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v0.13.0
rfc-0012superseded

Attributes, Metadata, Macros, and Derived Aspects

Superseded 2026-07-09 by four smaller, independently reviewable RFCs, split out after this RFC grew to cover generics-as-comptime-sugar, reflection, derive registration, general metaprogramming, and attributes/metadata all in one document — too much for one review, and with a real dependency-ordering problem this split resolves: comptime derive (this RFC) needed structural records' row concept, while structural records' ToRecord/FromRecord derive convenience needed comptime derive — circular only if the row concept and the derive-sugar convenience are conflated. Splitting them dissolved the cycle into a DAG. This document is kept as historical record; all decisions, worked examples, and open questions have been carried forward into the four successors below, plus RFC-0089/0090/0091 (also split out the same day, from the parallel linear-types.md/structural-records.md circular dependency).

  • RFC-0092 (Comptime Core) — type-as-value, typeinfo reflection, single-declaration emit. This RFC's §1-3.
  • RFC-0093 (Derive Registration) — @derive(Aspect) as request and registration, the Derivable Aspects table (with Linear corrected out of it — see RFC-0089 §2), Path A/B/C alternatives. This RFC's §9, Derivable Aspects, and the derive-specific Alternatives Considered entries.
  • RFC-0094 (Comptime Metaprogramming) — generalized multi-declaration/expression- position emit, comptime-callable parsing, span-tracked diagnostics, body reflection scoping. This RFC's §4-7, the Macros motivation subsection, and the Lisp-style-macros alternative.
  • RFC-0095 (Attributes and Metadata) — the @ attribute system, attributes as comptime-visible metadata. This RFC's Motivation "Attributes and metadata" subsection, §8, and "Preferred Syntax."

RFC-0080 (Standard Library Aspects) now depends on RFC-0093 for Clone's derive mechanism, not this RFC directly.

Summary​

(Superseded — see the four successor RFCs above for current content. The original summary, motivation, and full text of this RFC as of 2026-07-09 are preserved in this repository's git history and in the successor RFCs' own revision-history blockquotes, which trace the design decisions made under this RFC number before the split.)

Decision​

Outcome: Superseded by RFC-0092, RFC-0093, RFC-0094, RFC-0095. Target: n/a — see successor RFCs.