another one

packages/svelte/src/compiler/phases/2-analyze/css/css-prune.js

@@ -825,9 +825,10 @@ function get_element_parent(node) {
 /**
  * @param {Compiler.AST.RegularElement | Compiler.AST.SvelteElement | Compiler.AST.RenderTag | Compiler.AST.Component | Compiler.AST.SvelteComponent | Compiler.AST.SvelteSelf} node
  * @param {boolean} adjacent_only
+ * @param {Set<Compiler.AST.SnippetBlock>} seen
  * @returns {Map<Compiler.AST.RegularElement | Compiler.AST.SvelteElement | Compiler.AST.SlotElement | Compiler.AST.RenderTag, NodeExistsValue>}
  */
-function get_possible_element_siblings(node, adjacent_only) {
+function get_possible_element_siblings(node, adjacent_only, seen = new Set()) {
 	/** @type {Map<Compiler.AST.RegularElement | Compiler.AST.SvelteElement | Compiler.AST.SlotElement | Compiler.AST.RenderTag, NodeExistsValue>} */
 	const result = new Map();
 	const path = node.metadata.path;
@@ -886,8 +887,11 @@ function get_possible_element_siblings(node, adjacent_only) {
 		}
 
 		if (current.type === 'SnippetBlock') {
+			if (seen.has(current)) break;
+			seen.add(current);
+
 			for (const site of current.metadata.sites) {
-				const siblings = get_possible_element_siblings(site, adjacent_only);
+				const siblings = get_possible_element_siblings(site, adjacent_only, seen);
 				add_to_map(siblings, result);
 
 				if (adjacent_only && current.metadata.sites.size === 1 && has_definite_elements(siblings)) {

packages/svelte/tests/css/samples/render-tag-loop/_config.js

@@ -0,0 +1,20 @@
+import { test } from '../../test';
+
+export default test({
+	warnings: [
+		{
+			code: 'css_unused_selector',
+			message: 'Unused CSS selector "div + div"',
+			start: {
+				line: 19,
+				column: 1,
+				character: 185
+			},
+			end: {
+				line: 19,
+				column: 10,
+				character: 194
+			}
+		}
+	]
+});

packages/svelte/tests/css/samples/render-tag-loop/expected.css

@@ -1,7 +1,10 @@
 
-	div.svelte-xyz div:where(.svelte-xyz) {
+	div div.svelte-xyz {
 		color: green;
 	}
+	/* (unused) div + div {
+		color: red; /* this is marked as unused, but only because we've written an infinite loop - worth fixing? *\/
+	}*/
 	div.svelte-xyz:has(div:where(.svelte-xyz)) {
 		color: green;
 	}

packages/svelte/tests/css/samples/render-tag-loop/input.svelte

@@ -1,10 +1,12 @@
 {#snippet a()}
+	{@render b()}
 	<div>
 		{@render b()}
 	</div>
 {/snippet}
 
 {#snippet b()}
+	{@render a()}
 	<div>
 		{@render a()}
 	</div>
@@ -14,6 +16,9 @@
 	div div {
 		color: green;
 	}
+	div + div {
+		color: red; /* this is marked as unused, but only because we've written an infinite loop - worth fixing? */
+	}
 	div:has(div) {
 		color: green;
 	}