17 unstable releases (7 breaking)

0.8.1 Jun 12, 2019
0.8.0 Dec 9, 2018
0.7.0 Dec 2, 2018
0.6.3 Nov 12, 2018
0.3.1 Nov 30, 2017

⚠️ Issues reported

#643 in Data structures

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Used in 7 crates (4 directly)


1.5K SLoC


This crate contains an Arena allocator, along with a few common data structures that can be used in tandem with it.

For all those times when you need to create a recursively nested tree of enums and find yourself in pain having to put everything in Boxes all the time.


  • Paginated Arena: internally preallocates 64KiB pages on the heap and allows Copy types to be put on that heap.

  • CopyCell: virtually identical to std::cell::Cell but requires that internal types implement Copy, and implements Copy itself.

  • List, Map and Set: your basic data structures that allocate on the Arena and use internal mutability via CopyCell. Never worry about sharing pointers again!

  • BloomMap and BloomSet: special variants of Map and Set with a very simple but very fast bloom filter. If a map / set is often queried for keys / elements it doesn't contain, the bloom filter check will reduce the need to do a full tree lookup, greatly increasing performance. The overhead compared to a regular Map or Set is also minimal.

  • All data structures implement expected traits, such as Debug or PartialEq.

  • Optional serde Serialize support behind a feature flag.


extern crate toolshed;

use toolshed::Arena;
use toolshed::map::Map;

// Only `Copy` types can be allocated on the `Arena`!
#[derive(Debug, PartialEq, Clone, Copy)]
enum Foo<'arena> {

    // Recursive enum without `Box`es!
    Nested(&'arena Foo<'arena>),

fn main() {
    // Create a new arena
    let arena = Arena::new();

    // We allocate first instance of `Foo` in the arena.
    // Please note that the `alloc` method returns a `&mut` reference.
    // Since we want to share our references around, we are going to
    // dereference and re-reference them to immutable ones with `&*`.
    let child: &Foo = &*arena.alloc(Foo::Integer(42));

    // Next instance of `Foo` will contain the child reference.
    let parent: &Foo = &*arena.alloc(Foo::Nested(child));

    // Empty map does not allocate
    let map = Map::new();

    // Inserting stuff in the map requires a reference to the `Arena`.
    // The reference can be shared, since `Arena` uses interior mutability.
    map.insert(&arena, "child", child);

    // We can put our `map` on the arena as well. Once again we use the `&*`
    // operation to change the reference to be immutable, just to demonstrate
    // that our `Map` implementation is perfectly happy with internal mutability.
    let map: &Map<&str, &Foo> = &*arena.alloc(map);

    // Each insert allocates a small chunk of data on the arena. Since arena is
    // preallocated on the heap, these inserts are very, very fast.
    // We only have a non-mutable reference to `map` now, however `Map` is also
    // using interior mutability on references to allow exactly this kind of
    // behavior in a safe manner.
    map.insert(&arena, "parent", parent);

    assert_eq!(map.get("child"), Some(&Foo::Integer(42)));
    assert_eq!(map.get("parent"), Some(&Foo::Nested(&Foo::Integer(42))));
    assert_eq!(map.get("heh"), None);


Here is a very biased benchmark of the different sets:

running 8 tests
test bloom_set_create  ... bench:          49 ns/iter (+/- 0)
test bloom_set_read    ... bench:         181 ns/iter (+/- 10)
test fxhash_set_create ... bench:          86 ns/iter (+/- 1)
test fxhash_set_read   ... bench:         312 ns/iter (+/- 4)
test hash_set_create   ... bench:         152 ns/iter (+/- 94)
test hash_set_read     ... bench:       1,105 ns/iter (+/- 1)
test set_create        ... bench:          37 ns/iter (+/- 0)
test set_read          ... bench:         440 ns/iter (+/- 1)
  • set and bloom_set are benchmarks of Set and BloomSet from this crate.
  • hash_set is the default stdlib HashSet.
  • fxhash_set is a HashSet using the fxhash crate hash.


This crate is distributed under the terms of both the MIT license and the Apache License (Version 2.0). Choose whichever one works best for you.