Function parse_selector 

fn parse_selector<'i, P, Impl>(
    parser: &P,
    input: &mut Parser<'i>,
    state: SelectorParsingState,
    parse_relative: ParseRelative,
) -> Result<Selector<Impl>, ParseError<P::Error>>
where
    P: Parser<'i, Impl = Impl>,
    Impl: SelectorImpl,

Build up a Selector. selector : simple_selector_sequence [ combinator simple_selector_sequence ]* ;

Err means invalid selector.