Getting Started

So you want to get started with snapshot testing? Here is the 5 minute introduction to how to be successful with insta.

Installation

The recommended way is to add the dependency with cargo add:

cargo add --dev insta --features yaml

Alternatively edit your Cargo.toml manually and add insta as manual dependency:

[dev-dependencies]
insta = { version = "1.22.0", features = ["yaml"] }

And for an improved review experience it's recommended to install the cargo-insta tool:

cargo install cargo-insta

Note that this documentation prefers the YAML format which is why the yaml feature is proposed by default. If you do not want to use it, you can omit it.

Optional: Faster Runs

Insta benefits from being compiled in release mode, even as dev dependency. It will compile slightly slower once, but use less memory, have faster diffs and just generally be more fun to use. To achieve that, opt insta and similar (the diffing library) into higher optimization in your Cargo.toml:

[profile.dev.package.insta]
opt-level = 3

[profile.dev.package.similar]
opt-level = 3

Writing Tests

Insta snapshots reference values which are fundamentally strings. However the method by which these strings are generated can be split in three different ways:

For most real world applications the recommendation is to use YAML snapshots of serializable values. This is because they look best under version control and the diff viewer and support redactions. To use this enabled the yaml feature of insta.

The following example demonstrates a very simple test case:

fn split_words(s: &str) -> Vec<&str> {
    s.split_whitespace().collect()
}

#[test]
fn test_split_words() {
    let words = split_words("hello from the other side");
    insta::assert_yaml_snapshot!(words);
}

Reviewing Snapshots

The recommended flow is to run the tests once, have them fail and check if the result is okay. By default the new snapshots are stored next to the old ones with the extra .new extension. Once you are satisifed move the new files over. To simplify this workflow you can use cargo insta review which will let you interactively review them:

cargo insta review

You can run your tests as normal with cargo test but if you have multiple snapshot assertions in a single function you might want to use cargo insta test instead which collects all snapshot changes in one go.

cargo insta test
cargo insta review

The above can be combined into a single command as well:

cargo insta test --review

For more information see cargo-insta documentation.

Inline Snapshots

Snapshots can also be stored inline. In that case the format for the snapshot macros is assert_snapshot!(reference_value, @"snapshot"). The leading at sign (@) indicates that the following string is the reference value. cargo-insta will then update that string with the new value on review.

This is the example above with inline snapshots:

fn split_words(s: &str) -> Vec<&str> {
    s.split_whitespace().collect()
}

#[test]
fn test_split_words() {
    let words = split_words("hello from the other side");
    insta::assert_yaml_snapshot!(words, @"");
}

After the initial test failure you can run cargo insta review to accept the change. The file will then be updated automatically and the reference value will be placed in the macro invocation like this:

fn split_words(s: &str) -> Vec<&str> {
    s.split_whitespace().collect()
}

#[test]
fn test_split_words() {
    let words = split_words("hello from the other side");
    insta::assert_yaml_snapshot!(words, @r###"
    ---
    - hello
    - from
    - the
    - other
    - side
    "###);
}

Annotating Snapshots

Particularly when reviewing a lot of snapshots, it can be hard to tell if a snapshot is correct or not from the default information alone. Out of the box, insta will include the expression that was asserted on with the snapshot, but that is often insufficient. Take for instance a practical example from the MiniJinja template engine. MiniJinja uses snapshots to assert the behavior of the template engine. Out of the box this is what cargo insta would show:

Reviewing [1/1] minijinja@0.20.0:
Snapshot file: minijinja/tests/snapshots/test_templates__vm@getattr.txt.snap
Snapshot: vm@getattr
Source: minijinja/tests/test_templates.rs:56
Input file: minijinja/tests/inputs/getattr.txt
──────────────────────────────────────────────────────────────────────────────────
template.render(ctx)
──────────────────────────────────────────────────────────────────────────────────
-old snapshot
+new results
────────────┬─────────────────────────────────────────────────────────────────────
    0       │-name: Peter
          0 │+name:
    1     1 │ active: true
────────────┴─────────────────────────────────────────────────────────────────────

It would be completely impossible for a reviewer to assess this snapshot without consulting the source code of the assertion. However by using with_settings! it is possible to provide additional information in the form of a description (a text field) and info which is a structured value. The former takes a string, the latter a serializable value.

insta::with_settings!({
    info => &ctx, // the template context
    description => source, // the template source code
    omit_expression => true // do not include the default expression
}, {
    insta::assert_snapshot!(template.render(ctx));
});

With this modification, the review dialog in cargo insta looks like this now:

Reviewing [1/1] minijinja@0.20.0:
Snapshot file: minijinja/tests/snapshots/test_templates__vm@getattr.txt.snap
Snapshot: vm@getattr
Source: minijinja/tests/test_templates.rs:56
Input file: minijinja/tests/inputs/getattr.txt
──────────────────────────────────────────────────────────────────────────────────
name: {{ user.name }}
active: {{ user.is_active }}
──────────────────────────────────────────────────────────────────────────────────
user:
  is_active: true
  username: Peter
──────────────────────────────────────────────────────────────────────────────────
-old snapshot
+new results
────────────┬─────────────────────────────────────────────────────────────────────
    0       │-name: Peter
          0 │+name:
    1     1 │ active: true
────────────┴─────────────────────────────────────────────────────────────────────

Now it's more obvious about why the snapshot is failing and what it should be. In this case we can clearly see that there is a mismatch between username and name between context and template.

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