Function insert_value_into_option_or_error 

fn insert_value_into_option_or_error(
    cx: &mut AcceptContext<'_, '_>,
    param: &MetaItemParser,
    item: &mut Option<Symbol>,
    name: Ident,
) -> Option<()>

Tries to insert the value of a key = value meta item into an option.

Emits an error when either the option was already Some, or the arguments weren’t of form name = value