Function prepare_call_hierarchy 

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

v0.34 (ADR 0067): prepareCallHierarchy — the symbol under the cursor and its definition site (the goto-def resolution; an item is anchored on the definition). None for out-of-scope positions.