JSON performance: Speed up encoding of submessages

The previous logic created a completely new encoder, encoded the
subobject, and then copied the result to append it.   The new
logic avoids that by traversing the subobject with the current
encoder.  (This is the same strategy that was already used by
the binary encoder.)

Author: tbkka

Sources/SwiftProtobuf/JSONEncoder.swift

@@ -154,6 +154,11 @@ internal struct JSONEncoder {
         separator = nil
     }
 
+    internal mutating func startNestedObject() {
+        data.append(asciiOpenCurlyBracket)
+        separator = nil
+    }
+
     /// Append a close curly brace `}` to the JSON.
     internal mutating func endObject() {
         data.append(asciiCloseCurlyBracket)

Sources/SwiftProtobuf/JSONEncodingVisitor.swift

@@ -166,8 +166,19 @@ internal struct JSONEncodingVisitor: Visitor {
 
   mutating func visitSingularMessageField<M: Message>(value: M, fieldNumber: Int) throws {
     try startField(for: fieldNumber)
-    let json = try value.jsonUTF8Data(options: options)
-    encoder.append(utf8Data: json)
+    if let m = value as? _CustomJSONCodable {
+      let json = try m.encodedJSONString(options: options)
+      encoder.append(text: json)
+    } else if let newNameMap = (M.self as? _ProtoNameProviding.Type)?._protobuf_nameMap {
+      encoder.startNestedObject()
+      let oldNameMap = self.nameMap
+      self.nameMap = newNameMap
+      try value.traverse(visitor: &self)
+      self.nameMap = oldNameMap
+      endObject()
+    } else {
+      throw JSONEncodingError.missingFieldNames
+    }
   }
 
   mutating func visitSingularGroupField<G: Message>(value: G, fieldNumber: Int) throws {
@@ -274,12 +285,35 @@ internal struct JSONEncodingVisitor: Visitor {
   }
 
   mutating func visitRepeatedMessageField<M: Message>(value: [M], fieldNumber: Int) throws {
-    let localOptions = options
-    try _visitRepeated(value: value, fieldNumber: fieldNumber) {
-      (encoder: inout JSONEncoder, v: M) throws in
-      let json = try v.jsonUTF8Data(options: localOptions)
-      encoder.append(utf8Data: json)
+    try startField(for: fieldNumber)
+    var comma = false
+    encoder.startArray()
+    if let _ = M.self as? _CustomJSONCodable.Type {
+      for v in value {
+        if comma {
+          encoder.comma()
+        }
+        comma = true
+        let json = try v.jsonString(options: options)
+        encoder.append(text: json)
+      }
+    } else if let newNameMap = (M.self as? _ProtoNameProviding.Type)?._protobuf_nameMap {
+      let oldNameMap = self.nameMap
+      self.nameMap = newNameMap
+      for v in value {
+        if comma {
+          encoder.comma()
+        }
+        comma = true
+        encoder.startNestedObject()
+        try v.traverse(visitor: &self)
+        encoder.endObject()
+      }
+      self.nameMap = oldNameMap
+    } else {
+      throw JSONEncodingError.missingFieldNames
     }
+    encoder.endArray()
   }
 
   mutating func visitRepeatedGroupField<G: Message>(value: [G], fieldNumber: Int) throws {