Filter out Svelte's unknown class prop console warnings (#7291)

* Filter out Svelte's unexpected class prop console warnings

Astro's hydration code passes a `class` prop to Svelte components, inducing Svelte to log a warning about an unknown prop. Preempting this by exporting a `class` prop from the Svelte component isn't a viable workaround since `class` is a reserved identifier in JS.

This PR implements the console-filtering workaround suggested by @HiDeoo in #5665, borrowing the `useConsoleFilter` approach from the [preact integration](https://github.com/withastro/astro/blob/a1c0cbe604c9f91cdc421b5606aab574999eba01/packages/integrations/preact/src/server.ts#L72-L94).

It would probably be better to generalize console filtering so it could be shared across multiple integrations.

Ideally there would be a way to handle this in Svelte, but as was pointed out in the issue thread even they resort to [similar cringe-inducing hackery](https://github.com/sveltejs/kit/blob/master/packages/kit/src/runtime/client/client.js#L1974-L1996) in sveltekit.

* Only filter Svelte console warnings in dev builds

* Add changeset

* Fix lint error.

---------

Co-authored-by: bluwy <[email protected]>
Co-authored-by: Nate Moore <[email protected]>

.changeset/big-elephants-drive.md

@@ -0,0 +1,5 @@
+---
+'@astrojs/svelte': patch
+---
+
+Filter unknown `class` prop warnings

packages/integrations/svelte/client.js

@@ -1,13 +1,19 @@
 const noop = () => {};
 
+let originalConsoleWarning;
+let consoleFilterRefs = 0;
+
 export default (target) => {
 	return (Component, props, slotted, { client }) => {
 		if (!target.hasAttribute('ssr')) return;
 		const slots = {};
 		for (const [key, value] of Object.entries(slotted)) {
 			slots[key] = createSlotDefinition(key, value);
 		}
+
 		try {
+			if (import.meta.env.DEV) useConsoleFilter();
+
 			new Component({
 				target,
 				props: {
@@ -18,7 +24,10 @@ export default (target) => {
 				hydrate: client !== 'only',
 				$$inline: true,
 			});
-		} catch (e) {}
+		} catch (e) {
+		} finally {
+			if (import.meta.env.DEV) finishUsingConsoleFilter();
+		}
 	};
 };
 
@@ -51,3 +60,52 @@ function createSlotDefinition(key, children) {
 		noop,
 	];
 }
+
+/**
+ * Reduces console noise by filtering known non-problematic warnings.
+ *
+ * Performs reference counting to allow parallel usage from async code.
+ *
+ * To stop filtering, please ensure that there always is a matching call
+ * to `finishUsingConsoleFilter` afterwards.
+ */
+function useConsoleFilter() {
+	consoleFilterRefs++;
+
+	if (!originalConsoleWarning) {
+		originalConsoleWarning = console.warn;
+		try {
+			console.warn = filteredConsoleWarning;
+		} catch (error) {
+			// If we're unable to hook `console.warn`, just accept it
+		}
+	}
+}
+
+/**
+ * Indicates that the filter installed by `useConsoleFilter`
+ * is no longer needed by the calling code.
+ */
+function finishUsingConsoleFilter() {
+	consoleFilterRefs--;
+
+	// Note: Instead of reverting `console.warning` back to the original
+	// when the reference counter reaches 0, we leave our hook installed
+	// to prevent potential race conditions once `check` is made async
+}
+
+/**
+ * Hook/wrapper function for the global `console.warning` function.
+ *
+ * Ignores known non-problematic errors while any code is using the console filter.
+ * Otherwise, simply forwards all arguments to the original function.
+ */
+function filteredConsoleWarning(msg, ...rest) {
+	if (consoleFilterRefs > 0 && typeof msg === 'string') {
+		// Astro passes a `class` prop to the Svelte component, which
+		// outputs the following warning, which we can safely filter out.
+		const isKnownSvelteError = msg.endsWith("was created with unknown prop 'class'");
+		if (isKnownSvelteError) return;
+	}
+	originalConsoleWarning(msg, ...rest);
+}