rust

Ownership is the design tool: encode invariants in types so illegal states do not compile.

Activate when

• Writing, reviewing, or testing Rust; choosing async runtime, web framework, serialization; Cargo workspace decisions.

Do NOT activate for

• Syntax-only questions (rustc + cargo doc answer those faster).

Procedure

1. Version: detect edition/toolchain from Cargo.toml/Cargo.lock/rust-toolchain.toml; research in https://doc.rust-lang.org/ (Book, std, Cargo) for that toolchain. Never assume latest-stable features on pinned toolchains.
2. Ownership design: borrow by default; lifetimes describe real relationships, not appeasement ('static sprawl is a smell); Clone only where duplication is semantically right (Arc for shared ownership, channels for handoff).
3. Errors: Result/Option with ?; thiserror for libraries, anyhow for applications (confirm versions first); no unwrap/expect outside tests and proven invariants.
4. Traits/generics: small trait bounds, behavior-first; generics with static dispatch preferred; dynamic dispatch (dyn) only at real heterogeneity points.
5. Async: Tokio runtime chosen deliberately; Send/Sync understood, not fought; no blocking calls inside async (use spawn_blocking); backpressure on channels.
6. Web/serde: Axum handlers as thin extractors + typed errors (see https://docs.rs/axum/ for the pinned version); Serde derives with explicit renames/deny-unknown-fields at trust boundaries (see https://serde.rs/).
7. Cargo: workspaces with shared lockfile; features additive; cargo test, clippy -D warnings, cargo fmt --check; benches with criterion or std harness for perf claims.
8. Unsafe/FFI: unsafe minimal, documented invariants, Miri-tested where feasible; FFI boundaries validated on both sides.
9. Verify: cargo test + clippy + fmt green; perf claims backed by bench numbers; unsafe blocks listed in the PR.

Failure modes

• Fighting the borrow checker instead of remodeling ownership; hidden blocking in async; unwrap in library code; feature unification surprises across workspace members.

References

• Registry: ../../research/documentation-search/references/framework-registry.yaml (rust, tokio, axum, serde).