rename BitSet to DenseBitSet

This should make it clearer that this bitset is dense, with the
advantages and disadvantages that it entails.

compiler/rustc_mir_transform/src/coverage/counters.rs

@@ -5,7 +5,7 @@ use rustc_data_structures::captures::Captures;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::graph::DirectedGraph;
 use rustc_index::IndexVec;
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::mir::coverage::{CounterId, CovTerm, Expression, ExpressionId, Op};
 use tracing::{debug, debug_span, instrument};
 
@@ -77,7 +77,7 @@ impl CoverageCounters {
     /// counters or counter expressions for nodes and edges as required.
     pub(super) fn make_bcb_counters(
         graph: &CoverageGraph,
-        bcb_needs_counter: &BitSet<BasicCoverageBlock>,
+        bcb_needs_counter: &DenseBitSet<BasicCoverageBlock>,
     ) -> Self {
         let mut builder = CountersBuilder::new(graph, bcb_needs_counter);
         builder.make_bcb_counters();
@@ -220,13 +220,16 @@ fn sibling_out_edge_targets(
 /// the set of nodes that need counters.
 struct CountersBuilder<'a> {
     graph: &'a CoverageGraph,
-    bcb_needs_counter: &'a BitSet<BasicCoverageBlock>,
+    bcb_needs_counter: &'a DenseBitSet<BasicCoverageBlock>,
 
     site_counters: FxHashMap<Site, SiteCounter>,
 }
 
 impl<'a> CountersBuilder<'a> {
-    fn new(graph: &'a CoverageGraph, bcb_needs_counter: &'a BitSet<BasicCoverageBlock>) -> Self {
+    fn new(
+        graph: &'a CoverageGraph,
+        bcb_needs_counter: &'a DenseBitSet<BasicCoverageBlock>,
+    ) -> Self {
         assert_eq!(graph.num_nodes(), bcb_needs_counter.domain_size());
         Self { graph, bcb_needs_counter, site_counters: FxHashMap::default() }
     }

compiler/rustc_mir_transform/src/coverage/graph.rs

@@ -8,7 +8,7 @@ use rustc_data_structures::fx::FxHashSet;
 use rustc_data_structures::graph::dominators::Dominators;
 use rustc_data_structures::graph::{self, DirectedGraph, StartNode};
 use rustc_index::IndexVec;
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::mir::{self, BasicBlock, Terminator, TerminatorKind};
 use tracing::debug;
 
@@ -27,7 +27,7 @@ pub(crate) struct CoverageGraph {
     /// their relative order is consistent but arbitrary.
     dominator_order_rank: IndexVec<BasicCoverageBlock, u32>,
     /// A loop header is a node that dominates one or more of its predecessors.
-    is_loop_header: BitSet<BasicCoverageBlock>,
+    is_loop_header: DenseBitSet<BasicCoverageBlock>,
     /// For each node, the loop header node of its nearest enclosing loop.
     /// This forms a linked list that can be traversed to find all enclosing loops.
     enclosing_loop_header: IndexVec<BasicCoverageBlock, Option<BasicCoverageBlock>>,
@@ -72,7 +72,7 @@ impl CoverageGraph {
             predecessors,
             dominators: None,
             dominator_order_rank: IndexVec::from_elem_n(0, num_nodes),
-            is_loop_header: BitSet::new_empty(num_nodes),
+            is_loop_header: DenseBitSet::new_empty(num_nodes),
             enclosing_loop_header: IndexVec::from_elem_n(None, num_nodes),
         };
         assert_eq!(num_nodes, this.num_nodes());

compiler/rustc_mir_transform/src/coverage/mappings.rs

@@ -3,7 +3,7 @@ use std::collections::BTreeSet;
 use rustc_data_structures::fx::FxIndexMap;
 use rustc_data_structures::graph::DirectedGraph;
 use rustc_index::IndexVec;
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::mir::coverage::{
     BlockMarkerId, BranchSpan, ConditionId, ConditionInfo, CoverageInfoHi, CoverageKind,
 };
@@ -128,7 +128,7 @@ pub(super) fn extract_all_mapping_info_from_mir<'tcx>(
 }
 
 impl ExtractedMappings {
-    pub(super) fn all_bcbs_with_counter_mappings(&self) -> BitSet<BasicCoverageBlock> {
+    pub(super) fn all_bcbs_with_counter_mappings(&self) -> DenseBitSet<BasicCoverageBlock> {
         // Fully destructure self to make sure we don't miss any fields that have mappings.
         let Self {
             num_bcbs,
@@ -140,7 +140,7 @@ impl ExtractedMappings {
         } = self;
 
         // Identify which BCBs have one or more mappings.
-        let mut bcbs_with_counter_mappings = BitSet::new_empty(*num_bcbs);
+        let mut bcbs_with_counter_mappings = DenseBitSet::new_empty(*num_bcbs);
         let mut insert = |bcb| {
             bcbs_with_counter_mappings.insert(bcb);
         };
@@ -172,8 +172,8 @@ impl ExtractedMappings {
     }
 
     /// Returns the set of BCBs that have one or more `Code` mappings.
-    pub(super) fn bcbs_with_ordinary_code_mappings(&self) -> BitSet<BasicCoverageBlock> {
-        let mut bcbs = BitSet::new_empty(self.num_bcbs);
+    pub(super) fn bcbs_with_ordinary_code_mappings(&self) -> DenseBitSet<BasicCoverageBlock> {
+        let mut bcbs = DenseBitSet::new_empty(self.num_bcbs);
         for &CodeMapping { span: _, bcb } in &self.code_mappings {
             bcbs.insert(bcb);
         }

compiler/rustc_mir_transform/src/coverage/query.rs

@@ -1,5 +1,5 @@
 use rustc_data_structures::captures::Captures;
-use rustc_index::bit_set::BitSet;
+use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags;
 use rustc_middle::mir::coverage::{
     CounterId, CovTerm, CoverageIdsInfo, CoverageKind, Expression, ExpressionId,
@@ -92,13 +92,13 @@ fn coverage_ids_info<'tcx>(
 
     let Some(fn_cov_info) = mir_body.function_coverage_info.as_deref() else {
         return CoverageIdsInfo {
-            counters_seen: BitSet::new_empty(0),
-            zero_expressions: BitSet::new_empty(0),
+            counters_seen: DenseBitSet::new_empty(0),
+            zero_expressions: DenseBitSet::new_empty(0),
         };
     };
 
-    let mut counters_seen = BitSet::new_empty(fn_cov_info.num_counters);
-    let mut expressions_seen = BitSet::new_filled(fn_cov_info.expressions.len());
+    let mut counters_seen = DenseBitSet::new_empty(fn_cov_info.num_counters);
+    let mut expressions_seen = DenseBitSet::new_filled(fn_cov_info.expressions.len());
 
     // For each expression ID that is directly used by one or more mappings,
     // mark it as not-yet-seen. This indicates that we expect to see a
@@ -148,23 +148,23 @@ fn is_inlined(body: &Body<'_>, statement: &Statement<'_>) -> bool {
     scope_data.inlined.is_some() || scope_data.inlined_parent_scope.is_some()
 }
 
-/// Identify expressions that will always have a value of zero, and note
-/// their IDs in a `BitSet`. Mappings that refer to a zero expression
-/// can instead become mappings to a constant zero value.
+/// Identify expressions that will always have a value of zero, and note their
+/// IDs in a `DenseBitSet`. Mappings that refer to a zero expression can instead
+/// become mappings to a constant zero value.
 ///
 /// This function mainly exists to preserve the simplifications that were
 /// already being performed by the Rust-side expression renumbering, so that
 /// the resulting coverage mappings don't get worse.
 fn identify_zero_expressions(
     fn_cov_info: &FunctionCoverageInfo,
-    counters_seen: &BitSet<CounterId>,
-    expressions_seen: &BitSet<ExpressionId>,
-) -> BitSet<ExpressionId> {
+    counters_seen: &DenseBitSet<CounterId>,
+    expressions_seen: &DenseBitSet<ExpressionId>,
+) -> DenseBitSet<ExpressionId> {
     // The set of expressions that either were optimized out entirely, or
     // have zero as both of their operands, and will therefore always have
     // a value of zero. Other expressions that refer to these as operands
     // can have those operands replaced with `CovTerm::Zero`.
-    let mut zero_expressions = BitSet::new_empty(fn_cov_info.expressions.len());
+    let mut zero_expressions = DenseBitSet::new_empty(fn_cov_info.expressions.len());
 
     // Simplify a copy of each expression based on lower-numbered expressions,
     // and then update the set of always-zero expressions if necessary.
@@ -228,8 +228,8 @@ fn identify_zero_expressions(
 /// into account knowledge of which counters are unused and which expressions
 /// are always zero.
 fn is_zero_term(
-    counters_seen: &BitSet<CounterId>,
-    zero_expressions: &BitSet<ExpressionId>,
+    counters_seen: &DenseBitSet<CounterId>,
+    zero_expressions: &DenseBitSet<ExpressionId>,
     term: CovTerm,
 ) -> bool {
     match term {