Stop using HybridBitSet in clippy.

The compiler uses `BitSet<Local>`, because the number of locals doesn't
get that high, so clippy should do likewise.

clippy_utils/src/mir/possible_borrower.rs

@@ -2,7 +2,7 @@ use super::possible_origin::PossibleOriginVisitor;
 use super::transitive_relation::TransitiveRelation;
 use crate::ty::is_copy;
 use rustc_data_structures::fx::FxHashMap;
-use rustc_index::bit_set::{BitSet, HybridBitSet};
+use rustc_index::bit_set::BitSet;
 use rustc_lint::LateContext;
 use rustc_middle::mir::visit::Visitor as _;
 use rustc_middle::mir::{self, Mutability};
@@ -21,14 +21,14 @@ struct PossibleBorrowerVisitor<'a, 'b, 'tcx> {
     possible_borrower: TransitiveRelation,
     body: &'b mir::Body<'tcx>,
     cx: &'a LateContext<'tcx>,
-    possible_origin: FxHashMap<mir::Local, HybridBitSet<mir::Local>>,
+    possible_origin: FxHashMap<mir::Local, BitSet<mir::Local>>,
 }
 
 impl<'a, 'b, 'tcx> PossibleBorrowerVisitor<'a, 'b, 'tcx> {
     fn new(
         cx: &'a LateContext<'tcx>,
         body: &'b mir::Body<'tcx>,
-        possible_origin: FxHashMap<mir::Local, HybridBitSet<mir::Local>>,
+        possible_origin: FxHashMap<mir::Local, BitSet<mir::Local>>,
     ) -> Self {
         Self {
             possible_borrower: TransitiveRelation::default(),
@@ -119,7 +119,7 @@ impl<'tcx> mir::visit::Visitor<'tcx> for PossibleBorrowerVisitor<'_, '_, 'tcx> {
             let mut mutable_variables: Vec<mir::Local> = mutable_borrowers
                 .iter()
                 .filter_map(|r| self.possible_origin.get(r))
-                .flat_map(HybridBitSet::iter)
+                .flat_map(BitSet::iter)
                 .collect();
 
             if ContainsRegion.visit_ty(self.body.local_decls[*dest].ty).is_break() {
@@ -171,7 +171,7 @@ fn rvalue_locals(rvalue: &mir::Rvalue<'_>, mut visit: impl FnMut(mir::Local)) {
 #[allow(clippy::module_name_repetitions)]
 pub struct PossibleBorrowerMap<'b, 'tcx> {
     /// Mapping `Local -> its possible borrowers`
-    pub map: FxHashMap<mir::Local, HybridBitSet<mir::Local>>,
+    pub map: FxHashMap<mir::Local, BitSet<mir::Local>>,
     maybe_live: ResultsCursor<'b, 'tcx, MaybeStorageLive<'tcx>>,
     // Caches to avoid allocation of `BitSet` on every query
     pub bitset: (BitSet<mir::Local>, BitSet<mir::Local>),

clippy_utils/src/mir/possible_origin.rs

@@ -1,7 +1,7 @@
 use super::transitive_relation::TransitiveRelation;
 use crate::ty::is_copy;
 use rustc_data_structures::fx::FxHashMap;
-use rustc_index::bit_set::HybridBitSet;
+use rustc_index::bit_set::BitSet;
 use rustc_lint::LateContext;
 use rustc_middle::mir;
 
@@ -22,7 +22,7 @@ impl<'a, 'tcx> PossibleOriginVisitor<'a, 'tcx> {
         }
     }
 
-    pub fn into_map(self, cx: &LateContext<'tcx>) -> FxHashMap<mir::Local, HybridBitSet<mir::Local>> {
+    pub fn into_map(self, cx: &LateContext<'tcx>) -> FxHashMap<mir::Local, BitSet<mir::Local>> {
         let mut map = FxHashMap::default();
         for row in (1..self.body.local_decls.len()).map(mir::Local::from_usize) {
             if is_copy(cx, self.body.local_decls[row].ty) {

clippy_utils/src/mir/transitive_relation.rs

@@ -1,5 +1,5 @@
 use rustc_data_structures::fx::FxHashMap;
-use rustc_index::bit_set::HybridBitSet;
+use rustc_index::bit_set::BitSet;
 use rustc_middle::mir;
 
 #[derive(Default)]
@@ -12,8 +12,8 @@ impl TransitiveRelation {
         self.relations.entry(a).or_default().push(b);
     }
 
-    pub fn reachable_from(&self, a: mir::Local, domain_size: usize) -> HybridBitSet<mir::Local> {
-        let mut seen = HybridBitSet::new_empty(domain_size);
+    pub fn reachable_from(&self, a: mir::Local, domain_size: usize) -> BitSet<mir::Local> {
+        let mut seen = BitSet::new_empty(domain_size);
         let mut stack = vec![a];
         while let Some(u) = stack.pop() {
             if let Some(edges) = self.relations.get(&u) {