As an example:

Given dbus type (si), does the corresponding Go struct have to be:

struct {
   A string
   B int64
}

Or would

struct {
   B int64
   A string
}

also be feasible?

I assume the latter is not feasible, as it'd lead to ambiguity. If my assumption is correct, then could it be mentioned in the Conversion rules of the docs that the ordering of the struct members in Go matter?