At a high-level, test-fuzz is a convenient front end for In more concrete terms, test-fuzz is a collection of Rust macros and a Cargo subcommand that automate certain fuzzing-related tasks, most notably:

  • generating a fuzzing corpus
  • implementing a fuzzing harness

test-fuzz accomplishes these (in part) using Rust's testing facilities. For example, to generate a fuzzing corpus, test-fuzz records a target's arguments each time it is called during an invocation of cargo test. Similarly, test-fuzz implements a fuzzing harness as an additional test in a cargo-test-generated binary. This tight integration with Rust's testing facilities is what motivates the name test-fuzz.


  1. Installation
  2. Usage
  3. Components
  4. test-fuzz Package Features
  5. Auto-generated Corpus Files
  6. Environment Variables
  7. Limitations
  8. Tips and Tricks


cargo install cargo-test-fuzz


Fuzzing with test-fuzz is essentially three steps:*

  1. Identify a fuzz target:

    • Add the following dependencies to the target crate's Cargo.toml file:
      serde = "1.0"
      test-fuzz = "1.0.0"
    • Precede the target function with the test_fuzz macro:
      fn foo(...) {
  2. Generate a corpus by running cargo test:

    $ cargo test
  3. Fuzz your target by running cargo test-fuzz:

    $ cargo test-fuzz --target foo

* Some additional steps may be necessary following a reboot. AFL requires the following commands to be run as root:

  • Linux

    echo core >/proc/sys/kernel/core_pattern
    cd /sys/devices/system/cpu
    echo performance | tee cpu*/cpufreq/scaling_governor
  • OSX

    launchctl unload -w ${SL}/LaunchAgents/${PL}.plist
    sudo launchctl unload -w ${SL}/LaunchDaemons/${PL}.Root.plist


test_fuzz macro

Preceding a function with the test_fuzz macro indicates that the function is a fuzz target.

The primary effects of the test_fuzz macro are:

  • Add instrumentation to the target to serialize its arguments and write them to a corpus file each time the target is called. The instrumentation is guarded by #[cfg(test)] so that corpus files are generated only when running tests (however, see enable_in_production below).
  • Add a test to read and deserialize arguments from standard input and apply the target to them. The test checks an environment variable, set by cargo test-fuzz, so that the test does not block trying to read from standard input during a normal invocation of cargo test. The test is enclosed in a module to reduce the likelihood of a name collision. Currently, the name of the module is target_fuzz, where target is the name of the target (however, see rename below).


  • bounds = "where_predicates" - Impose where_predicates (e.g., trait bounds) on the struct used to serialize/deserialize arguments. This may be necessary, e.g., if a target's argument type is an associated type. For an example, see in this repository.

  • concretize = "parameters" - Use parameters as the target's type parameters when fuzzing. Example:

    #[test_fuzz(concretize = "String")]
    fn foo<T: Clone + Debug + Serialize>(x: &T) {

    Note: The target's arguments must be serializable for every instantiation of its type parameters. But the target's arguments are required to be deserializable only when the target is instantiated with parameters.

  • concretize_impl = "parameters" - Use parameters as the target's Self type parameters when fuzzing. Example:

    impl<T: Clone + Debug + Serialize> for Foo {
        #[test_fuzz(concretize_impl = "String")]
        fn bar(&self, x: &T) {

    Note: The target's arguments must be serializable for every instantiation of its Self type parameters. But the target's arguments are required to be deserializable only when the target's Self is instantiated with parameters.

  • convert = "X, Y" - When serializing the target's arguments, convert values of type X to type Y using Y's implementation of From<X>. When deserializing, convert those values back to type X using Y's implementation of the non-standard trait test_fuzz::Into<X>.

    That is, use of convert = "X, Y" should be accompanied by the following implementations:

    impl From<X> for Y {
        fn from(x: X) -> Self {
    impl test_fuzz::Into<X> for Y {
        fn into(self) -> X {

    The definition of test_fuzz::Into is identical to that of std::conversion::Into. The reason for using a non-standard trait is to avoid conflicts that could arise from blanket implementations of standard traits.

  • enable_in_production - Generate corpus files when not running tests, provided the environment variable TEST_FUZZ_WRITE is set. The default is to generate corpus files only when running tests, regardless of whether TEST_FUZZ_WRITE is set. When running a target from outside its package directory, set TEST_FUZZ_MANIFEST_PATH to the path of the package's Cargo.toml file.

    WARNING: Setting enable_in_production could introduce a denial-of-service vector. For example, setting this option for a function that is called many times with different arguments could fill up the disk. The check of TEST_FUZZ_WRITE is meant to provide some defense against this possibility. Nonetheless, consider this option carefully before using it.

  • execute_with = "function" - Rather than call the target directly:

    • construct a closure of type FnOnce() -> R, where R is the target's return type, so that calling the closure calls the target;
    • call function with the closure.

    Calling the target in this way allows function to set up the call's environment. This can be useful, e.g., for fuzzing Substrate externalities.

  • no_auto_generate - Do not try to auto-generate corpus files for the target.

  • only_concretizations - Record the target's concretizations when running tests, but do not generate corpus files and do not implement a fuzzing harness. This can be useful when the target is a generic function, but it is unclear what type parameters should be used for fuzzing.

    The intended workflow is: enable only_concretizations, then run cargo test followed by cargo test-fuzz --display-concretizations. One of the resulting concretizations might be usable as concretize's parameters. Similarly, a concretization resulting from cargo test-fuzz --display-imply-concretizations might be usable as concretize_impl's parameters.

    Note, however, that just because a target was concretized with certain parameters during tests, it does not imply the target's arguments are serializable/deserializable when so concretized. The results of --display-concretizations/--display-impl-concretizations are merely suggestive.

  • rename = "name" - Treat the target as though its name is name when adding a module to the enclosing scope. Expansion of the test_fuzz macro adds a module definition to the enclosing scope. Currently, the module is named target_fuzz, where target is the name of the target. Use of this option causes the module to instead be be named name_fuzz. Example:

    #[test_fuzz(rename = "bar")]
    fn foo() {}
    // Without the use of `rename`, a name collision and compile error would result.
    mod foo_fuzz {}

test_fuzz_impl macro

Whenever the test_fuzz macro is used in an impl block, the impl must be preceded with the test_fuzz_impl macro. Example:

impl Foo {
    fn bar(&self, x: &str) {

The reason for this requirement is as follows. Expansion of the test_fuzz macro adds a module definition to the enclosing scope. However, a module definition cannot appear inside an impl block. Preceding the impl with the test_fuzz_impl macro causes the module to be added outside the impl block.

If you see an error like the following, it likely means a use of the test_fuzz_impl macro is missing:

error: module is not supported in `trait`s or `impl`s

test_fuzz_impl currently has no options.

cargo test-fuzz command

The cargo test-fuzz command is used to interact with fuzz targets, and to manipulate their corpora, crashes, hangs, and work queues. Example invocations include:

  1. List fuzz targets

    cargo test-fuzz --list
  2. Display target foo's corpus

    cargo test-fuzz --target foo --display-corpus
  3. Fuzz target foo

    cargo test-fuzz --target foo
  4. Replay crashes found for target foo

    cargo test-fuzz --target foo --replay-crashes


    cargo test-fuzz [FLAGS] [OPTIONS] [-- <args>...]


    <args>...    Arguments for the fuzzer


            Display backtraces

            Move one target's crashes, hangs, and work queue to its corpus; to consolidate all
            targets, use --consolidate-all

            Display concretizations

            Display corpus using uninstrumented fuzz target; to display with instrumentation, use

            Display crashes

            Display hangs

            Display `impl` concretizations

            Display work queue

            Target name is an exact name rather than a substring

    -h, --help
            Print help information

            List fuzz targets

            Do not activate the `default` feature

            Compile without instrumentation (for testing build process)

            Compile, but don't fuzz

            Disable user interface

            Enable persistent mode fuzzing

            Pretty-print debug output when displaying/replaying

            Replay corpus using uninstrumented fuzz target; to replay with instrumentation, use

            Replay crashes

            Replay hangs

            Replay work queue

            Clear fuzzing data for one target, but leave corpus intact; to reset all targets, use

            Resume target's last fuzzing session

            Stop fuzzing once a crash is found

    -V, --version
            Print version information


        --features <FEATURES>...    Space or comma separated list of features to activate
        --manifest-path <PATH>      Path to Cargo.toml
    -p, --package <PACKAGE>         Package containing fuzz target
        --target <TARGET>           String that fuzz target's name must contain
        --test <NAME>               Integration test containing fuzz target
        --timeout <TIMEOUT>         Number of milliseconds to consider a hang when fuzzing or
                                    replaying (equivalent to `-- -t <timeout>` when fuzzing)

dont_care! macro

The dont_care! macro can be used to implement serde::Serialize/serde::Deserialize for types that are easy to construct and whose values you do not care to record. Intuitively, dont_care!($ty, $expr) says:

  • Skip values of type $ty when serializing.
  • Initialize values of type $ty with $expr when deserializing.

More specifically, dont_care!($ty, $expr) expands to the following:

impl serde::Serialize for $ty {
    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
        S: serde::Serializer,

impl<'de> serde::Deserialize<'de> for $ty {
    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
        D: serde::Deserializer<'de>,
        <()>::deserialize(deserializer).map(|_| $expr)

If $ty is a unit struct, then $expr can be be omitted. That is, dont_care!($ty) is equivalent to dont_care!($ty, $ty).

Warning: dont_care! is provided for convenience and may be removed in future versions of test-fuzz.

test-fuzz Package Features

The features in this section apply to the test-fuzz package as a whole. Enable them in test-fuzz's dependency specification as described in the The Cargo Book. For example, to enable the auto_concretize feature, use:

test-fuzz = { version = "1.0.0", features = ["auto_concretize"] }

The test-fuzz package currently supports the following features:

  • auto_concretize - When this feature is enabled, test-fuzz tries to infer impl and non-impl concretizations. Success requires that a target be called with exactly one impl concretization and exactly one non-impl concretization during tests. Success is not guaranteed by these conditions, however.

    The implementation of auto_concretize uses the unstable language feature proc_macro_span. So enabling auto_concretize requires that targets be built with a nightly compiler.

  • Serde formats - test-fuzz can serialize target arguments in multiple Serde formats. The following are the features used to select a format. Note that if a format other than the default is selected, then default-feature = false must be specified.

Auto-generated Corpus Files

cargo-test-fuzz can auto-generate values for types that implement certain traits. If all of a target's argument types implement such traits, cargo-test-fuzz can auto-generate corpus files for the target.

The traits that cargo-test-fuzz currently supports and the values generated for them are as follows:

Trait(s) Value(s)
Bounded T::min_value(), T::max_value()
Bounded + Add + One T::min_value() + T::one()
Bounded + Add + Div + Two T::min_value() / T::two() + T::max_value() / T::two()
Bounded + Add + Div + Two + One T::min_value() / T::two() + T::max_value() / T::two() + T::one()
Bounded + Sub + One T::max_value() - T::one()
Default T::default()


Environment Variables

  • TEST_FUZZ_LOG - During macro expansion:

    • If TEST_FUZZ_LOG is set to 1, write all instrumented fuzz targets and module definitions to standard output.
    • If TEST_FUZZ_LOG is set to a crate name, write that crate's instrumented fuzz targets and module definitions to standard output.

    This can be useful for debugging.

  • TEST_FUZZ_MANIFEST_PATH - When running a target from outside its package directory, find the package's Cargo.toml file at this location. One may need to set this environment variable when enable_in_production is used.

  • TEST_FUZZ_WRITE - Generate corpus files when not running tests for those targets for which enable_in_production is set.


  • Clonable arguments - A target's arguments must implement the Clone trait. The reason for this requirement is that the arguments are needed in two places: in a test-fuzz-internal function that writes corpus files, and in the body of the target function. To resolve this conflict, the arguments are cloned before being passed to the former.

  • Serializable / deserializable arguments - In general, a target's arguments must implement the serde::Serialize and serde::Deserialize traits, e.g., by deriving them. We say "in general" because test-fuzz knows how to handle certain special cases that wouldn't normally be serializable/deserializable. For example, an argument of type &str is converted to String when serializing, and back to a &str when deserializing. See also concretize and concretize_impl above.

  • Global variables - The fuzzing harnesses that test-fuzz implements do not initialize global variables. No general purpose solution for this problem currently exists. So, to fuzz a function that relies on global variables using test-fuzz, ad-hoc methods must be used.

Tips and tricks

  • #[cfg(test)] is not enabled for integration tests. If your target is tested only by integration tests, then consider using enable_in_production and TEST_FUZZ_WRITE to generate a corpus. (Note the warning accompanying enable_in_production, however.)

  • If you know the package in which your target resides, passing -p <package> to cargo test/cargo test-fuzz can significantly reduce build times.

  • Rust won't allow you to implement serde::Serialize for other repositories' types. But you may be able to patch other repositories to make their types serializeble. Also, cargo-clone can be useful for grabbing dependencies' repositories.