Refactor the looking for existing container code and change the error message on the condition

Author: pitiphong-p

stdlib/public/Darwin/Foundation/JSONEncoder.swift

@@ -500,11 +500,17 @@ fileprivate struct _JSONKeyedEncodingContainer<K : CodingKey> : KeyedEncodingCon
 
     public mutating func nestedContainer<NestedKey>(keyedBy keyType: NestedKey.Type, forKey key: Key) -> KeyedEncodingContainer<NestedKey> {
         let containerKey = _converted(key).stringValue
-        let existingContainer = self.container[containerKey]
-        precondition(existingContainer is NSMutableDictionary?,
-                     "Attempt to request for keyed container with the key that previously unkeyed container already requested.")
-        let dictionary = existingContainer as? NSMutableDictionary ?? NSMutableDictionary()
-        self.container[containerKey] = dictionary
+        let dictionary: NSMutableDictionary
+        if let existingContainer = self.container[containerKey] {
+            precondition(
+                existingContainer is NSMutableDictionary,
+                "Attempt to re-encode into nested KeyedEncodingContainer<\(Key.self)> for key \"\(containerKey)\" is invalid: non-keyed container already encoded for this key"
+            )
+            dictionary = existingContainer as! NSMutableDictionary
+        } else {
+            dictionary = NSMutableDictionary()
+            self.container[containerKey] = dictionary
+        }
 
         self.codingPath.append(key)
         defer { self.codingPath.removeLast() }
@@ -515,11 +521,17 @@ fileprivate struct _JSONKeyedEncodingContainer<K : CodingKey> : KeyedEncodingCon
 
     public mutating func nestedUnkeyedContainer(forKey key: Key) -> UnkeyedEncodingContainer {
         let containerKey = _converted(key).stringValue
-        let existingContainer = self.container[containerKey]
-        precondition(existingContainer is NSMutableArray?,
-                     "Attempt to request for unkeyed container with the key that previously keyed container already requested.")
-        let array = existingContainer as? NSMutableArray ?? NSMutableArray()
-        self.container[containerKey] = array
+        let array: NSMutableArray
+        if let existingContainer = self.container[containerKey] {
+            precondition(
+                existingContainer is NSMutableArray,
+                "Attempt to re-encode into nested UnkeyedEncodingContainer for key \"\(containerKey)\" is invalid: keyed container/single value already encoded for this key"
+            )
+            array = existingContainer as! NSMutableArray
+        } else {
+            array = NSMutableArray()
+            self.container[containerKey] = array
+        }
 
         self.codingPath.append(key)
         defer { self.codingPath.removeLast() }

stdlib/public/Darwin/Foundation/PlistEncoder.swift

@@ -266,12 +266,18 @@ fileprivate struct _PlistKeyedEncodingContainer<K : CodingKey> : KeyedEncodingCo
 
     public mutating func nestedContainer<NestedKey>(keyedBy keyType: NestedKey.Type, forKey key: Key) -> KeyedEncodingContainer<NestedKey> {
         let containerKey = key.stringValue
-        let existingContainer = self.container[containerKey]
-        precondition(existingContainer is NSMutableDictionary?,
-                     "Attempt to request for keyed container with the key that previously unkeyed container already requested.")
-        let dictionary = existingContainer as? NSMutableDictionary ?? NSMutableDictionary()
-        self.container[containerKey] = dictionary
-
+        let dictionary: NSMutableDictionary
+        if let existingContainer = self.container[containerKey] {
+            precondition(
+                existingContainer is NSMutableDictionary,
+                "Attempt to re-encode into nested KeyedEncodingContainer<\(Key.self)> for key \"\(containerKey)\" is invalid: non-keyed container already encoded for this key"
+            )
+            dictionary = existingContainer as! NSMutableDictionary
+        } else {
+            dictionary = NSMutableDictionary()
+            self.container[containerKey] = dictionary
+        }
+        
         self.codingPath.append(key)
         defer { self.codingPath.removeLast() }
 
@@ -281,11 +287,17 @@ fileprivate struct _PlistKeyedEncodingContainer<K : CodingKey> : KeyedEncodingCo
 
     public mutating func nestedUnkeyedContainer(forKey key: Key) -> UnkeyedEncodingContainer {
         let containerKey = key.stringValue
-        let existingContainer = self.container[containerKey]
-        precondition(existingContainer is NSMutableArray?,
-                     "Attempt to request for unkeyed container with the key that previously keyed container already requested.")
-        let array = existingContainer as? NSMutableArray ?? NSMutableArray()
-        self.container[containerKey] = array
+        let array: NSMutableArray
+        if let existingContainer = self.container[containerKey] {
+            precondition(
+                existingContainer is NSMutableArray, 
+                "Attempt to re-encode into nested UnkeyedEncodingContainer for key \"\(containerKey)\" is invalid: keyed container/single value already encoded for this key"
+            )
+            array = existingContainer as! NSMutableArray
+        } else {
+            array = NSMutableArray()
+            self.container[containerKey] = array
+        }
 
         self.codingPath.append(key)
         defer { self.codingPath.removeLast() }