Function expand_map 

pub fn expand_map(
    root: &mut Value,
    lookup: &dyn Lookup,
    skip: Option<&'_ dyn Fn(&[String]) -> bool>,
) -> Result<Vec<Unresolved>, ExpandError>

Walks a parsed config mapping and expands references in every string value, in place. The top-level "variables" subtree is left untouched.