fix: noop on the server for custom renderer components

packages/svelte/src/compiler/phases/3-transform/server/transform-server.js

@@ -6,7 +6,7 @@ import { walk } from 'zimmerframe';
 import { set_scope } from '../../scope.js';
 import { extract_identifiers } from '../../../utils/ast.js';
 import * as b from '#compiler/builders';
-import { component_name, dev, filename } from '../../../state.js';
+import { component_name, custom_renderer, dev, filename } from '../../../state.js';
 import { render_stylesheet } from '../css/index.js';
 import { AssignmentExpression } from './visitors/AssignmentExpression.js';
 import { AwaitBlock } from './visitors/AwaitBlock.js';
@@ -90,6 +90,19 @@ const template_visitors = {
  * @returns {ESTree.Program}
  */
 export function server_component(analysis, options) {
+	// Components compiled with a custom renderer have no meaningful server
+	// representation — they only render on the client through the custom
+	// renderer — so we compile them down to a no-op.
+	if (custom_renderer) {
+		const component_function = b.function_declaration(b.id(analysis.name), [], b.block([]));
+
+		return {
+			type: 'Program',
+			sourceType: 'module',
+			body: [b.export_default(component_function)]
+		};
+	}
+
 	/** @type {ComponentServerTransformState} */
 	const state = {
 		analysis,

packages/svelte/tests/snapshot/samples/custom-renderer-server-noop/_config.js

@@ -0,0 +1,13 @@
+import { test } from '../../test';
+
+export default test({
+	compileOptions: {
+		experimental: {
+			// The actual renderer module is irrelevant for this snapshot — we only
+			// care that the server output is a no-op while the client output still
+			// imports/uses the custom renderer. Using a fixed path keeps the
+			// snapshot stable across machines.
+			customRenderer: 'my-custom-renderer'
+		}
+	}
+});

packages/svelte/tests/snapshot/samples/custom-renderer-server-noop/_expected/client/main.svelte.js

@@ -0,0 +1,17 @@
+import $renderer from 'my-custom-renderer';
+import 'svelte/internal/disclose-version';
+import * as $ from 'svelte/internal/client';
+
+var root = $.from_tree([['div', { class: 'greeting' }, ' ']]);
+
+export default function Main($$anchor, $$props) {
+	var $$pop_renderer = $.push_renderer($renderer);
+	let name = $.prop($$props, 'name', 3, 'world');
+	var div = root();
+	var text = $.child(div);
+
+	$.reset(div);
+	$.template_effect(() => $.set_text(text, `Hello ${name() ?? ''}!`));
+	$.append($$anchor, div);
+	$$pop_renderer();
+}

packages/svelte/tests/snapshot/samples/custom-renderer-server-noop/_expected/server/main.svelte.js

@@ -0,0 +1 @@
+export default function Main() {}

packages/svelte/tests/snapshot/samples/custom-renderer-server-noop/main.svelte

@@ -0,0 +1,5 @@
+<script>
+	let { name = 'world' } = $props();
+</script>
+
+<div class="greeting">Hello {name}!</div>