encoding/json/v2: allow streaming JSON methods to return errors.ErrUnsupported

Allow the MarshalJSONTo and UnmarshalJSONFrom methods
to return errors.ErrUnsupported to be skipped in a similar manner
to how the caller-specified functions can be skipped as well.

Note that the v1 MarshalJSON and UnmarshalJSON methods may not
return errors.ErrUnsupported as that would be a breaking change.
Also, we couldn't implement it for UnmarshalJSON since
that requires consuming the value, which changes the state of
the underlying jsontext.Decoder.

A side-effect of this change is that MarshalJSONTo and UnmarshalJSONFrom
methods may now return sentinel errors. We document that users
should avoid calling this methods directly and instead
rely on MarshalEncode and UnmarshalDecode,
which can handle the ErrUnsupported sentinel error and
others that we may add in the future.

Fixes #74324
Fixes #76712

Change-Id: I851e907ef8d25e31964148515879a243cb5069c5
Reviewed-on: https://go-review.googlesource.com/c/go/+/744941
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>

0 files changed, 0 insertions, 0 deletions