Rollup merge of #122322 - Zalathar:branch, r=oli-obk

coverage: Initial support for branch coverage instrumentation

(This is a review-ready version of the changes that were drafted in #118305.)

This PR adds support for branch coverage instrumentation, gated behind the unstable flag value `-Zcoverage-options=branch`. (Coverage instrumentation must also be enabled with `-Cinstrument-coverage`.)

During THIR-to-MIR lowering (MIR building), if branch coverage is enabled, we collect additional information about branch conditions and their corresponding then/else blocks. We inject special marker statements into those blocks, so that the `InstrumentCoverage` MIR pass can reliably identify them even after the initially-built MIR has been simplified and renumbered.

The rest of the changes are mostly just plumbing needed to gather up the information that was collected during MIR building, and include it in the coverage metadata that we embed in the final binary.

Note that `llvm-cov show` doesn't print branch coverage information in its source views by default; that needs to be explicitly enabled with `--show-branches=count` or similar.

---

The current implementation doesn't have any support for instrumenting `if let` or let-chains. I think it's still useful without that, and adding it would be non-trivial, so I'm happy to leave that for future work.

2 files changed, 11 insertions, 5 deletions

diff --git a/compiler/rustc_codegen_llvm/src/coverageinfo/ffi.rs b/compiler/rustc_codegen_llvm/src/coverageinfo/ffi.rs
index 017843c7e7d..2af28146a51 100644
--- a/compiler/rustc_codegen_llvm/src/coverageinfo/ffi.rs
+++ b/compiler/rustc_codegen_llvm/src/coverageinfo/ffi.rs
@@ -164,6 +164,15 @@ impl CounterMappingRegion {
                 end_line,
                 end_col,
             ),
+            MappingKind::Branch { true_term, false_term } => Self::branch_region(
+                Counter::from_term(true_term),
+                Counter::from_term(false_term),
+                local_file_id,
+                start_line,
+                start_col,
+                end_line,
+                end_col,
+            ),
         }
     }
 
@@ -188,9 +197,6 @@ impl CounterMappingRegion {
         }
     }
 
-    // This function might be used in the future; the LLVM API is still evolving, as is coverage
-    // support.
-    #[allow(dead_code)]
     pub(crate) fn branch_region(
         counter: Counter,
         false_counter: Counter,
diff --git a/compiler/rustc_codegen_llvm/src/coverageinfo/mod.rs b/compiler/rustc_codegen_llvm/src/coverageinfo/mod.rs
index 733a77d24c2..133084b7c12 100644
--- a/compiler/rustc_codegen_llvm/src/coverageinfo/mod.rs
+++ b/compiler/rustc_codegen_llvm/src/coverageinfo/mod.rs
@@ -88,7 +88,7 @@ impl<'tcx> CoverageInfoBuilderMethods<'tcx> for Builder<'_, '_, 'tcx> {
         match coverage.kind {
             // Marker statements have no effect during codegen,
             // so return early and don't create `func_coverage`.
-            CoverageKind::SpanMarker => return,
+            CoverageKind::SpanMarker | CoverageKind::BlockMarker { .. } => return,
             // Match exhaustively to ensure that newly-added kinds are classified correctly.
             CoverageKind::CounterIncrement { .. } | CoverageKind::ExpressionUsed { .. } => {}
         }
@@ -108,7 +108,7 @@ impl<'tcx> CoverageInfoBuilderMethods<'tcx> for Builder<'_, '_, 'tcx> {
 
         let Coverage { kind } = coverage;
         match *kind {
-            CoverageKind::SpanMarker => unreachable!(
+            CoverageKind::SpanMarker | CoverageKind::BlockMarker { .. } => unreachable!(
                 "unexpected marker statement {kind:?} should have caused an early return"
             ),
             CoverageKind::CounterIncrement { id } => {