Address exclusivity warnings in JSONEncoder

An idiom used in JSONEncoder is violating Swift's rules about exclusive
access to memory. The 'with(pushedKey:)' family of mutating functions
on EncodingContainers modify 'self' by pushing a key, executing a
passed-in closure, and popping the key. However, when the passed-in closure
refers to 'self' this violates the rules for exclusive access since the
closure reads self while with(pushedKey:) has an already in-progress
modification of 'self'.

return self.with(pushedKey: _JSONKey(index: self.count - 1)) {
  // Referring to 'self.codingPath' violates rules for exclusive access.
  ...
}

The compiler is warning about this. To address these warnings, change
the callback to with(pushKey:) to accept an additional parameter: the
modified encoding container. This enables the closure to safely refer to it
without accessing via the captured `self` and avoids the violation of
exclusive access to memory. This also makes it explicit that in the body the
body of the closure the value of .codingPath read is the mutated one and
not the original.

Author: devincoughlin

Foundation/JSONEncoder.swift

@@ -318,10 +318,10 @@ fileprivate struct _JSONKeyedEncodingContainer<K : CodingKey> : KeyedEncodingCon
     /// Performs the given closure with the given key pushed onto the end of the current coding path.
     ///
     /// - parameter key: The key to push. May be nil for unkeyed containers.
-    /// - parameter work: The work to perform with the key in the path.
-    fileprivate mutating func with<T>(pushedKey key: CodingKey, _ work: () throws -> T) rethrows -> T {
+    /// - parameter work: The work to perform with the key in the path. The closure parameter is the container with the pushed path.
+    fileprivate mutating func with<T>(pushedKey key: CodingKey, _ work: (inout _JSONKeyedEncodingContainer) throws -> T) rethrows -> T {
         self.codingPath.append(key)
-        let ret: T = try work()
+        let ret: T = try work(&self)
         self.codingPath.removeLast()
         return ret
     }
@@ -367,7 +367,7 @@ fileprivate struct _JSONKeyedEncodingContainer<K : CodingKey> : KeyedEncodingCon
         self.container[key.stringValue._bridgeToObjectiveC()] = dictionary
 
         return self.with(pushedKey: key) {
-            let container = _JSONKeyedEncodingContainer<NestedKey>(referencing: self.encoder, codingPath: self.codingPath, wrapping: dictionary)
+            let container = _JSONKeyedEncodingContainer<NestedKey>(referencing: $0.encoder, codingPath: $0.codingPath, wrapping: dictionary)
             return KeyedEncodingContainer(container)
         }
     }
@@ -377,7 +377,7 @@ fileprivate struct _JSONKeyedEncodingContainer<K : CodingKey> : KeyedEncodingCon
         self.container[key.stringValue._bridgeToObjectiveC()] = array
 
         return self.with(pushedKey: key) {
-            return _JSONUnkeyedEncodingContainer(referencing: self.encoder, codingPath: self.codingPath, wrapping: array)
+            return _JSONUnkeyedEncodingContainer(referencing: $0.encoder, codingPath: $0.codingPath, wrapping: array)
         }
     }
 
@@ -421,10 +421,10 @@ fileprivate struct _JSONUnkeyedEncodingContainer : UnkeyedEncodingContainer {
     /// Performs the given closure with the given key pushed onto the end of the current coding path.
     ///
     /// - parameter key: The key to push. May be nil for unkeyed containers.
-    /// - parameter work: The work to perform with the key in the path.
-    fileprivate mutating func with<T>(pushedKey key: CodingKey, _ work: () throws -> T) rethrows -> T {
+    /// - parameter work: The work to perform with the key in the path. The closure parameter is the container with the pushed path.
+    fileprivate mutating func with<T>(pushedKey key: CodingKey, _ work: (inout _JSONUnkeyedEncodingContainer) throws -> T) rethrows -> T {
         self.codingPath.append(key)
-        let ret: T = try work()
+        let ret: T = try work(&self)
         self.codingPath.removeLast()
         return ret
     }
@@ -471,7 +471,7 @@ fileprivate struct _JSONUnkeyedEncodingContainer : UnkeyedEncodingContainer {
 
         // self.count - 1 to accommodate the fact that we just pushed a container.
         return self.with(pushedKey: _JSONKey(index: self.count - 1)) {
-            let container = _JSONKeyedEncodingContainer<NestedKey>(referencing: self.encoder, codingPath: self.codingPath, wrapping: dictionary)
+            let container = _JSONKeyedEncodingContainer<NestedKey>(referencing: $0.encoder, codingPath: $0.codingPath, wrapping: dictionary)
             return KeyedEncodingContainer(container)
         }
     }
@@ -482,7 +482,7 @@ fileprivate struct _JSONUnkeyedEncodingContainer : UnkeyedEncodingContainer {
 
         // self.count - 1 to accommodate the fact that we just pushed a container.
         return self.with(pushedKey: _JSONKey(index: self.count - 1)) {
-            return _JSONUnkeyedEncodingContainer(referencing: self.encoder, codingPath: self.codingPath, wrapping: array)
+            return _JSONUnkeyedEncodingContainer(referencing: $0.encoder, codingPath: $0.codingPath, wrapping: array)
         }
     }