Function eval_with_file 

pub fn eval_with_file(
    input: &str,
    file: Option<PathBuf>,
) -> Result<Value, EvalError>

Evaluate a Nix expression string, optionally tagged with the path of the source file. The file is stored on the root Env so that any closure created during evaluation captures it and can resolve relative path literals (./foo.nix) in function defaults that fire after control has left the file’s scope.