Rollup merge of #119842 - Zalathar:kind, r=oli-obk

coverage: Add enums to accommodate other kinds of coverage mappings

Extracted from  #118305.

LLVM supports several different kinds of coverage mapping regions, but currently we only ever emit ordinary “code” regions.  This PR performs the plumbing required to add other kinds of regions as enum variants, but does not add any specific variants other than `Code`.

The main motivation for this change is branch coverage, but it will also allow separate experimentation with gap regions and skipped regions, which might help in producing more accurate and useful coverage reports.

---

``@rustbot`` label +A-code-coverage

compiler/rustc_mir_transform/src/coverage/mod.rs

@@ -9,7 +9,7 @@ mod tests;
 
 use self::counters::{BcbCounter, CoverageCounters};
 use self::graph::{BasicCoverageBlock, CoverageGraph};
-use self::spans::CoverageSpans;
+use self::spans::{BcbMapping, BcbMappingKind, CoverageSpans};
 
 use crate::MirPass;
 
@@ -141,22 +141,21 @@ impl<'a, 'tcx> Instrumentor<'a, 'tcx> {
         let file_name =
             Symbol::intern(&source_file.name.for_codegen(self.tcx.sess).to_string_lossy());
 
+        let term_for_bcb = |bcb| {
+            coverage_counters
+                .bcb_counter(bcb)
+                .expect("all BCBs with spans were given counters")
+                .as_term()
+        };
+
         coverage_spans
-            .bcbs_with_coverage_spans()
-            // For each BCB with spans, get a coverage term for its counter.
-            .map(|(bcb, spans)| {
-                let term = coverage_counters
-                    .bcb_counter(bcb)
-                    .expect("all BCBs with spans were given counters")
-                    .as_term();
-                (term, spans)
-            })
-            // Flatten the spans into individual term/span pairs.
-            .flat_map(|(term, spans)| spans.iter().map(move |&span| (term, span)))
-            // Convert each span to a code region, and create the final mapping.
-            .filter_map(|(term, span)| {
+            .all_bcb_mappings()
+            .filter_map(|&BcbMapping { kind: bcb_mapping_kind, span }| {
+                let kind = match bcb_mapping_kind {
+                    BcbMappingKind::Code(bcb) => MappingKind::Code(term_for_bcb(bcb)),
+                };
                 let code_region = make_code_region(source_map, file_name, span, body_span)?;
-                Some(Mapping { term, code_region })
+                Some(Mapping { kind, code_region })
             })
             .collect::<Vec<_>>()
     }

compiler/rustc_mir_transform/src/coverage/spans.rs

@@ -1,5 +1,5 @@
 use rustc_data_structures::graph::WithNumNodes;
-use rustc_index::IndexVec;
+use rustc_index::bit_set::BitSet;
 use rustc_middle::mir;
 use rustc_span::{BytePos, Span, DUMMY_SP};
 
@@ -8,9 +8,21 @@ use crate::coverage::ExtractedHirInfo;
 
 mod from_mir;
 
+#[derive(Clone, Copy, Debug)]
+pub(super) enum BcbMappingKind {
+    /// Associates an ordinary executable code span with its corresponding BCB.
+    Code(BasicCoverageBlock),
+}
+
+#[derive(Debug)]
+pub(super) struct BcbMapping {
+    pub(super) kind: BcbMappingKind,
+    pub(super) span: Span,
+}
+
 pub(super) struct CoverageSpans {
-    /// Map from BCBs to their list of coverage spans.
-    bcb_to_spans: IndexVec<BasicCoverageBlock, Vec<Span>>,
+    bcb_has_mappings: BitSet<BasicCoverageBlock>,
+    mappings: Vec<BcbMapping>,
 }
 
 impl CoverageSpans {
@@ -23,36 +35,42 @@ impl CoverageSpans {
         hir_info: &ExtractedHirInfo,
         basic_coverage_blocks: &CoverageGraph,
     ) -> Option<Self> {
+        let mut mappings = vec![];
+
         let coverage_spans = CoverageSpansGenerator::generate_coverage_spans(
             mir_body,
             hir_info,
             basic_coverage_blocks,
         );
+        mappings.extend(coverage_spans.into_iter().map(|CoverageSpan { bcb, span, .. }| {
+            // Each span produced by the generator represents an ordinary code region.
+            BcbMapping { kind: BcbMappingKind::Code(bcb), span }
+        }));
 
-        if coverage_spans.is_empty() {
+        if mappings.is_empty() {
             return None;
         }
 
-        // Group the coverage spans by BCB, with the BCBs in sorted order.
-        let mut bcb_to_spans = IndexVec::from_elem_n(Vec::new(), basic_coverage_blocks.num_nodes());
-        for CoverageSpan { bcb, span, .. } in coverage_spans {
-            bcb_to_spans[bcb].push(span);
+        // Identify which BCBs have one or more mappings.
+        let mut bcb_has_mappings = BitSet::new_empty(basic_coverage_blocks.num_nodes());
+        let mut insert = |bcb| {
+            bcb_has_mappings.insert(bcb);
+        };
+        for &BcbMapping { kind, span: _ } in &mappings {
+            match kind {
+                BcbMappingKind::Code(bcb) => insert(bcb),
+            }
         }
 
-        Some(Self { bcb_to_spans })
+        Some(Self { bcb_has_mappings, mappings })
     }
 
     pub(super) fn bcb_has_coverage_spans(&self, bcb: BasicCoverageBlock) -> bool {
-        !self.bcb_to_spans[bcb].is_empty()
+        self.bcb_has_mappings.contains(bcb)
     }
 
-    pub(super) fn bcbs_with_coverage_spans(
-        &self,
-    ) -> impl Iterator<Item = (BasicCoverageBlock, &[Span])> {
-        self.bcb_to_spans.iter_enumerated().filter_map(|(bcb, spans)| {
-            // Only yield BCBs that have at least one coverage span.
-            (!spans.is_empty()).then_some((bcb, spans.as_slice()))
-        })
+    pub(super) fn all_bcb_mappings(&self) -> impl Iterator<Item = &BcbMapping> {
+        self.mappings.iter()
     }
 }