fix: deconflict multiple snippets of the same name (#12221)

* fix: deconflict multiple snippets of the same name

* address feedback

* only create BlockStatement when necessary

---------

Co-authored-by: Rich Harris <rich.harris@vercel.com>

.changeset/witty-hornets-think.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: deconflict multiple snippets of the same name

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

@@ -1098,13 +1098,12 @@ function serialize_update(statement) {
 }
 
 /**
- *
- * @param {import('../types.js').ComponentClientTransformState} state
+ * @param {import('estree').Statement[]} update
  */
-function serialize_render_stmt(state) {
-	return state.update.length === 1
-		? serialize_update(state.update[0])
-		: b.stmt(b.call('$.template_effect', b.thunk(b.block(state.update))));
+function serialize_render_stmt(update) {
+	return update.length === 1
+		? serialize_update(update[0])
+		: b.stmt(b.call('$.template_effect', b.thunk(b.block(update))));
 }
 
 /**
@@ -1732,7 +1731,7 @@ export const template_visitors = {
 		}
 
 		if (state.update.length > 0) {
-			body.push(serialize_render_stmt(state));
+			body.push(serialize_render_stmt(state.update));
 		}
 
 		body.push(...state.after_update);
@@ -2179,8 +2178,15 @@ export const template_visitors = {
 			state.options.preserveComments
 		);
 
+		/** Whether or not we need to wrap the children in `{...}` to avoid declaration conflicts */
+		const has_declaration = node.fragment.nodes.some((node) => node.type === 'SnippetBlock');
+
+		const child_state = has_declaration
+			? { ...state, init: [], update: [], after_update: [] }
+			: state;
+
 		for (const node of hoisted) {
-			context.visit(node, state);
+			context.visit(node, child_state);
 		}
 
 		process_children(
@@ -2193,9 +2199,19 @@ export const template_visitors = {
 						: context.state.node
 				),
 			true,
-			{ ...context, state }
+			{ ...context, state: child_state }
 		);
 
+		if (has_declaration) {
+			context.state.init.push(
+				b.block([
+					...child_state.init,
+					child_state.update.length > 0 ? serialize_render_stmt(child_state.update) : b.empty,
+					...child_state.after_update
+				])
+			);
+		}
+
 		if (has_direction_attribute) {
 			// This fixes an issue with Chromium where updates to text content within an element
 			// does not update the direction when set to auto. If we just re-assign the dir, this fixes it.
@@ -2292,7 +2308,7 @@ export const template_visitors = {
 		/** @type {import('estree').Statement[]} */
 		const inner = inner_context.state.init;
 		if (inner_context.state.update.length > 0) {
-			inner.push(serialize_render_stmt(inner_context.state));
+			inner.push(serialize_render_stmt(inner_context.state.update));
 		}
 		inner.push(...inner_context.state.after_update);
 		inner.push(
@@ -2757,7 +2773,7 @@ export const template_visitors = {
 			snippet = b.call('$.wrap_snippet', snippet, b.id(context.state.analysis.name));
 		}
 
-		const declaration = b.var(node.expression, snippet);
+		const declaration = b.const(node.expression, snippet);
 
 		// Top-level snippets are hoisted so they can be referenced in the `<script>`
 		if (context.path.length === 1 && context.path[0].type === 'Fragment') {

packages/svelte/tests/migrate/samples/svelte-element/output.svelte

@@ -3,4 +3,4 @@
 <svelte:element this={"div"} />
 
 <!-- we don't try to fix this bug, we just leave it as-is -->
-<svelte:element this="h{n}" />
+<svelte:element this="h{n}" />

packages/svelte/tests/parser-modern/samples/snippets/output.json

@@ -27,8 +27,8 @@
 				"parameters": [
 					{
 						"type": "Identifier",
-						"name": "msg",
 						"start": 43,
+						"end": 46,
 						"loc": {
 							"start": {
 								"line": 3,
@@ -39,7 +39,7 @@
 								"column": 25
 							}
 						},
-						"end": 46,
+						"name": "msg",
 						"typeAnnotation": {
 							"type": "TSTypeAnnotation",
 							"start": 46,

packages/svelte/tests/runtime-runes/samples/snippet-deconflict/_config.js

@@ -0,0 +1,24 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	mode: ['client'],
+
+	test({ assert, target }) {
+		const btn = target.querySelector('button');
+
+		btn?.click();
+		btn?.click();
+		btn?.click();
+		flushSync();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>push</button><div style="display: grid; grid-template-columns: 1fr 1fr"><div><p style="color: red">1</p><p style="color: red">2</p><p style="color: red">3</p>
+				<p style="color: red">4</p><p style="color: red">5</p><p style="color: red">6</p></div><div><p style="color: blue">1</p><p style="color: blue">2</p><p style="color: blue">3</p>
+				<p style="color: blue">4</p><p style="color: blue">5</p><p style="color: blue">6</p></div></div>
+			`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/snippet-deconflict/main.svelte

@@ -0,0 +1,29 @@
+<script>
+	let numbers = $state([1, 2, 3]);
+</script>
+
+<button onclick={() => numbers.push(numbers.length + 1)}>
+	push
+</button>
+
+<div style="display: grid; grid-template-columns: 1fr 1fr">
+	<div>
+		{#snippet x(n)}
+			<p style="color: red">{n}</p>
+		{/snippet}
+
+		{#each numbers as n}
+			{@render x(n)}
+		{/each}
+	</div>
+
+	<div>
+		{#snippet x(n)}
+			<p style="color: blue">{n}</p>
+		{/snippet}
+
+		{#each numbers as n}
+			{@render x(n)}
+		{/each}
+	</div>
+</div>

packages/svelte/tests/snapshot/samples/bind-component-snippet/_expected/client/index.svelte.js

@@ -6,7 +6,7 @@ var root_1 = $.template(`Something`, 1);
 var root = $.template(`<!> `, 5);
 
 export default function Bind_component_snippet($$anchor) {
-	var snippet = ($$anchor) => {
+	const snippet = ($$anchor) => {
 		var fragment = root_1();
 
 		$.append($$anchor, fragment);