9.2 Field numbers and schema

Every field of a data and every variant of an enum has a stable tag (the number in the wire format).
The tag is set explicitly by @tag(n) or assigned by the compiler. The assigned tags are recorded in the package file oxl.schema.lock (type name → field name → tag). A new field gets max(tag) + 1. The lock file MUST be kept in version control. Format and reconciliation with the registry — appendix C.
Renaming a field without @renamed(from: "old") is treated as a deletion and an addition.
Schema version: @v(N) on data, enum or service (1 by default). @since(N) — the field appeared in version N; @removed(N) — the field was removed in version N, its tag is reserved forever.

Breaking changes (the compiler and oxl publish MUST reject them relative to the last published schema):

Change	Why
removing a field without `@removed`	old readers silently lose data
reusing a tag	wrong interpretation of data
changing a field type, except the widenings `i32→i64`, `u32→u64`, `f32→f64`, `T→T?`	incompatible encoding
adding a required field (without a default value and not optional)	old writers do not send it
`T?` → `T`	old writers may not send it
removing an enum variant or adding a variant to an enum without `@open`	old readers will not be able to parse it
removing or renaming an rpc, changing its parameters or result	breaks clients

Related diagnostics