coverage: Separate branch pairs from other mapping kinds

This clears the way for larger changes to how branches are handled by the
coverage instrumentor, in order to support branch coverage for more language
constructs.

compiler/rustc_mir_transform/src/coverage/spans.rs

@@ -15,8 +15,10 @@ mod from_mir;
 pub(super) enum BcbMappingKind {
     /// Associates an ordinary executable code span with its corresponding BCB.
     Code(BasicCoverageBlock),
-    /// Associates a branch span with BCBs for its true and false arms.
-    Branch { true_bcb: BasicCoverageBlock, false_bcb: BasicCoverageBlock },
+
+    // Ordinary branch mappings are stored separately, so they don't have a
+    // variant in this enum.
+    //
     /// Associates a mcdc branch span with condition info besides fields for normal branch.
     MCDCBranch {
         true_bcb: BasicCoverageBlock,
@@ -35,9 +37,20 @@ pub(super) struct BcbMapping {
     pub(super) span: Span,
 }
 
+/// This is separate from [`BcbMappingKind`] to help prepare for larger changes
+/// that will be needed for improved branch coverage in the future.
+/// (See <https://github.com/rust-lang/rust/pull/124217>.)
+#[derive(Debug)]
+pub(super) struct BcbBranchPair {
+    pub(super) span: Span,
+    pub(super) true_bcb: BasicCoverageBlock,
+    pub(super) false_bcb: BasicCoverageBlock,
+}
+
 pub(super) struct CoverageSpans {
     bcb_has_mappings: BitSet<BasicCoverageBlock>,
-    mappings: Vec<BcbMapping>,
+    pub(super) mappings: Vec<BcbMapping>,
+    pub(super) branch_pairs: Vec<BcbBranchPair>,
     test_vector_bitmap_bytes: u32,
 }
 
@@ -46,10 +59,6 @@ impl CoverageSpans {
         self.bcb_has_mappings.contains(bcb)
     }
 
-    pub(super) fn all_bcb_mappings(&self) -> impl Iterator<Item = &BcbMapping> {
-        self.mappings.iter()
-    }
-
     pub(super) fn test_vector_bitmap_bytes(&self) -> u32 {
         self.test_vector_bitmap_bytes
     }
@@ -65,6 +74,7 @@ pub(super) fn generate_coverage_spans(
     basic_coverage_blocks: &CoverageGraph,
 ) -> Option<CoverageSpans> {
     let mut mappings = vec![];
+    let mut branch_pairs = vec![];
 
     if hir_info.is_async_fn {
         // An async function desugars into a function that returns a future,
@@ -86,7 +96,7 @@ pub(super) fn generate_coverage_spans(
             BcbMapping { kind: BcbMappingKind::Code(bcb), span }
         }));
 
-        mappings.extend(from_mir::extract_branch_mappings(
+        branch_pairs.extend(from_mir::extract_branch_pairs(
             mir_body,
             hir_info,
             basic_coverage_blocks,
@@ -99,7 +109,7 @@ pub(super) fn generate_coverage_spans(
         ));
     }
 
-    if mappings.is_empty() {
+    if mappings.is_empty() && branch_pairs.is_empty() {
         return None;
     }
 
@@ -112,8 +122,7 @@ pub(super) fn generate_coverage_spans(
     for BcbMapping { kind, span: _ } in &mappings {
         match *kind {
             BcbMappingKind::Code(bcb) => insert(bcb),
-            BcbMappingKind::Branch { true_bcb, false_bcb }
-            | BcbMappingKind::MCDCBranch { true_bcb, false_bcb, .. } => {
+            BcbMappingKind::MCDCBranch { true_bcb, false_bcb, .. } => {
                 insert(true_bcb);
                 insert(false_bcb);
             }
@@ -126,8 +135,12 @@ pub(super) fn generate_coverage_spans(
             }
         }
     }
+    for &BcbBranchPair { true_bcb, false_bcb, .. } in &branch_pairs {
+        insert(true_bcb);
+        insert(false_bcb);
+    }
 
-    Some(CoverageSpans { bcb_has_mappings, mappings, test_vector_bitmap_bytes })
+    Some(CoverageSpans { bcb_has_mappings, mappings, branch_pairs, test_vector_bitmap_bytes })
 }
 
 #[derive(Debug)]