Implement the private attribute for assignments

[adsnaider]

This PR fixes #2299. When a variable is specified as [private] or is named with _* the variable will be hidden from just --variables and forbidden from being overridden. I purposefully didn't change the behavior of the evaluator on private variables since it's likely useful to be able to view the values of private variables for debugging & introspection.

[casey]

The [private] attribute on recipes doesn't prevent running private them, only listing them, and I'm inclined to think the [private] variable attribute should similar, i.e., only prevent listing them, but still allow overriding them.

[casey]

I would also filter them out of --evaluate. We can think about some kind of --all flag in a follow-up PR, which un-hides them. (--all should probably also allow listing all recipes, including private recipes)

[adsnaider]

I believe this is ready again. Haven't implemented the --all flag but the other changes should now be working properly and as you point out that flag can be added later

[casey]

Sorry I missed this the first time! I would remove the allow-private-variables setting, since I think an --all flag, which can be added later, is more useful and general.

[adsnaider]

Ah sure, I assumed that would be a backwards-incompatible change

[laniakea64]

Ah sure, I assumed that would be a backwards-incompatible change

Using the [private] attribute on variables is backwards-compatible, since it's an error to do that in current just, so it won't change behavior of any working justfiles. So no need for a setting for that.

But wouldn't making all underscore-prefixed variables private be backwards-incompatible, and therefore that aspect would need an opt-in setting?

[adsnaider]

@laniakea64 Yeah, that's what I thought. I could take out the _ prefix for private variables 🤷

[casey]

It could be argued either way, but I think making variables private if they start with _ is probably fine. It won't actually break any justfiles, just remove those variables from the output of just --evaluate and just --variables, which are, I think, mostly for human consumption.

[adsnaider]

Sorry I missed this the first time! I would remove the allow-private-variables setting, since I think an --all flag, which can be added later, is more useful and general.

Removed the setting @casey

[casey]

LGTM!

[casey]

Available in 1.35.0, just released: https://github.com/casey/just/releases/tag/1.35.0