Insta

What is Snapshot Testing?

Snapshot testing is just a fancy term for asserting that something equals a reference value in a test. However the reference value is stored in a separate file and support is provided to update and diff this value.

Insta supports snapshotting the output of the Debug or serde::Serialize trait:

use insta::assert_debug_snapshot_matches;

#[test]
fn test_snapshots() {
    let value = vec![1, 2, 3];
    assert_debug_snapshot_matches!("snapshot_name", value);
}

API

This crate exports two basic macros for snapshot testing: assert_snapshot_matches! for comparing basic string snapshots and assert_debug_snapshot_matches! for snapshotting the debug print output of a type. Additionally if the serialization feature is enabled the assert_serialized_snapshot_matches! macro becomes available which serializes an object with serde to yaml before snapshotting.

Snapshots are stored in the snapshots folder right next to the test file where this is used. The name of the file is <module>__<name>.snap where the name of the snapshot has to be provided to the assertion macro.

To update the snapshots export the INSTA_UPDATE environment variable and set it to 1. The snapshots can then be committed.

• Documentation
• Crate

Workflow

The recommended flow is to run the tests once, have them fail and check if the result is okay. Once you are satisifed run the tests again with INSTA_UPDATE set to 1 and updates will be stored:

$ INSTA_UPDATE=1 cargo test

License

Insta is licensed under the Apache 2 license.