coverage: Add LLVM plumbing for expansion regions

This is currently unused, but paves the way for future work on expansion
regions without having to worry about the FFI parts.

compiler/rustc_codegen_llvm/src/coverageinfo/ffi.rs

@@ -146,6 +146,7 @@ pub(crate) struct CoverageSpan {
 #[derive(Clone, Debug, Default)]
 pub(crate) struct Regions {
     pub(crate) code_regions: Vec<CodeRegion>,
+    pub(crate) expansion_regions: Vec<ExpansionRegion>,
     pub(crate) branch_regions: Vec<BranchRegion>,
     pub(crate) mcdc_branch_regions: Vec<MCDCBranchRegion>,
     pub(crate) mcdc_decision_regions: Vec<MCDCDecisionRegion>,
@@ -154,10 +155,16 @@ pub(crate) struct Regions {
 impl Regions {
     /// Returns true if none of this structure's tables contain any regions.
     pub(crate) fn has_no_regions(&self) -> bool {
-        let Self { code_regions, branch_regions, mcdc_branch_regions, mcdc_decision_regions } =
+        let Self {
-            self;
+            code_regions,
+            expansion_regions,
+            branch_regions,
+            mcdc_branch_regions,
+            mcdc_decision_regions,
+        } = self;
 
         code_regions.is_empty()
+            && expansion_regions.is_empty()
             && branch_regions.is_empty()
             && mcdc_branch_regions.is_empty()
             && mcdc_decision_regions.is_empty()
@@ -172,6 +179,14 @@ pub(crate) struct CodeRegion {
     pub(crate) counter: Counter,
 }
 
+/// Must match the layout of `LLVMRustCoverageExpansionRegion`.
+#[derive(Clone, Debug)]
+#[repr(C)]
+pub(crate) struct ExpansionRegion {
+    pub(crate) cov_span: CoverageSpan,
+    pub(crate) expanded_file_id: u32,
+}
+
 /// Must match the layout of `LLVMRustCoverageBranchRegion`.
 #[derive(Clone, Debug)]
 #[repr(C)]

compiler/rustc_codegen_llvm/src/coverageinfo/llvm_cov.rs

@@ -63,8 +63,18 @@ pub(crate) fn write_function_mappings_to_buffer(
     expressions: &[ffi::CounterExpression],
     regions: &ffi::Regions,
 ) -> Vec<u8> {
-    let ffi::Regions { code_regions, branch_regions, mcdc_branch_regions, mcdc_decision_regions } =
+    let ffi::Regions {
-        regions;
+        code_regions,
+        expansion_regions,
+        branch_regions,
+        mcdc_branch_regions,
+        mcdc_decision_regions,
+    } = regions;
+
+    // SAFETY:
+    // - All types are FFI-compatible and have matching representations in Rust/C++.
+    // - For pointer/length pairs, the pointer and length come from the same vector or slice.
+    // - C++ code does not retain any pointers after the call returns.
     llvm::build_byte_buffer(|buffer| unsafe {
         llvm::LLVMRustCoverageWriteFunctionMappingsToBuffer(
             virtual_file_mapping.as_ptr(),
@@ -73,6 +83,8 @@ pub(crate) fn write_function_mappings_to_buffer(
             expressions.len(),
             code_regions.as_ptr(),
             code_regions.len(),
+            expansion_regions.as_ptr(),
+            expansion_regions.len(),
             branch_regions.as_ptr(),
             branch_regions.len(),
             mcdc_branch_regions.as_ptr(),

compiler/rustc_codegen_llvm/src/coverageinfo/mapgen/covfun.rs

@@ -120,13 +120,18 @@ fn fill_region_tables<'tcx>(
     // Associate that global file ID with a local file ID for this function.
     let local_file_id = covfun.virtual_file_mapping.local_id_for_global(global_file_id);
 
-    let ffi::Regions { code_regions, branch_regions, mcdc_branch_regions, mcdc_decision_regions } =
-        &mut covfun.regions;
-
     let make_cov_span =
         |span: Span| spans::make_coverage_span(local_file_id, source_map, &source_file, span);
     let discard_all = tcx.sess.coverage_discard_all_spans_in_codegen();
 
+    let ffi::Regions {
+        code_regions,
+        expansion_regions: _, // FIXME(Zalathar): Fill out support for expansion regions
+        branch_regions,
+        mcdc_branch_regions,
+        mcdc_decision_regions,
+    } = &mut covfun.regions;
+
     // For each counter/region pair in this function+file, convert it to a
     // form suitable for FFI.
     for &Mapping { ref kind, span } in &fn_cov_info.mappings {

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -2019,6 +2019,8 @@ unsafe extern "C" {
         NumExpressions: size_t,
         CodeRegions: *const crate::coverageinfo::ffi::CodeRegion,
         NumCodeRegions: size_t,
+        ExpansionRegions: *const crate::coverageinfo::ffi::ExpansionRegion,
+        NumExpansionRegions: size_t,
         BranchRegions: *const crate::coverageinfo::ffi::BranchRegion,
         NumBranchRegions: size_t,
         MCDCBranchRegions: *const crate::coverageinfo::ffi::MCDCBranchRegion,

compiler/rustc_llvm/llvm-wrapper/CoverageMappingWrapper.cpp

@@ -77,6 +77,13 @@ struct LLVMRustCoverageCodeRegion {
   LLVMRustCounter Count;
 };
 
+// Must match the layout of
+// `rustc_codegen_llvm::coverageinfo::ffi::ExpansionRegion`.
+struct LLVMRustCoverageExpansionRegion {
+  LLVMRustCoverageSpan Span;
+  uint32_t ExpandedFileID;
+};
+
 // Must match the layout of
 // `rustc_codegen_llvm::coverageinfo::ffi::BranchRegion`.
 struct LLVMRustCoverageBranchRegion {
@@ -151,6 +158,8 @@ extern "C" void LLVMRustCoverageWriteFunctionMappingsToBuffer(
     const unsigned *VirtualFileMappingIDs, size_t NumVirtualFileMappingIDs,
     const LLVMRustCounterExpression *RustExpressions, size_t NumExpressions,
     const LLVMRustCoverageCodeRegion *CodeRegions, size_t NumCodeRegions,
+    const LLVMRustCoverageExpansionRegion *ExpansionRegions,
+    size_t NumExpansionRegions,
     const LLVMRustCoverageBranchRegion *BranchRegions, size_t NumBranchRegions,
     const LLVMRustCoverageMCDCBranchRegion *MCDCBranchRegions,
     size_t NumMCDCBranchRegions,
@@ -179,6 +188,13 @@ extern "C" void LLVMRustCoverageWriteFunctionMappingsToBuffer(
         Region.Span.ColumnStart, Region.Span.LineEnd, Region.Span.ColumnEnd));
   }
 
+  // Expansion regions:
+  for (const auto &Region : ArrayRef(ExpansionRegions, NumExpansionRegions)) {
+    MappingRegions.push_back(coverage::CounterMappingRegion::makeExpansion(
+        Region.Span.FileID, Region.ExpandedFileID, Region.Span.LineStart,
+        Region.Span.ColumnStart, Region.Span.LineEnd, Region.Span.ColumnEnd));
+  }
+
   // Branch regions:
   for (const auto &Region : ArrayRef(BranchRegions, NumBranchRegions)) {
     MappingRegions.push_back(coverage::CounterMappingRegion::makeBranchRegion(