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/from_mir.rs

@@ -13,7 +13,7 @@ use rustc_span::{ExpnKind, MacroKind, Span, Symbol};
 use crate::coverage::graph::{
     BasicCoverageBlock, BasicCoverageBlockData, CoverageGraph, START_BCB,
 };
-use crate::coverage::spans::{BcbMapping, BcbMappingKind};
+use crate::coverage::spans::{BcbBranchPair, BcbMapping, BcbMappingKind};
 use crate::coverage::ExtractedHirInfo;
 
 /// Traverses the MIR body to produce an initial collection of coverage-relevant
@@ -388,16 +388,16 @@ fn resolve_block_markers(
 }
 
 // FIXME: There is currently a lot of redundancy between
-// `extract_branch_mappings` and `extract_mcdc_mappings`. This is needed so
+// `extract_branch_pairs` and `extract_mcdc_mappings`. This is needed so
 // that they can each be modified without interfering with the other, but in
 // the long term we should try to bring them together again when branch coverage
 // and MC/DC coverage support are more mature.
 
-pub(super) fn extract_branch_mappings(
+pub(super) fn extract_branch_pairs(
     mir_body: &mir::Body<'_>,
     hir_info: &ExtractedHirInfo,
     basic_coverage_blocks: &CoverageGraph,
-) -> Vec<BcbMapping> {
+) -> Vec<BcbBranchPair> {
     let Some(branch_info) = mir_body.coverage_branch_info.as_deref() else { return vec![] };
 
     let block_markers = resolve_block_markers(branch_info, mir_body);
@@ -420,7 +420,7 @@ pub(super) fn extract_branch_mappings(
             let true_bcb = bcb_from_marker(true_marker)?;
             let false_bcb = bcb_from_marker(false_marker)?;
 
-            Some(BcbMapping { kind: BcbMappingKind::Branch { true_bcb, false_bcb }, span })
+            Some(BcbBranchPair { span, true_bcb, false_bcb })
         })
         .collect::<Vec<_>>()
 }