[][src]Crate insta

insta: a snapshot testing library for Rust

How it Operates

This crate exports multiple macros for snapshot testing:

  • assert_snapshot_matches! for comparing basic string snapshots.
  • assert_debug_snapshot_matches! for comparing Debug outputs of values.
  • assert_yaml_snapshot_matches! for comparing YAML serialized output of types implementing serde::Serialize.
  • assert_ron_snapshot_matches! for comparing RON serialized output of types implementing serde::Serialize.
  • assert_json_snapshot_matches! for comparing JSON serialized output of types implementing serde::Serialize.

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.

Additionally snapshots can also be stored inline. In that case the cargo-insta tool is necessary. See inline snapshots for more information.

For macros that work with serde::Serialize this crate also permits redacting of partial values. See redactions for more information.

Example

Install insta and cargo-insta:

This example is not tested
$ cargo add --dev insta
$ cargo install cargo-insta
This example is not tested
use insta::assert_debug_snapshot_matches;

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

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. You can also use cargo insta review which will let you interactively review them:

This example is not tested
$ cargo test
$ cargo insta review

For more information on updating see Snapshot Updating.

Snapshot files

The committed snapshot files will have a header with some meta information that can make debugging easier and the snapshot:

This example is not tested
---
created: "2019-01-21T22:03:13.792906+00:00"
creator: insta@0.3.0
expression: "&User{id: Uuid::new_v4(), username: \"john_doe\".to_string(),}"
source: tests/test_redaction.rs
---
[
    1,
    2,
    3
]

Snapshot Updating

During test runs snapshots will be updated according to the INSTA_UPDATE environment variable. The default is auto which will write all new snapshots into .snap.new files if no CI is detected.

INSTA_UPDATE modes:

  • auto: the default. no for CI environments or new otherwise
  • always: overwrites old snapshot files with new ones unasked
  • new: write new snapshots into .snap.new files.
  • no: does not update snapshot files at all (just runs tests)

When new is used as mode the cargo-insta command can be used to review the snapshots conveniently:

This example is not tested
$ cargo install cargo-insta
$ cargo test
$ cargo insta review

"enter" or "a" accepts a new snapshot, "escape" or "r" rejects, "space" or "s" skips the snapshot for now.

For more information invoke cargo insta --help.

Test Assertions

By default the tests will fail when the snapshot assertion fails. However if a test produces more than one snapshot it can be useful to force a test to pass so that all new snapshots are created in one go.

This can be enabled by setting INSTA_FORCE_PASS to 1:

This example is not tested
$ INSTA_FORCE_PASS=1 cargo test --no-fail-fast

A better way to do this is to run cargo insta test --review which will run all tests with force pass and then bring up the review tool:

This example is not tested
$ cargo insta test --review

Redactions

For all snapshots created based on serde::Serialize output insta supports redactions. This permits replacing values with hardcoded other values to make snapshots stable when otherwise random or otherwise changing values are involved.

Redactions can be defined as the third argument to those macros with the syntax { selector => replacement_value }.

The following selectors exist:

  • .key: selects the given key
  • ["key"]: alternative syntax for keys
  • [index]: selects the given index in an array
  • []: selects all items on an array
  • [:end]: selects all items up to end (excluding, supports negative indexing)
  • [start:]: selects all items starting with start
  • [start:end]: selects all items from start to end (end excluding, supports negative indexing).
  • .*: selects all keys on that depth

Example usage:

This example is not tested
#[derive(Serialize)]
pub struct User {
    id: Uuid,
    username: String,
    extra: HashMap<String, String>,
}

assert_yaml_snapshot_matches!("user", &User {
    id: Uuid::new_v4(),
    username: "john_doe".to_string(),
    extra: {
        let mut map = HashMap::new();
        map.insert("ssn".to_string(), "123-123-123".to_string());
        map
    },
}, {
    ".id" => "[uuid]",
    ".extra.ssn" => "[ssn]"
});

Inline Snapshots

Additionally snapshots can also be stored inline. In that case the format for the snapshot macros is assert_snapshot_matches!(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.

Example:

This example is not tested
#[derive(Serialize)]
pub struct User {
    username: String,
}

assert_yaml_snapshot_matches!(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.

Macros

assert_debug_snapshot_matches

Assets a Debug snapshot.

assert_json_snapshot_matches

Assets a Serialize snapshot in JSON format.

assert_ron_snapshot_matches

Assets a Serialize snapshot in RON format.

assert_serialized_snapshot_matches[
Deprecated
]

Alias for assert_yaml_snapshot_matches.

assert_snapshot_matches

Assets a string snapshot.

assert_yaml_snapshot_matches

Assets a Serialize snapshot in YAML format.

Structs

MetaData

Snapshot metadata information.

Snapshot

A helper to work with stored snapshots.