Race condition around compiler warnings

[Rich-Harris]

Hard to repro completely reliably, but there's some race condition that allows out of date compiler warnings to be shown. This issue is mostly a placeholder until I have time to look into it more closely

[tomoam]

I found a way to reproduce it.

1. open https://learn.svelte.dev/tutorial/slot-props
2. First, add let:item={row} to App.svelte
3. After that, add type {it} to FilterableList.svelte

warnings_.mov

---

Presumably, if making changes that cause require a reload, the iframe is not listening for events during the reload, and therefore cannot receive the svelte:warnings event.

[tomoam]

As I said above, if a user makes a change that requires the iframe to reload, the iframe is not listening for any events while reloading, which means that the iframe will not receive the svelte:warnings event.

To verify, I tried the following:

I added console.log to __client.js like this...

	import.meta.hot.on('svelte:warnings', (data) => {
+		console.group('svelte:warnings', data.normalizedFilename, data.allWarnings.length);
+		data.allWarnings.forEach((warnings) => console.log(warnings.message));
+		console.groupEnd();
		post({ type: 'warnings', data });
	});

...then built and previewed learn.svelte.dev.

First, for changes that do not require reloading:

not_require_reloading.mov

We can see that the warnings in the terminal match the warnings in the browser's console.

Second, for changes that require reloading:

require_reloading.mov

The terminal shows the latest warnings, which is the log output just before sending svelte:warnings.
On the other hand, the browser console does not show the warnings. Instead, we see that [HMR:Svelte] Reload.

I think this issue is probably inevitable as long as we use import.meta.hot.on('svelte:warnings', function). I will try to receive svelte compile warnings in some different way. Perhaps we can take advantage of Vite's stdout.