source_gen is a Dart library that provides a framework for writing source code generators that integrate with the `package:build` build system. It defines abstract `Generator` and `GeneratorForAnnotation` classes, and provides concrete `Builder` implementations (`PartBuilder`, `SharedPartBuilder`, `LibraryBuilder`) that wrap generators into build-compatible builders. The library also includes utilities for type checking (`TypeChecker`), reading constant annotation values (`ConstantReader`), reviving constant expressions (`Revivable`), and navigating library elements (`LibraryReader`). A `CombiningBuilder` merges multiple generated part files into a single `.g.dart` output. The codebase is well-structured, thoroughly tested, and actively maintained by the Dart team. It is not a standalone application but a framework dependency for packages that need to generate Dart code at build time.