encoding: support weak fields in text and JSON unmarshaling

If the message for a weak field is linked in,
we treat it as if it were identical to a normal known field.
However, if the weak field is not linked in,
we treat it as if the field were not known.

Change-Id: I576d911deec98e13211304024a6353734d055465
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/185457
Reviewed-by: Herbie Ong <herbie@google.com>
diff --git a/encoding/protojson/decode_test.go b/encoding/protojson/decode_test.go
index 00b1a54..03bd10a 100644
--- a/encoding/protojson/decode_test.go
+++ b/encoding/protojson/decode_test.go
@@ -9,12 +9,15 @@
 	"testing"
 
 	"google.golang.org/protobuf/encoding/protojson"
-	"google.golang.org/protobuf/encoding/testprotos/pb2"
-	"google.golang.org/protobuf/encoding/testprotos/pb3"
+	"google.golang.org/protobuf/internal/flags"
 	pimpl "google.golang.org/protobuf/internal/impl"
 	"google.golang.org/protobuf/proto"
 	preg "google.golang.org/protobuf/reflect/protoregistry"
 
+	"google.golang.org/protobuf/encoding/testprotos/pb2"
+	"google.golang.org/protobuf/encoding/testprotos/pb3"
+	testpb "google.golang.org/protobuf/internal/testprotos/test"
+	weakpb "google.golang.org/protobuf/internal/testprotos/test/weak1"
 	"google.golang.org/protobuf/types/known/anypb"
 	"google.golang.org/protobuf/types/known/durationpb"
 	"google.golang.org/protobuf/types/known/emptypb"
@@ -33,6 +36,7 @@
 		wantMessage  proto.Message
 		// TODO: verify expected error message substring.
 		wantErr bool
+		skip    bool
 	}{{
 		desc:         "proto2 empty message",
 		inputMessage: &pb2.Scalars{},
@@ -2440,10 +2444,29 @@
 		wantMessage: &anypb.Any{
 			TypeUrl: "type.googleapis.com/google.protobuf.Empty",
 		},
+	}, {
+		desc:         "weak fields",
+		inputMessage: &testpb.TestWeak{},
+		inputText:    `{"weak_message1":{"a":1}}`,
+		wantMessage: func() *testpb.TestWeak {
+			m := new(testpb.TestWeak)
+			m.SetWeakMessage1(&weakpb.WeakImportMessage1{A: proto.Int32(1)})
+			return m
+		}(),
+		skip: !flags.Proto1Legacy,
+	}, {
+		desc:         "weak fields; unknown field",
+		inputMessage: &testpb.TestWeak{},
+		inputText:    `{"weak_message1":{"a":1}, "weak_message2":{"a":1}}`,
+		wantErr:      true, // weak_message2 is unknown since the package containing it is not imported
+		skip:         !flags.Proto1Legacy,
 	}}
 
 	for _, tt := range tests {
 		tt := tt
+		if tt.skip {
+			continue
+		}
 		t.Run(tt.desc, func(t *testing.T) {
 			err := tt.umo.Unmarshal([]byte(tt.inputText), tt.inputMessage)
 			if err != nil && !tt.wantErr {