Wrap ast.Expr into a valid ast.File?

[cueckoo]

Originally opened by @shykes in cuelang/cue#444

I'm playing with the Cue parser and ast packages. Note: the package docs are very helpful, thank you.

I am trying to wrap a ast.Expr into a valid ast.File. My definition of "valid" is that I can append the file to build.Instance.Files, and the build will correctly pick it up. But currently it does not: all other files are correctly used in the build, except my little frankenstein wrapper...

Here is the relevant wrapping code. Can anyone point me to a mistake I'm making?

import (
    cueAst "cuelang.org/go/cue/ast"
)

func exprToFile (filename string, expr cueAst.Expr) (*cueAst.File) {
    return &cueAst.File{
        Filename: filename,
        Decls: []cueAst.Decl{expr},
    }
}

Not sure it's relevant, but in my case expr is generated by cuelang.org/go/encoding/json.Extract. The original json contents is {"hello": "world"}.

[cueckoo]

Original reply by @shykes in cuelang/cue#444 (comment)

I got the answer from @mpvl on slack: "cuelang.org/go/cue/ast/astutil".ToFile. Thank you!