commit ce496b5d4dc139b4d1e85d8afc56349b34b3571f
author Joe Tsai <joetsai@digital-static.net> Thu Jan 02 15:47:15 2020 -0800
committer Joe Tsai <joetsai@google.com> Mon Jan 06 20:26:20 2020 +0000
tree e1a16712c45efad79f712b699ea120a5ba07828e
parent 96a44732e0b1ca7ffd992bf5ed657345a06a3aac

proto: add MergeOptions.Shallow option

A shallow copy of a message is a common operation with over 10k
usages inside Google. However, the semantics of a shallow copy
on the struct is ill-defined and not officially supported by
the generated protobuf API.

To reduce improper usages, add an official implementation of
shallow merging that does something similar where messages, lists,
and maps are shallow copied into the destination if it does not
already have one populated.

In the common case where the destination is empty, this equivalent to:
	src.Range(func(fd protoreflect.FieldDescriptor, v protoreflect.Value) bool {
		dst.Set(fd, v)
	})
	if len(src.GetUnknown()) > 0 {
		dst.SetUnknown(src.GetUnknown())
	}
which is as simple of a shallow copy definition as you can get.

A future CL will add a fast-path implementation of both
deep and shallow merges.

Change-Id: Ic4a5503dd1b11b505738f5e503f97d55997e9418
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/213131
Reviewed-by: Damien Neil <dneil@google.com>