Add ESP to --preview for stable

[felix-hilden]

A preview flag is added in #2751, which could also contain experimental string processing. So shall we convert ESP there? Or are we still thinking about making it stable very soon?

[JelleZijlstra]

I don't think there's enough time to turn it on by default in this month's release. We should include it in the --preview feature set though.

[felix-hilden]

Do we want to keep the existing ESP flag simultaneously with its --preview addition?

Personally, I'd want to remove it and a first stable release seems like a good excuse, but Jelle raised a good point about keeping compatibility on Discord. This could also affect the feature flag machinery implementation in #2752.

[Shivansh-007]

I would be fine with removing the ESP flag as we are doing a major stable release, so "breaking" a bit of compatibility is fine.

If we decide to keep it for compatibility reasons, we could deprecate the flag and add some sort of "warning" log when someone uses an ESP flag. The deprecation could either still preserve the flag's functionality or direct the user to the new --preview flag (without actually doing anything, just outputting a simple message).

[felix-hilden]

Hmm, redirecting users sounds even better! I'm thinking: catch the use of ESP and throw before doing anything with a message telling people to use --preview.

But a softer deprecation works as well!

[JelleZijlstra]

I disagree with the idea that "we're doing a stable release, so we can break things". We've spent too many years in "beta" to keep that distinction meaningful.

My preference is to keep the behavior of the existing flag the same, but deprecate it and fold it into --preview in a few releases.

[Shivansh-007]

catch the use of ESP and throw before doing anything with a message telling people to use --preview.

Yeah, that's what I meant 👍🏻

My preference is to keep the behavior of the existing flag the same, but deprecate it and fold it into --preview in a few releases.

By few releases, do we mean major releases or 22.X?

[felix-hilden]

I imagine yes, in a few minor releases. "In a few years" sounds too much 😄 So we'll do the soft deprecation! I'll work on it later today.

[JelleZijlstra]

There is no such thing as a major or minor release on this project. We use calendar versioning.

[felix-hilden]

Yeah, I meant the second number place, I don't really know what they are called in calver. Anyways, as you said, "in a couple of releases".