Add documentation how to creat a SwiftSyntax release

Author: ahoppen

Documentation/Create SwiftSyntax Release.md

@@ -0,0 +1,93 @@
+# Create SwiftSyntax snapshot release with parser library
+
+## Create release tag
+1. Check out the tag you want to release in the SwiftSyntax repository
+2. Download the matching Swift release from <https://swift.org> and install it
+3. Create an `.xcframework` by running 
+```bash
+xcodebuild -create-xcframework -library /Library/Developer/Toolchains/<toolchain-name>.xctoolchain/usr/lib/swift/macosx/lib_InternalSwiftSyntaxParser.dylib -output _InternalSwiftSyntaxParser.xcframework
+```
+4. Zip up the framework
+```bash
+zip -r _InternalSwiftSyntaxParser.xcframework.zip _InternalSwiftSyntaxParser.xcframework
+```
+5. Compute the checksum for the generated `.xcframework.zip`
+```bash
+/Library/Developer/Toolchains/<toolchain-name>.xctoolchain/usr/bin/swift package --package-path /path/to/swift-syntax/ compute-checksum _InternalSwiftSyntaxParser.xcframework.zip
+```
+6. Apply the following diff to `Package.swift`, adjusting `<release tag>` and `<checksum>`
+```diff
+diff --git a/Package.swift b/Package.swift
+index 4c6db83a..7a953fc6 100644
+--- a/Package.swift
++++ b/Package.swift
+@@ -23,6 +23,21 @@ if ProcessInfo.processInfo.environment["SWIFT_BUILD_SCRIPT_ENVIRONMENT"] != nil
+   swiftSyntaxSwiftSettings = []
+ }
+ 
++// Include the parser library as a binary dependency if both the host and the target are macOS.
++//  - We need to require that the host is macOS (checked by `#if os(macOS)`) because package resolve will download and unzip the referenced framework, which requires `unzip` to be installed. Only macOS guarantees that `unzip` is installed, the Swift Docker images don’t have unzip installed, so package resolve would fail.
++//  - We need to require that the target is macOS (checked by `.when`) because binary dependencies are only supported by SwiftPM on macOS.
++#if os(macOS)
++let parserLibraryTarget: [Target] = [.binaryTarget(
++  name: "_InternalSwiftSyntaxParser",
++  url: "https://github.com/apple/swift-syntax/releases/download/<release tag>/_InternalSwiftSyntaxParser.xcframework.zip",
++  checksum: "<checksum>"
++)]
++let parserLibraryDependency: [Target.Dependency] = [.target(name: "_InternalSwiftSyntaxParser", condition: .when(platforms: [.macOS]))]
++#else
++let parserLibraryTarget: [Target] = []
++let parserLibraryDependency: [Target.Dependency] = []
++#endif
++
+ let package = Package(
+   name: "SwiftSyntax",
+   products: [
+@@ -65,7 +80,7 @@ let package = Package(
+         "Tokens.swift.gyb",
+       ]
+     ),
+-    .target(name: "SwiftSyntaxParser", dependencies: ["SwiftSyntax"], exclude: [
++    .target(name: "SwiftSyntaxParser", dependencies: ["SwiftSyntax"] + parserLibraryDependency, exclude: [
+       "NodeDeclarationHash.swift.gyb"
+     ]),
+     .target(
+@@ -90,5 +105,5 @@ let package = Package(
+       dependencies: ["SwiftSyntax", "SwiftSyntaxParser"],
+       exclude: ["Inputs"]
+     ),
+-  ]
++  ] + parserLibraryTarget
+ )
+```
+7. Commit the changes
+8. Create a tag and push it to your personal GitHub repository
+9. On your personal GitHub repo create new release, select the tag, name it and upload `_InternalSwiftSyntaxParser.xcframework.zip`
+
+## Smoke test on macOS
+
+1. Change the binary target in `Package.swift` to point to your personal GitHub repo
+2. Open the SwiftSyntax package with an Xcode version that contains no or an incompatible parser library
+3. Update to Latest Package Versions in Xcode
+4. Run tests
+
+## Smoke test on Linux
+
+1. Pull the latest development snapshot docker image for the release, e.g. `docker pull swiftlang/swift:nightly-5.6-focal`
+2. Start a development docker image `docker run -it --rm swiftlang/swift:nightly-5.6-focal`
+3. `git clone https://github.com/<your github account>/swift-syntax`
+4. `cd swift-syntax`
+5. `git checkout <release tag>`
+6. `swift test`
+
+## Publish Release
+
+1. Push tag to apple/swift-syntax
+2. On apple/swift-syntax create a new release using the just-pushed tag and upload the parser library as `_InternalSwiftSyntaxParser.xcframework.zip`
+3. If this is a snapshot release, check “This is a pre-release”
+4. Publish 🎉
+
+## Integration test
+
+1. Create a new Swift package on macOS that depends on SwiftSyntax and verify that it can parser Swift source into a syntax tree
+2. Do the same on Linux