breaking: prevent unparenthesized sequence expressions in attributes

Author: Rich-Harris

.changeset/long-humans-repair.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+breaking: prevent unparenthesized sequence expressions in attributes

packages/svelte/src/compiler/errors.js

@@ -282,7 +282,9 @@ const attributes = {
 	},
 	'invalid-let-directive-placement': () => 'let directive at invalid position',
 	'invalid-style-directive-modifier': () =>
-		`Invalid 'style:' modifier. Valid modifiers are: 'important'`
+		`Invalid 'style:' modifier. Valid modifiers are: 'important'`,
+	'invalid-sequence-expression': () =>
+		`Sequence expressions are not allowed as attribute/directive values in runes mode, unless wrapped in parentheses`
 };
 
 /** @satisfies {Errors} */

packages/svelte/src/compiler/index.js

@@ -41,7 +41,7 @@ export function compile(source, options) {
 			};
 		}
 
-		const analysis = analyze_component(parsed, combined_options);
+		const analysis = analyze_component(parsed, source, combined_options);
 
 		const result = transform_component(analysis, source, combined_options);
 		return result;

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

@@ -258,10 +258,11 @@ export function analyze_module(ast, options) {
 
 /**
  * @param {import('#compiler').Root} root
+ * @param {string} source
  * @param {import('#compiler').ValidatedCompileOptions} options
  * @returns {import('../types.js').ComponentAnalysis}
  */
-export function analyze_component(root, options) {
+export function analyze_component(root, source, options) {
 	const scope_root = new ScopeRoot();
 
 	const module = js(root.module, scope_root, false, null);
@@ -396,7 +397,8 @@ export function analyze_component(root, options) {
 					})
 				: '',
 			keyframes: []
-		}
+		},
+		source
 	};
 
 	if (!options.customElement && root.options?.customElement) {

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

@@ -50,6 +50,18 @@ function validate_component(node, context) {
 		}
 
 		if (attribute.type === 'Attribute') {
+			if (context.state.analysis.runes && is_expression_attribute(attribute)) {
+				const expression = attribute.value[0].expression;
+				if (expression.type === 'SequenceExpression') {
+					let i = /** @type {number} */ (expression.start);
+					while (--i > 0) {
+						const char = context.state.analysis.source[i];
+						if (char === '(') break; // parenthesized sequence expressions are ok
+						if (char === '{') error(expression, 'invalid-sequence-expression');
+					}
+				}
+			}
+
 			validate_attribute_name(attribute, context);
 
 			if (attribute.name === 'slot') {
@@ -81,12 +93,21 @@ function validate_element(node, context) {
 
 	for (const attribute of node.attributes) {
 		if (attribute.type === 'Attribute') {
+			const is_expression = is_expression_attribute(attribute);
+
+			if (context.state.analysis.runes && is_expression) {
+				const expression = attribute.value[0].expression;
+				if (expression.type === 'SequenceExpression') {
+					error(expression, 'invalid-sequence-expression');
+				}
+			}
+
 			if (regex_illegal_attribute_character.test(attribute.name)) {
 				error(attribute, 'invalid-attribute-name', attribute.name);
 			}
 
 			if (attribute.name.startsWith('on') && attribute.name.length > 2) {
-				if (!is_expression_attribute(attribute)) {
+				if (!is_expression) {
 					error(attribute, 'invalid-event-attribute-value');
 				}
 

packages/svelte/src/compiler/phases/types.d.ts

@@ -73,6 +73,7 @@ export interface ComponentAnalysis extends Analysis {
 		hash: string;
 		keyframes: string[];
 	};
+	source: string;
 }
 
 declare module 'estree' {

packages/svelte/tests/compiler-errors/samples/attribute-sequence-expression/_config.js

@@ -0,0 +1,10 @@
+import { test } from '../../test';
+
+export default test({
+	error: {
+		code: 'invalid-sequence-expression',
+		message:
+			'Sequence expressions are not allowed as attribute/directive values in runes mode, unless wrapped in parentheses',
+		position: [163, 170]
+	}
+});

packages/svelte/tests/compiler-errors/samples/attribute-sequence-expression/main.svelte

@@ -0,0 +1,10 @@
+<script>
+	import Child from './Child.svelte';
+	let { x, y, z } = $props();
+</script>
+
+<!-- allowed -->
+<Child foo={(x, y, z)} />
+
+<!-- not allowed -->
+<Child foo={x, y, z} />