collection_validation.cpp isn't used anywhere in the 4.2 branch; it simply appears to duplicate the 'validate' command logic in collection_impl.cpp. The file should either be removed or the duplicated logic should be removed from collection_impl.cpp and collection_validation.cpp should be included within db/catalog/SConscript.
- is related to
SERVER-42312 Validate during rollback can cause count mismatch
SERVER-43081 validate should report when an index's 'multikeyPaths' are set but the 'multikey' flag is false