Explicit Module Builds, the new Swift Driver, and SwiftPM

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Hi everyone,

A few months have passed since this announcement was made and I wanted to provide a quick update on the progress that @Xi_Ge, @Douglas_Gregor and I have been making on this project, spanning the Swift compiler, new Swift driver, and Swift Package Manager.

Status

SwiftPM can now self-host using Explicit Module Builds. The package manager itself is a reasonably complex Swift package, and building it exercises most of the new machinery across the involved components. This was an important milestone for getting the basics of the new compilation flow functional.

Highlights

  • The new Swift driver's Swift library architecture made it simple to integrate the basics of explicit modules into SwiftPM by providing APIs for the new build planning flow.
  • Relying exclusively on explicit module dependencies has been a powerful learning tool for understanding the fine details of how today's Implicit Module Builds work as we try to achieve functional parity between the two flows. Interesting example:
    • Dependencies on Clang modules in a build graph mean that each Clang module must potentially be built and scanned multiple times (once for each depending Swift module with a distinct target).
  • Explicit Module Builds are allowing us to diagnose a class of problems at build plan time that would otherwise be caught much later in the compilation process or even cause compiler hangs and crashes.

Call to action

While the new build flow is still very much experimental, there are several ways to get your hands on it to start contributing bug reports and patches, please give it a try!

  • When using the new Swift driver as a drop-in replacement for today's driver [how-to] on the command line, Explicit Module Build of the target module is enabled with the -experimental-explicit-module-build flag.
  • SwiftPM packages can be built using Explicit Modules with the --use-integrated-swift-driver --experimental-explicit-module-build flag combination.

In expectation, the above flags should not affect the result of compilation; however, explicit module build jobs are not yet capable of interacting with the module cache, which can result in more computation and slower build times than their implicit counterparts.

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