[go-nuts] Why can't struct field assignments be on the LHS of the short variable declaration (when actual declarations are present)?

Normally, if I have some free variable that's already been declared, I can
have it on the left side of := as long as at least one other variable on
the LHS has not been declared, for instance

var a int
// ...
a,err := DoSomething()
// ...

I recently found out this apparently doesn't work with struct fields

var x SomeStruct
x.a,err := DoSomething()

(Playground: http://play.golang.org/p/qidzSkDbzA )

If it's just an "=" operator and you forward declare err, it works just fine

var x SomeStruct
var err error
x.a,err = DoSomething()

(Playground: http://play.golang.org/p/IlIUERzbk6 )

It's kind of obnoxious, honestly. Is there a good reason for it? Does it
complicate the grammar too much to allow this? I mean, it only takes a few
extra lines (depending on how many LHS variables you have), so it's not the
end of the world to work around it, but I don't really see a rationale for
not allowing this. I can see someone making the argument "you can never
declare a struct field, therefore a struct field has no business on the LHS
of a short form variable declaration", but given that non-declarations are
already allowed on the LHS of one, it feels inconsistent to not allow
struct fields there under the same rules as already-declared variables
(that is: at least one other variable must actually be a declaration).

Again, it's not a huge deal, but I would like to understand the rationale.