0006 — Facade features default to thin

• Status: Accepted
• Date: 2026-08-26
• Deciders: openbimrs contributors
• Supersedes: —

Context

openbim-ifc is a facade over nineteen crates. Its default feature set decides what a casual cargo add openbim-ifc compiles.

The tempting default is generous — enable the domains, the schema, geometry — so that the crate "just works" on first use. The cost is borne invisibly by every downstream build: a web viewer that only needs to parse and draw would compile domain interpreters and a geometry lowering stack it never calls.

Because domain crates are optional views over a graph that round-trips regardless (ADR 0003), a thin build is not a degraded build. It reads and writes every entity in the file. It simply does not interpret the ones it was not asked about.

Decision

We will default to step only. Every domain, the schema metadata, both alternative codecs, and the geometry bridge are opt-in features. Convenience bundles (codecs, domains, full) exist for consumers who genuinely want breadth.

We will enforce this with a test rather than a convention, because a default feature set drifts the moment someone adds a dependency for convenience.

Alternatives considered

Option	Why not
Enable all domains by default	Every consumer pays compile time and binary size for interpreters they never call
No default features at all	cargo add openbim-ifc then fails to do the single most common thing — read a .ifc file
Split into separate published facades	Multiplies published crates; users must choose before understanding the axes

Consequences

Positive

• A thin consumer compiles the model plus the STEP codec and nothing else.
• Feature choice maps directly onto the architecture, so the dependency graph is predictable from the feature list.
• Round-tripping is unaffected by feature selection, so thin builds are safe to use on files containing data they do not interpret.

Negative / costs

• Users must discover features to reach functionality; a missing feature presents as a missing symbol rather than a runtime error.
• Feature combinations multiply, so CI must build several explicitly rather than relying on --all-features alone.

Follow-ups / risks to watch

• Any future dependency added unconditionally to the facade silently fattens every downstream build. The thin-build test is the guard; keep it meaningful.

Relation to existing code

• openbim-ifc/Cargo.toml — default = ["step"] with a comment recording this decision
• openbim-ifc/tests/thin_build.rs — enforcement
• scripts/gate.sh — builds --no-default-features, --features step, --features ifcxml, and --all-features with clippy denials on each