Update DeclGroupSyntax to refine DeclSyntaxProtocol (#2366) (#2372)

Updates the swift-syntax code generator to support traits refining a syntax base type. Uses this new code generator feature to change DeclGroupSyntax to refine DeclSyntaxProtocol.

Author: rauhul

CodeGeneration/Sources/SyntaxSupport/Traits.swift

@@ -14,12 +14,19 @@ import SwiftSyntax
 
 public class Trait {
  public let traitName: String
+ /// The kind of syntax the trait refines.
+ ///
+ /// Base kind _must_ be a base syntax node, e.g. `.decl`, `.expr` , or
+ /// others. See ``SyntaxNodeKind/isBase`` for more details.
+ public let baseKind: SyntaxNodeKind?
  public let protocolName: TokenSyntax
  public let documentation: SwiftSyntax.Trivia
  public let children: [Child]
 
- init(traitName: String, documentation: String? = nil, children: [Child]) {
+ init(traitName: String, baseKind: SyntaxNodeKind? = nil, documentation: String? = nil, children: [Child]) {
+ precondition(baseKind?.isBase != false, "`baseKind` must be a base syntax node kind")
  self.traitName = traitName
+ self.baseKind = baseKind
  self.protocolName = .identifier("\(traitName)Syntax")
  self.documentation = docCommentTrivia(from: documentation)
  self.children = children
@@ -36,6 +43,7 @@ public let TRAITS: [Trait] = [
  ),
  Trait(
  traitName: "DeclGroup",
+ baseKind: .decl,
  children: [
  Child(name: "attributes", kind: .node(kind: .attributeList)),
  Child(name: "modifiers", kind: .node(kind: .declModifierList)),

CodeGeneration/Sources/generate-swift-syntax/templates/swiftsyntax/SyntaxTraitsFile.swift

@@ -22,7 +22,11 @@ let syntaxTraitsFile = SourceFileSyntax(leadingTrivia: copyrightHeader) {
  // MARK: - \(trait.protocolName)
 
  \(trait.documentation)
- public protocol \(trait.protocolName): SyntaxProtocol
+ public protocol \(trait.protocolName): SyntaxProtocol\(raw:
+ trait.baseKind != nil
+ ? ", \(trait.baseKind!.protocolType)"
+ : ""
+ )
  """
  ) {
  for child in trait.children {

Sources/SwiftSyntax/generated/SyntaxTraits.swift

@@ -57,7 +57,7 @@ public extension SyntaxProtocol {
 // MARK: - DeclGroupSyntax
 
 
-public protocol DeclGroupSyntax: SyntaxProtocol {
+public protocol DeclGroupSyntax: SyntaxProtocol, DeclSyntaxProtocol {
  var attributes: AttributeListSyntax {
  get
  set