How to inject go struct into cue build instance?

[cueckoo]

Originally opened by @joscha-alisch in cuelang/cue#710

Say, I have a cue file like this

test: fromGo.value

I want to inject a struct from go into this file:

struct FromGo {
   Value: string
}
...
fromGo := FromGo{Value: "something"}

So that the the cue file evaluates to

test: "something"

If I create another cue file that acts a a schema for the struct, load both files and use instance.Fill, it works fine, like so

b := &build.Instance{}

err := b.AddFile("test.cue", "test: fromGo.value")
panicErr(err)

err = b.AddFile("schema.cue", "fromGo: { value: string }")
panicErr(err)

r := cue.Runtime{}
i, err := r.Build(b)
panicErr(err)

fromGo := FromGo{Value: "something"}
i, err = i.Fill(&fromGo, "fromGo")
panicErr(err)

s, err := i.Lookup("test").String()
panicErr(err)
println(s) // prints "something"

But I wonder whether there is a way to skip having to handcraft this second schema file? I.e. replace b.AddFile("schema.cue", "fromGo: { value: string }") with something automatically generated from the struct type. Given that there is the gocodec package I assume there is some way?

[cueckoo]

Original reply by @mpvl in cuelang/cue#710 (comment)

The API could be improved. But from gocodec you can get a cue.Value, which has a Syntax method to get an AST, from which you can get a File using astutil.ToFile.

[cueckoo]

Original reply by @joscha-alisch in cuelang/cue#710 (comment)

Ah perfect, I got to the point where I found the Syntax method but wasn't able to find out how to make a file out of it.
Thanks @mpvl !