[SE-0306] Enable actors by default.

With the acceptance of SE-0306, enable actors by default and add a
ChangeLog entry.

Tracked by rdar://77223763.

Author: DougGregor

CHANGELOG.md

@@ -29,6 +29,25 @@ CHANGELOG
 Swift 5.5
 ---------
 
+* [SE-0306][]:
+
+	Swift 5.5 includes support for actors, a new kind of type that isolates its instance data to protect it from concurrent access. Accesses to an actor's instance declarations from outside the must be asynchronous:
+
+	```swift
+  actor Counter {
+    var value = 0
+
+    func increment() {
+      value = value + 1
+    }
+  }
+
+  func useCounter(counter: Counter) async {
+    print(await counter.value) // interaction must be async
+    await counter.increment()  // interaction must be async
+  }
+	```
+
 * The determination of whether a call to a `rethrows` function can throw now considers default arguments of `Optional` type.
 
   In Swift 5.4, such default arguments were ignored entirely by `rethrows` checking. This meant that the following example was accepted:
@@ -8434,6 +8453,7 @@ Swift 1.0
 [SE-0297]: <https://github.com/apple/swift-evolution/blob/main/proposals/0297-concurrency-objc.md>
 [SE-0298]: <https://github.com/apple/swift-evolution/blob/main/proposals/0298-asyncsequence.md>
 [SE-0299]: <https://github.com/apple/swift-evolution/blob/main/proposals/0299-extend-generic-static-member-lookup.md>
+[SE-0306]: <https://github.com/apple/swift-evolution/blob/main/proposals/0306-actors.md>
 
 [SR-75]: <https://bugs.swift.org/browse/SR-75>
 [SR-106]: <https://bugs.swift.org/browse/SR-106>

include/swift/Basic/Features.def

@@ -37,7 +37,7 @@
 LANGUAGE_FEATURE(StaticAssert, 0, "#assert", langOpts.EnableExperimentalStaticAssert)
 LANGUAGE_FEATURE(AsyncAwait, 296, "async/await", true)
 LANGUAGE_FEATURE(MarkerProtocol, 0, "@_marker protocol", true)
-LANGUAGE_FEATURE(Actors, 0, "actors", langOpts.EnableExperimentalConcurrency)
+LANGUAGE_FEATURE(Actors, 0, "actors", true)
 LANGUAGE_FEATURE(ConcurrentFunctions, 0, "@concurrent functions", true)
 LANGUAGE_FEATURE(RethrowsProtocol, 0, "@rethrows protocol", true)
 LANGUAGE_FEATURE(GlobalActors, 0, "Global actors", langOpts.EnableExperimentalConcurrency)

lib/Parse/ParseDecl.cpp

@@ -3604,7 +3604,7 @@ bool Parser::parseDeclModifierList(DeclAttributes &Attributes,
       if (Kind == DAK_Count)
         break;
 
-      if (Kind == DAK_Actor && shouldParseExperimentalConcurrency()) {
+      if (Kind == DAK_Actor) {
         // If the next token is a startOfSwiftDecl, we are part of the modifier
         // list and should consume the actor token (e.g, actor public class Foo)
         // otherwise, it's the decl keyword (e.g. actor Foo) and shouldn't be.
@@ -4075,8 +4075,7 @@ bool Parser::isStartOfSwiftDecl() {
     return isStartOfSwiftDecl();
   }
 
-  if (shouldParseExperimentalConcurrency() &&
-      Tok.isContextualKeyword("actor")) {
+  if (Tok.isContextualKeyword("actor")) {
     if (Tok2.is(tok::identifier)) // actor Foo {}
       return true;
     BacktrackingScope Scope(*this);
@@ -4428,8 +4427,7 @@ Parser::parseDecl(ParseDeclOptions Flags,
   // Obvious nonsense.
   default:
 
-    if (shouldParseExperimentalConcurrency() &&
-        Tok.isContextualKeyword("actor") && peekToken().is(tok::identifier)) {
+    if (Tok.isContextualKeyword("actor") && peekToken().is(tok::identifier)) {
       Tok.setKind(tok::contextual_keyword);
       DeclParsingContext.setCreateSyntax(SyntaxKind::ClassDecl);
       DeclResult = parseDeclClass(Flags, Attributes);

test/decl/class/actor/noconcurrency.swift

@@ -1,4 +1,8 @@
 // RUN: %target-typecheck-verify-swift
 
-actor class C { } // expected-error{{'actor' modifier is only valid when experimental concurrency is enabled}}
+actor C {
+  nonisolated func f() { } // expected-error{{'nonisolated' modifier is only valid when experimental concurrency is enabled}}
+}
+
+