Thank you for this report.
I didn't know that svelte-ignore comments can suppress runtime warnings. But eslint-plugin can't get runtime warnings statically, so I don't know how to solve that 😓
I hope someone has some good ideas 🤔

For now, I think the only workaround is to disable the svelte/no-unused-svelte-ignore rule.

If it's not a lint that the plugin concerns itself with, then I think the sensible thing is to whitelist it

This workaround does indeed work:

<!-- eslint-disable-next-line svelte/no-unused-svelte-ignore -->
<!-- svelte-ignore reactive-component -->

I think it would be a good idea to have a whitelist. Is it possible to create the whitelist?

Yeah I'm sure it could be done in here
https://github.com/sveltejs/eslint-plugin-svelte/blob/ad528c9b9136825cf2003206df105cc87153bdc6/packages/eslint-plugin-svelte/src/rules/no-unused-svelte-ignore.ts

I guess I didn't explain it well.
Is it possible to list rules to be whitelisted?
What rules should be listed besides the reactive-component rule? Or are there no rules other than reactive-component?

I'm not sure. I found this list of compiler warnings here (from Svelte 3), but I assume it's hard to tell which would be runtime warnings: https://github.com/sveltejs/svelte/blob/v3.59.2/src/compiler/compile/compiler_warnings.ts

Yeah. I wish we could figure out which ones are runtime warnings...

I think there's no easy way tbh

@baseballyama Do you have any ideas?

reactive-component is a compile-time warning, and we can actually get the warning when compiling with the Svelte Compiler without comments.
Therefore, I suspect that it’s simply a bug in eslint-plugin-svelte.

Also, I don’t think there’s a way to suppress runtime warnings using comments.
If the official documentation mentions such a method, could you please share the link?

I said it was a runtime warning, but that's wrong. It's actually a rendering warning.
However, this warning doesn't appear at compile() time, so svelte-check doesn't warn we about it either.

https://github.com/sveltejs/svelte/blob/d8b31333ec4afeab7dd178aedc7c187bfb971531/packages/svelte/src/compiler/compile/render_dom/wrappers/InlineComponent/index.js#L121

I confirmed that I could actually get the warning by running the following code after removing the comment.

import { compile } from 'svelte/compiler';
import { readFileSync } from 'fs';

const main = () => {
    const file = readFileSync('./src/routes/+page.svelte', {encoding: 'utf8'});
    console.log(compile(file))
}

main();

Hmm... something seems different from what I've tried 😓 I'll look into it some more.

It looks like I was trying with the following code, I forgot what generate: false meant so I'll look into it a bit more.

import { compile } from 'svelte/compiler';
import { readFileSync } from 'fs';

const main = () => {
    const file = readFileSync('./src/routes/+page.svelte', {encoding: 'utf8'});
    console.log(compile(file, {generate: false}))
}

main();

The documentation says that generate: false is useful for tooling.

Useful for tooling that is only interested in warnings.

https://svelte.dev/docs/svelte/svelte-compiler#ModuleCompileOptions

I think I probably used generate: false based on the code in eslint-plugin-svelte3.

https://github.com/sveltejs/eslint-plugin-svelte3/blob/92671bee83b1d38979e1b0f2897a987cbd61b808/src/preprocess.js#L245

Shouldn't we use generate: false? 🤔

The Svelte compiler mainly consists of three steps: parsing, validation, and transform, and it seems that this warning is generated during the transform step.
So, it might be necessary to set generate: true even though it comes with a performance cost.

The documentation says that generate: false is useful for tooling.

This is Svelte5 document, but the OP uses Svelte3/4.
(The same thing might happen in Svelte 5 as well, but I haven’t tested it yet.)

Just sharing: It seems that language-tools also uses generate: false.

https://github.com/sveltejs/language-tools/blob/52bf7d8461dcc5ff96771de656546a2d316885a1/packages/language-server/src/lib/documents/configLoader.ts#L38

I tried the same thing in Svelte 5, but no warning was emitted.
(It seems that the reactive_component warning doesn’t exist in Svelte 5.)

I think reactive-component is probably the only special warning emitted in the render (transform) phase in Svelte 4.

I think it would make sense for us to change the svelte/no-unused-svelte-ignore rule to ignore only reactive-component.

We might be able to lint correctly by removing generate: false, but I think that's probably not what Svelte core intends.