Snapshot Testing

Snapshot testing makes it easy to compare large outputs from a function. Instead of asserting against the raw output directly:

assert_eq!(my_really_large_6mb_string, my_func());

you save the expected output in a file and compare against that. But manually saving, loading, and comparing snapshot files is a lot of work! Snapshot testing frameworks make this process easy. In Rust, I have used insta:

assert_debug_snapshot!(my_func());

I don’t need to explicitly save or load snapshot files. The framework handles it automatically!

The workflow is:

1. Write a test and include the assert macro.
2. Since this is the first run, there is no snapshot file yet. The framework generates one.
3. [Optional] Modify the snapshot file as needed.
4. Run the tests again - now the framework will compare against the saved snapshot and the test will pass.
5. If there is a regression or intentional change to the output, running the tests will generate a new snapshot file. The old one is preserved so you can review the change.

Here is a small svg from insta’s doc showing the interactive review:

You may check my pull request on libsql-server which adds a bunch of snapshot tests.

Jane Street has a nice blog post on the same - What if writing tests was a joyful experience?.