feat: allow const tag inside `svelte:boundary` (#14993)

* feat: allow const tag inside `svelte:boundary`

* chore: add better test

* docs: update docs for `@const`

.changeset/curvy-lies-rush.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+feat: allow const tag inside `svelte:boundary`

documentation/docs/03-template-syntax/09-@const.md

@@ -11,4 +11,4 @@ The `{@const ...}` tag defines a local constant.
 {/each}
 ```
 
-`{@const}` is only allowed as an immediate child of a block — `{#if ...}`, `{#each ...}`, `{#snippet ...}` and so on — or a `<Component />`.
+`{@const}` is only allowed as an immediate child of a block — `{#if ...}`, `{#each ...}`, `{#snippet ...}` and so on — a `<Component />` or a `<svelte:boundary`.

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

@@ -24,6 +24,7 @@ export function ConstTag(node, context) {
 			grand_parent?.type !== 'EachBlock' &&
 			grand_parent?.type !== 'AwaitBlock' &&
 			grand_parent?.type !== 'SnippetBlock' &&
+			grand_parent?.type !== 'SvelteBoundary' &&
 			((grand_parent?.type !== 'RegularElement' && grand_parent?.type !== 'SvelteElement') ||
 				!grand_parent.attributes.some((a) => a.type === 'Attribute' && a.name === 'slot')))
 	) {

packages/svelte/src/compiler/phases/3-transform/client/visitors/SvelteBoundary.js

@@ -33,21 +33,34 @@ export function SvelteBoundary(node, context) {
 	const nodes = [];
 
 	/** @type {Statement[]} */
-	const snippet_statements = [];
+	const external_statements = [];
+
+	const snippets_visits = [];
 
 	// Capture the `failed` implicit snippet prop
 	for (const child of node.fragment.nodes) {
 		if (child.type === 'SnippetBlock' && child.expression.name === 'failed') {
+			// we need to delay the visit of the snippets in case they access a ConstTag that is declared
+			// after the snippets so that the visitor for the const tag can be updated
+			snippets_visits.push(() => {
 				/** @type {Statement[]} */
 				const init = [];
 				context.visit(child, { ...context.state, init });
 				props.properties.push(b.prop('init', child.expression, child.expression));
-			snippet_statements.push(...init);
+				external_statements.push(...init);
+			});
+		} else if (child.type === 'ConstTag') {
+			/** @type {Statement[]} */
+			const init = [];
+			context.visit(child, { ...context.state, init });
+			external_statements.push(...init);
 		} else {
 			nodes.push(child);
 		}
 	}
 
+	snippets_visits.forEach((visit) => visit());
+
 	const block = /** @type {BlockStatement} */ (context.visit({ ...node.fragment, nodes }));
 
 	const boundary = b.stmt(
@@ -56,6 +69,6 @@ export function SvelteBoundary(node, context) {
 
 	context.state.template.push('<!>');
 	context.state.init.push(
-		snippet_statements.length > 0 ? b.block([...snippet_statements, boundary]) : boundary
+		external_statements.length > 0 ? b.block([...external_statements, boundary]) : boundary
 	);
 }

packages/svelte/tests/runtime-runes/samples/const-tag-boundary/FlakyComponent.svelte

@@ -0,0 +1,3 @@
+<script>
+	throw new Error();
+</script>

packages/svelte/tests/runtime-runes/samples/const-tag-boundary/_config.js

@@ -0,0 +1,17 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	html: '<button></button><p>2</p>',
+	mode: ['client'],
+	test({ target, assert }) {
+		const btn = target.querySelector('button');
+		const p = target.querySelector('p');
+
+		flushSync(() => {
+			btn?.click();
+		});
+
+		assert.equal(p?.innerHTML, '4');
+	}
+});

packages/svelte/tests/runtime-runes/samples/const-tag-boundary/main.svelte

@@ -0,0 +1,14 @@
+<script>
+	import FlakyComponent from "./FlakyComponent.svelte";
+	let test=$state(1);
+</script>
+
+<button onclick={()=>test++}></button>
+
+<svelte:boundary>
+	{@const double = test * 2}
+	{#snippet failed()}
+		<p>{double}</p>
+	{/snippet}
+	<FlakyComponent />
+</svelte:boundary>

packages/svelte/tests/validator/samples/const-tag-placement-svelte-boundary/errors.json

@@ -0,0 +1 @@
+[]

packages/svelte/tests/validator/samples/const-tag-placement-svelte-boundary/input.svelte

@@ -0,0 +1,11 @@
+<script>
+  let a = $state("");
+</script>
+
+<svelte:boundary>
+	{@const x = a}
+	{#snippet failed()}
+		{x}
+	{/snippet}
+	<FlakyComponent />
+</svelte:boundary>