rc-rust

Rust Best Practices

Unified Rust guidelines covering coding style, ownership, error handling, async patterns, traits, testing, performance, linting, and documentation. Apply when writing or reviewing Rust code.

Pair with

This skill owns Rust the language. The stack's layers live elsewhere — reach for them instead of duplicating their guidance here:

• rc-axum — HTTP routing, extractors, Tower middleware, IntoResponse error mapping, WebSockets
• rc-sqlx — PgPool, query macros, transactions, migrations, #[sqlx::test]
• rc-sveltekit — the SSR front end that consumes these APIs
• rc-fullstack-axum-svelte — the router when a change spans more than one layer
• rc-final-verify — after cargo fmt/clippy/test come back green

When to Apply

• Writing new Rust code or designing APIs
• Reviewing or refactoring existing Rust code
• Implementing async systems with Tokio
• Designing error hierarchies with thiserror/anyhow
• Choosing between borrowing, cloning, or ownership transfer
• Setting up tests, benchmarks, or snapshot testing
• Configuring clippy lints and workspace settings
• Optimizing Rust code for performance

Reference Guide

Load detailed guidance based on context. Read the relevant file when the topic arises:

Topic	Reference	Load When
Coding Style	references/coding-style.md	Naming, imports, iterators, comments, string handling, macros
Error Handling	references/error-handling.md	Result, Option, ?, thiserror, anyhow, custom errors, async errors
Ownership & Pointers	references/ownership-and-pointers.md	Lifetimes, borrowing, smart pointers, Pin, Cow, interior mutability
Traits & Generics	references/traits-and-generics.md	Trait design, dispatch, GATs, sealed traits, type state pattern
Async & Concurrency	references/async-and-concurrency.md	Tokio, channels, streams, shutdown, runtime config, async traits
Sync Concurrency	references/concurrency-sync.md	Atomics, Mutex, RwLock, lock ordering, Send/Sync, memory ordering
Testing	references/testing.md	Unit/integration/doc tests, snapshot, proptest, mockall, benchmarks, fuzz
Performance	references/performance.md	Profiling, flamegraph, cloning, stack vs heap, iterators, allocation
Clippy & Linting	references/clippy-and-linting.md	Clippy config, key lints, workspace setup, #[expect] vs #[allow]
Documentation	references/documentation.md	Doc comments, rustdoc, doc lints, coverage checklist

Quick Reference: Coding Style

• Prefer &T over .clone() unless ownership transfer is required
• Use &str over String, &[T] over Vec<T> in function parameters
• No get_ prefix on getters: fn name() not fn get_name()
• Conversion naming: as_ (cheap borrow), to_ (expensive/cloning), into_ (ownership transfer)
• Iterator methods: iter() / iter_mut() / into_iter()
• Import ordering: std -> external crates -> workspace crates -> super:: -> crate::
• Comments explain why (safety, workarounds), not what
• Use format! over string concatenation with +
• Prefer s.bytes() over s.chars() for ASCII-only operations
• Avoid macros unless necessary; prefer functions or generics

Quick Reference: Error Handling

• Return Result<T, E> for fallible operations; reserve panic! for unrecoverable bugs
• No unwrap() in production. Use expect() with descriptive message only when the value is logically guaranteed. Prefer ?, if let, let...else for all other cases
• Use thiserror for library/crate errors, anyhow for binaries only
• Prefer ? operator over match chains for error propagation
• Use _else variants (ok_or_else, unwrap_or_else) to prevent eager allocation
• Use inspect_err and map_err for logging and transforming errors
• assert! at function entry for invariant checking (debug builds)

Quick Reference: Ownership & Pointers

• Small Copy types (<=24 bytes, all fields Copy, no heap) pass by value
• Use Cow<'_, T> when data may or may not need ownership
• Meaningful lifetime names: 'src, 'ctx, 'conn — not just 'a
• Use try_borrow() on RefCell to avoid panics; prefer over direct .borrow_mut()
• Shadowing for transformations: let x = x.parse()?

Pointer	When to Use
Box<T>	Single ownership, heap allocation, recursive types
Rc<T>	Shared ownership, single-threaded
Arc<T>	Shared ownership, multi-threaded
Cell<T> / RefCell<T>	Interior mutability, single-threaded
Mutex<T> / RwLock<T>	Interior mutability, multi-threaded

Quick Reference: Traits & Generics

• Prefer generics (static dispatch) by default for zero-cost abstractions
• Use dyn Trait only when heterogeneous collections or plugin architectures are needed
• Box at API boundaries, not internally
• Object safety: no generic methods, no Self: Sized, methods use &self/&mut self/self
• Use sealed traits to prevent external implementors
• Type state pattern encodes valid states in the type system:

struct Connection<S> { _state: PhantomData<S> }
struct Disconnected;
struct Connected;
impl Connection<Connected> { fn send(&self, data: &[u8]) { /* ... */ } }

Quick Reference: Async & Concurrency

• Async for I/O-bound work, sync for CPU-bound work
• Never hold locks across .await points — use scoped guards
• Never use std::thread::sleep in async — use tokio::time::sleep
• Never spawn unboundedly — use semaphores for limits
• Ensure Send bounds on spawned futures
• Use JoinSet for managing multiple concurrent tasks
• Use CancellationToken (from tokio_util) for graceful shutdown
• Instrument with tracing + #[instrument] for async debugging

Channel	Use Case
mpsc	Multi-producer, single-consumer message passing
broadcast	Multi-producer, multi-consumer event fan-out
oneshot	Single value, single use (request-response)
watch	Latest-value-only, change notification

• Sync channels: crossbeam::channel over std::sync::mpsc
• Async channels: tokio::sync::{mpsc, broadcast, oneshot, watch}
• Atomics (AtomicBool, AtomicUsize) over Mutex for primitive types
• Choose memory ordering carefully: Relaxed / Acquire / Release / SeqCst

Quick Reference: Testing

• Name tests descriptively: process_should_return_error_when_input_empty()
• One assertion per test when possible; include formatted failure messages
• Group tests in mod blocks by unit of work
• Use doc tests (///) for public API examples; run separately with cargo test --doc
• Snapshot testing: cargo insta test then cargo insta review; redact unstable fields
• rstest for parameterized tests with #[case::name] labels
• proptest for property-based testing with custom strategies
• mockall with #[automock] for mocking traits
• criterion for benchmarks with iter_batched and BenchmarkId
• cargo-fuzz with libfuzzer_sys for fuzz testing
• cargo-tarpaulin or cargo-llvm-cov for code coverage
• sqlx::test for database integration tests with automatic pool injection
• Use #[should_panic] and #[ignore] attributes where appropriate

Quick Reference: Performance

• Golden rule: don't guess, measure. Always benchmark with --release
• Run cargo clippy -- -D clippy::perf for performance-related hints
• Use cargo flamegraph or samply (macOS) for profiling
• Avoid cloning in loops; clone at the last moment only
• Pre-allocate: Vec::with_capacity(), String::with_capacity()
• Prefer iterators over manual for loops; avoid intermediate .collect()
• Stack for small types, heap for large/recursive; use smallvec for large const arrays
• Use Cow<'_, T> to avoid unnecessary allocation
• Prefer s.bytes() over s.chars() for ASCII-only string operations

Quick Reference: Clippy & Linting

Run regularly:

cargo clippy --all-targets --all-features --locked -- -D warnings

Lint	Catches
redundant_clone	Unnecessary .clone() calls
needless_borrow	Unnecessary & borrows
large_enum_variant	Oversized variants (consider Box)
needless_collect	Premature .collect() before iteration
map_unwrap_or	.map().unwrap_or() chains
unnecessary_wraps	Functions always returning Ok/Some
clone_on_copy	.clone() on Copy types

• Use #[expect(clippy::lint)] over #[allow(...)] — expect warns when lint no longer applies
• Add justification comment on every suppression
• Set #![warn(clippy::all)] as workspace minimum
• Configure workspace lints in Cargo.toml with priority levels

Quick Reference: Documentation

• // comments explain why: safety invariants, workarounds, design rationale
• /// doc comments explain what and how for all public items
• //! for module-level and crate-level documentation at top of lib.rs/mod.rs
• Every TODO needs a linked issue: // TODO(#42): description
• Enable #![deny(missing_docs)] for libraries
• Include # Examples, # Errors, # Panics, # Safety sections in doc comments

Doc Lint	Purpose
missing_docs	Ensure all public items documented
broken_intra_doc_links	Catch dead cross-references
missing_panics_doc	Document panic conditions
missing_errors_doc	Document error conditions
missing_safety_doc	Document unsafe safety requirements

Quick Reference: Data Types & Patterns

• Use newtypes for domain semantics: struct Email(String)
• Prefer slice patterns: if let [first, .., last] = slice
• Use arrays for fixed sizes; avoid Vec when length is known at compile time
• Shadowing for transformation: let x = x.parse()?
• Cow<str> when data might need modification of borrowed data
• contains() on strings is O(n*m) — avoid nested string iteration

Deprecated to Modern Migration

Deprecated	Better	Since
lazy_static!	std::sync::OnceLock	Rust 1.70
once_cell::Lazy	std::sync::LazyLock	Rust 1.80
std::sync::mpsc	crossbeam::channel (sync)	—
std::sync::Mutex	parking_lot::Mutex (recommended)	—
failure / error-chain	thiserror / anyhow	—
try!()	? operator	Rust 2018
async-trait crate	Native async fn in traits (1.75+, limited)	Rust 1.75

Cargo.toml Essentials

Recommended dependencies:

[dependencies]
thiserror = "2"
anyhow = "1"
tokio = { version = "1", features = ["full"] }
serde = { version = "1", features = ["derive"] }
tracing = "0.1"
tracing-subscriber = "0.3"

[dev-dependencies]
rstest = "0.25"
proptest = "1"
mockall = "0.13"
criterion = { version = "0.5", features = ["html_reports"] }
insta = { version = "1", features = ["yaml"] }

Workspace lints (Cargo.toml):

[workspace.lints.clippy]
all = { level = "warn", priority = -1 }
pedantic = { level = "warn", priority = -1 }

rustfmt.toml:

reorder_imports = true
imports_granularity = "Crate"
group_imports = "StdExternalCrate"

Constraints

MUST DO

1. When reviewing or writing code, suggest a testing approach using the recommended tools (rstest, proptest, insta, mockall, criterion) — even if the user did not ask for tests