fix: add compiler error for each block mutations in runes mode

[trueadm]

Fixes #9400

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Latest commit: 9f6bf0d

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svelte	Patch

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[dummdidumm]

I vaguely remember that Rich wanted this kind of mutation to the each entry directly to be disallowed in runes mode - but I'm not sure if this was more due to the fact that this wasn't possible to achieve correctly before we added the proxy, so may be ok now. @Rich-Harris do you remember?

[trueadm]

Ah yeah. Well we can go that route if needed instead.

[trueadm]

Changed the PR to reflect that instead.

[dummdidumm]

I meant this in the sense of "back then we may have taken the stance that this should be disallowed, but now that we use proxies that might have changed, so your change is correct" and wanted to double-check with Rich because he might remember more. Let's wait on him to see whether or not this should be a compiler error or if this should work (personally I see no reason why it shouldn't work).

[trueadm]

We can always revert that commits lol

[Rich-Harris]

I think it should be disallowed, because otherwise things like this break down:

-{#each numbers as n}
+{#each numbers.filter((n) => n % 2 === 0) as n}
  <button onclick={() => n += 1}>{n}</button>
{/each}

It's better to enforce consistency (e.g. 'you have to do numbers[i] += 1') than to have something that is superficially more convenient, but which is pretty confusing on the face of it (it's basically like reassigning a function parameter, and yet it somehow works?) and which requires you to understand non-obvious properties of the system before you can wield it successfully.

It doesn't work in Svelte 4, FWIW, so we should disallow it regardless of runes mode. I think it would be better to do this work in AssignmentExpression and UpdateExpression validation visitors — lemme push up some code real quick

[Rich-Harris]

lol ok ignore the svelte 4 comment, i was doing onclick instead of on:click 🤦

[Rich-Harris]

though that said, this is a useful demo of 'works until it doesn't'

[Antonio-Bennett]

Hey @Rich-Harris what exactly doesn’t work in this demo/examples?