Allow custom derivate registration for _modify accessors

CodeGeneration/Sources/SyntaxSupport/AttributeNodes.swift

@@ -385,7 +385,7 @@ public let ATTRIBUTE_NODES: [Node] = [
       ),
       Child(
         name: "accessorSpecifier",
-        kind: .token(choices: [.keyword(.get), .keyword(.set)]),
+        kind: .token(choices: [.keyword(.get), .keyword(.set), .keyword(._modify)]),
         documentation: "The accessor name.",
         isOptional: true
       ),

Sources/SwiftParser/Attributes.swift

@@ -650,7 +650,12 @@ extension Parser {
     let unexpectedBeforeAccessor: RawUnexpectedNodesSyntax?
     let accessor: RawTokenSyntax?
     if period != nil {
-      (unexpectedBeforeAccessor, accessor) = self.expect(.keyword(.get), .keyword(.set), default: .keyword(.get))
+      (unexpectedBeforeAccessor, accessor) = self.expect(
+        .keyword(.get),
+        .keyword(.set),
+        .keyword(._modify),
+        default: .keyword(.get)
+      )
     } else {
       (unexpectedBeforeAccessor, accessor) = (nil, nil)
     }

Sources/SwiftParser/generated/Parser+TokenSpecSet.swift

@@ -1374,13 +1374,16 @@ extension DerivativeAttributeArgumentsSyntax {
   public enum AccessorSpecifierOptions: TokenSpecSet {
     case get
     case set
+    case _modify
 
     init?(lexeme: Lexer.Lexeme, experimentalFeatures: Parser.ExperimentalFeatures) {
       switch PrepareForKeywordMatch(lexeme) {
       case TokenSpec(.get):
         self = .get
       case TokenSpec(.set):
         self = .set
+      case TokenSpec(._modify):
+        self = ._modify
       default:
         return nil
       }
@@ -1392,6 +1395,8 @@ extension DerivativeAttributeArgumentsSyntax {
         self = .get
       case TokenSpec(.set):
         self = .set
+      case TokenSpec(._modify):
+        self = ._modify
       default:
         return nil
       }
@@ -1403,6 +1408,8 @@ extension DerivativeAttributeArgumentsSyntax {
         return .keyword(.get)
       case .set:
         return .keyword(.set)
+      case ._modify:
+        return .keyword(._modify)
       }
     }
 
@@ -1416,6 +1423,8 @@ extension DerivativeAttributeArgumentsSyntax {
         return .keyword(.get)
       case .set:
         return .keyword(.set)
+      case ._modify:
+        return .keyword(._modify)
       }
     }
   }

Sources/SwiftSyntax/generated/raw/RawSyntaxValidation.swift

@@ -950,7 +950,7 @@ func validateLayout(layout: RawSyntaxBuffer, as kind: SyntaxKind) {
     assertNoError(kind, 6, verify(layout[6], as: RawUnexpectedNodesSyntax?.self))
     assertNoError(kind, 7, verify(layout[7], as: RawTokenSyntax?.self, tokenChoices: [.tokenKind(.period)]))
     assertNoError(kind, 8, verify(layout[8], as: RawUnexpectedNodesSyntax?.self))
-    assertNoError(kind, 9, verify(layout[9], as: RawTokenSyntax?.self, tokenChoices: [.keyword("get"), .keyword("set")]))
+    assertNoError(kind, 9, verify(layout[9], as: RawTokenSyntax?.self, tokenChoices: [.keyword("get"), .keyword("set"), .keyword("_modify")]))
     assertNoError(kind, 10, verify(layout[10], as: RawUnexpectedNodesSyntax?.self))
     assertNoError(kind, 11, verify(layout[11], as: RawTokenSyntax?.self, tokenChoices: [.tokenKind(.comma)]))
     assertNoError(kind, 12, verify(layout[12], as: RawUnexpectedNodesSyntax?.self))

Sources/SwiftSyntax/generated/syntaxNodes/SyntaxNodesD.swift

@@ -1239,7 +1239,7 @@ public struct DeinitializerEffectSpecifiersSyntax: SyntaxProtocol, SyntaxHashabl
 ///  - `colon`: `:`
 ///  - `originalDeclName`: ``ExprSyntax``
 ///  - `period`: `.`?
-///  - `accessorSpecifier`: (`get` | `set`)?
+///  - `accessorSpecifier`: (`get` | `set` | `_modify`)?
 ///  - `comma`: `,`?
 ///  - `arguments`: ``DifferentiabilityWithRespectToArgumentSyntax``?
 ///
@@ -1439,6 +1439,7 @@ public struct DerivativeAttributeArgumentsSyntax: SyntaxProtocol, SyntaxHashable
   /// For syntax trees generated by the parser, this is guaranteed to be one of the following kinds:
   ///  - `get`
   ///  - `set`
+  ///  - `_modify`
   public var accessorSpecifier: TokenSyntax? {
     get {
       return Syntax(self).child(at: 9)?.cast(TokenSyntax.self)