QuickCheck is a way to do property based testing using randomly generated input. This crate comes with the ability to randomly generate and shrink integers, floats, tuples, booleans, lists, strings, options and results. All QuickCheck needs is a property function—it will then randomly generate inputs to that function and call the property for each set of inputs. If the property fails (whether by a runtime error like index out-of-bounds or by not satisfying your property), the inputs are "shrunk" to find a smaller counter-example.
The shrinking strategies for lists and numbers use a binary search to cover the input space quickly. (It should be the same strategy used in Koen Claessen's QuickCheck for Haskell.)
Dual-licensed under MIT or the UNLICENSE.
The API is fully documented: https://docs.rs/quickcheck.
Here's an example that tests a function that reverses a vector:
extern crate quickcheck;
This example uses the
quickcheck! macro, which is backwards compatible with
old versions of Rust.
To make it easier to write QuickCheck tests, the
will convert a property function into a
To use the
#[quickcheck] attribute, you must import the
extern crate quickcheck; extern crate quickcheck_macros;
quickcheck is on
crates.io, so you can include it in your project like so:
 = "1"
If you're only using
quickcheck in your test code, then you can add it as a
development dependency instead:
 = "1"
If you want to use the
#[quickcheck] attribute, then add
 = "1" = "1"
N.B. When using
quickcheck (either directly or via the attributes),
info! so that it shows useful output
(like the number of tests passed). This is not needed to show
witnesses for failures.
"use_logging": (Enabled by default.) Enables the log messages governed
"regex": (Enabled by default.) Enables the use of regexes with
Minimum Rust version policy
This crate's minimum supported
rustc version is
The current policy is that the minimum Rust version required to use this crate
can be increased in minor version updates. For example, if
crate 1.0 requires
Rust 1.20.0, then
crate 1.0.z for all values of
z will also require Rust
1.20.0 or newer. However,
crate 1.y for
y > 0 may require a newer minimum
version of Rust.
In general, this crate will be conservative with respect to the minimum supported version of Rust.
With all of that said, currently,
rand is a public dependency of
quickcheck. Therefore, the MSRV policy above only applies when it is more
rand's MSRV policy. Otherwise,
quickcheck will defer to
rand's MSRV policy.
In general, this crate considers the
Arbitrary implementations provided as
implementation details. Strategies may or may not change over time, which may
cause new test failures, presumably due to the discovery of new bugs due to a
new kind of witness being generated. These sorts of changes may happen in
semver compatible releases.
Alternative Rust crates for property testing
proptest crate is inspired by the
Hypothesis framework for Python.
You can read a comparison between
proptest improves on the concept of shrinking. So if you've
ever had problems/frustration with shrinking in
might be worth a try!
Alternatives for fuzzing
Discarding test results (or, properties are polymorphic!)
Sometimes you want to test a property that only holds for a subset of the possible inputs, so that when your property is given an input that is outside of that subset, you'd discard it. In particular, the property should neither pass nor fail on inputs outside of the subset you want to test. But properties return boolean values—which either indicate pass or fail.
To fix this, we need to take a step back and look at the type of the
quickcheck can test any value with a type that satisfies the
trait. Great, so what is this
This trait states that a type is testable if it can produce a
given a source of randomness. (A
TestResult stores information about the
results of a test, like whether it passed, failed or has been discarded.)
bool satisfies the
But in the example, we gave a function to
quickcheck. Yes, functions can
Which says that a function satisfies
Testable if and only if it has a single
parameter type (whose values can be randomly generated and shrunk) and returns
any type (that also satisfies
Testable). So a function with type
fn(usize) -> bool satisfies
So to discard a test, we need to return something other than
bool. What if we
just returned a
TestResult directly? That should work, but we'll need to
Now we can test functions that return a
As an example, let's test our reverse function to make sure that the reverse of a vector of length 1 is equal to the vector itself.
(A full working program for this example is in
So now our property returns a
TestResult, which allows us to encode a bit
more information. There are a few more
convenience functions defined for the
For example, we can't just return a
bool, so we convert a
bool value to a
(The ability to discard tests allows you to get similar functionality as
N.B. Since discarding a test means it neither passes nor fails,
will try to replace the discarded test with a fresh one. However, if your
condition is seldom met, it's possible that
quickcheck will have to settle
for running fewer tests than usual. By default, if
quickcheck can't find
100 valid tests after trying
10,000 times, then it will give up.
These parameters may be changed using
or by setting the
There is also
QUICKCHECK_MIN_TESTS_PASSED which sets the minimum number of
valid tests that need pass (defaults to
0) in order for it to be considered a
Shrinking is a crucial part of QuickCheck that simplifies counter-examples for your properties automatically. For example, if you erroneously defined a function for reversing vectors as: (my apologies for the contrived example)
And a property to test that
xs == reverse(reverse(xs)):
Then without shrinking, you might get a counter-example like:
[quickcheck] TEST FAILED. Arguments: ([-17, 13, -12, 17, -8, -10, 15, -19, -19, -9, 11, -5, 1, 19, -16, 6])
Which is pretty mysterious. But with shrinking enabled, you're nearly guaranteed to get this counter-example every time:
[quickcheck] TEST FAILED. Arguments: ()
Which is going to be much easier to debug.
More Thorough Checking
Quickcheck uses random input to test, so it won't always find bugs that could be uncovered with a particular property. You can improve your odds of finding these latent bugs by spending more CPU cycles asking quickcheck to find them for you. There are a few different ways to do this, and which one you choose is mostly a matter of taste.
If you are finding yourself doing this sort of thing a
lot, you might also be interested in trying out
which runs in a loop by default.
Running in a Loop
One approach is to run your quickcheck properties in a loop that just keeps going until you tell it to stop or it finds a bug. For example, you could use a bash script such as the following one.
#!/usr/bin/bash while do if ; then fi done
One thing to note is that this script passes the
qc_ filter to
cargo test. This assumes that you've prefixed all your quickcheck
qc_. You could leave off the filter, but then
you would be running all your deterministic tests as well, which
would take time away from quickcheck!
Checking the return code and exiting is also important. Without that test, you won't ever notice when a failure happens.
Cranking the Number of Tests
Another approach is to just ask quickcheck to run properties more
times. You can do this either via the
method, or via the
QUICKCHECK_TESTS environment variable.
This will cause quickcheck to run for a much longer time. Unlike,
the loop approach this will take a bounded amount of time, which
makes it more suitable for something like a release cycle that
wants to really hammer your software.
Making Arbitrary Smarter
This approach entails spending more time generating interesting inputs in your implementations of Arbitrary. The idea is to focus on the corner cases. This approach can be tricky because programmers are not usually great at intuiting corner cases, and the whole idea of property checking is to take that burden off the programmer. Despite the theoretical discomfort, this approach can turn out to be practical.
It is very simple to generate structs in QuickCheck. Consider the following
example, where the struct
Point is defined:
In order to generate a random
Point instance, you need to implement
Arbitrary for the struct
Case study: The Sieve of Eratosthenes
The Sieve of Eratosthenes
is a simple and elegant way to find all primes less than or equal to
Briefly, the algorithm works by allocating an array with
N slots containing
booleans. Slots marked with
false correspond to prime numbers (or numbers
not known to be prime while building the sieve) and slots marked with
are known to not be prime. For each
n, all of its multiples in this array
are marked as true. When all
n have been checked, the numbers marked
are returned as the primes.
As you might imagine, there's a lot of potential for off-by-one errors, which makes it ideal for randomized testing. So let's take a look at my implementation and see if we can spot the bug:
Let's try it on a few inputs by hand:
sieve(3) => [2, 3] sieve(5) => [2, 3, 5] sieve(8) => [2, 3, 5, 7, 8] # !!!
Something has gone wrong! But where? The bug is rather subtle, but it's an easy one to make. It's OK if you can't spot it, because we're going to use QuickCheck to help us track it down.
Even before looking at some example outputs, it's good to try and come up with
some properties that are always satisfiable by the output of the function. An
obvious one for the prime number sieve is to check if all numbers returned are
prime. For that, we'll need an
All this is doing is checking to see if any number in
[2, sqrt(n)] divides
n with base cases for
Now we can write our QuickCheck property:
And finally, we need to invoke
quickcheck with our property:
A fully working source file with this code is in
The output of running this program has this message:
[quickcheck] TEST FAILED. Arguments: (4)
Which says that
sieve failed the
prop_all_prime test when given
n = 4.
Because of shrinking, it was able to find a (hopefully) minimal counter-example
for our property.
With such a short counter-example, it's hopefully a bit easier to narrow down
where the bug is. Since
4 is returned, it's likely never marked as being not
4 is a multiple of
2, its slot should be marked as
p = 2 on these lines:
for i in .filter
Ah! But does the
.. (range) operator include
n? Nope! This particular
operator is a half-open interval.
2*p..n range will never yield
n = 4. When we change this to
2*p..n+1, all tests pass.
In addition, if our bug happened to result in an index out-of-bounds error,
quickcheck can handle it just like any other failure—including
shrinking on failures caused by runtime errors.
But hold on... we're not done yet. Right now, our property tests that all
the numbers returned by
sieve are prime but it doesn't test if the list is
complete. It does not ensure that all the primes between
n are found.
Here's a property that is more comprehensive:
It tests that for each number between 0 and n, inclusive, the naive primality test yields the same result as the sieve.
Now, if we run it:
we see that it fails immediately for value n = 2.
[quickcheck] TEST FAILED. Arguments: (2)
If we inspect
sieve() once again, we see that we mistakenly mark
non-prime. Removing the line
marked = true; results in both properties
What's not in this port of QuickCheck?
I think I've captured the key features, but there are still things missing:
- Only functions with 8 or fewer parameters can be quickchecked. This
limitation can be lifted to some
N, but requires an implementation for each
- Functions that fail because of a stack overflow are not caught by QuickCheck. Therefore, such failures will not have a witness attached to them. (I'd like to fix this, but I don't know how.)
Coarbitrarydoes not exist in any form in this package. It's unlikely that it ever will.
Arbitraryis not implemented for closures. See issue #56 for more details on why.