Rust Code Review

Review Workflow

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

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.
Check dependencies — Note key crates (thiserror vs anyhow, tokio features, serde features). These inform which patterns are expected.
Scan changed files — Read full functions, not just diffs. Many Rust bugs hide in ownership flow across a function.
Check each category — Work through the checklist below, loading references as needed.
Verify before reporting — Load beagle-rust:review-verification-protocol before submitting findings.

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, error context, Error trait	references/error-handling.md
Async pitfalls, Send/Sync bounds, runtime blocking	references/async-concurrency.md
Send/Sync semantics, atomics, memory ordering, lock patterns	references/concurrency-primitives.md
Type layout, alignment, repr, PhantomData, generics vs dyn Trait	references/types-layout.md
Unsafe code, API design, derive patterns, clippy patterns	references/common-mistakes.md
Safety contracts, raw pointers, MaybeUninit, soundness, 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

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

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)]

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)
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, Error trait impls → error-handling.md
Reviewing async code, tokio usage, task management → async-concurrency.md
Reviewing Send/Sync, atomics, memory ordering, mutexes, lock patterns → concurrency-primitives.md
Reviewing type layout, alignment, repr, PhantomData, generics vs dyn → types-layout.md
Reviewing unsafe code, API design, derive macros, clippy patterns → common-mistakes.md
Reviewing safety contracts, raw pointers, MaybeUninit, soundness → 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

Load and follow beagle-rust:review-verification-protocol before reporting any issue.

rust-code-review