Insta Snapshots

This skill guides you through working with insta snapshot tests in the Oxc codebase without requiring terminal interaction.

What are Insta Snapshots?

Insta is a snapshot testing library for Rust. Oxc uses it extensively for:

• Linter rule tests (crates/oxc_linter/src/snapshots/)
• Semantic analysis tests (crates/oxc_semantic/tests/integration/snapshots/)
• Other crate-specific snapshot tests

Snapshots track expected test outputs (often failures or errors). When code changes, new snapshots are generated as .snap.new files for review.

Running Tests and Generating Snapshots

Run tests for a specific crate:

cargo test -p <crate_name>

This generates .snap.new files if test outputs have changed.

Reviewing Snapshots Non-Interactively

IMPORTANT: Avoid using cargo insta review (the interactive terminal UI). Instead, follow these steps:

1. List all pending snapshots

cargo insta pending-snapshots
# Or for workspace-wide:
cargo insta pending-snapshots --workspace

This shows all .snap.new files waiting for review.

2. Read the snapshot files directly

New snapshots are stored as .snap.new files next to their corresponding .snap files. You can use cargo insta pending-snapshots to view the changes to the snapshot files.

3. Accept or reject changes

Accept all pending snapshots:

cargo insta accept
# Or workspace-wide:
cargo insta accept --workspace

Accept specific snapshot(s):

cargo insta accept --snapshot <snapshot_name>

Reject all pending snapshots:

cargo insta reject
# Or workspace-wide:
cargo insta reject --workspace

This deletes all .snap.new files.

4. Verify the changes

After accepting, the .snap.new files become .snap files. Check with git:

git diff <path/to/snapshots/>

Common Workflows

After fixing a bug or adding new snapshot tests:

1. Run tests: cargo test -p oxc_linter (or relevant crate)
2. Check pending: cargo insta pending-snapshots
3. Read new snapshots to verify they match expected behavior
4. Accept if correct: cargo insta accept
5. Commit the updated .snap files

Working with specific test files:

1. Run: cargo test -p oxc_linter specific_test_name
2. Read: cargo insta pending-snapshots - look for specific_test_name.snap.new
3. Accept: cargo insta accept -p oxc_linter

Snapshot File Formats

• .snap - Current expected output
• .snap.new - New output from recent test run (pending review), do not commit if these files are present

Note that .snap.md files are from Vitest, and not Insta. They are used in conformance tests, and are not handled by the instructions in this skill.

Tips

• Always review snapshots manually before accepting - don't blindly accept all changes
• Snapshot files are checked into git - they're part of the test suite
• Large snapshot changes may indicate breaking changes or bugs
• Use git diff after accepting to see what actually changed
• Use the CLI commands. DO NOT just copy-paste or manually edit .snap files.

Example: Complete Workflow

# 1. Make a code change to a linter rule
# 2. Run tests
cargo test -p oxc_linter

# 3. See what changed
cargo insta pending-snapshots

# 4. Review specific snapshot (using Read tool)
# Read: crates/oxc_linter/src/snapshots/some_test.snap.new

# 5. Accept if correct
cargo insta accept -p oxc_linter

# 6. Verify with git
git diff crates/oxc_linter/src/snapshots/

# 7. Run tests again to ensure everything passes
cargo test -p oxc_linter

When to use this skill: When tests fail due to snapshot mismatches or after changing code that affects test outputs.