ADR 0002 — Platform gems build default cargo features only

Status

Accepted (0.4.0 removed the vendored C++ stack; 0.5.x added the opt-in pgp feature on top).

Context

The 0.1–0.3 extension vendored the full librnp + Botan + json-c C/C++ stack to get ASCII armor — every install paid for a toolchain it did not use, and Windows/musl could not ship at all.

Decision

The extension is pure Rust by default. OpenPGP armor is pure Ruby. Anything needing a C/C++ vendored build (currently OpenPGP signature verification via rnp-rs) goes behind an opt-in cargo feature (pgp) selected at build time via RB_SYS_CARGO_FEATURES=pgp bundle exec rake compile (extconf.rb seeds r.features from that variable — the env var alone is a no-op in rb_sys).

Ruby-facing contract: Confium::OpenPGP::PGP_AVAILABLE reports whether the feature was compiled in; the verify methods raise with rebuild instructions when it was not. A dedicated CI job (pgp-verify) compiles the feature so the path cannot rot.

Consequences

• Source builders can opt in; platform gems stay C-toolchain-free.
• Feature-gated code needs a runtime availability constant and a stub that raises, or gem users hit silent NameErrors.