Function prepare_rename 

pub fn prepare_rename<'a>(
    index: &'a ProjectIndex,
    path: &Path,
    offset: usize,
) -> Option<(&'a SymbolKey, &'a SiteRef)>

prepareRename: the renameable range under the cursor, or None for out-of-scope symbols (locals, unit names) — the request is refused rather than falling through to a partial rename.