Auto merge of #133891 - nnethercote:MixedBitSet, r=Mark-Simulacrum

Introduce `MixedBitSet`

`ChunkedBitSet` is good at avoiding excessive memory usage for programs with very large functgions where dataflow bitsets have very large domain sizes. But it's overly heavyweight for small bitsets, because any non-empty `ChunkedBitSet` takes up at least 256 bytes.

This PR introduces `MixedBitSet`, which is a simple bitset that uses `BitSet` for small/medium bitsets and `ChunkedBitSet` for large bitsets. It's a speed and memory usage win.

r? `@Mark-Simulacrum`

1 files changed, 2 insertions, 2 deletions

diff --git a/compiler/rustc_mir_dataflow/src/framework/lattice.rs b/compiler/rustc_mir_dataflow/src/framework/lattice.rs
index e2b56aedca3..e063eaf74bd 100644
--- a/compiler/rustc_mir_dataflow/src/framework/lattice.rs
+++ b/compiler/rustc_mir_dataflow/src/framework/lattice.rs
@@ -40,7 +40,7 @@
 
 use std::iter;
 
-use rustc_index::bit_set::{BitSet, ChunkedBitSet};
+use rustc_index::bit_set::{BitSet, MixedBitSet};
 use rustc_index::{Idx, IndexVec};
 
 use crate::framework::BitSetExt;
@@ -126,7 +126,7 @@ impl<T: Idx> JoinSemiLattice for BitSet<T> {
     }
 }
 
-impl<T: Idx> JoinSemiLattice for ChunkedBitSet<T> {
+impl<T: Idx> JoinSemiLattice for MixedBitSet<T> {
     fn join(&mut self, other: &Self) -> bool {
         self.union(other)
     }