Comptime

Superseded 2026-07-09, discovered via public/rfcs/INDEX.md — the first RFC that index surfaced as a silent duplicate. This RFC sat in draft since 2026-06-05; RFC-0092/0093/0094 were drafted independently starting 2026-07-09 without anyone checking whether comptime had prior art in this repository, because no index existed to check against. Building the index the same day surfaced the overlap immediately.

This RFC's foundational execution model (comptime let, comptime fun's restrictions, comptime if) was real, correct, and missing from RFC-0092 as originally drafted — folded into RFC-0092 §0 rather than silently dropped. Its Open Questions 1-2 (recursion/termination, comptime and allocation) and OQ-5 (error message quality) are folded into RFC-0092's Open Questions 6-8. Its OQ-4 (comptime aspect inspection, "could replace some uses of conditional impl blocks") is answered more precisely by RFC-0093's @derive(Aspect) registration than by this RFC's own comptime has_aspect(T, Aspect) sketch. Its motivating observation about conditional compilation folding into comptime if with no separate mechanism needed is independently corroborated by, not merged into, RFC-0095's Open Question 4 — the two were written from opposite directions and arrived at the same place.

• RFC-0092 (Comptime Core) — the primary successor; type-as-value, typeinfo reflection, emit, and (as of this reconciliation) this RFC's own execution-model foundation.
• RFC-0093 (Derive Registration) — answers this RFC's OQ-4.
• RFC-0095 (Attributes and Metadata) — independently corroborates this RFC's @cfg/comptime-if observation.

Kept as historical record. Read it in full before touching the comptime cluster again — this reconciliation pass was not exhaustive, only targeted at the gaps found on one read-through.

Summary

(Superseded — see the three successor RFCs above. Original content preserved below for historical reference.)

Introduce a comptime evaluation phase that allows expressions, constants, and function calls to be evaluated at compile time. A comptime expression is guaranteed to produce its result before any generated code runs. Comptime values may be used wherever a compile-time constant is required — array sizes, type arguments, conditional compilation — and comptime functions may manipulate types as first-class values.

(Full original Motivation, Proposal, Alternatives Considered, Open Questions, Timing Recommendation, and References sections are preserved in this repository's git history prior to 2026-07-09, and their substance is carried forward into RFC-0092/0093/0095 per the supersession note above.)

Decision

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