Function generate_method_body 

fn generate_method_body(
    cls: &Type,
    spec: &FnSpec<'_>,
    calling_convention: &SlotCallingConvention,
    extract_error_mode: ExtractErrorMode,
    holders: &mut Holders,
    return_mode: Option<&ReturnMode>,
    ctx: &Ctx,
) -> Result<MethodBody>