fix: prevent infinite loops when pruning CSS

Author: Rich-Harris

.changeset/itchy-timers-relax.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: prevent infinite loops when pruning CSS

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

@@ -43,6 +43,12 @@ const nesting_selector = {
 	}
 };
 
+/**
+ * Snippets encountered already (avoids infinite loops)
+ * @type {Set<Compiler.AST.SnippetBlock>}
+ */
+const seen = new Set();
+
 /**
  *
  * @param {Compiler.Css.StyleSheet} stylesheet
@@ -60,6 +66,8 @@ export function prune(stylesheet, element) {
 		ComplexSelector(node) {
 			const selectors = get_relative_selectors(node);
 
+			seen.clear();
+
 			if (
 				apply_selector(selectors, /** @type {Compiler.Css.Rule} */ (node.metadata.rule), element)
 			) {
@@ -193,6 +201,9 @@ function apply_combinator(relative_selector, parent_selectors, rule, node) {
 				const parent = path[i];
 
 				if (parent.type === 'SnippetBlock') {
+					if (seen.has(parent)) return true;
+					seen.add(parent);
+
 					for (const site of parent.metadata.sites) {
 						if (apply_combinator(relative_selector, parent_selectors, rule, site)) {
 							return true;

packages/svelte/src/compiler/phases/2-analyze/visitors/shared/component.js

@@ -51,7 +51,7 @@ export function visit_component(node, context) {
 			if (binding?.initial?.type === 'SnippetBlock') {
 				node.metadata.snippets.add(binding.initial);
 			}
-		} else {
+		} else if (expression.type !== 'Literal') {
 			resolved = false;
 		}
 	}

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

@@ -0,0 +1,4 @@
+
+	div.svelte-xyz div:where(.svelte-xyz) {
+		color: red;
+	}

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

@@ -0,0 +1,17 @@
+{#snippet a()}
+	<div>
+		{@render b()}
+	</div>
+{/snippet}
+
+{#snippet b()}
+	<div>
+		{@render a()}
+	</div>
+{/snippet}
+
+<style>
+	div div {
+		color: red;
+	}
+</style>