Project Model This is the Swift Build representation of a package graph in SwiftPM or workspace in Xcode, including all model objects and all data that they contain. This representation very closely matches the higher-level representations, although some transforms or derived information are present in some cases where it is considered beneficial. The term "PIF" (Project Interchange Format) is used both to describe the serialized representation used to transfer the model from clients to Swift Build, and as a shorthand for the Swift Build-side representation of the model.
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SwiftBuild.docc/Architecture/build-system-architecture.md