created_at2020-10-12 15:44:50.407502
updated_at2024-05-10 15:03:33.272443
Samuel Moelius (smoelius)




test-fuzz is a Cargo subcommand and a collection of Rust macros to automate certain tasks related to fuzzing with, including:

  • 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. Overview
  3. Components
  4. test-fuzz package features
  5. Auto-generated corpus files
  6. Environment variables
  7. Limitations
  8. Tips and tricks
  9. License


Install cargo-test-fuzz and with the following command:

cargo install cargo-test-fuzz cargo-afl


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 = "*"
      test-fuzz = "*"
    • 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 foo

* An additional, preliminary step may be necessary following a reboot:

cargo afl system-config

Note that the above command runs sudo internally. Hence, you may be prompted to enter your password.


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.

generic_args = "parameters"

Use parameters as the target's type parameters when fuzzing. Example:

#[test_fuzz(generic_args = "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.

impl_generic_args = "parameters"

Use parameters as the target's Self type parameters when fuzzing. Example:

impl<T: Clone + Debug + Serialize> for Foo {
    #[test_fuzz(impl_generic_args = "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>, or of type &X to type Y using Y's implementation of the non-standard trait test_fuzz::FromRef<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" must be accompanied by certain implementations. If X implements Clone, then Y may implement the following:

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

If X does not implement Clone, then Y must implement the following:

impl test_fuzz::FromRef<X> for Y {
    fn from_ref(x: &X) -> Self {

Additionally, Y must implement the following (regardless of whether X implements Clone):

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

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


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.


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


Record the target's generic args 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_generic_args, then run cargo test followed by cargo test-fuzz --display generic-args. One of the resulting generic args might be usable as generic_args's parameters. Similarly, generic args resulting from cargo test-fuzz --display impl-generic-args might be usable as impl_generic_args's parameters.

Note, however, that just because a target was called with certain parameters during tests, it does not imply the target's arguments are serializable/deserializable when those parameters are used. The results of --display generic-args/--display impl-generic-args 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 foo --display corpus
  3. Fuzz target foo

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

    cargo test-fuzz foo --replay crashes


Usage: cargo test-fuzz [OPTIONS] [TARGETNAME] [-- <ARGS>...]

  [TARGETNAME]  String that fuzz target's name must contain
  [ARGS]...     Arguments for the fuzzer

      --backtrace                 Display backtraces
      --consolidate               Move one target's crashes, hangs, and work queue to its corpus; to
                                  consolidate all targets, use --consolidate-all
      --cpus <N>                  Fuzz using at most <N> cpus; default is all but one
      --display <OBJECT>          Display corpus, crashes, generic args, `impl` generic args, hangs,
                                  or work queue. By default, corpus uses an uninstrumented fuzz
                                  target; the others use an instrumented fuzz target. To display the
                                  corpus with instrumentation, use --display corpus-instrumented.
      --exact                     Target name is an exact name rather than a substring
      --exit-code                 Exit with 0 if the time limit was reached, 1 for other
                                  programmatic aborts, and 2 if an error occurred; implies --no-ui,
                                  does not imply --run-until-crash or --max-total-time <SECONDS>
      --features <FEATURES>       Space or comma separated list of features to activate
      --list                      List fuzz targets
      --manifest-path <PATH>      Path to Cargo.toml
      --max-total-time <SECONDS>  Fuzz at most <SECONDS> of time (equivalent to -- -V <SECONDS>)
      --no-default-features       Do not activate the `default` feature
      --no-instrumentation        Compile without instrumentation (for testing build process)
      --no-run                    Compile, but don't fuzz
      --no-ui                     Disable user interface
  -p, --package <PACKAGE>         Package containing fuzz target
      --persistent                Enable persistent mode fuzzing
      --pretty-print              Pretty-print debug output when displaying/replaying
      --replay <OBJECT>           Replay corpus, crashes, hangs, or work queue. By default, corpus
                                  uses an uninstrumented fuzz target; the others use an instrumented
                                  fuzz target. To replay the corpus with instrumentation, use
                                  --replay corpus-instrumented.
      --reset                     Clear fuzzing data for one target, but leave corpus intact; to
                                  reset all targets, use --reset-all
      --resume                    Resume target's last fuzzing session
      --run-until-crash           Stop fuzzing once a crash is found
      --slice <SECONDS>           If there are not sufficiently many cpus to fuzz all targets
                                  simultaneously, fuzz them in intervals of <SECONDS> [default:
      --test <NAME>               Integration test containing fuzz target
      --timeout <TIMEOUT>         Number of seconds to consider a hang when fuzzing or replaying
                                  (equivalent to -- -t <TIMEOUT * 1000> when fuzzing)
      --verbose                   Show build output when displaying/replaying
  -h, --help                      Print help
  -V, --version                   Print version

Try `cargo afl fuzz --help` to see additional fuzzer options.

Convenience functions and macros

Warning: These utilties are excluded from semantic versioning and may be removed in future versions of test-fuzz.


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) -> std::result::Result<S::Ok, S::Error>
        S: serde::Serializer,

impl<'de> serde::Deserialize<'de> for $ty {
    fn deserialize<D>(deserializer: D) -> std::result::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).


The leak! macro can help to serialize target arguments that are references and whose types implement the ToOwned trait. It is meant to be used with the convert option.

Specifically, an invocation of the following form declares a type LeakedX, and implements the From and test_fuzz::Into traits for it:

leak!(X, LeakedX);

One can then use LeakedX with the convert option as follows:

#[test_fuzz::test_fuzz(convert = "&X, LeakedX")

An example where X is Path appears in in this repository.

More generally, an invocation of the form leak!($ty, $ident) expands to the following:

#[derive(Clone, std::fmt::Debug, serde::Deserialize, serde::Serialize)]
struct $ident(<$ty as ToOwned>::Owned);

impl From<&$ty> for $ident {
    fn from(ty: &$ty) -> Self {

impl test_fuzz::Into<&$ty> for $ident {
    fn into(self) -> &'static $ty {

serialize_ref / deserialize_ref

serialize_ref and deserialize_ref function similar to leak!, but they are meant to be used wth Serde's serialize_with and deserialize_with field attributes (respectively).

fn serialize_ref<S, T>(x: &&T, serializer: S) -> Result<S::Ok, S::Error>
    S: serde::Serializer,
    T: serde::Serialize,
    <T as serde::Serialize>::serialize(*x, serializer)

fn deserialize_ref<'de, D, T>(deserializer: D) -> Result<&'static T, D::Error>
    D: serde::Deserializer<'de>,
    T: serde::de::DeserializeOwned + std::fmt::Debug,
    let x = <T as serde::de::Deserialize>::deserialize(deserializer)?;

serialize_ref_mut / deserialize_ref_mut

serialize_ref_mut and deserialize_ref_mut are similar to serialize_ref and deserialize_ref (respectively), expect they operate on mutable references instead of immutable ones.

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 self_ty_in_mod_name feature, use:

test-fuzz = { version = "*", features = ["self_ty_in_mod_name"] }

The test-fuzz package currently supports the following features:


Use cast_checks to automatically check target functions for invalid casts.


Incorporate an impl's Self type into the names of modules generated for the impl. Expansion of the test_fuzz macro adds a module definition to the enclosing scope. By default, the module is named target_fuzz, where target is the name of the target. If the target appears in an impl block, then use of this feature causes the module to instead be named path_target_fuzz, where path is the path of the impl's Self type converted to snake case and joined with _. (See also rename above.)

In a future version of test-fuzz, this behavior will be the default.

Serde formats

test-fuzz can serialize target arguments in multiple Serde formats. The following are the features used to select a format.

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


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.


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.


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 generic_args and impl_generic_args above.

Global variables

The fuzzing harnesses that test-fuzz implements do not initialize global variables. While execute_with provides some remedy, it is not a complete solution. In general, fuzzing a function that relies on global variables requires ad-hoc methods.

convert and generic_args / impl_generic_args

These options are incompatible in the following sense. If a fuzz target's argument type is a type parameter, convert will try to match the type parameter, not the type to which the parameter is set. Supporting the latter would seem to require simulating type substitution as the compiler would perform it. However, this is not currently implemented.

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. Similarly, if you know your target is called from only one integration test, passing --test <name> can 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 serializable. Also, cargo-clone can be useful for grabbing dependencies' repositories.

  • Serde attributes can be helpful in implementing serde::Serialize/serde::Deserialize for difficult types.


test-fuzz is licensed and distributed under the AGPLv3 license with the Macros and Inline Functions Exception. In plain language, using the test_fuzz macro, the test_fuzz_impl macro, or test-fuzz's convenience functions and macros in your software does not require it to be covered by the AGPLv3 license.

Commit count: 830

cargo fmt