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#218 in Procedural macros

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K9 - Rust Testing Library

Crates.io Docs.rs Rust CI


Available macros

  • assert_equal
  • assert_greater_than
  • assert_greater_than_or_equal
  • assert_lesser_than
  • assert_lesser_than_or_equal
  • assert_matches_regex
  • assert_err_matches_regex
  • assert_matches_snapshot
  • assert_matches_inline_snapshot
  • assert_ok
  • assert_err

See https://docs.rs/k9 for API documentation

Rust testing library that provides pretty assertion macros and snapshot testing.

Rust already provides a good built-in test runner and a set of assertion macros like assert! and assert_eq!. They work great for for quick unit tests, but once the codebase and test suites grows to a certain point it gets harder and harder to test things and keep tests readable.

This crate is aiming to solve two issues:

  • Provide better output when a test fails
  • Provide a set of assertion macros for non trivial testing use cases.

For example, when testing that two structs are equal using assert_eq! macro the output does not provide a lot of help in understanding why exactly this test failed.

#[derive(PartialEq, Debug)]
struct Person {
    name: &'static str,
    age: usize,

fn test_eq() {
    let person1 = Person {name: "Bob", age: 12 };
    let person2 = Person {name: "Alice", age: 20 };
    assert_eq!(person1, person2, "These two must be the same person!");

All we get is usually a wall of wite text collapsed into a single line and you have to find the difference between two structs yourself. Which becomes very time consuming when structs are 10+ fields.

---- eq::test_eq stdout ----
thread 'eq::test_eq' panicked at 'assertion failed: `(left == right)`
  left: `Person { name: "Bob", age: 12 }`,
 right: `Person { name: "Alice", age: 20 }`: These two must be the same person!', src/eq.rs:13:5

using k9::assert_equal macro improves this output and prints the difference between two structs:

use k9::assert_equal;
assert_equal!(person1, person2, "These two must be the same person!");


Non-equality based assertions

Testing equality is very simple and can definitely work for most of the cases, but one of the disadvantages of only using assert! and assert_eq! is the error messages when something fails. For example, if you're testing that your code produces valid URL

let url = generate_some_url();
assert_eq!(URL_REGEX.is_match(url), true);

What you get is

thread 'eq::test_eq3' panicked at 'assertion failed: `(left == right)`
  left: `false`,
 right: `true`', src/eq.rs:19:5

Which doesn't help much. Especially, if you're new to the code base, seeing things like expected 'true' but got 'false' will make you go and look at the code before you even know what the problem can be, which can be very time consuming.

What we probably want to see is:


Which gives us enough context on what the problem is and how to fix it without for us having to go and run/debug the test first.

Snapshot testing

When testing high level APIs some data structures that are returned by functions can become pretty large and manually testing every single field can become impossible

let response = make_api_call();
assert_eq!(response.field_a, "some value");
assert_eq!(response.field_b, "some value");
assert_eq!(response.field_c, "some value");
assert_eq!(response.field_d, "some value");
// 100 more response fields

This is very unmaintainable and even small refactoring will end up in changing a lot of tests manually.

Snapshot tests provide an automated way of testing that these large structs don't change their values over time, or, if they change, pinpoint to the exact difference between the value before code changes and after code changes.

Snapshot testing involves multiple stages. First thing you need to do when creating a new test is using assert_matches_snapshot! macro that expects a string argument

assert_matches_snapshot!(format!("{:#?}", response));

First time you run the tests it will fail, saying that there is no existing snapshot found in the project.

To create a snapshot you need to run tests with K9_UPDATE_SNAPSHOTS=1 environment variable

K9_UPDATE_SNAPSHOTS=1 cargo test

this will create a snapshot file for this test shat will hold the context of the passed string:

├── __k9_snapshots__
│   └── my_snapshot_test
│       └── snapshot_test.snap

These files are expected to be checked into the repository and go through a code review.

Then, after modifying your code (and potentially the value of returned response object) you can run the tests again. It will serialized a new response object into string and compare it with the previous (stored in the repo) snapshot while highlighting the difference


During this step you can examine whether the changes to the response object are intended or whether it's a newly introduced bug that needs to be fixed.

Once the snapshot looks correct, it can be updated again using K9_UPDATE_SNAPSHOTS=1 variable.

Inline Snapshots

Sometimes assert_matches_snapshot! has its limitations, for example, there can't be more than one snapshot per test, snapshot file needs to be opened separately to see its content and sometimes too many files are hard to maintain.

To simplify things a little bit, there is assert_matches_inline_snapshot! macro. Instead of storing snapshot string in a separate file it will update rust source file directly with a string literal containing snapshot.

use k9::*;


This will fail with an error saying that snapshot is missing. But running

K9_UPDATE_SNAPSHOTS=1 cargo test

will update the source code directly to:

use k9::*;

assert_matches_inline_snapshot!(dbg!("hello".chars().rev().collect::<String>(), "olleh);

And next time cargo test is run the tests will pass.



~64K SLoC