Auto merge of #109458 - Nilstrieb:smol-cute-little-bits, r=wesleywiser

Use `SmallVec` in bitsets

This doesn't increase their size and means that we don't have to heap allocate for small sets.

Author: bors

compiler/rustc_index/src/bit_set.rs

@@ -1,5 +1,6 @@
 use crate::vec::{Idx, IndexVec};
 use arrayvec::ArrayVec;
+use smallvec::{smallvec, SmallVec};
 use std::fmt;
 use std::iter;
 use std::marker::PhantomData;
@@ -111,7 +112,7 @@ macro_rules! bit_relations_inherent_impls {
 #[derive(Eq, PartialEq, Hash, Decodable, Encodable)]
 pub struct BitSet<T> {
     domain_size: usize,
-    words: Vec<Word>,
+    words: SmallVec<[Word; 2]>,
     marker: PhantomData<T>,
 }
 
@@ -127,14 +128,15 @@ impl<T: Idx> BitSet<T> {
     #[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 }
+        BitSet { domain_size, words: smallvec![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 };
+        let mut result =
+            BitSet { domain_size, words: smallvec![!0; num_words], marker: PhantomData };
         result.clear_excess_bits();
         result
     }
@@ -1571,7 +1573,7 @@ impl<T: Idx> From<BitSet<T>> for GrowableBitSet<T> {
 pub struct BitMatrix<R: Idx, C: Idx> {
     num_rows: usize,
     num_columns: usize,
-    words: Vec<Word>,
+    words: SmallVec<[Word; 2]>,
     marker: PhantomData<(R, C)>,
 }
 
@@ -1584,7 +1586,7 @@ impl<R: Idx, C: Idx> BitMatrix<R, C> {
         BitMatrix {
             num_rows,
             num_columns,
-            words: vec![0; num_rows * words_per_row],
+            words: smallvec![0; num_rows * words_per_row],
             marker: PhantomData,
         }
     }