I use CMake to generate Visual Studio solution in a separate directory (out of source build)

For example:

mkdir build
cd build
cmake .. -GNinja

Compiler prints warnings in the following format:

[14/139] Building C object modules\Lib\CMakeFiles\Lib.dir\file.c.obj
 ..\modules\Lib\file.c(608): warning C4101: 'i': unreferenced local variable
 ..\modules\Lib\file.c(725): warning C4267: '=': conversion from 'size_t' to 'int', possible loss of data

And the Warnings-NG plugin, called at the post-build step, cannot find these files and outputs errors:

[MSBuild] [ERROR] Can't resolve absolute paths for some files:
[MSBuild] [ERROR] - ../modules/Lib/file.c
 ...
[MSBuild] [ERROR] Can't create fingerprints for some files:
[MSBuild] [ERROR] - '../modules/Lib/file.c', IO exception has been thrown: java.nio.file.NoSuchFileException: ..\modules\Lib\file.c
 ...
[MSBuild] [ERROR] Can't create blame requests for some affected files:
[MSBuild] [ERROR] - Skipping non-workspace file ../modules/Lib/file.c
 ...

Suggestion:

Add option for path mapping, relative to workspace root.