> Go should really have an analogue to Rust's `?`, but you can't really do that in a sane way without sum types

It could be just some simple (hey, that's what go wants to be, right?) macro thing, that just does the everyday `if err!=nil{return ..., err}` for you without having to juggle (and think about) vars.

    b := g(f()?)?
    // var b B
    // {
    //     var a A
    //     var err error
    //     a, err = f()
    //     if err != nil {
    //         return *new(A), *new(B), err
    //     }
    //     var b B
    //     b, err = g(a)
    //     if err != nil {
    //         return *new(A), *new(B), err
    //     }
    //     // no accidental reuse of err
    // }

I mean look how much utterly useless noise this is, and count all the opportunities for mistakes that wouldn't get caught by the compiler.