#assert #assertion #testing #test #tests


Fluent, easy-to-extend test assertion library

4 stable releases

1.3.0 Oct 2, 2021
1.2.0 Oct 2, 2021
1.1.0 Oct 2, 2021
1.0.0 Sep 30, 2021

#116 in Testing

MIT license

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Fluent, easy-to-extend testing assertions for Rust.

rassert uses The MIT License. Current crates.io version. Continuous Integration status.


  • Fluent Expectations. When writing tests, we strive for clarity and readability. rassert supports this goal by providing a fluent expectation interface allowing one to write assertions as close to natural language as possible. See Writing Assertions.
  • Human-friendly Failures. Instead of cryptic failure messages, rassert displays the tested expression, its location, the actual value and the expected value/condition on failure to aid debugging.
  • Pattern Matching Support. Use the expect_matches! macro to check if an expression matches some pattern, such as expect_matches!(result, Ok(..)).
  • Lazy and Soft Evaluation. Assertions are evaluated lazily, only when closing an expectation chain with a conclude_panic or conclude_result function call. In soft mode (turn on via the soft() call), xpectation chain evaluation does not stop on the first failure. See Writing Assertions.
  • Easy-to-Extend. Extend rassert with custom expectations easily and succinctly. See Custom Expectations.

Up and Running

First, add rassert to the dev-dependencies section of your Cargo.toml file:

rassert = "1"

Then simply import the prelude module in your tests:

mod tests {
    use rassert::prelude::*;

    fn rassert_works() {

Writing Assertions

Expectation Chains

In rassert one can write assertions in the form of expectation chains. Such chains allow for writing multiple expectations against the same expression. There are two so-called entry points with which one can start a chain:

  • expect!(expression)
    • Creates a new chain asserting against the provided expression.
  • expect_matches!(expression, pattern)
    • Creates a new chain from the provided expression and automatically adds an expectation to the chain, asserting that expression matches pattern.

Once a chain is started, one can subsequently call expectations on it, as follows:

let v = vec![10];


Note, that the and() call is not mandatory, as it only serves readability purposes.

Concluding Chains

Since rassert evaluates expectations lazily, a chain like the above one will do nothing. A chain will only assert the specified expectations when concluded:

// Will panic on a failed expectation.

// Will return Result<(), String> containing the error
// message on failure.
let res = expect!(&true)

Soft Mode

A chain can be put into soft mode by calling soft() prior to concluding the chain:

let v = vec![10];


Soft chains will not panic/return on the first failure, instead they will run each assertion and present a merged report of every failure that occurred.

Negating Expectations

One can negate a single subsequent expectation using the not() function:


If one wishes to negate additionals expectations, then not() has to be applied again.

Available Expectations

  • T
    • to
  • T where T: Debug + PartialEq
    • to_equal, to_be
    • to_not_equal, to_not_be
  • boolean
    • to_be_true
    • to_be_false
  • Option<T>
    • to_be_some
    • to_be_none
  • Option<T> where T: Debug + PartialEq
    • to_contain
  • Result<T, E>
    • to_be_ok
    • to_be_err˙
  • Result<T, E> where T: Debug + PartialEq
    • to_be_ok_with
  • Vec<T>
    • to_have_length
    • to_be_empty
    • to_be_non_empty
  • Vec<T> where T: Debug + PartialEq
    • to_contain

Custom Expectations

Custom expectations can be written as extension traits on the ExpectationChain type, provided by rassert. In what follows, we show how to write custom expectations through an example.

Let's assume, that we want to write an expectation against a custom struct, Pizza:

pub struct Pizza {
    pub flavor: String

In rassert, expectations are actually structs, following something like a Command pattern. Given, we want to check what flavor of Pizza we have, we can create something as follows:

use rassert::Expectation;

struct ExpectPizzaFlavor {
    expected_flavor: String

impl Expectation<Pizza> for ExpectPizzaFlavor {
    fn test(&self, actual: &Pizza) -> bool {

    fn message(&self, expression: &str, actual: &Pizza) -> String {
        format!("Expected {:?}\n  to have flavor {}\n  but had {}.", expression, self.expected_flavor, actual.flavor)

Implementing the Expectation trait comes with two functions:

  • fn(&self, actual) -> bool
    • The actual assertion which returns true on success and false otherwise. The actual parameter corresponds to the value being tested in the chain.
  • fn(&self, expression, actual) -> String
    • The message shown if the expectation fails. The expression parameter is the stringified expression argument of the expect!/expect_matches! macro.

Once we got our expectation struct written, we can finally extend the ExpectationChain type:

use rassert::ExpectationChain;

pub trait PizzaExpectationsExt<'a> {
    fn to_have_flavor(self, expected: &str) -> ExpectationChain<'a, Pizza>;

impl<'a> PizzaExpectationsExt for ExpectationChain<'a, Pizza> {
    fn to_have_flavor(self, expected: &str) -> ExpectationChain<'a, Pizza> {
        self.expecting(ExpectPizzaFlavor {
            expected_flavor: expected.to_owned()

The most important bits of the above snippet are the following:

  • Extension traits must take a generic lifetime parameter and use it in the ExpectationChain type returned from expectation functions. This parameter corresponds to the lifetime of the immutable reference held inside the chain. This refernece then refers to the actual tested value.
  • Expectation functions must take self since expectation chains are Consuming builders.
  • Expectation functions can extend the chain with a new expectation using the expecting() function. This function takes an Expectation which will be executed when concluding the chain. The fields of the expectation can be used to parameterize the actual assertion.

Then, using the above expectation is as easy as

let pizza = Pizza {
  flavor: "Hawaii".to_owned(),


The built-in expectations of the src/expectations directory also use the above facilities, therefore they serve as a great starting point for writing custom expectations.


Licensed under The MIT License

No runtime deps