A New Swift Parser for SwiftSyntax

[taylorswift]

@gwendal.roue

I'm talking about successfully building a package with swift-tools-version:5.8, and a dependency on SwiftSyntax for Swift 5.8, with a later compiler (Swift 5.8.1, Swift 5.9, and maybe more): this is a scenario where the version of SwiftSyntax lags behind the compiler version. Since this scenario goes further than what is strictly needed for a compiler, I feel the need to ask.

If this scenario is not supported, package plugins would need to have their dependency on SwiftSyntax depend on the compiler version, and this would make them very difficult to maintain, and frankly a PITA from the point of view of their end users (read: I wouldn't use them if a minor Xcode upgrade breaks a build)

these are all valid points, except for the // swift-tools-version: which is not really involved here.

using a plugin that does not match the version of the toolchain you are using is not correct usage. so source generators should never run (or ideally, be built at all) as part of a CI matrix, or be enabled by default for end users. this precludes using them as buildTools; all source generators should run as command plugins, using them otherwise is fundamentally unsafe.

right now in swift-json, i am using:

#if swift(>=5.8) && (os(Linux) || os(macOS))
    plugins.append(.package(url: "https://github.com/kelvin13/swift-package-factory", 
        branch: "swift-DEVELOPMENT-SNAPSHOT-2022-08-18-a"))
#endif 

in the package manifest. this effectively requires having the swift-DEVELOPMENT-SNAPSHOT-2022-08-18-a installed in order to develop (as opposed to just use) the package. this makes it harder to contribute to the project, and ideally SPM would be able to detect the currently-installed toolchain, and choose the appropriate tag of swift-package-factory to include as a dependency. but right now SPM cannot do this.

related: Allow SPM to specify a toolchain dependency