Function parse_inner_compound_selector 

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

Parses one compound selector suitable for nested stuff like :-moz-any, etc.