toBuilder() makes only a shallow copy of the object. It does not copy the
contents of individual Strings, ByteStrings, sub-messages, etc.; it just
reuses the existing values. This means that it's generally reasonably
cheap, although obviously not quite as cheap as if the builders for
sub-objects
Yes, this seems nice:
Foo.Builder builder;
builder.setBar(builder.getBar().toBuilder().setA(...).build());
But I guess I'm a little worried about its hidden cost if I use it a
lot. If message Bar was very large, then would converting it to a
builder, setting one value, and converting it back to a
