List of crates that improves or experiments with Rust, but may be hard to find

Let's list here crates that enhance Rust as a language.

Note: this post has reached its 32k technical limit and will not be updated anymore. Primary copy is now at GIthub Gists

It not "batteries" like in stdx, but Rust-specific crates for workarounds for various missing features and experimental ideals from non-accepted/postponed RFCs, or just hacky tricks.

The list is supposed to contain (mostly) crates that are internal to Rust, not ones for making Rust deal with "external world" like other languages bindings, file formats, protocols and so on.

Primary focus should be on crates that are not easy to find by conventional means (e.g. no algorithm name, format or protocol to search for).

Note that quality of the listed crates may vary from proof-of-concept to stable-and-widely-used.

Procedural macros and custom derive:

  • auto-impl - Derive Box (and other pointers) wrappers with all methods delegated.
  • derive_more - derive standard traits like Add or Index that lack built-in derive.
  • enum-primitive-derive, derive_builder, strum, enum-unitary - derivers for enums; enum-map - "map literal" macro for enums.
  • smart-default - #[derive(Default)] with your default values.
  • derivative - Some additinoal derives and adjustable analogues of std derives. Debug with ignored fields, Clone with overridden clone function, Default with alternative value like in smart-default.
  • couted-array - Automatix size for constant fixed size arrays before RFC 2545 goes in.
  • sugar-rs - List comprehensions, hashmap construction.
  • match_all - A match-like block that executes multiple branches if needed.
  • if_chain - A macro crate that makes embedding a lot of if let nicer. Related: guard for if !let or Swift's guard let.
  • delegate - Proxy struct methods to a field.
  • overflower - proc macro that lets you annotate a function to make arithmetic operations in it wrapping, saturating or panicking.
  • def_mod - forward declare mod's interface, asserting that implementation (included by specifying #[path] based on #[cfg]) matches the declared interface.
  • derive_utils - helpers for implementing your own custom derive / macros for enums.
  • interpolate - string interpolation: let world = "world."; s!("Hello, {world}")
  • taken - Sugar for let x = x; or let y = y.clone();, for use in closures.
  • momo - a prototype of automatic introduction of dynamic dispatch in a function using proc macro.
  • ifmt - "inline string {interpolation}"
  • shrinkwraprs - Create Newtype(u32), automatically implementing traits to make as convenient as original type.
  • custom_debug_derive - Derive Debug with a custom format per field.
  • cxx - Generates a bridge between C++ and Rust based on schema defined in Rust code.
  • genawaiter - generators for stable Rust. 3 Types.
  • vecmerge - merge compile-time-known slices of vector literals using + sign.
  • log-derive - auto insert logging for functions
  • cascade - cascade expressions - multiple actions on one object.
  • overloadable - macro function overloading.
  • include-flate - compress literal inlucde_str! / include_bytes! buffer before compilation, lazily decompress in runtime.
  • ambassador - Delegate trait implementations
  • inline-proc - Procedure macros declared and used within the same crate. #[inline_proc] mod ... { }

Enum trait


  • test-case-derive - Nightly-only. Allows adding arguments to test functions and listing sets of inputs in attributes.
  • quickcheck, proptest - Fuzz testing. Also mutagen for alternative test coverate method. dicetest.
  • mockiato - Mocking of traits: .expect_myfunc(...).times(2).returns(...)
  • loom - test concurrent code

Threading and sleeping:

  • spin_sleep - precise sleep.
  • desync Desync<T> where you can post operations to be executed, maybe in background. Alternative approach to threading/syncronosation.
  • crossbeam, rayon - Advanced threading.
  • parking_lot - Synchronisation primitives (Mutex, RwLock, Once) speed up.
  • atomic - generic Atomic<T> type.
  • arc-swap - Arc meets AtomicPtr - Like crossbeam::ArcCell or optimized Mutex<Arc<...>>. Readers don't block.
  • atomic_refcell - Threadsafe RefCell.
  • arbalest - Like Arc<AtomicRefCell<T>>, but without some runtime check.
  • qcell - Less flexible, but statically checked RefCell analogue. QCell, LCell, TCell with detached owners for borrowing.
  • flume - mpsc channel, competes with std and crossbeam's. Supports async (including mixing it with sync). Also mp2c.
  • bus - spmc broadcast lock-free channel.
  • conquer-once - lazy and one-time initialisation, including for nostd

Software transactional memory (STM)

  • swym - transactional memory.
  • stm-core - Clojure-like software transaction memory in Rust. TVar, atomically.
  • lever - STM primitives, also some key-value tables

General list:

  • rental - Deal with self-referential structs. Related: owning_ref. Related: ouroboros
  • take_mut, replace_with - Apply Fn(T) -> T to &mut T.
  • type_num / peano - Simulate missing type-level integers.
  • slice - Make a "view" into a Read+Write+Seek with offset and length.
  • mopa - implement customized Any-like trait with your own additional methods. See also: query_interface.
  • streaming_iterator - like Iterator, but with another ownership rules.
  • num, num_traits, alga - Traits for denoting algebraic properties of things and other math; rug - long arithmetic (bigint and friends); caldyn - evaluate strings like "2+2"; nalgebra - linear algebra.
  • failure, error_chain - Dealing with errors in more organized way.
  • downcast-rs, vec_box, traitobject - Tricks with trait objects.
  • optional - a space efficient Option replacement. Related: nanbox.
  • lazy_static - declaring lazily evaluated statics (global variables). See also: core_memo.
  • bytes, byteorder - Work with byte arrays (big-endian/little-endian).
  • static_assertions - compile-time assertions.
  • reflect - A proof-of-concept for making Java/Go-like reflections in Rust using procedural macros.
  • boolinator - Option or Result-style methods for bool. Also there's crate bool_ext.
  • noisy_float - Floats that panic on NaN.
  • decorum - Ordered, NonNan and Finite wrappers for floats.
  • smallvec - On-stack small vectors. Tuner: smallvectune.
  • array_tool - More methods for arrays and strings, including a Unicode grapheme iterator.
  • nom, combine, lalrpop - parser combinators
  • serde - serialization. Utils: serde_with crate.
  • mitochondria - MoveCell and OnceCell. Also: once_cell, lazycell.
  • either - Like Result, but without error semantics.
  • const-concat - A concat! that accepts non-literal strings.
  • void - A crate with a "canonical" uninhabited (zero, bottom) type, to use instead of the ! never type before it goes stable.
  • itertools - More iterator adapters like and related functions like unfold.
  • reexport-proc-macro - Re-export a custom derive.
  • array-macro - Like [42; 3] array literal, but with some restrictions removed.
  • m - pure Rust "libm" - primitive mathematical functions for nostd.
  • interpolate_idents - more powerful concat_idents!. Also paste.
  • null-terminated - NULL-terminated arrays for C interop.
  • bus - Single writer, multiple reader broadcast lockfree communication channel. std::io::{Read,Write} wrapper: bus-writer.
  • frunk - Functinal programming toolkit: HList (heterogenous list of cons cell), Coproduct for ad-hoc enums, Semigroup/Monoid like in alga above.
  • fragile - Fragile<T> and Sticky<T> wrappers for sending non-Send things across threads.
  • double - mocking (mock testing) library.
  • joinery - join_with for joining to a string with separators from an Iterator.
  • pipe - Generate a byte stream-based pipe, like std::sync::mpsc, but for bytes. Combined with my crate readwrite you can make it bi-directional, like a socket. Also newer crate duplexify.
  • uninitialized - Choose std::mem::zeroed or std::mem::uninitialized based on Cargo feature flag.
  • failsafe - wrap function in a supervisor that prohibits execution (returns error early) for some time in case of consequtive failures of the wrapped function. Also recloser.
  • corona - stackful coroutines.
  • subtle - constant-time primitive ops for crypto
  • ctor - life before main() - auto-initialization of things before main() using linker tricks.
  • inventory - Typed distributed plugin registration using ctor crate above.
  • strfmt - Like format!(), but with dynamically specified format string and your hashmap for available variables.
  • panic-never - Panic handler for no_std code that causes linker error if panics are not statically excluded from your code.
  • stdio-override - Override std::io::std{in,out,err} using dup2 on Unix.
  • bstr - Text processing methods for UTF-8-esque byte strings.
  • async-trait - Traits for async fn using dynamic dispatch. See also See also: polling-async-trait, async-trait-ext
  • pow - insert proof-of-work requirements using serde
  • zerocopy - Utilities for zero-copy parsing and serialization
  • scopeguard - defer!, defer_on_unwind!, defer_on_success!.
  • bytemuck - Zeroable, Pod (plain old data) trait, safe reinterpret_casts for some types.

Allocation or data structures:

  • slab - Hands out integer handles instead of actual pointers. My own additions: slab_typesafe, compactmap.
  • non-empty, vec1 - A non-empty vector or maybe other things. See also a reddit thread.
  • lru_time_cache - A cache. timed_cache - another cache.
  • uluru - Another LRU cache.
  • im, rpds - Immutable collection data types.
  • fst - Specialized compact data structure for pre-ordered data.
  • internment - Interning strings or data. Stores data permanently/to Rc/to Arc and gives a handle. Same data => same handle.
  • tendril - compact string/buffer type, optimized for zero-copy parsing.
  • slotmap - like slab, but handles are protected against accidental reuse (there is counter inside).
  • froggy - like slab, but with iteration, reference counting and weak pointers.
  • containers - containers like in std, but with fallible allocation and custom allocators.
  • sdset - A wrapper around a sorted+deduplicated slice providing faster set operations (union, intersection, etc.).
  • intrusive-collections - no_std single- and double-linked lists, red-black tree without allocating dedicated memory for them, but by attaching things to contained objects instead.
  • phf - compile-time-constructed static perfect hash table maps and sets.
  • generational-arena - Safe allocator with deletions.
  • rctree - DOM-like tree with ordered children, parents and siblings, based on strong and weak Rc links.
  • heapless - allocation-free data structures: fixed-size vec, priority queue, hash table/set.
  • voluntary-servitude - thread-safe appendable list with lock-free iterator.
  • copyless - Pre-allocate Box or Vec entry to fill it in later. Also maybe prevents extra memcpy call.
  • ccl - Fast concurrent hashmap and timed cache, also for no_std. See also dashmap and chashmap. shard_lock.
  • contrie - Lock-free concurrent map and set with interface similar to HashMap.
  • Evmap - A lock-free, eventually consistent, concurrent multi-value map.
  • sharded_slab - Slab mutable without exclusive access, lockfree
  • arrayvec - Array-backed fixed-capacity vector
  • vec-utils - map and zip vector while reusing allocation.
  • elsa - append-only collections for getting references to elements. Insertion does not require exclusive (&mut) access.
  • shawshank - generic internment.
  • concache - Two fast concurrent shared hash maps.
  • hashcow - Hash table with copy-on-write for storing duplicated data.

Cargo-specific or code analysers:

  • cargo bloat - Show what takes space in your binary
  • cargo license, cargo-lichking - List all dependencies by licence.
  • cargo outdated - Show which packages have newer version available.
  • cargo clippy - Additional warnings
  • cargo fmt (rustfmt) - Automatically format the code. Also installable by rustup.
  • cargo geiger - Measure "unsafeness" of the project by counting expressions in unsafe{} blocks and other unsafe things. Prototype: cargo-osha
  • cargo make - Define and run targets in a toml file like in Makefile.
  • cargo add, cargo rm and so on - Manage dependencies from command line.
  • cargo tarpaulin - code coverage for tests.
  • cargo asm - display the assembly or llvm-ir generated for Rust source code (a single function can be specified).
  • cargo audit - Scan dependencies (Cargo.lock) for reported security vulnerabilities.
  • cargo expand - Show source code as one big file with expanded macros, like in gcc -E.
  • rustig - Show places where your program can theoretically panic.
  • cargo cache - Shows disk size of Cargo cache and allows cleaning it.
  • cargo web - Client-side web for Rust (wasm?).
  • cargo tally - Plot reverse dependencies over time.
  • cargo rerast - Apply cargo fix-like modifications according to specifiable rules.
  • cargo-workspaces
  • - list source files used in compilation
  • cargo-incversion - Increment version in Cargo.toml from CLI

Hosted languages designed to interoperate with Rust

  • Rhai - small, inspired by Chai.
  • Rune - async-friendly
  • Dyon
  • Mun

Unsorted links dump

(to be moved up and described properly)

"If you're looking to get structs (possibly dynamically sized) consider using zerocopy (or scroll, or structview). If you need to turn bytes into trivial types such as u16 or arrays thereof, use bytemuck (or safe-transmute)."

Please suggest additions, replacements or removals.


There's a peano crate, succeeded by typenum.


optional - a space efficient Option replacement.


lazy_static - declaring lazily evaluated statics.


Is this still needed with the new notch optimizations? The readme says

Not only does OptionBool improve space efficiency as opposed to Option<bool>

But nowadays bool, Option<bool>, and even Option<Option<bool>> and beyond are all the same size.


I think the original bool use case is gone, but I could see myself using that crate for storing optional floats, using NaN as the None value.


That's exactly what I plan to use it for.


typenum has no underscore nor does it use a lowly Peano encoding (it's a binary encoding!)

(edit: okay the peano thing was fixed, but that underscore is still there silently taunting at me)

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I tried to be careful how I mentioned peano followed by typenum... :man_shrugging:

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Extended the list from relevant "Crate of the Week" posts.

Can you please link the 'cargo-specific or related tools' section?


cargo license - List all dependencies by licence. There is also some alternative implementation, but I forgot the name.

cargo-lichking. Haven't researched them for their strengths or weaknesses.

proptest probably derserves a mention alongside quickcheck :slight_smile:


Maintainer of optional here. I was about to deprecate OptionBool when re-running the benchmarks still showed some measurable differences. So yes, it no longer has a space advantage, but it may sometimes have a time advantage.

Even without OptionBool, the crate still has option replacements for other types.

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Have you filed bugs for those?


cargo-make rust build tool and task runner.
comes with many default tasks to make any project development easier and automated.

rules was an incomplete experiment to bring Perl 6 style regex to rust. There is a lot which could be done to extend the project. I'm not sure if this is the type of thing you're looking for.

I definitely think this type of thing might be really positive in the long run but it would have be more fully implemented to verify in any fashion.

I made some crates that I think could fit onto the list.

  • array-macro - vec![something; 4] for arrays, not requiring a Copy implementation
  • reexport-proc-macro - probably useless with Rust 2018 where you can go pub use crate_derive::Trait, but allows reexporting a procedural derive, not requiring an user of a crate to have a dependency on derive crate
  • - just like EnumMap in Java, except for Rust, a map implementation optimized for enums implemented as an array