The path to the top level of the source tree.
This is the full path to the top level of the current CMake source tree. For an in-source build, this would be the same as CMAKE_BINARY_DIR.
When run in -P script mode, CMake sets the variables CMAKE_BINARY_DIR, CMAKE_SOURCE_DIR, CMAKE_CURRENT_BINARY_DIR and CMAKE_CURRENT_SOURCE_DIR to the current working directory.
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https://cmake.org/cmake/help/v3.19/variable/CMAKE_SOURCE_DIR.html