orders: #825 review follow-ups

[wboayue]

Addresses the review on #825. Six findings land here; two are deferred to issues. Plan and reasoning: plans/orders-wire-enum-followups.md.

1. The required-action invariant leaked one layer up

#825 made a present-but-empty action Error::Parse. But decode_open_order_proto / decode_completed_order_proto / decode_execution_data_proto did

let order = p.order.as_ref().map(decode_order).transpose()?.unwrap_or_default();

so a wholly absent order submessage still produced Order::default() — action == Buy, exactly the mishandling §14 says it removed. Present-but-empty errored; absent defaulted.

The reference client does neither: EDecoder.cs's OpenOrderEventProtoBuf (:2652) and ExecutionDataEventProtoBuf (:2918) drop the frame, returning before eWrapper.openOrder(..) / execDetails(..) if a submessage is null. They never synthesize a default. This crate has no skip channel, so a missing submessage is now Error::Parse. Defaulting is the one option neither client takes — it hands the caller a phantom BUY order over an empty contract, which reads as real data.

decode_order_bound's local required helper folds into the shared one.

2. #[repr(i32)] and the discriminants were decorative

Once From<T> for i32 is hand-written, = 0 / = 1 drive nothing — and variant as i32 does not compile either way, once a payload variant exists. They were the only reason #[repr(i32)] was needed (explicit discriminants on a data-carrying enum require a primitive repr), so it was an ABI commitment in the public API buying nothing.

Worse, they were a second source of truth. A missing match arm is a compile error; Foo = 5 paired with Foo => 6 is not.

Dropped from all eight, Liquidity included — it has no From<Liquidity> for i32 at all, so its discriminants were never read either, and leaving it out would have split enum-typing.md's shape description in two. Each variant's wire code is now rustdoc (Wire code `0`.), which is strictly more visible than a discriminant.

3. *_round_trips_every_wire_code did not check "every"

Seven tests so named verified only the rows listed; adding OcaType::Reserved = 7 broke none of them. #822 hit this and answered it with all_tifs_covers_every_variant.

Integer enums admit a cheaper guard than a per-enum modeled_index match: From<i32> is total, so every code outside the table must be Unknown(code). check_wire_code_round_trip now takes the Unknown constructor and probes -8..=64, skipping listed codes. Verified by temporarily adding a Reserved = 7 variant:

assertion `left == right` failed: code 7 is modeled by a variant the table does not list

5. enum-typing.md's AuctionStrategy claim was false

The node said From<i32> for AuctionStrategy "converts nothing but the i32 a caller hands OrderBuilder". OrderBuilder::auction_strategy was a private field with no setter — the only writes were None at construction, so the .into() reading it was unreachable. Field deleted, claim corrected. The missing typed setter is the builder-enum-coverage gap the dead field was hiding: issue #828.

6. "All eight macros in src/" is nine

grep -rn "macro_rules!" src --include=*.rs | wc -l → 9. impl_proto_payload! had no row. #825 re-derived the impl_wire_enum! count on the line below but left the total.

7. impl_wire_enum! empty-input message asymmetry

The closed arm reported "unknown Action" for empty input where the fallback arm reported "empty $name". Both now distinguish no-value from unrecognized-value. Only observable through a direct "".parse() — parse_required intercepts empty first with "missing {label}", which names the field rather than the type.

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Deferred

• orders: OrderCondition discriminator still collapses an unknown type to a default price condition #827 — decode_order_condition's _ => OrderCondition::Price(PriceCondition::default()) still decodes an unmodeled condition type as a zeroed price condition, and From<i32> for OrderCondition still panics. orders: parse wire enums through FromStr, preserving unknown values #825 called this "a separate change" and that holds: preserving the value means an OrderCondition::Unknown(i32) variant on an exhaustive public enum.
• Unknown-code aliasing — OcaType::Unknown(1) != OcaType::CancelWithBlock under derived PartialEq, yet both encode as 1. Only reachable by hand-constructing Unknown with a known code; decoders never produce it. Inherited from the Liquidity / TimeInForce precedent, so no action.

Gate

cargo fmt --check; clippy × 3 configs; just test (6 legs, 4393 tests, 0 failed); RUSTDOCFLAGS="-D warnings" cargo doc × 3; cargo build --examples × 2; cargo build -p ibapi-integration-{sync,async} --tests; just rules-check — all clean.