)
+    }
+
+    #[inline]
+    pub fn resize_to_elem(&mut self, elem: I, fill_value: impl FnMut() -> T) {
+        let min_new_len = elem.index() + 1;
+        self.raw.resize_with(min_new_len, fill_value);
+    }
+}
+
+impl<I: Idx, T: Ord> IndexVec<I, T> {
+    #[inline]
+    pub fn binary_search(&self, value: &T) -> Result<I, I> {
+        match self.raw.binary_search(value) {
+            Ok(i) => Ok(Idx::new(i)),
+            Err(i) => Err(Idx::new(i)),
+        }
+    }
+}
+
+impl<I: Idx, T> Index<I> for IndexVec<I, T> {
+    type Output = T;
+
+    #[inline]
+    fn index(&self, index: I) -> &T {
+        &self.raw[index.index()]
+    }
+}
+
+impl<I: Idx, T> IndexMut<I> for IndexVec<I, T> {
+    #[inline]
+    fn index_mut(&mut self, index: I) -> &mut T {
+        &mut self.raw[index.index()]
+    }
+}
+
+impl<I: Idx, T> Default for IndexVec<I, T> {
+    #[inline]
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl<I: Idx, T> Extend<T> for IndexVec<I, T> {
+    #[inline]
+    fn extend<J: IntoIterator<Item = T>>(&mut self, iter: J) {
+        self.raw.extend(iter);
+    }
+
+    #[inline]
+    fn extend_one(&mut self, item: T) {
+        self.raw.push(item);
+    }
+
+    #[inline]
+    fn extend_reserve(&mut self, additional: usize) {
+        self.raw.reserve(additional);
+    }
+}
+
+impl<I: Idx, T> FromIterator<T> for IndexVec<I, T> {
+    #[inline]
+    fn from_iter<J>(iter: J) -> Self
+    where
+        J: IntoIterator<Item = T>,
+    {
+        IndexVec { raw: FromIterator::from_iter(iter), _marker: PhantomData }
+    }
+}
+
+impl<I: Idx, T> IntoIterator for IndexVec<I, T> {
+    type Item = T;
+    type IntoIter = vec::IntoIter<T>;
+
+    #[inline]
+    fn into_iter(self) -> vec::IntoIter<T> {
+        self.raw.into_iter()
+    }
+}
+
+impl<'a, I: Idx, T> IntoIterator for &'a IndexVec<I, T> {
+    type Item = &'a T;
+    type IntoIter = slice::Iter<'a, T>;
+
+    #[inline]
+    fn into_iter(self) -> slice::Iter<'a, T> {
+        self.raw.iter()
+    }
+}
+
+impl<'a, I: Idx, T> IntoIterator for &'a mut IndexVec<I, T> {
+    type Item = &'a mut T;
+    type IntoIter = slice::IterMut<'a, T>;
+
+    #[inline]
+    fn into_iter(self) -> slice::IterMut<'a, T> {
+        self.raw.iter_mut()
+    }
+}
+
+pub struct IntoIdx<I: Idx> {
+    _marker: PhantomData<fn(&I)>,
+}
+impl<I: Idx, T> FnOnce<((usize, T),)> for IntoIdx<I> {
+    type Output = (I, T);
+
+    extern "rust-call" fn call_once(self, ((n, t),): ((usize, T),)) -> Self::Output {
+        (I::new(n), t)
+    }
+}
+
+impl<I: Idx, T> FnMut<((usize, T),)> for IntoIdx<I> {
+    extern "rust-call" fn call_mut(&mut self, ((n, t),): ((usize, T),)) -> Self::Output {
+        (I::new(n), t)
+    }
+}
+
+impl<I: Idx> FnOnce<(usize,)> for IntoIdx<I> {
+    type Output = I;
+
+    extern "rust-call" fn call_once(self, (n,): (usize,)) -> Self::Output {
+        I::new(n)
+    }
+}
+
+impl<I: Idx> FnMut<(usize,)> for IntoIdx<I> {
+    extern "rust-call" fn call_mut(&mut self, (n,): (usize,)) -> Self::Output {
+        I::new(n)
+    }
+}
+
+#[cfg(test)]
+mod tests;
diff --git a/compiler/rustc_index/src/vec/tests.rs b/compiler/rustc_index/src/vec/tests.rs
new file mode 100644
index 00000000000..15c43c72c7b
--- /dev/null
+++ b/compiler/rustc_index/src/vec/tests.rs
@@ -0,0 +1,51 @@
+#![allow(dead_code)]
+newtype_index!(struct MyIdx { MAX = 0xFFFF_FFFA });
+
+#[test]
+fn index_size_is_optimized() {
+    use std::mem::size_of;
+
+    assert_eq!(size_of::<MyIdx>(), 4);
+    // Uses 0xFFFF_FFFB
+    assert_eq!(size_of::<Option<MyIdx>>(), 4);
+    // Uses 0xFFFF_FFFC
+    assert_eq!(size_of::<Option<Option<MyIdx>>>(), 4);
+    // Uses 0xFFFF_FFFD
+    assert_eq!(size_of::<Option<Option<Option<MyIdx>>>>(), 4);
+    // Uses 0xFFFF_FFFE
+    assert_eq!(size_of::<Option<Option<Option<Option<MyIdx>>>>>(), 4);
+    // Uses 0xFFFF_FFFF
+    assert_eq!(size_of::<Option<Option<Option<Option<Option<MyIdx>>>>>>(), 4);
+    // Uses a tag
+    assert_eq!(size_of::<Option<Option<Option<Option<Option<Option<MyIdx>>>>>>>(), 8);
+}
+
+#[test]
+fn range_iterator_iterates_forwards() {
+    let range = MyIdx::from_u32(1)..MyIdx::from_u32(4);
+    assert_eq!(
+        range.collect::<Vec<_>>(),
+        [MyIdx::from_u32(1), MyIdx::from_u32(2), MyIdx::from_u32(3)]
+    );
+}
+
+#[test]
+fn range_iterator_iterates_backwards() {
+    let range = MyIdx::from_u32(1)..MyIdx::from_u32(4);
+    assert_eq!(
+        range.rev().collect::<Vec<_>>(),
+        [MyIdx::from_u32(3), MyIdx::from_u32(2), MyIdx::from_u32(1)]
+    );
+}
+
+#[test]
+fn range_count_is_correct() {
+    let range = MyIdx::from_u32(1)..MyIdx::from_u32(4);
+    assert_eq!(range.count(), 3);
+}
+
+#[test]
+fn range_size_hint_is_correct() {
+    let range = MyIdx::from_u32(1)..MyIdx::from_u32(4);
+    assert_eq!(range.size_hint(), (3, Some(3)));
+}