Rollup merge of #135205 - lqd:bitsets, r=Mark-Simulacrum

Rename `BitSet` to `DenseBitSet`

r? `@Mark-Simulacrum` as you requested this in https://github.com/rust-lang/rust/pull/134438#discussion_r1890659739 after such a confusion.

This PR renames `BitSet` to `DenseBitSet` to make it less obvious as the go-to solution for bitmap needs, as well as make its representation (and positives/negatives) clearer. It also expands the comments there to hopefully make it clearer when it's not a good fit, with some alternative bitsets types.

(This migrates the subtrees cg_gcc and clippy to use the new name in separate commits, for easier review by their respective owners, but they can obvs be squashed)

src/tools/clippy/clippy_lints/src/needless_borrows_for_generic_args.rs

@@ -9,7 +9,7 @@ use rustc_errors::Applicability;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_hir::{Body, Expr, ExprKind, Mutability, Path, QPath};
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_infer::infer::TyCtxtInferExt;
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_middle::mir::{Rvalue, StatementKind};
@@ -390,7 +390,7 @@ fn replace_types<'tcx>(
     projection_predicates: &[ProjectionPredicate<'tcx>],
     args: &mut [GenericArg<'tcx>],
 ) -> bool {
-    let mut replaced = BitSet::new_empty(args.len());
+    let mut replaced = DenseBitSet::new_empty(args.len());
 
     let mut deque = VecDeque::with_capacity(args.len());
     deque.push_back((param_ty, new_ty));

src/tools/clippy/clippy_utils/src/mir/mod.rs

@@ -1,5 +1,5 @@
 use rustc_hir::{Expr, HirId};
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::mir::visit::{MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor};
 use rustc_middle::mir::{
     BasicBlock, Body, InlineAsmOperand, Local, Location, Place, START_BLOCK, StatementKind, TerminatorKind, traversal,
@@ -88,7 +88,7 @@ impl<'tcx> Visitor<'tcx> for V<'_> {
 
 /// Checks if the block is part of a cycle
 pub fn block_in_cycle(body: &Body<'_>, block: BasicBlock) -> bool {
-    let mut seen = BitSet::new_empty(body.basic_blocks.len());
+    let mut seen = DenseBitSet::new_empty(body.basic_blocks.len());
     let mut to_visit = Vec::with_capacity(body.basic_blocks.len() / 2);
 
     seen.insert(block);

src/tools/clippy/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;
+use rustc_index::bit_set::DenseBitSet;
 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, BitSet<mir::Local>>,
+    possible_origin: FxHashMap<mir::Local, DenseBitSet<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, BitSet<mir::Local>>,
+        possible_origin: FxHashMap<mir::Local, DenseBitSet<mir::Local>>,
     ) -> Self {
         Self {
             possible_borrower: TransitiveRelation::default(),
@@ -56,7 +56,7 @@ impl<'a, 'b, 'tcx> PossibleBorrowerVisitor<'a, 'b, 'tcx> {
             }
         }
 
-        let bs = BitSet::new_empty(self.body.local_decls.len());
+        let bs = DenseBitSet::new_empty(self.body.local_decls.len());
         PossibleBorrowerMap {
             map,
             maybe_live,
@@ -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(BitSet::iter)
+                .flat_map(DenseBitSet::iter)
                 .collect();
 
             if ContainsRegion.visit_ty(self.body.local_decls[*dest].ty).is_break() {
@@ -171,10 +171,10 @@ 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, BitSet<mir::Local>>,
+    pub map: FxHashMap<mir::Local, DenseBitSet<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>),
+    // Caches to avoid allocation of `DenseBitSet` on every query
+    pub bitset: (DenseBitSet<mir::Local>, DenseBitSet<mir::Local>),
 }
 
 impl<'b, 'tcx> PossibleBorrowerMap<'b, 'tcx> {
@@ -184,7 +184,7 @@ impl<'b, 'tcx> PossibleBorrowerMap<'b, 'tcx> {
             vis.visit_body(mir);
             vis.into_map(cx)
         };
-        let maybe_storage_live_result = MaybeStorageLive::new(Cow::Owned(BitSet::new_empty(mir.local_decls.len())))
+        let maybe_storage_live_result = MaybeStorageLive::new(Cow::Owned(DenseBitSet::new_empty(mir.local_decls.len())))
             .iterate_to_fixpoint(cx.tcx, mir, Some("redundant_clone"))
             .into_results_cursor(mir);
         let mut vis = PossibleBorrowerVisitor::new(cx, mir, possible_origin);

src/tools/clippy/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::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 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, BitSet<mir::Local>> {
+    pub fn into_map(self, cx: &LateContext<'tcx>) -> FxHashMap<mir::Local, DenseBitSet<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) {

src/tools/clippy/clippy_utils/src/mir/transitive_relation.rs

@@ -1,5 +1,5 @@
 use rustc_data_structures::fx::FxHashMap;
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 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) -> BitSet<mir::Local> {
-        let mut seen = BitSet::new_empty(domain_size);
+    pub fn reachable_from(&self, a: mir::Local, domain_size: usize) -> DenseBitSet<mir::Local> {
+        let mut seen = DenseBitSet::new_empty(domain_size);
         let mut stack = vec![a];
         while let Some(u) = stack.pop() {
             if let Some(edges) = self.relations.get(&u) {