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new 0.1.0-alpha.15 May 3, 2021
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Used in cargo-test-fuzz


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At a high-level, test-fuzz is a convenient front end for afl.rs. 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. Environment Variables
  5. Limitations


$ cargo install cargo-test-fuzz --version '>=0.1.0-alpha'


Fuzzing with test-fuzz is essentially three easy steps:

  1. Identify a fuzz target:

    • Add the following dependencies to the target crate's Cargo.toml file:
      serde = "1.0"
      test-fuzz = "0.1.0-alpha"
    • 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


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).


  • 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.

  • 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 {}
  • skip - Disable the use of the test_fuzz macro in which skip appears.

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

    #[test_fuzz(specialize = "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.

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

    impl<T: Clone + Debug + Serialize> for Foo {
        #[test_fuzz(specialize_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.

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


  • --backtrace - Display backtraces

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

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

  • --display-crashes - Display crashes

  • --display-queue - Display work queue

  • --exact - Target name is an exact name rather than a substring

  • --list - List fuzz targets

  • --no-instrumentation - Compile without instrumentation (for testing build process)

  • --no-run - Compile, but don't fuzz

  • --no-ui - Disable user interface

  • --persistent - Enable persistent mode fuzzing

  • --pretty-print - Pretty-print debug output when displaying/replaying

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

  • --replay-crashes - Replay crashes

  • --replay-queue - Replay work queue

  • --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


  • -- <args>... - Arguments for the fuzzer

  • -p, --package = <package> - Package containing fuzz target

  • --target = <target> - String that fuzz target's name must contain

  • --timeout <timeout> - Number of milliseconds to consider a hang when fuzzing or replaying (equivalent to -- -t <timeout> when fuzzing)

Environment Variables

  • TEST_FUZZ_LOG - During macro expansion, write instrumented fuzz targets and their associated 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 - If an argument is passed by reference to a target function, then the argument's type must implement the Clone trait. The reason for this requirement is that the referent is 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 argument is cloned before being passed to the former.

  • Serializable / deserializable arguments - In general, a target's arguments must implement the serde::Deserialize and serde::Serialize 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 specialize and specialize_impl above.

  • Global variables - The fuzzing harnesses that test-fuzz generates 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.


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