Snapshot types

Insta has two major types of snapshots: file and inline snapshots. The former are placed as separate .snap files, the latter are stored inline in string literals in the .rs files.

File 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. Snapshots can either be explicitly named or the name is derived from the test name.

The format of snapshot files is documented here.

File snapshots

Unnamed snapshots

All snapshot assertion functions let you leave out the snapshot name in which case the snapshot name is derived from the function name (with an optional leading test_ prefix removed, as it is typically the test name).

These are two assertions that use implicitly named snapshots:

#[test]
fn test_something() {
    assert_snapshot!("first value");
    assert_snapshot!("second value");
}

This creates two snapshots named something and something-2.

Named snapshots

Explicit snapshot naming can be useful to be more explicit when multiple snapshots are tested within one function as the default behavior would be to just count up the snapshot names.

To provide an explicit name provide the name of the snapshot as first argument to the macro:

#[test]
fn test_something() {
    assert_snapshot!("first_snapshot", "first value");
    assert_snapshot!("second_snapshot", "second value");
}

This will create two snapshots: first_snapshot for the first value and second_snapshot for the second value.

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:

#[derive(Serialize)]
pub struct User {
    username: String,
}

assert_yaml_snapshot!(User {
    username: "john_doe".to_string(),
}, @"");

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. Note that inline snapshots require the use of cargo-insta.

Debug expressions

Snapshots optionally accept a debug expression, which can be helpful for adding contextual information to the snapshot. Where this isn't provided, insta defaults to the stringified expression.

#[test]
fn test_something() {
    // Will use tha value of `description_of_1`
    assert_snapshot!("snapshot", description_of_1, expr_1);
    // No debug expr — will use the literal `"expr_1"` as the debug expr
    assert_snapshot!("snapshot_no_debug", expr_1);
}

Snapshot types list

Here's a full list of formats that snapshots macros accept: