ADR-0067: The playground's wasm facade, and the JavaScript engine as an optional feature

• Status: Accepted (2026-10-11, by the owner: "Accept ADRs 0067, 0072 and 0073"; proposed 2026-10-10)
• Date: 2026-10-10
• Derives from: plan 0004, Step 2 and its decision rule in § 1.7 ("Every crate that compiles to wasm32-unknown-unknown without a patch; one that does not (the candidates are rquickjs and any notify or process code in rb-cli) is feature-gated out of rb-wasm, and the playground page says which inputs it therefore cannot take"); ADR-0002 (the playground as the consequence of Rust)
• Amends: ADR-0010's crate table, by one row. ADR-0010's rules are unchanged.
• Constrains: crates/rb-wasm, crates/rb-config (js-config feature), the workspace's rb-config dependency, crates/rb-cli/tests/crate_boundary.rs
• Requirements: FR-REACH-01, FR-CFG-03

Context

Plan 0004 adds a browser playground: the library crates compiled to wasm32-unknown-unknown behind a thin facade, rb-wasm, whose exports take and return strings. ADR-0010's table names every crate and the crates it may depend on, and the crate-boundary test holds the workspace to it. rb-wasm is not in the table.

Every library crate builds for the target. One links badly: rquickjs-sys 0.14, the QuickJS engine behind JavaScript configurations and plugin: reporters (ADR-0006). Its wasm bindings declare JS_DupValue and JS_FreeContext as functions, but QuickJS's C source emits neither as a symbol for that target, so the module fails to link. Fixing that means patching the dependency, which the plan's rule says not to do.

Decision

rb-wasm joins ADR-0010's table as a facade. It may depend on rb-model, rb-config, rb-rules, rb-report and rb-ingest, and no crate depends on it. ADR-0073 adds rb-extract-ts and rb-extract-python to the row, for cruise_ts and cruise_py. It never links rb-cli. The logic it needs from rb-cli's cruise --graph path moves into rb-rules, so both call the same code:

• reset, which clears an earlier evaluation from a saved graph;
• report_format, the report's filters and collapse.

QuickJS becomes rb-config's js-config feature, on by default in rb-config and asked for explicitly by rb-cli.

• The workspace's rb-config dependency turns default features off, so that rb-rules does not pull the engine into the playground.
• Without the feature, every type, constant and policy of the JavaScript module stays. Evaluating a JavaScript configuration is JsError::Unavailable, exit 3, naming the file and the way out: "write it as YAML or JSON".
• JSON5, which the engine reads, is refused in the same words.
• rb_config::js::JS_ENGINE says which build is running.

The playground page states that it takes YAML and JSON configurations only.

Consequences

• The command line is unchanged: rb-cli always has the engine, and its tests, the sandbox-escape tests among them, run with it.
• cargo test -p rb-config --no-default-features runs in the wasm CI job, so the build without the engine is tested, and its refusals have tests of their own.
• Tested on its own, rb-wasm has no engine, as in the browser. Tested beside rb-cli, Cargo's feature unification gives it one. Its test of a JavaScript configuration asserts whichever build it is in.
• When a later rquickjs release links for the target, turning js-config on in rb-wasm brings JavaScript configurations to the playground with no other change.

Alternatives considered