pub trait Arbitrary<'a>: Sized {
    fn arbitrary(u: &mut Unstructured<'a>) -> Result<Self>;

    fn arbitrary_take_rest(u: Unstructured<'a>) -> Result<Self> { ... }
    fn size_hint(depth: usize) -> (usize, Option<usize>) { ... }
Expand description

Generate arbitrary structured values from raw, unstructured data.

The Arbitrary trait allows you to generate valid structured values, like HashMaps, or ASTs, or MyTomlConfig, or any other data structure from raw, unstructured bytes provided by a fuzzer.

Deriving Arbitrary

Automatically deriving the Arbitrary trait is the recommended way to implement Arbitrary for your types.

Using the custom derive requires that you enable the "derive" cargo feature in your Cargo.toml:

arbitrary = { version = "1", features = ["derive"] }

Then, you add the #[derive(Arbitrary)] annotation to your struct or enum type definition:

use arbitrary::Arbitrary;
use std::collections::HashSet;

pub struct AddressBook {
    friends: HashSet<Friend>,

#[derive(Arbitrary, Hash, Eq, PartialEq)]
pub enum Friend {
    Buddy { name: String },
    Pal { age: usize },

Every member of the struct or enum must also implement Arbitrary.

Implementing Arbitrary By Hand

Implementing Arbitrary mostly involves nested calls to other Arbitrary arbitrary implementations for each of your struct or enum’s members. But sometimes you need some amount of raw data, or you need to generate a variably-sized collection type, or something of that sort. The Unstructured type helps you with these tasks.

use arbitrary::{Arbitrary, Result, Unstructured};

impl<'a, T> Arbitrary<'a> for MyCollection<T>
    T: Arbitrary<'a>,
    fn arbitrary(u: &mut Unstructured<'a>) -> Result<Self> {
        // Get an iterator of arbitrary `T`s.
        let iter = u.arbitrary_iter::<T>()?;

        // And then create a collection!
        let mut my_collection = MyCollection::new();
        for elem_result in iter {
            let elem = elem_result?;


Required Methods

Generate an arbitrary value of Self from the given unstructured data.

Calling Arbitrary::arbitrary requires that you have some raw data, perhaps given to you by a fuzzer like AFL or libFuzzer. You wrap this raw data in an Unstructured, and then you can call <MyType as Arbitrary>::arbitrary to construct an arbitrary instance of MyType from that unstructured data.

Implementations may return an error if there is not enough data to construct a full instance of Self, or they may fill out the rest of Self with dummy values. Using dummy values when the underlying data is exhausted can help avoid accidentally “defeating” some of the fuzzer’s mutations to the underlying byte stream that might otherwise lead to interesting runtime behavior or new code coverage if only we had just a few more bytes. However, it also requires that implementations for recursive types (e.g. struct Foo(Option<Box<Foo>>)) avoid infinite recursion when the underlying data is exhausted.

use arbitrary::{Arbitrary, Unstructured};

pub struct MyType {
    // ...

// Get the raw data from the fuzzer or wherever else.
let raw_data: &[u8] = get_raw_data_from_fuzzer();

// Wrap that raw data in an `Unstructured`.
let mut unstructured = Unstructured::new(raw_data);

// Generate an arbitrary instance of `MyType` and do stuff with it.
if let Ok(value) = MyType::arbitrary(&mut unstructured) {

See also the documentation for Unstructured.

Provided Methods

Generate an arbitrary value of Self from the entirety of the given unstructured data.

This is similar to Arbitrary::arbitrary, however it assumes that it is the last consumer of the given data, and is thus able to consume it all if it needs. See also the documentation for Unstructured.

Get a size hint for how many bytes out of an Unstructured this type needs to construct itself.

This is useful for determining how many elements we should insert when creating an arbitrary collection.

The return value is similar to Iterator::size_hint: it returns a tuple where the first element is a lower bound on the number of bytes required, and the second element is an optional upper bound.

The default implementation return (0, None) which is correct for any type, but not ultimately that useful. Using #[derive(Arbitrary)] will create a better implementation. If you are writing an Arbitrary implementation by hand, and your type can be part of a dynamically sized collection (such as Vec), you are strongly encouraged to override this default with a better implementation. The size_hint module will help with this task.


It must be possible to construct every possible output using only inputs of lengths bounded by these parameters. This applies to both Arbitrary::arbitrary and Arbitrary::arbitrary_take_rest.

This is trivially true for (0, None). To restrict this further, it must be proven that all inputs that are now excluded produced redundant outputs which are still possible to produce using the reduced input space.

The depth Parameter

If you 100% know that the type you are implementing Arbitrary for is not a recursive type, or your implementation is not transitively calling any other size_hint methods, you can ignore the depth parameter. Note that if you are implementing Arbitrary for a generic type, you cannot guarantee the lack of type recursion!

Otherwise, you need to use arbitrary::size_hint::recursion_guard(depth) to prevent potential infinite recursion when calculating size hints for potentially recursive types:

use arbitrary::{Arbitrary, Unstructured, size_hint};

// This can potentially be a recursive type if `L` or `R` contain
// something like `Box<Option<MyEither<L, R>>>`!
enum MyEither<L, R> {

impl<'a, L, R> Arbitrary<'a> for MyEither<L, R>
    L: Arbitrary<'a>,
    R: Arbitrary<'a>,
    fn arbitrary(u: &mut Unstructured) -> arbitrary::Result<Self> {
        // ...

    fn size_hint(depth: usize) -> (usize, Option<usize>) {
        // Protect against potential infinite recursion with
        // `recursion_guard`.
        size_hint::recursion_guard(depth, |depth| {
            // If we aren't too deep, then `recursion_guard` calls
            // this closure, which implements the natural size hint.
            // Don't forget to use the new `depth` in all nested
            // `size_hint` calls! We recommend shadowing the
            // parameter, like what is done here, so that you can't
            // accidentally use the wrong depth.
                <L as Arbitrary>::size_hint(depth),
                <R as Arbitrary>::size_hint(depth),

Implementations on Foreign Types