Rust Best Practices

Apply these guidelines when writing or reviewing Rust code. Based on Apollo GraphQL's Rust Best Practices Handbook.

Best Practices Reference

Before reviewing, familiarize yourself with Apollo's Rust best practices. Read ALL relevant chapters in the same turn in parallel. Reference these files when providing feedback:

• Chapter 1 - Coding Styles and Idioms: Borrowing vs cloning, Copy trait, Option/Result handling, iterators, comments
• Chapter 2 - Clippy and Linting: Clippy configuration, important lints, workspace lint setup
• Chapter 3 - Performance Mindset: Profiling, avoiding redundant clones, stack vs heap, zero-cost abstractions
• Chapter 4 - Error Handling: Result vs panic, thiserror vs anyhow, error hierarchies
• Chapter 5 - Automated Testing: Test naming, one assertion per test, snapshot testing
• Chapter 6 - Generics and Dispatch: Static vs dynamic dispatch, trait objects
• Chapter 7 - Type State Pattern: Compile-time state safety, when to use it
• Chapter 8 - Comments vs Documentation: When to comment, doc comments, rustdoc
• Chapter 9 - Understanding Pointers: Thread safety, Send/Sync, pointer types

Quick Reference

Borrowing & Ownership

• Prefer

  &T

  over

  .clone()

  unless ownership transfer is required
• Use

  &str

  over

  String

  ,

  &[T]

  over

  Vec<T>

  in function parameters
• Small

  Copy

  types (≤24 bytes) can be passed by value
• Use

  Cow<'_, T>

  when ownership is ambiguous

Error Handling

• Return

  Result<T, E>

  for fallible operations; avoid

  panic!

  in production
• Never use

  unwrap()

  /

  expect()

  outside tests
• Use

  thiserror

  for library errors,

  anyhow

  for binaries only
• Prefer

  ?

  operator over match chains for error propagation

Performance

• Always benchmark with

  --release

  flag
• Run

  cargo clippy -- -D clippy::perf

  for performance hints
• Avoid cloning in loops; use

  .iter()

  instead of

  .into_iter()

  for Copy types
• Prefer iterators over manual loops; avoid intermediate

  .collect()

  calls

Linting

Run regularly:

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

Key lints to watch:

• redundant_clone

  - unnecessary cloning

• large_enum_variant

  - oversized variants (consider boxing)

• needless_collect

  - premature collection

Use

#[expect(clippy::lint)]

over

#[allow(...)]

with justification comment.

Testing

• Name tests descriptively:

  process_should_return_error_when_input_empty()

• One assertion per test when possible
• Use doc tests (

  ///

  ) for public API examples
• Consider

  cargo insta

  for snapshot testing generated output

Generics & Dispatch

• Prefer generics (static dispatch) for performance-critical code
• Use

  dyn Trait

  only when heterogeneous collections are needed
• Box at API boundaries, not internally

Type State Pattern

Encode valid states in the type system to catch invalid operations at compile time:

struct Connection<State> { /* ... */ _state: PhantomData<State> }
struct Disconnected;
struct Connected;

impl Connection<Connected> {
    fn send(&self, data: &[u8]) { /* only connected can send */ }
}

Documentation

• //

  comments explain why (safety, workarounds, design rationale)

• ///

  doc comments explain what and how for public APIs
• Every

  TODO

  needs a linked issue:

  // TODO(#42): ...

• Enable

  #![deny(missing_docs)]

  for libraries