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
Clippy	Run cargo clippy -- -D warnings on every commit; configure workspace lints	references/clippy-config.md
Performance	Don't guess, measure. Profile with --release first. Watch monomorphization + cache-line alignment at scale	references/performance.md
Generics	Static dispatch by default, dynamic dispatch when you need mixed types	references/generics-dispatch.md
Type State	Encode state in the type system when invalid operations should be compile errors	references/type-state-pattern.md
Documentation	// for why, /// for what and how, //! for module/crate purpose	references/documentation.md
Pointers	Choose pointer types based on ownership needs and threading model	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
Wild Patterns	Drop guards, extension traits, index pointers, crate preludes — battle-tested idioms from mature crates	references/coding-idioms.md
Ecosystem	Evaluate crates, pick error handling strategy, stay current	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 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 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 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 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 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 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 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 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, documentation quality, and transitive dependency weight. Use thiserror for library error types, anyhow for binaries, and eyre when you need custom error reporters. Prefer vendoring or writing code yourself when a crate pulls heavy dependencies for a small feature. Run cargo-deny for license and vulnerability auditing and cargo-udeps to trim unused dependencies. See references/ecosystem-patterns.md for crate evaluation criteria, edition migration, and essential tooling.

Pointer Types

Choose pointer types based on ownership and threading: Box<T> for single-owner heap allocation, Rc<T>/Arc<T> for shared ownership, Cell/RefCell/Mutex/RwLock for interior mutability. Use LazyLock/LazyCell (stable since 1.80) instead of lazy_static or once_cell. See references/pointer-types.md for the full single-thread vs multi-thread decision table and migration guidance.

Destructors and Cleanup

Drop::drop(&mut self) cannot return an error or .await. For fallible cleanup (I/O flush, network shutdown, async commit), expose an explicit close() or shutdown() method returning Result<(), Error> (or impl Future) and run best-effort cleanup in Drop as a fallback. Patterns for "consume self in Drop": Option<T>-newtype with mem::take, per-field mem::take, or ManuallyDrop<T>. Never block_on(...) in Drop (deadlock under async runtimes). For scoped state changes (toggle, restore, run on panic), use a drop guard bound with let _guard = ... — never let _ = ..., which drops immediately. scopeguard::defer! is the battle-tested option; note that drop guards do NOT run under panic = "abort". See references/coding-idioms.md and references/api-design.md for the explicit-destructor pattern.

Async APIs

async fn lowers to a state machine returning an anonymous impl Future. Lock down the Send-ness contract on public APIs with -> impl Future<...> + Send + '_ (or an explicit lifetime tied to &self) — auto-trait propagation through -> impl Trait is silent and a single Rc<...> or std::sync::MutexGuard held across .await downgrades the whole future to !Send, breaking downstream callers. Add + 'static only when the future must be spawned onto a multi-threaded executor (e.g. tokio::spawn); a blanket 'static on every signature forbids borrowing from &self and is the wrong default. Drop equals cancel: when a future is dropped mid-poll, locals drop, no cleanup runs. Document cancel-safety on every public async fn: cancel-safe (recv, observation-only) vs cancel-unsafe (read_exact, write_all, anything holding cross-poll invariants). For runtime-agnostic library code, take impl Future or use futures crate primitives — do NOT spawn internally, and document the required runtime. Use std::pin::pin! macro for stack-pinned local futures; Box::pin only when heap allocation is acceptable.