Rust Code Review

Reviews Rust code for ownership, borrowing, lifetime, error handling, trait design, unsafe usage, and common mistakes. Use when reviewing .rs files, checking borrow checker issues, error handling patterns, or trait implementations. Covers Rust 2024 edition patterns and modern idioms.

Review Workflow

Follow this sequence to avoid false positives and catch edition-specific issues:

1. Check Cargo.toml — Note the Rust edition (2018, 2021, 2024) and MSRV if set. Edition 2024 introduces breaking changes to unsafe semantics, RPIT lifetime capture, temporary scoping, and ! type fallback. This determines which patterns apply. Check workspace structure if present.
2. Check dependencies — Note key crates (thiserror vs anyhow, tokio features, serde features). These inform which patterns are expected.
3. Scan changed files — Read full functions, not just diffs. Many Rust bugs hide in ownership flow across a function.
4. Check each category — Work through the checklist below, loading references as needed.
5. Verify before reporting — Complete Gates (below), including the verification-protocol gate, before submitting findings.

Gates

These steps are sequenced: do not skip ahead with “mental verification.” Each step has an objective Pass you can satisfy from files on disk and your own read path.

1. Crate context — Before relying on edition-specific checklist rows (Edition 2024, MSRV-sensitive APIs) or dependency assumptions. Pass: You opened the relevant Cargo.toml (package or workspace manifest) and can state edition and rust-version (if set) in one line.
2. Expanded read — Before reporting a Major or Critical finding. Pass: You read the full function, unsafe block, or impl / trait item that contains the cited line (not only a diff hunk).
3. Severity match — Before each finding line in the report. Pass: The Severity label matches Severity Calibration for that issue class, or you use Informational and give a one-line rationale.
4. Verification protocol — Before finalizing the report. Pass: beagle-rust:review-verification-protocol is loaded and every step in it that applies to this review is completed (do not substitute a vague “I checked”).

Output Format

Report findings as:

[FILE:LINE] ISSUE_TITLE
Severity: Critical | Major | Minor | Informational
Description of the issue and why it matters.

Quick Reference

Issue Type	Reference
Ownership transfers, borrowing, lifetimes, clone traps, iterators	references/ownership-borrowing.md
Lifetime variance, covariance/invariance, memory regions	references/lifetime-variance.md
Result/Option handling, thiserror, anyhow, opaque vs enumerated errors, deferred-cleanup with ?	references/error-handling.md
Async pitfalls, Send/Sync bounds, poll contract, Pin mechanics, cancellation soundness	references/async-concurrency.md
Send/Sync semantics, atomics, memory ordering, lock patterns	references/concurrency-primitives.md
Memory ordering decision tree, fences, ABA, out-of-thin-air	references/memory-ordering.md
Hand-rolled spinlocks, channels, Arc, seqlock, CAS retry patterns	references/lock-free-patterns.md
Shared-memory vs worker-pool vs actor design, async vs threads, race-condition vs data race	references/concurrency-models.md
Type layout, alignment, repr, PhantomData, generics vs dyn Trait, wide pointers, auto-trait leakage	references/types-layout.md
Object safety, ergonomic trait impls, Deref discipline, fallible destructors, hidden contracts, is_normal	references/interface-design.md
Index pointers, drop guards, extension traits, crate preludes	references/patterns-in-the-wild.md
Unsafe code, API design, derive patterns, clippy patterns	references/common-mistakes.md
Validity vs safety, drop check, may_dangle, provenance, panic safety in unsafe, MaybeUninit, Miri	references/unsafe-deep.md

For development guidance on performance, pointer types, type state, clippy config, iterators, generics, and documentation, use the beagle-rust:rust-best-practices skill.

Review Checklist

Ownership and Borrowing

• No unnecessary .clone() to silence the borrow checker (hiding design issues)
• No .clone() inside loops — prefer .cloned() or .copied() on iterators
• No cloning to avoid lifetime annotations (take ownership explicitly or restructure)
• References have appropriate lifetimes (not overly broad 'static when shorter lifetime works)
• Edition 2024: RPIT (-> impl Trait) captures all in-scope lifetimes by default; use + use<'a> for precise capture control
• &str preferred over String, &[T] over Vec<T> in function parameters
• impl AsRef<T> or Into<T> used for flexible API parameters
• No dangling references or use-after-move
• Interior mutability (Cell, RefCell, Mutex) used only when shared mutation is genuinely needed
• Small types (≤24 bytes) derive Copy and are passed by value
• Cow<'_, T> used when ownership is ambiguous
• Iterator chains preferred over index-based loops for collection transforms
• No premature .collect() — pass iterators directly when the consumer accepts them
• .sum() preferred over .fold() for summation (compiler optimizes better)
• _or_else variants used when fallbacks involve allocation
• Edition 2024: if let temporaries drop at end of the if let — code relying on temporaries living through the else branch needs restructuring
• Edition 2024: Box<[T]> implements IntoIterator — prefer direct iteration over into_vec() first

Error Handling

• Result<T, E> used for recoverable errors, not panic!/unwrap/expect
• Error types provide context (thiserror with #[error("...")] or manual Display)
• ? operator used with proper From implementations or .map_err()
• unwrap() / expect() only in tests, examples, or provably-safe contexts
• Error variants are specific enough to be actionable by callers
• anyhow used in applications, thiserror in libraries (or clear rationale for alternatives)
• _or_else variants used when fallbacks involve allocation (ok_or_else, unwrap_or_else)
• let-else used for early returns on failure (let Ok(x) = expr else { return ... })
• inspect_err used for error logging, map_err for error transformation

Traits and Types

• Traits are minimal and cohesive (single responsibility)
• derive macros appropriate for the type (Clone, Debug, PartialEq used correctly)
• Newtypes used to prevent primitive obsession (e.g., struct UserId(Uuid) not bare Uuid)
• From/Into implementations are lossless and infallible; TryFrom for fallible conversions
• Sealed traits used when external implementations shouldn't be allowed
• Default implementations provided where they make sense
• Send + Sync bounds verified for types shared across threads
• #[diagnostic::on_unimplemented] used on public traits to provide clear error messages when users forget to implement them

Interface Design

Detailed guidance: references/interface-design.md

• Methods on dyn-intended traits don't use Self by value, generic params, or associated constants (or are gated where Self: Sized)
• New traits ship with blanket impls for &T, &mut T, Box<T> so reference and smart-pointer arguments work
• Iterable types implement IntoIterator for &Self and &mut Self, not just Self
• Deref only used for transparent forwarding, never as "inheritance" — inherent-method ambiguity is a real bug class
• Fallible cleanup uses an explicit close()/shutdown() returning Result; Drop is best-effort fallback only
• No block_on(...) or new runtime in Drop (deadlock under async runtimes)
• Public types have a compile-time fn is_normal<T: Sized + Send + Sync + Unpin>() {} test so auto-trait regressions surface at build time
• Re-exported foreign types in public API are flagged — downstream major bumps become this crate's breaking change
• Getter methods follow the convention fn name(&self) not fn get_name(&self) (reserve get_* for Option-returning or interesting lookups)
• as_* is cheap reference-to-reference, to_* may allocate, into_* consumes — verify the cost matches the prefix
• Standard derives (Debug, Clone, Default, PartialEq, Eq, Hash) considered for every public type; Copy only when truly cheap and value-like (removing it later is breaking)

Patterns in the Wild

Detailed guidance: references/patterns-in-the-wild.md

• Index-pointer graphs use generational indices (slotmap::DefaultKey, petgraph::NodeIndex) — bare usize orphans after Vec::swap_remove
• Drop guards bound to let _guard = ..., never let _ = ... (the second drops immediately, not at scope end)
• Drop-guard cleanup not relied upon under panic = "abort" (destructors do not run)
• Extension traits used only when the type is foreign; for owned types use inherent impl directly
• Extension-trait methods don't shadow popular existing methods on the same type (ambiguity at call sites)
• Crate prelude module additions treated as semver-minor (RFC 1105); reserve for major releases when possible

Concurrency Design (Models)

Detailed guidance: references/concurrency-models.md

• Concurrency model (shared memory vs worker pool vs actor) named explicitly in the design; primitives match the model
• Mutex critical section is short, measurable, and excludes I/O / network calls / .await points
• Worker-pool queues are bounded; backpressure strategy is named
• Actor mailbox channels are sized to expected load; cross-actor cycles reviewed for deadlock
• async is not conflated with parallelism — join! interleaves on one thread; only tokio::spawn (or equivalent on a multi-threaded runtime) parallelizes
• CAS retry loops on a hot atomic considered for replacement with fetch_add/sharding (CAS is O(N²) under contention)
• println! / dbg! not used as a debugging tool for race conditions (the Stdout mutex changes the race)

Unsafe Code

• unsafe blocks have safety comments explaining invariants
• unsafe is minimal — only the truly unsafe operation is inside the block
• Safety invariants are documented and upheld by surrounding safe code
• No undefined behavior (null pointer deref, data races, invalid memory access)
• unsafe trait implementations justify why the contract is upheld
• Edition 2024: unsafe fn bodies use explicit unsafe {} blocks around unsafe ops (unsafe_op_in_unsafe_fn is deny)
• Edition 2024: extern "C" {} blocks written as unsafe extern "C" {}
• Edition 2024: #[no_mangle] and #[export_name] written as #[unsafe(no_mangle)] and #[unsafe(export_name)]

Concurrency (Memory Ordering and Lock-Free Patterns)

Detailed guidance: references/memory-ordering.md, references/lock-free-patterns.md

• Types shared across threads have correct Send / Sync bounds; unsafe impl Send/Sync carries a comment naming the invariant
• Each atomic operation pairs with a named happens-before edge (spawn/join, Release/Acquire on the same atomic, or a fence); Release publishes data, Acquire observes it
• No SeqCst by default — only when two or more independent atomics need a single global total order, with a comment naming the requirement
• No store(.., Acquire) / load(.., Release) / load(.., AcqRel) (rejected by the type half they occupy)
• Relaxed not used to publish or observe non-atomic data (use Release / Acquire)
• compare_exchange_weak used inside retry loops; strong compare_exchange reserved for one-shot updates; success ordering at least Acquire when acquiring a critical section
• Hand-rolled spinlocks include std::hint::spin_loop() in the busy wait, exponential backoff, and an eventual thread::yield_now(); not used in normal user-space binaries without a documented reason a Mutex is unsuitable
• Hand-rolled Arc clones with Relaxed, drops with Release + fence(Acquire) on the last decrement, and includes an overflow guard
• Arc<Mutex<...>> cycles broken with Weak; Arc<Mutex<Copy>> reviewed for Arc<AtomicT> replacement
• Hand-rolled lock-free primitives have a #[cfg(loom)] test module and a Miri-runnable test (no blanket cfg_attr(miri, ignore))
• OnceLock / LazyLock preferred over once_cell / lazy_static for new code (MSRV ≥ 1.80)
• No MutexGuard held across .await (use tokio::sync::Mutex or drop the guard first)
• Hot atomics on contended cache lines wrapped with CachePadded or #[repr(align(64))] to avoid false sharing
• Shared mutation goes through UnsafeCell<T> (not bare *mut T or transmuted & to &mut)
• Pointer-based CAS (AtomicPtr<Node>) uses epoch / hazard-pointer / tagged-pointer reclamation; ABA hazards considered

Naming and Style

• Types are PascalCase, functions/methods snake_case, constants SCREAMING_SNAKE_CASE
• Modules use snake_case
• is_, has_, can_ prefixes for boolean-returning methods
• Builder pattern methods take and return self (not &mut self) for chaining
• Public items have doc comments (///)
• #[must_use] on functions where ignoring the return value is likely a bug
• Imports ordered: std → external crates → workspace → crate/super
• #[expect(clippy::...)] preferred over #[allow(...)] for lint suppression

Performance

Detailed guidance: beagle-rust:rust-best-practices skill (references/performance.md)

• No unnecessary allocations in hot paths (prefer &str over String, &[T] over Vec<T>)
• collect() type is specified or inferable
• Iterators preferred over indexed loops for collection transforms
• Vec::with_capacity() used when size is known
• No redundant .to_string() / .to_owned() chains
• No intermediate .collect() when passing iterators directly works
• .sum() preferred over .fold() for summation
• Static dispatch (impl Trait) used over dynamic (dyn Trait) unless flexibility required

Clippy Configuration

Detailed guidance: beagle-rust:rust-best-practices skill (references/clippy-config.md)

• Workspace-level lints configured in Cargo.toml ([workspace.lints.clippy] or [lints.clippy])
• #[expect(clippy::lint)] used over #[allow(...)] — warns when suppression becomes stale
• Justification comment present when suppressing any lint
• Key lints enforced: redundant_clone, large_enum_variant, needless_collect, perf group
• cargo clippy --all-targets --all-features -- -D warnings passes
• Doc lints enabled for library crates (missing_docs, broken_intra_doc_links)

Type State Pattern

Detailed guidance: beagle-rust:rust-best-practices skill (references/type-state-pattern.md)

• PhantomData<State> used for zero-cost compile-time state machines (not runtime enums/booleans)
• State transitions consume self and return new state type (prevents reuse of old state)
• Only applicable methods available per state (invalid operations are compile errors)
• Pattern used where it adds safety value (builders with required fields, connection states, workflows)
• Not overused for trivial state (simple enums are fine when runtime flexibility needed)

Severity Calibration

Critical (Block Merge)

• unsafe code with unsound invariants or undefined behavior
• Use-after-free or dangling reference patterns
• unwrap() on user input or external data in production code
• Data races (concurrent mutation without synchronization)
• Wrong memory ordering on an atomic that gates other shared data (data race)
• Memory leaks via circular Arc<Mutex<...>> without weak references

Major (Should Fix)

• Errors returned without context (bare return err equivalent)
• .clone() masking ownership design issues in hot paths
• Missing Send/Sync bounds on types used across threads
• panic! for recoverable errors in library code
• Overly broad 'static lifetimes hiding API design issues

Minor (Consider Fixing)

• Missing doc comments on public items
• String parameter where &str or impl AsRef<str> would work
• Derive macros missing for types that should have them
• Unused feature flags in Cargo.toml
• Suboptimal iterator chains (multiple allocations where one suffices)

Informational (Note Only)

• Suggestions to introduce newtypes for domain modeling
• Refactoring ideas for trait design
• Performance optimizations without measured impact
• Suggestions to add #[must_use] or #[non_exhaustive]

When to Load References

• Reviewing ownership, borrows, lifetimes, clone traps → ownership-borrowing.md
• Reviewing lifetime variance, covariance/invariance, multiple lifetime params → lifetime-variance.md
• Reviewing Result/Option handling, error types, opaque vs enumerated, deferred-cleanup, Error trait impls → error-handling.md
• Reviewing async code, poll contract, Pin mechanics, cancellation soundness, cross-runtime → async-concurrency.md
• Reviewing concurrency design decisions (shared memory vs worker pool vs actor, async vs threads), data race vs race condition → concurrency-models.md
• Reviewing Send/Sync, atomics, mutexes, lock patterns → concurrency-primitives.md
• Reviewing memory ordering decisions, fences, ABA, out-of-thin-air → memory-ordering.md
• Reviewing hand-rolled spinlocks, channels, Arc, seqlock, CAS retry patterns → lock-free-patterns.md
• Reviewing type layout, alignment, repr, PhantomData, wide pointers, auto-trait leakage, Sized/?Sized → types-layout.md
• Reviewing interface design — object safety, ergonomic blanket impls, Deref discipline, fallible/blocking destructors, hidden contracts, naming → interface-design.md
• Reviewing index-pointer graphs, drop guards, extension traits, crate preludes → patterns-in-the-wild.md
• Reviewing unsafe code, API design, derive macros, clippy patterns → common-mistakes.md
• Reviewing validity vs safety, drop check + may_dangle, provenance, panic safety in unsafe, MaybeUninit → unsafe-deep.md
• Reviewing performance, pointer types, type state, generics, iterators, documentation → beagle-rust:rust-best-practices skill

Valid Patterns (Do NOT Flag)

These are acceptable Rust patterns — reporting them wastes developer time:

• .clone() in tests — Clarity over performance in test code
• unwrap() in tests and examples — Acceptable where panicking on failure is intentional
• Box<dyn Error> in simple binaries — Not every application needs custom error types
• String fields in structs — Owned data in structs is correct; &str fields require lifetime parameters
• #[allow(dead_code)] during development — Common during iteration
• todo!() / unimplemented!() in new code — Valid placeholder during active development
• .expect("reason") with clear message — Self-documenting and acceptable for invariants
• use super::* in test modules — Standard pattern for #[cfg(test)] modules
• Type aliases for complex types — type Result<T> = std::result::Result<T, MyError> is idiomatic
• impl Trait in return position — Zero-cost abstraction, standard pattern
• Turbofish syntax — collect::<Vec<_>>() is idiomatic when type inference needs help
• _ prefix for intentionally unused variables — Compiler convention
• #[expect(clippy::...)] with justification — Self-cleaning lint suppression
• Arc::clone(&arc) — Explicit Arc cloning is idiomatic and recommended
• std::sync::Mutex for short critical sections in async — Tokio docs recommend this
• for loops over iterators — When early exit or side effects are needed
• async fn in trait definitions — Stable since 1.75; async-trait crate only needed for dyn Trait or pre-1.75 MSRV
• LazyCell / LazyLock from std — Stable since 1.80; replaces once_cell and lazy_static for new code
• + use<'a, T> precise capture syntax — Edition 2024 syntax for controlling RPIT lifetime capture

Context-Sensitive Rules

Only flag these issues when the specific conditions apply:

Issue	Flag ONLY IF
Missing error context	Error crosses module boundary without context
Unnecessary .clone()	In hot path or repeated call, not test/setup code
Missing doc comments	Item is pub and not in a #[cfg(test)] module
unwrap() usage	In production code path, not test/example/provably-safe
Missing Send + Sync	Type is actually shared across thread/task boundaries
Overly broad lifetime	A shorter lifetime would work AND the API is public
Missing #[must_use]	Function returns a value that callers commonly ignore
Stale #[allow] suppression	Should be #[expect] for self-cleaning lint management
Missing Copy derive	Type is ≤24 bytes with all-Copy fields and used frequently
Edition 2024: ! type fallback	Match on Result<T, !> or diverging expressions where () fallback was assumed — ! now falls back to ! not ()
Edition 2024: r#gen identifier	Code uses gen as an identifier — must be r#gen in edition 2024 (reserved keyword)

Before Submitting Findings

Satisfy Gates § verification protocol (step 4). Load and follow beagle-rust:review-verification-protocol before reporting any issue.