ADR-0002: Rust as the implementation language, one static binary

• Status: Accepted
• Date: 2026-09-20
• Derives from: design.md § Language decision, § Why Rust wins, § What Rust does not solve
• Constrains: architecture.md § Technology choices, § Distribution
• Implemented by: Wave 0 plan

Context

The tool must run inside a TypeScript pipeline, a .NET pipeline and a Python pipeline, and inside an agent's Stop hook in under two seconds. The three candidate languages were Rust, C# with NativeAOT, and TypeScript by extending dependency-cruiser. The decisive facts from the design's comparison table:

• dependency-cruiser's resolver, enhanced-resolve, has a maintained Rust port (oxc_resolver) and no .NET port. A C# implementation would spawn Node for TypeScript, which is the two-toolchain outcome the single binary exists to avoid.
• Python parsing has a maintained Rust crate (ruff_python_parser) and nothing equivalent in .NET.
• The only Rust gap is reading ECMA-335 metadata and portable PDBs; the usable crates are GPL-3 or decode no IL, so a reader of about 3,000 lines against two stable specifications is required.
• Only Rust yields one artefact with no runtime dependency on every host; npm, dotnet tool and pip wrappers are thin shells over it.
• The regex crate is linear-time by construction, which removes the safe-regex class of complaint heavy users hit.

Decision

Rulebearing is written in Rust as a Cargo workspace producing one static binary, rulebearing, with Node (napi-rs), NuGet and pip wrappers over that binary. The .NET metadata reader is written in Rust behind the extractor boundary, subject to the fallback in ADR-0003.

Consequences

• The maintainer's Rust familiarity is the day-to-day cost; the conformance suites (ADR-0009) are the reviewer of record so that contributors in any language can change an extractor safely.
• Every language-specific dependency is MIT-compatible (oxc, ruff, rquickjs), which ADR-0019 relies on.
• A WebAssembly playground (wave 4) is nearly free because the crates compile to wasm32.

Alternatives considered

• C# with NativeAOT. System.Reflection.Metadata makes the .NET side trivial, but TypeScript and Python would shell out. Kept as the extractor-only fallback.
• TypeScript, forking dependency-cruiser. Native for TypeScript, but requires Node in .NET and Python repositories and keeps the 13-second cruise.