by mike@mccartney.net:

I am seeing an inconsistency between the way encoding/xml and encoding/json handle
unmarshaling into structs containing interface{} fields.  Specifically, if the
interface{} field is non-nil and pointing to a struct with an appropriate schema, the
xml will not unmarshal into the pointed-to struct but the json unmarshaler will.  I want
the xml unmarshaler to behave like the json unmarshaler.

I'm using go version devel +f4d1cb8d9a91 Thu Sep 19 22:34:33 2013 +1000 darwin/amd64

The following code demonstrates the different behavior:
http://play.golang.org/p/-i96HoWP_A

Notice that the jsonResult contains an Address with City and State filled out but the
xmlResult does not.

I believe the code that is doing the right thing in json is here:
http://code.google.com/p/go/source/browse/src/pkg/encoding/json/decode.go#307

I don't know how best to fix the xml code, but i think the fix needs to go here:
http://code.google.com/p/go/source/browse/src/pkg/encoding/xml/read.go#287