Don't directly expose coverage::CounterMappingRegion via FFI

The definition of this struct changes in LLVM 12 due to the addition
of branch coverage support. To avoid future mismatches, declare our
own struct and then convert between them.

compiler/rustc_codegen_llvm/src/coverageinfo/mod.rs

@@ -162,7 +162,7 @@ pub(crate) fn write_filenames_section_to_buffer<'a>(
 pub(crate) fn write_mapping_to_buffer(
     virtual_file_mapping: Vec<u32>,
     expressions: Vec<CounterExpression>,
-    mut mapping_regions: Vec<CounterMappingRegion>,
+    mapping_regions: Vec<CounterMappingRegion>,
     buffer: &RustString,
 ) {
     unsafe {
@@ -171,7 +171,7 @@ pub(crate) fn write_mapping_to_buffer(
             virtual_file_mapping.len() as c_uint,
             expressions.as_ptr(),
             expressions.len() as c_uint,
-            mapping_regions.as_mut_ptr(),
+            mapping_regions.as_ptr(),
             mapping_regions.len() as c_uint,
             buffer,
         );

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -676,9 +676,7 @@ pub mod coverageinfo {
     /// array", encoded separately), and source location (start and end positions of the represented
     /// code region).
     ///
-    /// Aligns with [llvm::coverage::CounterMappingRegion](https://github.com/rust-lang/llvm-project/blob/rustc/11.0-2020-10-12/llvm/include/llvm/ProfileData/Coverage/CoverageMapping.h#L224-L227)
-    /// Important: The Rust struct layout (order and types of fields) must match its C++
-    /// counterpart.
+    /// Matches LLVMRustCounterMappingRegion.
     #[derive(Copy, Clone, Debug)]
     #[repr(C)]
     pub struct CounterMappingRegion {
@@ -1796,7 +1794,7 @@ extern "C" {
         NumVirtualFileMappingIDs: c_uint,
         Expressions: *const coverage_map::CounterExpression,
         NumExpressions: c_uint,
-        MappingRegions: *mut coverageinfo::CounterMappingRegion,
+        MappingRegions: *const coverageinfo::CounterMappingRegion,
         NumMappingRegions: c_uint,
         BufferOut: &RustString,
     );