fix: remove memory leak from bind:this

[trueadm]

This removes a nasty memory leak to do with bind:this. The PR also adds a runtime invariant for trying to create effects inside the teardown phase of an effect – as people shouldn't be creating new effects in something that's meant to be getting garbage collected.

[changeset-bot]

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Latest commit: 0b1a78a

The changes in this PR will be included in the next version bump.

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[dummdidumm]

Under what circumstance was this leaking memory? Always, because the effect is created in the cleanup phase of another effect?

[trueadm]

Under what circumstance was this leaking memory? Always, because the effect is created in the cleanup phase of another effect?

Yep. Plus it was an effect, so it built a relationship with the signals.

[Rich-Harris]

It really feels like creating effects inside teardown should be forbidden, even if that makes the implementation of bind:this slightly more finicky. Is there any place where it's necessary?

[trueadm]

@Rich-Harris Agreed. Updated PR to add error for that use-case.

[trueadm]

I've fixed the issues and came up with using the task queue instead which resolves of the issues and avoids us having to change that test.