Let's assume we have the following structure:

message Alpha {

  message Beta {
    optional int32 mercury = 1;
    optional int32 venus = 2;

  optional int32 earth = 1;

  optional Beta mars = 2;

The Java generated code allows to get a sub-builder for "mars" field from a 
builder for an Alpha message by calling 
Alpha.newBuilder().getMarsBuilder(). This is nice because I can keep 
builders for structured messages and update them as needed and build the 
Alpha message as needed, but still hold on to the big builder which I plan 
to update further.

But I do not think there is a way to do this in a generic way, using 
Message.Builder interface. Is there any plan for something like this?

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