TODO: add matching when bulk optional fields are allowed
alongside other fields.
-- in.cue --
{[X=string]: baz: X}
bar: {}
-- out/eval --
(struct){
  bar: (struct){
    baz: (string){ "bar" }
  }
}
-- out/compile --
--- in.cue
{
  {
    [string]: {
      baz: 〈1;-〉
    }
  }
  bar: {}
}