Use BitSet in SparseBitMatrix.

A `ChunkedBitSet` has to be at least 2048 bits for it to outperform a
`BitSet`, because that's the chunk size. The largest `SparseBitMatrix`
encountered when compiling the compiler and the entire rustc-perf
benchmark suite is less than 600 bits.

This change is a tiny perf win, but the motivation is more about
avoiding uses of `ChunkedBitSet` outside of `MixedBitSet`.

Author: nnethercote

compiler/rustc_index/src/bit_set.rs

@@ -1545,7 +1545,7 @@ where
     C: Idx,
 {
     num_columns: usize,
-    rows: IndexVec<R, Option<ChunkedBitSet<C>>>,
+    rows: IndexVec<R, Option<BitSet<C>>>,
 }
 
 impl<R: Idx, C: Idx> SparseBitMatrix<R, C> {
@@ -1554,10 +1554,10 @@ impl<R: Idx, C: Idx> SparseBitMatrix<R, C> {
         Self { num_columns, rows: IndexVec::new() }
     }
 
-    fn ensure_row(&mut self, row: R) -> &mut ChunkedBitSet<C> {
-        // Instantiate any missing rows up to and including row `row` with an empty ChunkedBitSet.
-        // Then replace row `row` with a full ChunkedBitSet if necessary.
-        self.rows.get_or_insert_with(row, || ChunkedBitSet::new_empty(self.num_columns))
+    fn ensure_row(&mut self, row: R) -> &mut BitSet<C> {
+        // Instantiate any missing rows up to and including row `row` with an empty `BitSet`.
+        // Then replace row `row` with a full `BitSet` if necessary.
+        self.rows.get_or_insert_with(row, || BitSet::new_empty(self.num_columns))
     }
 
     /// Sets the cell at `(row, column)` to true. Put another way, insert
@@ -1631,7 +1631,7 @@ impl<R: Idx, C: Idx> SparseBitMatrix<R, C> {
         self.row(row).into_iter().flat_map(|r| r.iter())
     }
 
-    pub fn row(&self, row: R) -> Option<&ChunkedBitSet<C>> {
+    pub fn row(&self, row: R) -> Option<&BitSet<C>> {
         self.rows.get(row)?.as_ref()
     }
 
@@ -1641,7 +1641,7 @@ impl<R: Idx, C: Idx> SparseBitMatrix<R, C> {
     /// Returns true if the row was changed.
     pub fn intersect_row<Set>(&mut self, row: R, set: &Set) -> bool
     where
-        ChunkedBitSet<C>: BitRelations<Set>,
+        BitSet<C>: BitRelations<Set>,
     {
         match self.rows.get_mut(row) {
             Some(Some(row)) => row.intersect(set),
@@ -1655,7 +1655,7 @@ impl<R: Idx, C: Idx> SparseBitMatrix<R, C> {
     /// Returns true if the row was changed.
     pub fn subtract_row<Set>(&mut self, row: R, set: &Set) -> bool
     where
-        ChunkedBitSet<C>: BitRelations<Set>,
+        BitSet<C>: BitRelations<Set>,
     {
         match self.rows.get_mut(row) {
             Some(Some(row)) => row.subtract(set),
@@ -1669,7 +1669,7 @@ impl<R: Idx, C: Idx> SparseBitMatrix<R, C> {
     /// Returns true if the row was changed.
     pub fn union_row<Set>(&mut self, row: R, set: &Set) -> bool
     where
-        ChunkedBitSet<C>: BitRelations<Set>,
+        BitSet<C>: BitRelations<Set>,
     {
         self.ensure_row(row).union(set)
     }