Add infrastructure for automated validation of syntax node structure

CodeGeneration/Package.swift

@@ -45,6 +45,12 @@ let package = Package(
         "SyntaxSupport",
       ]
     ),
+    .testTarget(
+      name: "ValidateSyntaxNodes",
+      dependencies: [
+        "SyntaxSupport"
+      ]
+    ),
   ]
 )
 

CodeGeneration/Sources/SyntaxSupport/Child.swift

@@ -12,7 +12,7 @@
 
 /// The kind of token a node can contain. Either a token of a specific kind or a
 /// keyword with the given text.
-public enum TokenChoice {
+public enum TokenChoice: Equatable {
   case keyword(text: String)
   case token(tokenKind: String)
 

CodeGeneration/Tests/ValidateSyntaxNodes/Utils.swift

@@ -0,0 +1,32 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2023 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+extension Collection {
+  /// If the collection contains a single element, return it, otherwise `nil`.
+  var only: Element? {
+    if !isEmpty && index(after: startIndex) == endIndex {
+      return self.first!
+    } else {
+      return nil
+    }
+  }
+}
+
+extension String {
+  func dropSuffix(_ suffix: String) -> String {
+    if hasSuffix(suffix) {
+      return String(self.dropLast(suffix.count))
+    } else {
+      return self
+    }
+  }
+}