[SE-0313] Enable nonisolated by default.

Author: DougGregor

CHANGELOG.md

@@ -6,6 +6,24 @@ _**Note:** This is in reverse chronological order, so newer entries are added to
 Swift 5.5
 ---------
 
+* [SE-0313][]:
+
+  Declarations inside an actor that would normally by actor-isolated can
+  explicitly become non-isolated using the `nonisolated` keyword. Non-isolated
+  declarations can be used to conform to synchronous protocol requirements:
+
+  ```swift
+  actor Account: Hashable {
+    let idNumber: Int
+    let balance: Double
+
+    nonisolated func hash(into hasher: inout Hasher) { // okay, non-isolated satisfies synchronous requirement
+      hasher.combine(idNumber) // okay, can reference idNumber from outside the let
+      hasher.combine(balance) // error: cannot synchronously access actor-isolated property
+    }
+  }
+  ```
+
 * [SE-0300][]:
 
   Async functions can now be suspended using the `withUnsafeContinuation`
@@ -8506,6 +8524,7 @@ Swift 1.0
 [SE-0300]: <https://github.com/apple/swift-evolution/blob/main/proposals/0300-continuation.md>
 [SE-0306]: <https://github.com/apple/swift-evolution/blob/main/proposals/0306-actors.md>
 [SE-0310]: <https://github.com/apple/swift-evolution/blob/main/proposals/0310-effectful-readonly-properties.md>
+[SE-0313]: <https://github.com/apple/swift-evolution/blob/main/proposals/0313-actor-isolation-control.md>
 
 [SR-75]: <https://bugs.swift.org/browse/SR-75>
 [SR-106]: <https://bugs.swift.org/browse/SR-106>

include/swift/AST/Attr.def

@@ -624,7 +624,6 @@ DECL_ATTR(completionHandlerAsync, CompletionHandlerAsync,
 
 CONTEXTUAL_SIMPLE_DECL_ATTR(nonisolated, Nonisolated,
   DeclModifier | OnFunc | OnConstructor | OnVar | OnSubscript |
-  ConcurrencyOnly |
   ABIStableToAdd | ABIStableToRemove |
   APIBreakingToAdd | APIStableToRemove,
   112)

test/SourceKit/CodeComplete/complete_override.swift.response

@@ -214,6 +214,16 @@
       key.typerelation: source.codecompletion.typerelation.notapplicable,
       key.num_bytes_to_erase: 0
     },
+    {
+      key.kind: source.lang.swift.keyword,
+      key.name: "nonisolated",
+      key.sourcetext: "nonisolated",
+      key.description: "nonisolated",
+      key.typename: "",
+      key.context: source.codecompletion.context.none,
+      key.typerelation: source.codecompletion.typerelation.notapplicable,
+      key.num_bytes_to_erase: 0
+    },
     {
       key.kind: source.lang.swift.keyword,
       key.name: "nonmutating",