Allow conditionally required fields (`required='C'`)

[kskalski]

From https://www.fixtrading.org/online-specification/introduction/#message-and-component-definitions
Messages are comprised of required, optional and conditionally required (fields which are required based on the presence or value of other fields) fields. Systems should be designed to operate when only the required and conditionally required fields are present.

So they should just be treated as optional for the code gen part

[ldanko]

Do you see any practical difference between Optional and Conditionally Required fields?

It's up to the user to decide if conditions for optional fields to be present are met.
Use InputResponder::reject

easyfix/easyfix-session/src/application.rs

Line 58 in 6b8cad4

pub fn reject(

(or just send Reject<3> message)

[kskalski]

From what I see, requirements for conditional fields are not formally specified, so it's application specific and should generate runtime error when in application logic the rules violation is detected.

So my take on that is:

• for the generated API messages this is the same as optional, since it should support cases of those fields omitted
• for the "reflection" way to inspect schema (easyfix-dictionary crate?), those fields might be good to mark as conditionally required, so user could treat them specially or at least know about them