Decision (recorded as an ADR-011 addendum): the packed-mode wire layout
for a field-name-discriminator TUnion is shared-then-variant — the
union's declared `fields` (disc + shared fields) first, then the
variant's own fields. Reader and materializer already implemented this;
LayoutBuilder was corrected from variant-only layout.
Enforcement in BastUnion::parse (the choke point every consumer
inherits — union roots at BastDoc::new, referenced unions at
resolve_ref):
- discriminator field must be declared in `fields`
- `fields` must not contain duplicate names
- variants must not re-declare shared fields (checked inline and
through $ref resolution — parse chain now threads the doc root)
- the discriminator field must be the FIRST entry in `fields` (the
reader reads the disc at the union start; a later position made it
dispatch on the wrong bytes — H3 item 2, probe-verified)
Schemas relying on the old variant-only builder convention (variants
re-declaring shared fields) are rejected with a clean Schema error
naming the convention. Breaking for 0.2.0-era re-declaring schemas;
announced with 0.3.x.
- L5: FieldValue::Union::variant_start doc now states per-kind
semantics (byte-disc: union_start + disc.offset + disc.size;
field-disc: after the shared walk).
- L6: roundtrip test added (poc_roundtrip.rs) — LayoutBuilder write →
SequentialReader read → materialize_packed → validate_bytes over a
field-disc union with a second shared field and non-redeclaring
variant; pins event.type@0/seq@1/handle@5, total 10.
- ADR-011: Status-block addendum recording the convention decision,
the no-re-declare rule, and the breaking-constraint note.
- Review #006 updated: H3/L5/L6 resolution blocks, resolution log,
recommended order.
Verified: 488 tests green (410+17+34+15+12, 2 pre-existing ignored),
clippy -D warnings clean, wasm32 build green, cargo doc zero warnings.
glm-5.3-flash
05a2a42983