💀 Mixing legacy (which is default if not using runes) and runes can hang the browser

[xl0]

Describe the bug

This one will hang your browser. This happens when App.svelte does not use runes (and this considered legacy mode by default), and tries to bind a $state (imported from another module) to a component:
https://svelte.dev/playground/6f867371b04e403e9f1b0f0a5dd34553?version=5.46.0#H4sIAAAAAAAAA41Sy27bMBD8lQ0RwBIqW_BVlQQEveSeY9kDLa1gojRJiKvYgcB_L5Zy_GhcoDdxZpaameUsrDqgqMQrGuPg6EbTQ4a9JuxzUYhBGwyi-jkL-vCsY0AUn1Mv3m_COxpibKcCPsI7ZwktBVGJegEr50k7G2CcLAYoW2mlrUM3ak_8Tfrg3Ujwwx28s2gJhtEdYLUpL8j5-tWNelbex4tSeR9IEZ6FGwpJW5ZgkGCf4jbwnDTZKp1X-fcbyceVlkIK5ury6tHWV3c7bfvq1LCBTbopniM9rdcgbb3ftku_M_cTn-pyv22lZUpbP50veFdmwmaRpE7q3UTkLDjbGd39buYsh6aFzk2W4FsD25jKSmSoYE5EZJvLIP9ivW5FIQhPJCoaJ4zFP1b5d7P3C33A3q31ujquboYTl8ep1M5glkPksx-dD1nOzUh6xmHAjrIl08yQJB6TQoVeCgZinuDb3mvfdp9e6tK30v5fvK_P4T7gQ_4mYmdUCPDi_Rur2C9AoK9PJHI6PKUHyVUo76EBi8fLbMaye9O_CkFKm6O2vagGZQLGPx2NHDeZAwAA

Remove the first line to hang the browser (Chrome/Brave) in a tight effect loop, can't even open dev tools or reload the page.

Reproduction

Playground

Logs

System Info

Playground

Severity

annoyance

[paoloricciuti]

So the reason this happen is because in legacy mode imports are reactive...so if you change imported.x it will invalidate the whole imported.

Now since you are binding imported.x you are effectively reading imported and setting imported in the same effect.

That's a very weird one, not sure how to fix this but I'll give it a thought.

[xl0]

Maybe just make runes mode the default? People can still run into this if they explicitly use legacy mode, but there will be a lot fewer people and the failure would be more explicit, "weird stuff happens if I mix legacy and effect".

But also, can we do something to not hang the browser on recursive effect updates? Maybe a yield is needed, or detect and disable the effect with a big fat warning? I definitely ran into similar issues more than once, everything just hangs, making debugging or even page reload impossible.

[paoloricciuti]

Maybe just make runes mode the default?

That would be a breaking change

But also, can we do something to not hang the browser on recursive effect updates?

There's technically a check where if you loop for more than 1000 times it will error out and bail but I never found that to be super reliable...but if you open the devtools and then remove the line you'll see a bunch of those errors popping out. Not sure why that doesn't stop the computation. We can't always always bail because sometimes you really have more than 1000 loops for legit reasons.