Function field_value 

pub fn field_value<'a, F: Filterable + ?Sized>(
    msg: &'a F,
    field: &ConditionField,
) -> &'a str

Resolve the string value of a condition field on a message.

HasAttachment resolves to "true"/"false"; missing fields resolve to the empty string. For ConditionField::Envelope this is the full envelope address (:all semantics); use evaluate_condition to apply :localpart/:domain extraction.