Function parse_shape_or_box 

fn parse_shape_or_box<R, ReferenceBox>(
    context: &ParserContext<'_>,
    input: &mut Parser<'_>,
    to_shape: impl FnOnce(Box<BasicShape>, ReferenceBox) -> R,
    to_reference_box: impl FnOnce(ReferenceBox) -> R,
    flags: AllowedBasicShapes,
) -> Result<R, ParseError>
where
    ReferenceBox: Default + Parse,

A helper for both clip-path and shape-outside parsing of shapes.