Move all checks for required fields into a proto.IsInitialized function.
Initial testing makes me confident that we can provide a fast-path
implementation of IsInitialized which will perform more than
acceptably. (In the degenerate-but-common case where a message
transitively contains no required fields, this check can be nearly
zero cost.)
Unifying checks into a single function provides consistent behavior
between the wire, text, and json codecs.
Performing the check after decoding eliminates the wire decoder bug
where a split message is incorrectly seen as missing required fields.
Performing the check after decoding also provides consistent and
arguably more correct behavior when the target message was partially
prepopulated.
Change-Id: I9478b7bebb263af00c0d9f66a1f26e31ff553522
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/170787
Reviewed-by: Herbie Ong <herbie@google.com>
