lru

lru is a minimal, dependency-light LRU (least-recently-used) cache for Rust, providing put, get, get_mut, and pop operations that all run in O(1) time. It was originally derived from the LRU cache implementation once found in an early version of Rust’s own std::collections before that module was removed, and remains one of the most widely depended-upon caching crates in the Rust ecosystem.

The crate is intentionally small in scope — a single LruCache type backed by an internal hash map and doubly linked list — making it easy to audit, embed, and use in both std and no_std (via the optional hashbrown backend) environments.

What You Get

• A generic LruCache<K, V> type with O(1) put/get/get_mut/pop/peek operations
• Optional hashbrown-backed hash map for faster lookups and no_std support
• Iterator support for inspecting cache contents in recency order
• A nightly feature flag for opting into nightly-only hashbrown optimizations
• Zero required runtime dependencies beyond the optional hashbrown backend

Common Use Cases

• Bounding memory use of an in-process cache (e.g. parsed config, compiled regexes, DB query results)
• Implementing eviction policy for a connection pool or object pool
• Building a simple in-memory HTTP response or asset cache inside a Rust web service
• Embedding a bounded cache inside a no_std or embedded Rust project via the hashbrown backend

Under The Hood

Architecture — The entire implementation lives in a single src/lib.rs (~3,200 lines including tests and docs), structured around an internal hash map from keys to nodes of a doubly-linked list; get/put move the touched node to the front of the list while pop/eviction removes from the back, giving the classic O(1) LRU behavior without a separate background thread or lock. Tech Stack — Pure Rust with a 1.85.0 MSRV, categorized under caching and no-std on crates.io; the only dependency is the optional hashbrown crate (enabled by default) used as a faster/no_std-compatible hash map, with scoped_threadpool and stats_alloc present only as dev-dependencies for benchmarking. Code Quality — The crate carries an extensive inline test suite directly in lib.rs covering capacity edge cases, eviction order, and the get_mut/peek variants, plus a CHANGELOG.md tracking version-by-version behavior changes, indicating a project that takes API stability seriously despite its small surface area. API Design — The API deliberately mirrors the ergonomics of std::collections::HashMap (put/get/get_mut/pop) so it requires almost no learning curve for anyone familiar with Rust’s standard collections, and the NonZeroUsize capacity parameter statically rules out the common zero-capacity-cache bug at the type level.