The current state of swift-driver and...roadmap?

Hello everyone from the Swift community! I’m really excited to see swift-driver which is far clearer and more readable than the existing driver written in C++. I’m a little curious about the state of this project, and the questions are:

  • In README, we’ve listed some development goals of swift-driver, which are really exciting. Are they all implemented by the existing driver, or they contains features that the existing one isn’t going to support?
  • Will swift-driver be shipped with Swift toolchain before all the items on the list are checked?
  • Is there any estimation on which Swift version will switch to the new driver? Are we expected to see this in Swift 5.x?
  • If I’d like to improve the driver, should I patch on swift-driver only, or patch on both drivers?

Thanks a lot!


With a couple of small exceptions (like typo correction for misspelled option names), everything on the development plan checklist is existing functionality of the C++ driver that needs to be reimplemented before swift-driver can replace it.

Starting fairly recently, nightly trunk snapshots from have started shipping with swift-driver. Using it requires some tinkering with build settings though, or using it via SPM with the --use-integrated-swift-driver flag. There's some more info on that in this post and in the swift-driver README.

Not really. swift-driver is in pretty good shape currently, but there are still a few big unimplemented features like incremental builds, and a lot of small differences in behavior and test failures that need fixing before it can ship.

It's really up to you. Any changes to the C++ driver will eventually need to be ported to swift-driver, and swift-driver shouldn't include changes that break compatibility with the C++ version, but there's no requirement that you change both at the same time or restrict changes to one or the other.

What’s included in the “compatibility” here? Must I keep the behavior of these two drivers the same?

Compatibility in this case means that if the C++ driver accepts a certain set of flags and inputs, swift-driver should also accept them, have the same observable behavior, and produce the same outputs. That said, swift-driver has a few major features, like explicit module builds, that don't exist in the C++ driver, so there's no need to match the behavior of those. Because the swift-driver work is still ongoing, it's also fine to change the behavior of the C++ driver and not update swift-driver's behavior to match at the same time :slight_smile:

Thanks a lot! I’ve been curious whether swift-driver has supported cross-compilation so far. I found the toolchain and build target still require to match?

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