There's lots of testing for Swift on Linux (e.g. every pull request runs tests on macOS and Linux), and Apple recently announced https://github.com/apple/swift-

There's lots of testing for Swift on Linux (e.g. every pull request runs tests on macOS and Linux), and Apple recently announced https://github.com/apple/swift-nio https://github.com/apple/swift-nio which seems to have been very well received, e.g. the Vapor framework is completely switching to it for Vapor 3.