IssueTracker

pwrev/135870 added a new path transformation of ,out/ that is past to the compiler using -ffile-prefix-map. The intention of this transformation was to append out/ to any path that didn't previously match one of the other transformations (which remove absolute paths to the build directory and root project directory). This applies both to debug and coverage information.

However, LLVM currently stores these path transformations in a std::map (https://llvm.googlesource.com/llvm-project/+/a45dd3d8140eab78a4554484c2b0435582ee262a/clang/include/clang/Basic/CodeGenOptions.h#183). The key for this std::map was the left-side of the comma, leading to the entries being ordered lexicographically. For ,out/, the left-hand path is "", which is sorted before any other string. Effectively, the ,out/ prefix map is always being applied.

The reason why this hasn't been an issue yet is that most people use single directory build dirs. This leads to paths to source files that are originally like ../pw_log/public/pw_log/log.h. This would then be prepended with out/, leading to out/../pw_log/public/pw_log/log.h. This hid the fact that the -ffile-prefix-map rebase_path("//", root_build_dir) + "=" option was not being used because prepending out is basically has the same outcome for single directory build dirs. The issue becomes apparent if you have a build dir like out/test, where the path would then become out/../../pw_log/public/pw_log/log.h, which will point to a non-existent file.