fix creating debugging comment for HTML comment (#4567)

CHANGELOG.md

@@ -1,5 +1,9 @@
 # Svelte changelog
 
+## Unreleased
+
+* Fix attaching of JS debugging comments to HTML comments ([#4565](https://github.com/sveltejs/svelte/issues/4565))
+
 ## 3.20.1
 
 * Fix compiler regression with slots ([#4562](https://github.com/sveltejs/svelte/issues/4562))

src/compiler/compile/render_dom/wrappers/shared/create_debugging_comment.ts

@@ -24,7 +24,7 @@ export default function create_debugging_comment(
 			while (source[d] !== '>') d += 1;
 			d += 1;
 		}
-	} else if (node.type === 'Text') {
+	} else if (node.type === 'Text' || node.type === 'Comment') {
 		d = node.end;
 	} else {
 		// @ts-ignore

test/runtime/samples/component-slot-fallback-4/Inner.svelte

@@ -0,0 +1,3 @@
+<slot name="a"><!-- placeholder--></slot>
+<slot name="b"><!-- placeholder--> </slot>
+<slot name="c"><!-- placeholder--> foobar </slot>

test/runtime/samples/component-slot-fallback-4/_config.js

@@ -0,0 +1,5 @@
+export default {
+	html: `
+	foobar
+	`,
+};

test/runtime/samples/component-slot-fallback-4/main.svelte

@@ -0,0 +1,5 @@
+<script>
+	import Inner from "./Inner.svelte";
+</script>
+
+<Inner></Inner>