Function handle_array_element

fn handle_array_element(
    cx: &ExtCtxt<'_>,
    guar: &mut Option<ErrorGuaranteed>,
    missing_literals: &mut Vec<Span>,
    expr: &P<Expr>,
) -> Option<u8>

Returns expr as a single byte literal if applicable.

Otherwise, returns None, and either pushes the expr’s span to missing_literals or updates guar accordingly.