fn maybe_supported_error(
    tcx: TyCtxt<'_>,
    sub: GenericConstantTooComplexSub,
    root_span: Span,
) -> Result<!, ErrorGuaranteed>fn maybe_supported_error(
    tcx: TyCtxt<'_>,
    sub: GenericConstantTooComplexSub,
    root_span: Span,
) -> Result<!, ErrorGuaranteed>