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

`yielding borrow` and `yielding mutate` are the final approved
names for the accessors previously known as `read` and `modify`
(which are themselves improvements over the earlier non-standard
`_read` and `_modify` accessors).

Since some people are already using these under the `read` and
`modify` names, we want to support the old and new names as synonyms
for a transition period until everyone has had a chance to migrate.

Author: tbkka

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 {
@@ -691,7 +692,9 @@ public enum Keyword: CaseIterable {
     case .wrt:
       return KeywordSpec("wrt")
     case .yield:
-      return KeywordSpec("yield")
+        return KeywordSpec("yield")
+    case .yielding:
+        return KeywordSpec("yielding")
     }
   }
 }

Sources/SwiftParser/Declarations.swift

@@ -1671,9 +1671,14 @@ extension Parser {
     var look = self.lookahead()
     let _ = look.consumeAttributeList()
     let hasModifier = look.consume(ifAnyIn: AccessorModifier.self) != nil
+    let yielding = 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()
 
@@ -1692,11 +1697,28 @@ extension Parser {
       modifier = nil
     }
 
+    if yielding {
+      _ = self.expect(TokenSpec(.yielding))
+    }
+
     let (unexpectedBeforeIntroducer, introducer) = self.expect(kind.spec)
+
+    // Map `yielding borrow` => `read`, etc.
+    let resolvedKind : AccessorDeclSyntax.AccessorSpecifierOptions
+    if yielding {
+      switch kind {
+      case .borrow: resolvedKind = .read // `yielding borrow` == `read`
+      case .mutate: resolvedKind = .modify // `yielding mutate` == `modify`
+      default: resolvedKind = kind
+      }
+    } else {
+      resolvedKind = kind
+    }
+
     return AccessorIntroducer(
       attributes: attrs,
       modifier: modifier,
-      kind: kind,
+      kind: resolvedKind,
       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

Sources/SwiftSyntax/generated/Keyword.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