fix: add warning when you read object property within attachments in legacy mode

[paoloricciuti]

This adds a warning when you are in legacy mode and you set the expression of an attachment reading from an object.

if you have a component like this

<script>
    let state = {
        count: 0,
        attachment: (node)=>{
            alert("attachment run");
        }
    };
</script>

<button onclick={()=>{
    state.count++;
}}>{state.count}</button>

<div {@attach state.attachment}></div>

and you don't realise you are in legacy mode the attachment will rerun every time you change count because you are updating state.count and in legacy mode that invalidate the whole state object (hence also state.attachment which means the attachment reruns).

While the above example is blatantly in legacy mode this is more subtle.

<script>
    import { State } from "some-lib";
    let state = new State();
</script>

<button onclick={()=>{
    state.count++;
}}>{state.count}</button>

<div {@attach state.attachment}></div>

This is up for discussion because i wonder if this warning could be too intrusive...an alternative @dummdidumm suggested is that we could opt in to runes mode if you use @attach...however this would be a breaking change (although probably not breaking for a lot of people) and since it's a new feature we can just label it as a bug. This would also hinder the adoption of attachments a bit and it would be cool if we could use them in legacy mode.

WDYT?

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[changeset-bot]

🦋 Changeset detected

Latest commit: 7d1e9f1

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-15948-svelte.vercel.app/

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@15948

[Rich-Harris]

Alternative: #15962

[Rich-Harris]

closing in favour of #15962