coverage: Store a copy of num_bcbs in ExtractedMappings

This makes it possible to allocate per-BCB data structures without needing
access to the whole graph.

Author: Zalathar

compiler/rustc_mir_transform/src/coverage/mappings.rs

@@ -56,6 +56,10 @@ pub(super) struct MCDCDecision {
 
 #[derive(Default)]
 pub(super) struct ExtractedMappings {
+    /// Store our own copy of [`CoverageGraph::num_nodes`], so that we don't
+    /// need access to the whole graph when allocating per-BCB data. This is
+    /// only public so that other code can still use exhaustive destructuring.
+    pub(super) num_bcbs: usize,
     pub(super) code_mappings: Vec<CodeMapping>,
     pub(super) branch_pairs: Vec<BranchPair>,
     pub(super) mcdc_bitmap_bytes: u32,
@@ -106,6 +110,7 @@ pub(super) fn extract_all_mapping_info_from_mir<'tcx>(
     );
 
     ExtractedMappings {
+        num_bcbs: basic_coverage_blocks.num_nodes(),
         code_mappings,
         branch_pairs,
         mcdc_bitmap_bytes,
@@ -115,12 +120,10 @@ pub(super) fn extract_all_mapping_info_from_mir<'tcx>(
 }
 
 impl ExtractedMappings {
-    pub(super) fn all_bcbs_with_counter_mappings(
-        &self,
-        basic_coverage_blocks: &CoverageGraph, // Only used for allocating a correctly-sized set
-    ) -> BitSet<BasicCoverageBlock> {
+    pub(super) fn all_bcbs_with_counter_mappings(&self) -> BitSet<BasicCoverageBlock> {
         // Fully destructure self to make sure we don't miss any fields that have mappings.
         let Self {
+            num_bcbs,
             code_mappings,
             branch_pairs,
             mcdc_bitmap_bytes: _,
@@ -129,7 +132,7 @@ impl ExtractedMappings {
         } = self;
 
         // Identify which BCBs have one or more mappings.
-        let mut bcbs_with_counter_mappings = BitSet::new_empty(basic_coverage_blocks.num_nodes());
+        let mut bcbs_with_counter_mappings = BitSet::new_empty(*num_bcbs);
         let mut insert = |bcb| {
             bcbs_with_counter_mappings.insert(bcb);
         };

compiler/rustc_mir_transform/src/coverage/mod.rs

@@ -88,8 +88,7 @@ fn instrument_function_for_coverage<'tcx>(tcx: TyCtxt<'tcx>, mir_body: &mut mir:
     // every coverage span has a `Counter` or `Expression` assigned to its `BasicCoverageBlock`
     // and all `Expression` dependencies (operands) are also generated, for any other
     // `BasicCoverageBlock`s not already associated with a coverage span.
-    let bcbs_with_counter_mappings =
-        extracted_mappings.all_bcbs_with_counter_mappings(&basic_coverage_blocks);
+    let bcbs_with_counter_mappings = extracted_mappings.all_bcbs_with_counter_mappings();
     if bcbs_with_counter_mappings.is_empty() {
         // No relevant spans were found in MIR, so skip instrumenting this function.
         return;
@@ -163,6 +162,7 @@ fn create_mappings<'tcx>(
 
     // Fully destructure the mappings struct to make sure we don't miss any kinds.
     let ExtractedMappings {
+        num_bcbs: _,
         code_mappings,
         branch_pairs,
         mcdc_bitmap_bytes: _,