ethercrab codegen backend

The one concrete backend (ethercat-esi-codegen-etherc... (BB_0062)) — the only crate in the toolchain that depends on ethercrab. Emits per-device structs that implement the runtime traits of Runtime trait surface.

Feature: ethercrab codegen backend FEAT_0053
status: open
satisfies: FEAT_0050
is satisfied by: REQ_0520, REQ_0521, REQ_0522, REQ_0523, REQ_0524, REQ_0525, REQ_0526, REQ_0527, REQ_0528
is refined by: CON_0011, ADR_0072
is implemented by: BB_0062

The opinionated, concrete backend that emits per-device structs implementing the runtime traits in Runtime trait surface (FEAT_0054). This is the only crate in the toolchain that depends on ethercrab.
Requirement: Backend crate is the sole ethercrab dependency REQ_0520
status: open
satisfies: FEAT_0053
is verified by: TEST_0472, TEST_0423

ethercat-esi-codegen-ethercrab shall be the only crate in the toolchain that declares ethercrab (any version) as a dependency. Neither ethercat-esi, ethercat-esi-codegen, ethercat-esi-build, nor ethercat-esi-verify shall depend on ethercrab.
Requirement: One device struct per ESI device entry REQ_0521
status: open
satisfies: FEAT_0053
is verified by: TEST_0420

For each <Device> element parsed from the input ESI files, the backend shall emit exactly one Rust struct named per the sanitised product ident (per Naming policy is owned by c... (REQ_0511) and Revision collision handled ... (REQ_0512)), deriving Debug + Default + Clone.
Requirement: Identity const emitted per device REQ_0522
status: open
satisfies: FEAT_0053
is verified by: TEST_0420

For each generated device struct, the backend shall emit an accompanying pub const <IDENT>_REV<REV>: Identity = Identity { vendor_id, product_code, revision }; so identity-driven dispatch (per Generated module root expos... (REQ_0525)) can use a static table. Identity is the shared taktora-fieldbus-od-core type (Lift OD IR to fieldbus-od-c... (ADR_0078)); the toolchain does not mint a separate SubDeviceIdentity. Mapping this triple onto ethercrab’s wire-read identity (which additionally carries serial) is the connector adapter’s concern (taktora-connector-ethercat ... (BB_0067)), not the generated code’s.
Requirement: Selectable PDO assignments emitted as a joint per-device OpMode enum REQ_0523
status: implemented
satisfies: FEAT_0053
is verified by: TEST_0870
links outgoing: BB_0062, TEST_0870
links incoming: ADR_0104

The backend shall resolve each device to a set of selectable PDO assignments and emit one joint <Dev>OpMode enum carrying a single variant per resolved assignment — one per <AlternativeSmMapping> (per AlternativeSmMapping captur... (REQ_0529)), or a single Default variant when the device declares none. The variant set spans both directions at once: a variant is a complete operating mode, not a per-direction choice. The device itself shall be emitted as struct <Dev> { mode: <Dev>OpMode }.

Modelling assignments with Option<…> fields on the device struct is rejected — every operating mode is a closed, named choice (Joint per-device OpMode enu... (ADR_0104), refining PDO assignment alternatives... (ADR_0072)). The earlier per-direction <IDENT>PdoAssignment sum type, which could not represent a multi-PDO assignment nor pair an RxPDO set with its TxPDO set, is retired in favour of this joint enum.
Requirement: Each OpMode variant carries a per-mode inputs/outputs data struct REQ_0524
status: implemented
satisfies: FEAT_0053
is verified by: TEST_0870
links outgoing: BB_0062, TEST_0870

Each <Dev>OpMode variant shall carry a { inputs, outputs } payload holding the typed process data for that mode. Each direction shall be flat-or-per-PDO: a single typed struct when the active mode assigns one PDO to that direction, or a struct composed of the per-PDO entry structs when it assigns several. Decode, encode, and the input_len / output_len lengths shall be computed per the active variant — the lengths of the variant currently held in mode — so a device’s process-image size follows its selected operating mode.
Requirement: Generated module root exposes a registry REQ_0525
status: open
satisfies: FEAT_0053
is verified by: TEST_0421

The module root emitted by emit_module_root shall expose a registry!() declarative macro (or equivalent generated static table) that maps each emitted device’s Identity to a factory closure returning Box<dyn EsiDevice>. Identity-based dispatch in downstream code (e.g. taktora-connector-ethercat) shall be reducible to a HashMap lookup against this table.
Requirement: Generated code compiles under no_std + alloc REQ_0526
status: open
satisfies: FEAT_0053
is verified by: TEST_0422

The emitted device modules shall compile under #![no_std] + alloc so generated drivers are usable from embedded contexts. The backend shall not emit std::-qualified paths in generated code.
Requirement: Default PDO assignment derived from Sm/Mandatory, not Fixed REQ_0527
status: implemented
satisfies: FEAT_0053
links outgoing: BB_0062, TEST_0870

When a device declares no <AlternativeSmMapping> (its Default mode, per Selectable PDO assignments ... (REQ_0523)), the backend shall derive that mode’s PDO assignment from the per-PDO Sm= attribute and the Mandatory flag — i.e. the PDOs the ESI assigns to a sync manager by default. The Beckhoff Fixed="1" attribute shall not be read as “always-on” / “always-assigned”: Fixed is orthogonal to assignment and means only that the PDO’s entry list is not editable (the wire layout is locked). A Fixed PDO that the default mapping does not assign to a sync manager is not part of the default assignment. This decoupling is Joint per-device OpMode enu... (ADR_0104). issue #70.
Requirement: Per-active-mode Rx/Tx PDO-index lists exposed for 0x1C12/0x1C13 REQ_0528
status: implemented
satisfies: FEAT_0053
is verified by: TEST_0870
links outgoing: BB_0062, TEST_0870

Each generated device shall expose, via an inherent fn pdo_assignment(&self) -> PdoAssignment (not a trait method), the Rx and Tx PDO index lists of the currently-held mode (per Selectable PDO assignments ... (REQ_0523)) so a connector can program the SM PDO-assignment objects 0x1C12 (RxPDO / SM2) and 0x1C13 (TxPDO / SM3). The indices shall be surfaced as plain u16 slices (&[u16]) — the raw PDO indices, not typed handles. The runtime EsiDevice trait surface (Object-safe EsiDevice, iden... (ADR_0098)) is unchanged; this is an inherent method on the concrete device struct, available to identity-dispatched bring-up code without widening the object-safe trait. issue #70.