Function lower_field_default_wire 

pub fn lower_field_default_wire(
    init: &Expr,
    expected: &TypeRef,
    types: &HashMap<String, Arc<TypeDecl>>,
) -> Result<String, String>

Events slice 3a (#972): the wire-form JSON literal an event field’s default deserialises from when its key is absent from the incoming JSON. Type-directed — given the field’s expected TypeRef and the visible types table, every syntactic ambiguity (a bare Ident that is really a sum variant, a FieldAccess that is a qualified nullary variant) resolves the same way the checker resolves it, just narrower (a bare name must match a variant of this expected sum specifically) — so this needs no per-expression Ty map (expr_types), which a subscriber regenerating a publisher’s own event codec (emit_consumed_context_helpers) has no way to obtain anyway (no cross-unit expr_types store exists).

Produces the value in its wire shape (kind discriminant for a sum, matching emit_sum_codec’s generated Option/Result instantiations exactly — never the in-memory tag discriminant, and never a qualified reference like Region.Domestic into a value namespace the emitting module may not import), so it can be spliced in as the field’s raw JSON access and re-enter emit_field_deserialise (bynk-emit) completely unchanged.

Err(reason) for anything not closed-form. The caller (bynk-emit/src/project/validate.rs’s event-field-default check) turns that into bynk.event.bad_field_default at check time, so a value this function cannot build should never reach emit_record in practice — the .ok() fallback there is a non-panicking safety net, not the intended rejection path.