Function format_reference 

pub fn format_reference(
    style: ReferenceStyle,
    from: &Path,
    to: &Path,
    id: Option<&Id>,
    title: &str,
) -> String

Render a durable reference from the document at from to to (titled title) in style. id must be Some when the style addresses by id (the caller registers the target first); it is ignored otherwise. Returns the exact scalar to store in metadata (a wikilink scalar keeps its [[…]] — the metadata writer is responsible for any format-level quoting).