Function selection_ranges 

pub fn selection_ranges(
    source: &str,
    positions: &[Position],
) -> Vec<SelectionRange>

textDocument/selectionRange — for each position, the chain of enclosing AST node ranges, innermost first (each .parent widens outward to the file). Falls back to an empty range at the cursor when no node contains it (e.g. trailing whitespace) or the file doesn’t parse.