Allow updating snapshots for individual tests or individual assertions

[ninevra]

What you're trying to do

Example use case:

• a developer writes tests using t.snapshot() assertions
• a regression or external change causes such a test to begin failing
• the developer marks the test .failing(), expecting the issue to remain for some time
• later, the developer makes changes to other tests in the same file, and runs --update-snapshots to record the changes
• this incidentally updates the snapshots for the .failing() test, causing it to begin passing, even though the underlying issue remains
• developer is now stuck: they have no way to update snapshots for just part of a file (not even manually, because AVA's snapshot format is an implementation detail)

Why you can't use AVA for this

AVA appears to provide no way to select individual tests or assertions for update. Selecting files can be accomplished by passing AVA a glob, but using --match or line number selection with --update-snapshots throws.

Fundamentally, this is because AVA implements --update-snapshots by entirely ignoring and overwriting the snapshot files. If AVA allowed users to select specific tests, then every other test in their files would have its snapshots deleted.

And maybe how you think AVA could handle this

From a ui perspective, AVA could handle this by providing an interactive --review-snapshots option, like the similar features in insta (implemented as a cli tool, cargo insta review) or jest (seemingly only available during watch mode).

AVA could also allow the use of --match and other existing forms of selection with --update-snapshots, but this would only allow per-test granularity, not per-snapshot.

From an implementation perspective, AVA would probably need to read snapshot files from disk even when run with --update-snapshots, so that the unselected tests/assertions could retain their preexisting snapshots.

It might be possible to implement this using AVA's currently-unused ability to append updates to .snap files:

https://github.com/avajs/ava/blob/3fe4c40f09fd8eef74ea998992a756525fca7b9f/lib/snapshot-manager.js#L184-L186

https://github.com/avajs/ava/blob/3fe4c40f09fd8eef74ea998992a756525fca7b9f/lib/snapshot-manager.js#L298-L299

but I think that would require (backwards-compatible!) changes to the snapshot file format.

[novemberborn]

If we knew the test was failing because of the snapshot, then we could retain that failing snapshot while updating everything else. We'd have to be able to regenerate the Markdown file based on the binary snapshot serialization, which should be possible.