Add IgnorePrefix to protojson.MarshalOptions

Author: rolancia

encoding/protojson/encode.go

@@ -7,6 +7,7 @@ package protojson
 import (
 	"encoding/base64"
 	"fmt"
+	"strings"
 
 	"google.golang.org/protobuf/internal/encoding/json"
 	"google.golang.org/protobuf/internal/encoding/messageset"
@@ -106,6 +107,10 @@ type MarshalOptions struct {
 	// as JSON numbers instead of strings.
 	EmitInt64sAsNumber bool
 
+	// IgnorePrefix is a prefix of the field names to ignore when marshaling.
+	// If empty, no fields are ignored.
+	IgnorePrefix string
+
 	// Resolver is used for looking up types when expanding google.protobuf.Any
 	// messages. If nil, this defaults to using protoregistry.GlobalTypes.
 	Resolver interface {
@@ -266,6 +271,10 @@ func (e encoder) marshalMessage(m protoreflect.Message, typeURL string) error {
 			name = fd.TextName()
 		}
 
+		if e.opts.IgnorePrefix != "" && strings.HasPrefix(name, e.opts.IgnorePrefix) {
+			return true
+		}
+
 		if err = e.WriteName(name); err != nil {
 			return false
 		}

encoding/protojson/encode_test.go

@@ -2520,6 +2520,61 @@ func TestMarshal(t *testing.T) {
   "optSint64": 3,
   "optFixed64": 4,
   "optSfixed64": 5
+}`,
+	}, {
+		desc: "IgnorePrefix",
+		mo:   protojson.MarshalOptions{IgnorePrefix: "sInt"},
+		input: &pb3.Scalars{
+			SBool:   true,
+			SInt32:  1,
+			SInt64:  2,
+			SUint32: 3,
+			SUint64: 4,
+			SSint32: 5,
+			SString: "abc",
+		},
+		want: `{
+  "sBool": true,
+  "sUint32": 3,
+  "sUint64": "4",
+  "sSint32": 5,
+  "sString": "abc"
+}`,
+	}, {
+		desc: "IgnorePrefix: json_name",
+		mo:   protojson.MarshalOptions{IgnorePrefix: "foo"},
+		input: &pb3.JSONNames{
+			SString: "abc",
+		},
+		want: `{}`,
+	}, {
+		desc: "IgnorePrefix: not on map keys",
+		mo:   protojson.MarshalOptions{IgnorePrefix: "-"},
+		input: &pb3.Maps{
+			Int32ToStr: map[int32]string{
+				-1: "a",
+				0:  "b",
+				1:  "c",
+			},
+			StrToNested: map[string]*pb3.Nested{
+				"-aa": {SString: "a"},
+				"bb":  {SString: "b"},
+			},
+		},
+		want: `{
+  "int32ToStr": {
+    "-1": "a",
+    "0": "b",
+    "1": "c"
+  },
+  "strToNested": {
+    "-aa": {
+      "sString": "a"
+    },
+    "bb": {
+      "sString": "b"
+    }
+  }
 }`,
 	}}