feat: disallow calling getContext inside element

[tanhauhau]

Fixes #7549

Throwing error when using getContext inside element, eg: attribute, text content, ...

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Tests

• Run the tests with npm test and lint the project with npm run lint

[baseballyama]

I think CI is actually failed.

[tanhauhau]

oh okay i'll take a look at it 🙈

[dummdidumm]

marked this as v4 because strictly speaking it's a breaking change

[vercel]

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[benmccann]

@tanhauhau if you get a chance, it looks like the tests will need to be updated on this one

[dummdidumm]

Wondering if we should just leave it be and not do this explicitly, instead stating in the docs that using getContext inside the markup is undefined behavior. That would free us from having to keep that check for Svelte 5, which may or may not be hard.

[benmccann]

If we added the check now, I don't think we'd be obligated to keep if for Svelte 5. For Svelte 4, it seems a bit safer than a warning the docs that people may or may not see.

[baseballyama]

I think this PR breaks {#await} block.

<script>
  let promise = Promise.resolve();
</script>

{#await promise}
  <p>waiting...</p>
{:then value}
  <p>then!</p>
{/await}

To fix this, I think we need variables scope analyzer and throw compile error if get_context is used in the template block.
But handling this by Svelte compiler is a bit difficult because we don't have a scope analyzer.

So to do this, an easier way is that we will implement this in ESLint plugin.

What do you think?

[dummdidumm]

Given that it looks to be really tricky to get this working properly I'm giving this the "one day" label. I think a note in the docs that you shouldn't use Svelte functions in the template would be enough.

[Rich-Harris]

Closing since we ended up going in the opposite direction: #11061