如何为rustc编译器自身收集代码覆盖率信息？

问题背景 #

尝试为rustc编译器自身收集代码覆盖率信息，目的是了解编译过程中执行的代码部分，确保关键区域测试覆盖全面。但直接设置RUSTFLAGS="-C instrument-coverage"后编译出错，不确定是否需要修改构建脚本/配置，也想知道适用的覆盖率工具或流程。

已做配置 #

1. config.toml修改内容：

assertions = true
profiler = true
prefix = "/path/to/install"
sysconfdir = "etc"
llvm-config = "/path/to/self-compile-llvm-config"
llvm-filecheck = "/path/to/self-compile-FileCheck"

2. 设置环境变量：

export RUSTFLAGS="-C instrument-coverage"

3. 执行构建命令：

./x build

遇到的错误 #

Compiling cc v1.0.99
Compiling std v0.0.0 (/path/to/rust/library/std)
warning: -C instrument-coverage requires symbol mangling version `v0`, but `-C symbol-mangling-version=legacy` was specified

Compiling profiler_builtins v0.0.0 (/path/to/rust/library/profiler_builtins)
error: `profiler_builtins` crate (required by compiler options) is not compatible with crate attribute `#![no_core]`

error[E0463]: can't find crate for `profiler_builtins`
  |
  = note: the compiler may have been built without the profiler runtime

For more information about this error, try `rustc --explain E0463`.

warning: `core` (lib) generated 1 warning
error: could not compile `core` (lib) due to 2 previous errors; 1 warning emitted
warning: build failed, waiting for other jobs to finish...

warning: profiler_builtins@0.0.0: In file included from /path/to/rust/src/llvm-project/compiler-rt/lib/profile/InstrProfiling.h:12,
warning: profiler_builtins@0.0.0:                  from /path/to/rust/src/llvm-project/compiler-rt/lib/profile/InstrProfilingFile.c:34:
warning: profiler_builtins@0.0.0: /path/to/rust/src/llvm-project/compiler-rt/lib/profile/InstrProfilingFile.c: In function ‘doProfileMerging’:
warning: profiler_builtins@0.0.0: /path/to/rust/src/llvm-project/compiler-rt/lib/profile/InstrProfilingPort.h:33:37: warning: ignoring return value of ‘ftruncate’ declared with attribute ‘warn_unused_result’ [-Wunused-result]
warning: profiler_builtins@0.0.0:    33 | #define COMPILER_RT_FTRUNCATE(f, l) ftruncate(fileno(f), l)
warning: profiler_builtins@0.0.0:       |                                     ^~~~~~~~~~~~~~~~~~~~~~~
warning: profiler_builtins@0.0.0: /path/to/rust/src/llvm-project/compiler-rt/lib/profile/InstrProfilingFile.c:445:9: note: in expansion of macro ‘COMPILER_RT_FTRUNCATE’
warning: profiler_builtins@0.0.0:   445 |   (void)COMPILER_RT_FTRUNCATE(ProfileFile,
warning: profiler_builtins@0.0.0:       |         ^~~~~~~~~~~~~~~~~~~~~
Build completed unsuccessfully in 0:00:31

解决思路和步骤 #

1. 错误根源分析 #

• 覆盖率插桩要求符号 mangling 版本为v0，但rustc默认构建使用legacy版本
• profiler_builtins crate 带有#![no_core]属性，与覆盖率插桩依赖的组件冲突，且编译器未正确加载profiler运行时

2. 调整构建配置与参数 #

不要全局设置RUSTFLAGS，避免影响core、profiler_builtins等底层核心库的编译，改用rustc构建系统的针对性参数：

修改config.toml

添加或调整以下配置项：

[rust]
# 适配覆盖率要求，启用v0符号mangling
symbol-mangling-version = "v0"

# 明确启用profiler和coverage组件
profiler = true
coverage = true

针对性传递覆盖率编译选项

仅对最终的stage2 rustc添加覆盖率插桩，执行构建命令：

./x build --stage 2 --rustc-extra-flags="-C instrument-coverage"

3. 收集覆盖率数据并生成报告 #

编译完成后，运行rustc编译目标代码或执行测试套件，会生成.profraw格式的原始覆盖率文件，后续通过以下步骤生成报告：

合并原始覆盖率数据

llvm-profdata merge -output=rustc.profdata *.profraw

生成lcov格式报告

llvm-cov export -format=lcov -instr-profile=rustc.profdata ./build/x86_64-unknown-linux-gnu/stage2/bin/rustc > rustc_coverage.lcov

生成HTML可视化报告

llvm-cov show -format=html -instr-profile=rustc.profdata ./build/x86_64-unknown-linux-gnu/stage2/bin/rustc -output-dir coverage_report

4. 注意事项 #

• 必须使用rustc构建时配套的llvm-profdata和llvm-cov工具，避免版本不兼容问题
• 若要收集rustc测试套件的覆盖率，可执行./x test --stage 2 --rustc-extra-flags="-C instrument-coverage"，再处理生成的.profraw文件
• 不要对stage0、stage1的编译器添加覆盖率插桩，会导致底层依赖库编译冲突

内容的提问来源于stack exchange，提问作者Surfer