warn module variables are not reactive and make them truely non reactive

[tanhauhau]

Fixes #5843

• added warning when using module variables as reactive declaration dependencies
• remove module variable as reactive declaration dependencies, making it the variable truly non-reactive.
  • Fixing the inconsistent behavior as seen in this REPL

Before submitting the PR, please make sure you do the following

- [ ] It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs

• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

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

[Conduitry]

What do you mean by making them truly non-reactive? What behavior is changing here besides adding the compiler warning?

[tanhauhau]

so, if u look at the REPL,

the value of a is undefined, where b is "moduleB".

if you look at the compiled code, svelte tries to set the value of a inside the $$self.$$.update, and when it checks that if moduleA is dirty before setting a.

but, moduleA is not an instance variable, it will never be dirty, therefore, the value of a is undefined.

moduleB on the other hand is different, it will not change, therefore, the $: b = moduleB is treated as static reactive declaration, and the value of b is "moduleB".

so i suggest that, if the reactive declaration depends only on module variable, then it should be a static reactive declaration.

[dummdidumm]

Is that technically a breaking change?
Also: some people might know this and still use module variables in the context of reactive assignments (knowing that something else triggers the recalculation). Is it possible to silence this warning like you can silence other warnings?

[oodavid]

@tanhauhau does this PR cover the following? tested in 3.31.0

<script context="module">
  let foo;
</script>
<script>
  let foo;
</script>

In vanilla html you'd see Uncaught SyntaxError: Identifier 'foo' has already been declared

[Conduitry]

This has been released in 3.32.1 - https://svelte.dev/repl/7eb62137060d4d53abe5e6a8b45074d5?version=3.32.1

[istarkov]

Is it possible to disable this rule? As of now now impossible to reuse imported constants because of this warning.
ie.

<script context="module" lang="ts">
  import { CIRCLE_GAP } from '../../components/carousel/consts';
  console.info(CIRCLE_GAP);
</script>

<script lang="ts">
  import Filters from '../../components/filters.svelte';
  $: {
    console.info(CIRCLE_GAP);
  }
</script>

gives "CIRCLE_GAP" is declared in a module script and will not be reactive.

Ok, constants can be not reactive.

[istarkov]

Same for heavy initializers at module level, how I can now init once something without this warning.

[istarkov]

I thought this should work <!-- svelte-ignore module-script-reactive-declaration -->,
but not.

[trmcnvn]

This warning also occurs when using exported functions.

https://svelte.dev/repl/7ddb4af47bbf4cdebbefcf5649e83ff1?version=3.32.1

[j3rem1e]

I have now a lot of warnings because of this rule.. Please add something to disable it globally.

[dummdidumm]

For svelte-check, do --compiler-warnings "module-script-reactive-declaration:ignore". A similar option is available for the VS Code extension.
I opened #5954 for a possibility to silence this through a comment.