Split enum visiting based on custom json support.

This should only do the check once per repeated enum, which should make the cost
much lower on the common case since enums don't normally support this.

Author: thomasvl

Sources/SwiftProtobuf/JSONEncodingVisitor.swift

@@ -280,17 +280,23 @@ internal struct JSONEncodingVisitor: Visitor {
   }
 
   mutating func visitRepeatedEnumField<E: Enum>(value: [E], fieldNumber: Int) throws {
-    let options = self.options
-    let alwaysPrintEnumsAsInts = options.alwaysPrintEnumsAsInts
-    try _visitRepeated(value: value, fieldNumber: fieldNumber) {
-      (encoder: inout JSONEncoder, v: E) throws in
-      if let e = v as? _CustomJSONCodable {
+    if let _ = E.self as? _CustomJSONCodable.Type {
+      let options = self.options
+      try _visitRepeated(value: value, fieldNumber: fieldNumber) {
+        (encoder: inout JSONEncoder, v: E) throws in
+        let e = v as! _CustomJSONCodable
         let json = try e.encodedJSONString(options: options)
         encoder.append(text: json)
-      } else if !alwaysPrintEnumsAsInts, let n = v.name {
-        encoder.appendQuoted(name: n)
-      } else {
-        encoder.putEnumInt(value: v.rawValue)
+      }
+    } else {
+      let alwaysPrintEnumsAsInts = options.alwaysPrintEnumsAsInts
+      try _visitRepeated(value: value, fieldNumber: fieldNumber) {
+        (encoder: inout JSONEncoder, v: E) throws in
+        if !alwaysPrintEnumsAsInts, let n = v.name {
+          encoder.appendQuoted(name: n)
+        } else {
+          encoder.putEnumInt(value: v.rawValue)
+        }
       }
     }
   }