[SE-0474] Support yielding borrow and yielding mutate accessors

CodeGeneration/Sources/SyntaxSupport/DeclNodes.swift

@@ -180,6 +180,14 @@ public let DECL_NODES: [Node] = [
     ]
   ),
 
+  Node(
+    kind: .accessorYieldingModifier,
+    base: .syntax,
+    nameForDiagnostics: nil,
+    traits: [],
+    children: [ ??? ],
+  ),
+
   Node(
     kind: .actorDecl,
     base: .decl,

CodeGeneration/Sources/SyntaxSupport/KeywordSpec.swift

@@ -279,6 +279,7 @@ public enum Keyword: CaseIterable {
   case willSet
   case wrt
   case yield
+  case yielding
 
   public var spec: KeywordSpec {
     switch self {
@@ -692,6 +693,8 @@ public enum Keyword: CaseIterable {
       return KeywordSpec("wrt")
     case .yield:
       return KeywordSpec("yield")
+    case .yielding:
+      return KeywordSpec("yielding")
     }
   }
 }

Sources/SwiftParser/Declarations.swift

@@ -1659,6 +1659,7 @@ extension Parser {
   struct AccessorIntroducer {
     var attributes: RawAttributeListSyntax
     var modifier: RawDeclModifierSyntax?
+    var yielding: RawAccessorYieldingModifierSyntax?
     var kind: AccessorDeclSyntax.AccessorSpecifierOptions
     var unexpectedBeforeToken: RawUnexpectedNodesSyntax?
     var token: RawTokenSyntax
@@ -1671,9 +1672,14 @@ extension Parser {
     var look = self.lookahead()
     let _ = look.consumeAttributeList()
     let hasModifier = look.consume(ifAnyIn: AccessorModifier.self) != nil
+    let hasYielding = look.consume(if: TokenSpec(.yielding)) != nil
     guard let (kind, _) = look.at(anyIn: AccessorDeclSyntax.AccessorSpecifierOptions.self) ?? forcedKind else {
       return nil
     }
+    guard !yielding || [AccessorDeclSyntax.AccessorSpecifierOptions.borrow, .mutate].contains(kind) else {
+      // `yielding` can only be followed by `borrow` or `mutate`
+      return nil
+    }
 
     let attrs = self.parseAttributeList()
 
@@ -1692,10 +1698,24 @@ extension Parser {
       modifier = nil
     }
 
+    let yielding: RawAccessorYieldingModifierSyntax?
+    if hasYielding {
+      let (unexpectedBeforeYieldingName, yieldingName) = self.expect(TokenSpec(.yielding))
+      yielding = RawAccessorYieldingModifierSyntax(
+        unexpectedBeforeYieldingName,
+        name: yieldingName,
+        arena: self.arena
+      )
+    } else {
+      yielding = nil
+    }
+
     let (unexpectedBeforeIntroducer, introducer) = self.expect(kind.spec)
+
     return AccessorIntroducer(
       attributes: attrs,
       modifier: modifier,
+      yielding: yielding,
       kind: kind,
       unexpectedBeforeToken: unexpectedBeforeIntroducer,
       token: introducer

Sources/SwiftParser/TokenPrecedence.swift

@@ -237,7 +237,7 @@ enum TokenPrecedence: Comparable {
       .dependsOn, .scoped, .sending,
       // Accessors
       .get, .set, .didSet, .willSet, .unsafeAddress, .addressWithOwner, .addressWithNativeOwner, .unsafeMutableAddress,
-      .mutableAddressWithOwner, .mutableAddressWithNativeOwner, ._read, .read, ._modify, .modify, .mutate,
+      .mutableAddressWithOwner, .mutableAddressWithNativeOwner, ._read, .read, ._modify, .modify, .mutate, .yielding,
       // Misc
       .import, .using:
       self = .declKeyword

Tests/SwiftParserTest/DeclarationTests.swift

@@ -3467,12 +3467,18 @@ final class DeclarationTests: ParserTestCase {
         read {
           yield _i
         }
+        yielding borrow {
+          yield _i
+        }
         _modify {
           yield &_i
         }
         modify {
           yield &_i
         }
+        yielding mutate {
+          yield &_i
+        }
       }
       """,
       experimentalFeatures: .coroutineAccessors