The Xcode 15b1 template for macros depends on SwiftSyntax like this:
// Depend on the latest Swift 5.9 prerelease of SwiftSyntax
.package(url: "https://github.com/apple/swift-syntax.git", from: "509.0.0-swift-5.9-DEVELOPMENT-SNAPSHOT-2023-04-25-b"),
But Doug Gregor’s examples simply depend on main. What's the best thing to depend on? There isn't a 5.9 release afaict.