fix: stricter validation for component exports

.changeset/odd-taxis-retire.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: disallow exporting props, derived and reassigned state from within components

packages/svelte/src/compiler/errors.js

@@ -169,8 +169,12 @@ const runes = {
 	'invalid-legacy-export': () => `Cannot use \`export let\` in runes mode — use $props instead`,
 	/** @param {string} rune */
 	'invalid-rune-usage': (rune) => `Cannot use ${rune} rune in non-runes mode`,
-	'invalid-state-export': () => `Cannot export state if it is reassigned`,
-	'invalid-derived-export': () => `Cannot export derived state`,
+	'invalid-state-export': () =>
+		`Cannot export state if it is reassigned. Either export a function returning the state value or only mutate the state value's properties`,
+	'invalid-derived-export': () =>
+		`Cannot export derived state. To expose the current derived value, export a function returning its value`,
+	'invalid-prop-export': () =>
+		`Cannot export properties. To expose the current value of a property, export a function returning its value`,
 	'invalid-props-id': () => `$props() can only be used with an object destructuring pattern`,
 	'invalid-props-pattern': () =>
 		`$props() assignment must not contain nested properties or computed keys`,

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

@@ -710,6 +710,10 @@ function validate_export(node, scope, name) {
 	const binding = scope.get(name);
 	if (!binding) return;
 
+	if (binding.kind === 'prop') {
+		error(node, 'invalid-prop-export');
+	}
+
 	if (binding.kind === 'derived') {
 		error(node, 'invalid-derived-export');
 	}
@@ -941,10 +945,20 @@ export const validation_runes = merge(validation, a11y_validators, {
 		if (node.label.name !== '$' || path.at(-1)?.type !== 'Program') return;
 		error(node, 'invalid-legacy-reactive-statement');
 	},
-	ExportNamedDeclaration(node, { state }) {
+	ExportNamedDeclaration(node, { state, next }) {
 		if (node.declaration?.type !== 'VariableDeclaration') return;
-		if (node.declaration.kind !== 'let') return;
+
+		// visit children, so bindings are correctly initialised
+		next();
+
+		for (const declarator of node.declaration.declarations) {
+			for (const id of extract_identifiers(declarator.id)) {
+				validate_export(node, state.scope, id.name);
+			}
+		}
+
 		if (state.analysis.instance.scope !== state.scope) return;
+		if (node.declaration.kind !== 'let') return;
 		error(node, 'invalid-legacy-export');
 	},
 	ExportSpecifier(node, { state }) {

packages/svelte/tests/compiler-errors/samples/export-derived-state/_config.js

@@ -3,7 +3,7 @@ import { test } from '../../test';
 export default test({
 	error: {
 		code: 'invalid-derived-export',
-		message: 'Cannot export derived state',
-		position: [24, 66]
+		message:
+			'Cannot export derived state. To expose the current derived value, export a function returning its value'
 	}
 });

packages/svelte/tests/compiler-errors/samples/export-derived-state/main.svelte

@@ -0,0 +1,4 @@
+<script>
+	let count = $state(0);
+	export const double = $derived(count * 2);
+</script>

packages/svelte/tests/compiler-errors/samples/export-state-2/_config.js

@@ -0,0 +1,10 @@
+import { test } from '../../test';
+
+export default test({
+	error: {
+		code: 'invalid-state-export',
+		message:
+			"Cannot export state if it is reassigned. Either export a function returning the state value or only mutate the state value's properties",
+		position: [59, 99]
+	}
+});

packages/svelte/tests/compiler-errors/samples/export-state-2/main.svelte

@@ -0,0 +1,15 @@
+<script>
+	export const object = $state({
+		ok: true
+	});
+
+	export const primitive = $state('nope');
+
+	export function update_object() {
+		object.ok = !object.ok;
+	}
+
+	export function update_primitive() {
+		primitive = 'yep';
+	}
+</script>

packages/svelte/tests/compiler-errors/samples/export-state/_config.js

@@ -3,7 +3,8 @@ import { test } from '../../test';
 export default test({
 	error: {
 		code: 'invalid-state-export',
-		message: 'Cannot export state if it is reassigned',
+		message:
+			"Cannot export state if it is reassigned. Either export a function returning the state value or only mutate the state value's properties",
 		position: [46, 86]
 	}
 });

packages/svelte/tests/compiler-errors/samples/runes-export-named-state/_config.js

@@ -3,7 +3,8 @@ import { test } from '../../test';
 export default test({
 	error: {
 		code: 'invalid-state-export',
-		message: 'Cannot export state if it is reassigned',
+		message:
+			"Cannot export state if it is reassigned. Either export a function returning the state value or only mutate the state value's properties",
 		position: [28, 53]
 	}
 });

packages/svelte/tests/compiler-errors/samples/runes-export-prop/_config.js

@@ -0,0 +1,9 @@
+import { test } from '../../test';
+
+export default test({
+	error: {
+		code: 'invalid-prop-export',
+		message:
+			'Cannot export properties. To expose the current value of a property, export a function returning its value'
+	}
+});

packages/svelte/tests/compiler-errors/samples/runes-export-prop/main.svelte

@@ -0,0 +1,4 @@
+<script>
+	let { foo } = $props();
+	export { foo };
+</script>