rust-best-practices

Rust Best Practices

Guidance for writing idiomatic, performant, and safe Rust code. This is a development skill, not a review skill -- use it when building, not reviewing.

Quick Reference

| Topic | Key Rule | Reference | |-------|----------|-----------| | Ownership | Borrow by default, clone only when you need a separate owned copy | [references/coding-idioms.md](references/coding-idioms.md) | | Clippy | Run cargo clippy -- -D warnings on every commit; configure workspace lints | [references/clippy-config.md](references/clippy-config.md) | | Performance | Don't guess, measure. Profile with --release first. Watch monomorphization + cache-line alignment at scale | [references/performance.md](references/performance.md) | | Generics | Static dispatch by default, dynamic dispatch when you need mixed types | [references/generics-dispatch.md](references/generics-dispatch.md) | | Type State | Encode state in the type system when invalid operations should be compile errors | [references/type-state-pattern.md](references/type-state-pattern.md) | | Documentation | // for why, /// for what and how, //! for module/crate purpose | [references/documentation.md](references/documentation.md) | | Pointers | Choose pointer types based on ownership needs and threading model | [references/pointer-types.md](references/pointer-types.md) | | API Design | Unsurprising, flexible, obvious, constrained — encode invariants in types; watch hidden contracts (re-exports, auto-traits) | [references/api-design.md](references/api-design.md) | | Wild Patterns | Drop guards, extension traits, index pointers, crate preludes — battle-tested idioms from mature crates | [references/coding-idioms.md](references/coding-idioms.md) | | Ecosystem | Evaluate crates, pick error handling strategy, stay current | [references/ecosystem-patterns.md](references/ecosystem-patterns.md) |

Gates

Short sequences with pass conditions before claiming outcomes that need evidence (not an internal “I checked”).

Clippy clean

1. From the workspace root (or with -p <crate>), run: cargo clippy --all-targets --all-features -- -D warnings.
2. Pass: exit status is 0 and the invocation finishes without Clippy-deny failures.

Performance claim

1. Build with cargo build --release (or your benchmark harness) under the same profile you ship or measure.
2. Capture a before and after number from the same tool and metric (name both), e.g. Criterion ns/iter, heaptrack allocations, or a flamegraph path on disk.
3. Pass: you can cite both measurements, or you explicitly state that only correctness or readability changed and you are not claiming a performance delta.

Docs for symbols you changed

1. Run cargo doc --no-deps for the crate you edited (add -p <crate> in workspaces).
2. Pass: the doc build succeeds; if #![deny(missing_docs)] (or crate policy) applies, there are no new missing-doc errors for those symbols.

Coding Idioms

Prefer &T over .clone(), use &str/&[T] in parameters, and chain iterators instead of index-based loops. For Option/Result, use let Ok(x) = expr else { return } for early returns and ? for propagation. For scoped state changes, use drop guards (let _guard = ..., never let _ = ...) with mem::replace or scopeguard::defer!. Add methods to foreign types via extension traits (trait MyExt; impl<T: Bound> MyExt for T). For graph and tree shapes, prefer index pointers (slotmap::DefaultKey, generational indices) over &T to side-step lifetimes without unsafe. Curate a lean crate prelude for ergonomic glob imports; prelude additions are semver-minor (RFC 1105). See [references/coding-idioms.md](references/coding-idioms.md) for ownership, iterator, import patterns, and these ecosystem-level idioms.

Error Handling

Return Result<T, E> for fallible operations. Use thiserror for library error types, anyhow for binaries. Propagate with ?, never unwrap() outside tests. See [references/coding-idioms.md](references/coding-idioms.md) for Option/Result patterns.

Clippy Discipline

Run cargo clippy --all-targets --all-features -- -D warnings on every commit. Configure workspace lints in Cargo.toml and use #[expect(clippy::lint)] (not #[allow]) as the standard for lint suppression -- it warns when the suppression becomes stale. See [references/clippy-config.md](references/clippy-config.md) for lint configuration and key lints.

Performance Mindset

Always benchmark with --release, profile before optimizing, and avoid cloning in loops or premature .collect() calls. Keep small types on the stack and heap-allocate only recursive structures and large buffers. For workspaces at scale, watch monomorphization budgets (extract type-independent inner functions; switch internal generics to dyn where peak inlining isn't critical) and false sharing (#[repr(align(64))] or crossbeam::utils::CachePadded on per-thread atomics; align(128) on Apple Silicon). Benchmark with criterion — persist a baseline (--save-baseline main) and compare in CI, use criterion::black_box (with as_ptr() for pointer inputs), and isolate I/O into iter_batched setup. See [references/performance.md](references/performance.md) for profiling tools, allocation guidance, monomorphization patterns, cache-line alignment, and criterion discipline.

Generics and Dispatch

Use static dispatch (impl Trait / <T: Trait>) by default for zero-cost monomorphization. Switch to dyn Trait only for heterogeneous collections or plugin architectures, preferring &dyn Trait over Box<dyn Trait> when ownership isn't needed. In edition 2024, -> impl Trait captures all in-scope lifetimes by default -- use + use<'a, T> for precise capture control. Prefer native async fn in traits over the async-trait crate for static dispatch. See [references/generics-dispatch.md](references/generics-dispatch.md) for dispatch trade-offs, RPIT capture rules, and async trait guidance.

Type State Pattern

Encode valid states in the type system so invalid operations become compile errors. Use for builders with required fields, protocol state machines, and workflow pipelines. See [references/type-state-pattern.md](references/type-state-pattern.md) for implementation patterns and when to avoid.

Documentation

Use // for why, /// for what/how on public APIs, and //! for module purpose. Every TODO needs a linked issue and library crates should enable #![deny(missing_docs)]. Use #[diagnostic::on_unimplemented] to provide custom compiler errors for your public traits. See [references/documentation.md](references/documentation.md) for doc test patterns, comment conventions, and diagnostic attributes.

API Design

Follow four principles: unsurprising (reuse standard names and traits), flexible (use generics and impl Trait to avoid unnecessary restrictions), obvious (encode invariants in the type system so misuse is a compile error), and constrained (expose only what you can commit to long-term). Use #[non_exhaustive] for types that may grow, seal traits you need to extend without breaking changes, and wrap foreign types in newtypes to control your SemVer surface. Watch for hidden contracts — re-exported foreign types, auto-trait propagation through -> impl Trait, and accidental !Send futures — and lock them down with a compile-time is_normal<T: Sized + Send + Sync + Unpin>() test for public types. Ship new traits with blanket impls for &T/Box<T> early (adding later is breaking). For fallible cleanup, expose an explicit close()/shutdown() returning Result; Drop cannot fail or .await. See [references/api-design.md](references/api-design.md) for builder patterns, sealed traits, object-safety mechanics, Deref discipline, fallible destructors, and SemVer implications.

Ecosystem Patterns

Evaluate crates by recent download trends, maintenance activity, documenta