Introduce `luau.collectgarbage` which runs `lua_gc`

[ffrostfall]

This is required for certain forms of automated testing, e.g. ensuring there are no references to something.

Luau forbids collectgarbage("collect") in sandboxed modes, but exposes lua_gc.

[aatxe]

cc @vegorov-rbx are there any significant concerns or considerations with this?

[vegorov-rbx]

I think the motivation is not great, GC is not as precise as one can imagine and tests will be flaky.

So in order to not shoot yourself in the foot, I advice not introducing this.

Consider luau-lang/luau#753 (comment)
and also luau-lang/luau#1917

[vegorov-rbx]

Also while the motivation is tests, what happens when it is placed outside the tests by people who don't know better.
The reason it's missing is not for sandboxing/security (all hackers know to just loop a coroutine creation to trigger a GC at a point they need), but so you don't bring your app to a halt by forcing full stop-the-world GC.
Generally it's best for GC to be a module you do not think about.

[ffrostfall]

Also while the motivation is tests, what happens when it is placed outside the tests by people who don't know better. The reason it's missing is not for sandboxing/security (all hackers know to just loop a coroutine creation to trigger a GC at a point they need), but so you don't bring your app to a halt by forcing full stop-the-world GC. Generally it's best for GC to be a module you do not think about.

Is there any way we can know that a value will be garbage collected, then?

It's pretty important to know that a reference to something is genuinely cleared. If we can get some kind of function to count references, then that would solve the use case.

[vegorov-rbx]

That function will be even worse as Luau doesn't use reference counting as a way to track and collect garbage (ARC).

[ffrostfall]

That function will be even worse as Luau doesn't use reference counting as a way to track and collect garbage (ARC).

Then do you have any alternative thoughts?

I'd like to be able to ensure my library isn't leaking resources/keeping any references around accidentally.