checks if the result is string, Buffer, or nully, and based on that either sets vfile.contents or vfile.result

This may work.
Thinking of edge cases

1. what if the return type is a class instance which implements a custom toString?

class Example {
  toString() {
    return "<h1>hello world</h1>";
  }
}

// where the compiler returns
new Example();

// which when used as a string
`${new Example()}`
// logs "<h1>hello world</h1>"

Is this something that would be considered a value or a result?

2. Depending on how you interpret "nully", would that include falsy values? Like number with 0, boolean with false, would those be split? (E.G. 0 would go to vfile.contents, 1 to vfile.result)

— @ChristianMurphy

Not handled differently. It’s a non-text (neither string nor buffer), so it goes into result. So, if some vdom node was returned that has a toString field or function in its __proto__, those will go into result.

Nully values are ignored (so a compiler could signal that the current file.value or file.result are not to be modified), string and buffer are new values for file.contents, and other values all go into file.result.