Introduce MixedBitSet.

It just uses `BitSet` for small/medium sizes (<= 2048 bits) and
`ChunkedBitSet` for larger sizes. This is good because `ChunkedBitSet`
is slow and memory-hungry at smaller sizes.

compiler/rustc_mir_dataflow/src/framework/mod.rs

@@ -35,7 +35,7 @@
 use std::cmp::Ordering;
 
 use rustc_data_structures::work_queue::WorkQueue;
-use rustc_index::bit_set::{BitSet, ChunkedBitSet};
+use rustc_index::bit_set::{BitSet, ChunkedBitSet, MixedBitSet};
 use rustc_index::{Idx, IndexVec};
 use rustc_middle::bug;
 use rustc_middle::mir::{self, BasicBlock, CallReturnPlaces, Location, TerminatorEdges, traversal};
@@ -77,6 +77,12 @@ impl<T: Idx> BitSetExt<T> for ChunkedBitSet<T> {
     }
 }
 
+impl<T: Idx> BitSetExt<T> for MixedBitSet<T> {
+    fn contains(&self, elem: T) -> bool {
+        self.contains(elem)
+    }
+}
+
 /// A dataflow problem with an arbitrarily complex transfer function.
 ///
 /// This trait specifies the lattice on which this analysis operates (the domain), its
@@ -337,6 +343,16 @@ impl<T: Idx> GenKill<T> for ChunkedBitSet<T> {
     }
 }
 
+impl<T: Idx> GenKill<T> for MixedBitSet<T> {
+    fn gen_(&mut self, elem: T) {
+        self.insert(elem);
+    }
+
+    fn kill(&mut self, elem: T) {
+        self.remove(elem);
+    }
+}
+
 impl<T, S: GenKill<T>> GenKill<T> for MaybeReachable<S> {
     fn gen_(&mut self, elem: T) {
         match self {