Use ProtoSize() in MarshalTo when enabled for oneof fields (#618)

Fixes #617
Adds test which generates a marshalTo for a oneof message. This will fail to compile if the wrong size method is generated in the marshalTo method.

Makefile

@@ -136,6 +136,7 @@ regenerate:
 	make -C test/issue498 regenerate
 	make -C test/issue503 regenerate
 	make -C test/issue530 regenerate
+	make -C test/issue617 regenerate
 
 	make gofmt
 

plugin/marshalto/marshalto.go

@@ -1010,7 +1010,12 @@ func (p *marshalto) Generate(file *generator.FileDescriptor) {
 			ccTypeName := p.OneOfTypeName(message, field)
 			p.P(`func (m *`, ccTypeName, `) MarshalTo(dAtA []byte) (int, error) {`)
 			p.In()
-			p.P(`return m.MarshalToSizedBuffer(dAtA[:m.Size()])`)
+			if gogoproto.IsProtoSizer(file.FileDescriptorProto, message.DescriptorProto) {
+				p.P(`size := m.ProtoSize()`)
+			} else {
+				p.P(`size := m.Size()`)
+			}
+			p.P(`return m.MarshalToSizedBuffer(dAtA[:size])`)
 			p.Out()
 			p.P(`}`)
 			p.P(``)

test/issue617/Makefile

@@ -0,0 +1,32 @@
+# Protocol Buffers for Go with Gadgets
+#
+# Copyright (c) 2019, The GoGo Authors. All rights reserved.
+# http://github.com/gogo/protobuf
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions are
+# met:
+#
+#     * Redistributions of source code must retain the above copyright
+# notice, this list of conditions and the following disclaimer.
+#     * Redistributions in binary form must reproduce the above
+# copyright notice, this list of conditions and the following disclaimer
+# in the documentation and/or other materials provided with the
+# distribution.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+# "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+# LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+# A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+# OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+# LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+# THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+# (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+# OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+regenerate:
+	go install github.com/gogo/protobuf/protoc-gen-gogo
+	go install github.com/gogo/protobuf/protoc-min-version
+	protoc-min-version --version="3.0.0" --gogo_out=. --proto_path=../../../../../:../../protobuf/:. *.proto