Skill: rust

Purpose

Apply the shared engineering style to Rust projects while preserving idiomatic ownership, error handling, module structure, and safety.

Inspect First

Before changing Rust code, inspect relevant files when present:

• Cargo.toml
• Cargo.lock
• .cargo/config.toml
• rust-toolchain.toml
• rustfmt.toml
• clippy.toml
• workspace layout
• existing modules and tests

Infer:

• edition;
• workspace structure;
• feature flags;
• dependency policy;
• error handling conventions;
• async runtime if present.

Tooling Defaults

• Prefer cargo nextest run over cargo test when cargo-nextest is installed and compatible with the workspace.
• Use bacon for local interactive feedback loops when available.
• Use cargo watch for focused watch commands when available and useful.
• Do not introduce these tools as required project dependencies unless the user asks or the repository already standardizes on them.

Style

• Follow existing module organization.
• Keep modules cohesive.
• Avoid giant mod.rs or lib.rs files.
• Prefer explicit domain names over generic utils modules.
• Keep public API surface intentionally small.
• Prefer simple ownership over lifetime acrobatics.

Error Handling

• Use Result for recoverable failures.
• Do not unwrap or expect in production paths unless the invariant is proven and the message explains it.
• In tests, unwrap/expect may be acceptable when failure context is clear.
• Preserve error context.
• Use the repository's established error type approach.
• Do not introduce anyhow, thiserror, or custom error stacks unless already used or justified.

Safety

• Avoid unsafe.
• If unsafe is necessary, isolate it, document invariants, and test the safe wrapper.
• Do not create undefined behavior for performance fantasies.

Ownership and APIs

• Prefer borrowing over cloning when simple.
• Clone intentionally when it improves clarity or avoids complex lifetimes.
• Avoid exposing unnecessary concrete types in public APIs.
• Use traits when they express real abstraction or test seams.
• Do not create traits for one implementation unless boundary value is clear.

Async

If async code is present:

• respect existing runtime;
• avoid blocking calls in async tasks;
• handle cancellation/drop behavior;
• avoid detached tasks without lifecycle management;
• use timeouts where external calls can hang.

Tests and Validation

Common commands, only when configured or clearly available:

cargo nextest run
cargo test
cargo test --workspace
cargo fmt --check
cargo clippy --workspace --all-targets --all-features
cargo check
bacon
cargo watch -x test

Report exact commands and results.

Shared Cross-Ecosystem Style

Apply the shared engineering style from:

• ../../engineering/05_design_principles.md
• ../../engineering/06_responsibility_model.md
• ../../engineering/07_complexity_budget.md
• ../../engineering/12_naming_bible.md

Language-specific idioms control syntax and ecosystem details, but they must not weaken the core rules: small cohesive units, explicit boundaries, honest validation, no dumping grounds, and no fake placeholders.