Add java conversions option (backported to 0.10.x)

CHANGELOG.md

@@ -1,5 +1,5 @@
 # Change Log
-## [0.11.0 (Unreleased)]
+[0.11.0 (Unreleased)]
 - Supports protobuf 3.12.2
 - Supports the new experimental optional fields introduced in proto3 at
   protobuf 3.12.0 (see https://github.com/protocolbuffers/protobuf/blob/v3.12.0/docs/field_presence.md). To enable, add the following to your `build.sbt`:

compiler-plugin/src/main/scala/scalapb/compiler/DescriptorImplicits.scala

@@ -847,7 +847,8 @@ class DescriptorImplicits private[compiler] (
     def scalaOptions: ScalaPbOptions =
       fileOptionsCache(file)
 
-    def javaConversions = params.javaConversions && !scalaOptions.getTestOnlyNoJavaConversions
+    def javaConversions =
+      (scalaOptions.getJavaConversions || params.javaConversions) && !scalaOptions.getTestOnlyNoJavaConversions
 
     def javaPackage: String = {
       if (file.getOptions.hasJavaPackage)

docs/src/main/markdown/customizations.md

@@ -26,6 +26,7 @@ option (scalapb.options) = {
   package_name: "com.example.myprotos"
   flat_package: true
   single_file: true
+  java_conversions: false
   import: "com.thesamet.pb.MyType"
   import: "com.thesamet.other._"
   preamble: "sealed trait BaseMessage"
@@ -54,6 +55,11 @@ globally to all files by adding it to your [ScalaPB SBT Settings]({{site.baseurl
 - The `single_file` option makes the generator output all messages and
 enums to a single Scala file.
 
+- The `java_conversions` options tells ScalaPB to generate converters to the
+  corresponding Java messages in this file. It does not automatically trigger
+  Java source code generation for the messages. If you need to generate source code
+  in Java, include `PB.gens.java` in the list of targets in sbt-protoc.
+
 - The `preamble` is a list of strings that is output at the top of the
   generated Scala file. This option requires `single_file` to be set. It is
   commonly used to define sealed traits that are extended using

protobuf/scalapb/scalapb.proto

@@ -96,6 +96,12 @@ message ScalaPbOptions {
   // Strip is applied before enum value naming changes.
   optional bool enum_strip_prefix = 17 [default=false];
 
+  // Scala type to use for bytes fields.
+  optional string bytes_type = 21;
+
+  // Enable java conversions for this file.
+  optional bool java_conversions = 23;
+
   // AuxMessageOptions enables you to set message-level options through package-scoped options.
   // This is useful when you can't add a dependency on scalapb.proto from the proto file that
   // defines the message.
@@ -141,11 +147,8 @@ message ScalaPbOptions {
   // List of message options to apply to some enums.
   repeated AuxEnumOptions aux_enum_options = 20;
 
-  // Scala type to use for bytes fields.
-  optional string bytes_type = 21;
-
   // Taken by ScalaPB 0.11.x branch
-  reserved 22, 23;
+  reserved 22;
 
   repeated string preprocessors = 24;