Add infrastructure for automated validation of syntax node structure

The goal here is to generalize the API inconsistency reported by @AnthonyLatsis and automatically detect them.

There are valid reasons for nodes or children to violate these base rules. My idea is that we note these exceptions in expected failures, ideally with a sort explanation that motivates the deviation from standard naming rules.

We aren’t running these tests at the moment yet. I’d like to merge them as a starting point for API improvement ideas at the moment.

The current rules are
- All nodes with base kind e.g. `ExprSyntax` should end with `ExprSyntax`.
- If a child has a single keyword as its choice, it should be named `*Keyword` (e.g. `ImportKeyword`)
- If a child has a single non-keyword token kind, name it the same as the the token choice (e.g. `LeftParen`)
- If a token only has keyword token choices, its name should end with `Keyword`.
- Children that have the same type also have the same child name.
- Every node that can conform to a trait does so

Author: ahoppen

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
+    }
+  }
+}