rust

name	rust
description	Rust coding conventions. Apply when writing, reviewing, or modifying Rust code. Covers error handling, types, ownership, testing, and anti-patterns.

Rust Coding Conventions

Error Handling

• Return Result<T, E> for fallible operations
• Use thiserror with #[derive(Error)] for library error enums. Structure: variant name describes the failure, #[error("...")] format string includes diagnostic context (sizes, IDs), use #[from] for transparent wrapping of upstream errors
• Propagate with ? operator — avoid match chains for error forwarding
• Never use .unwrap() or .expect() outside of tests
• anyhow for binary/CLI code where specific error types don't matter; never in library crates

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 at compile time
• If you're adding .clone() to satisfy the borrow checker, step back and reconsider the data flow

Type Design

• Use newtypes for semantic distinction: pub struct UserId(pub Uuid), pub struct Port(pub u16)
• Derive ordering: Debug, Clone, Copy, PartialEq, Eq, Hash, Serialize, Deserialize — include only what's semantically correct
• Copy for small value types (IDs, enums). Clone but not Copy for larger types
• Implement Display manually when the type appears in logs or user-facing output

Documentation Style

• /// doc comments on all public types, functions, and enum variants — explain what and how
• // inline comments only to explain why (safety invariants, workarounds, design rationale)
• Function docs should include format specs when relevant (e.g., wire format, encoding details)
• Group related constants with // --- Section Name --- separators
• Every TODO needs a linked issue: // TODO(#42): description
• No module-level //! doc comments unless the module is a public API entry point

Testing Conventions

• Tests live inline: #[cfg(test)] mod tests { } at the bottom of each module
• Test naming: snake_case_description — be descriptive (e.g., buffer_duplicate_insert_ignored)
• For serialization testing, use round-trip helpers that encode then decode and assert equality
• #[tokio::test] for async tests — always with timeouts
• One assertion per test when possible; multiple related assertions are fine if testing one logical behavior
• Test error conditions explicitly — assert the specific error variant, not just "is error"
• Use polling with backoff for async assertions, never fixed sleep

Iterators & Collections

• Prefer iterators (.iter(), .map(), .filter()) over index-based loops
• Avoid intermediate .collect() — chain iterators directly
• Use .iter() for Copy types, .into_iter() when consuming ownership

Clippy & Linting

• Run cargo clippy after every change
• Use #[expect(clippy::lint_name)] over #[allow(...)] — expect warns if the lint no longer triggers
• Key lints to watch: redundant_clone, large_enum_variant (consider Box), needless_collect

Anti-Patterns

Anti-Pattern	Why Bad	Better
.clone() everywhere	Hides ownership issues	Proper references or restructure data flow
.unwrap() in library code	Runtime panics	?, or handle the error
String in function params	Unnecessary allocation	&str, Cow<str>
Index-based loops	Error-prone, unidiomatic	Iterators
Rc/Arc when single owner	Unnecessary overhead	Simple ownership
Giant match arms	Unmaintainable	Extract to methods
Ignoring #[must_use]	Silently dropped errors	Handle or let _ =
unsafe without SAFETY comment	UB risk, no audit trail	Document invariants or find safe pattern