error early on invalid nesting selector

packages/svelte/src/compiler/errors.js

@@ -107,7 +107,8 @@ const css = {
 	'invalid-css-global-selector-list': () =>
 		`:global(...) must not contain type or universal selectors when used in a compound selector`,
 	'invalid-css-selector': () => `Invalid selector`,
-	'invalid-css-identifier': () => 'Expected a valid CSS identifier'
+	'invalid-css-identifier': () => 'Expected a valid CSS identifier',
+	'invalid-nesting-selector': () => `Nesting selectors can only be used inside a rule`
 };
 
 /** @satisfies {Errors} */

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

@@ -132,6 +132,12 @@ const validation_visitors = {
 				}
 			}
 		}
+	},
+	NestingSelector(node, context) {
+		const rule = /** @type {import('#compiler').Css.Rule} */ (context.state.rule);
+		if (!rule.metadata.parent_rule) {
+			error(node, 'invalid-nesting-selector');
+		}
 	}
 };
 

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

@@ -353,8 +353,7 @@ function relative_selector_might_apply_to_node(relative_selector, rule, node, st
 			case 'NestingSelector': {
 				let matched = false;
 
-				const parent = rule.metadata.parent_rule;
-				if (!parent) error(selector, 'TODO', 'invalid nesting selector'); // TODO error sooner
+				const parent = /** @type {import('#compiler').Css.Rule} */ (rule.metadata.parent_rule);
 
 				for (const complex_selector of parent.prelude.children) {
 					if (apply_selector(truncate(complex_selector), parent, node, stylesheet)) {