fix: prevent nullish snippet for rendering empty content

.changeset/tame-frogs-shave.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: prevent nullish snippet for rendering empty content

packages/svelte/messages/client-errors/errors.md

@@ -48,6 +48,10 @@
 
 > Failed to hydrate the application
 
+## invalid_snippet
+
+> Could not `{@render}` snippet due to the expression being `null` or `undefined`. Consider using optional chaining `{@render snippet?.()}`
+
 ## lifecycle_legacy_only
 
 > `%name%(...)` cannot be used in runes mode

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

@@ -30,6 +30,11 @@ export function RenderTag(node, context) {
 	let snippet_function = /** @type {Expression} */ (context.visit(callee));
 
 	if (node.metadata.dynamic) {
+		// If we have a chain expression then ensure a nullish snippet function gets turned into an empty one
+		if (node.expression.type === 'ChainExpression') {
+			snippet_function = b.logical('??', snippet_function, b.id('$.noop'));
+		}
+
 		context.state.init.push(
 			b.stmt(b.call('$.snippet', context.state.node, b.thunk(snippet_function), ...args))
 		);

packages/svelte/src/internal/client/dom/blocks/snippet.js

@@ -11,8 +11,10 @@ import { hydrate_next, hydrate_node, hydrating } from '../hydration.js';
 import { create_fragment_from_html } from '../reconciler.js';
 import { assign_nodes } from '../template.js';
 import * as w from '../../warnings.js';
+import * as e from '../../errors.js';
 import { DEV } from 'esm-env';
 import { get_first_child, get_next_sibling } from '../operations.js';
+import { noop } from '../../../shared/utils.js';
 
 /**
  * @template {(node: TemplateNode, ...args: any[]) => void} SnippetFn
@@ -25,7 +27,8 @@ export function snippet(node, get_snippet, ...args) {
 	var anchor = node;
 
 	/** @type {SnippetFn | null | undefined} */
-	var snippet;
+	// @ts-ignore
+	var snippet = noop;
 
 	/** @type {Effect | null} */
 	var snippet_effect;
@@ -38,9 +41,11 @@ export function snippet(node, get_snippet, ...args) {
 			snippet_effect = null;
 		}
 
-		if (snippet) {
-			snippet_effect = branch(() => /** @type {SnippetFn} */ (snippet)(anchor, ...args));
+		if (DEV && snippet == null) {
+			e.invalid_snippet();
 		}
+
+		snippet_effect = branch(() => /** @type {SnippetFn} */ (snippet)(anchor, ...args));
 	}, EFFECT_TRANSPARENT);
 
 	if (hydrating) {

packages/svelte/src/internal/client/errors.js

@@ -210,6 +210,22 @@ export function hydration_failed() {
 	}
 }
 
+/**
+ * Could not `{@render}` snippet due to the expression being `null` or `undefined`. Consider using optional chaining `{@render snippet?.()}`
+ * @returns {never}
+ */
+export function invalid_snippet() {
+	if (DEV) {
+		const error = new Error(`invalid_snippet\nCould not \`{@render}\` snippet due to the expression being \`null\` or \`undefined\`. Consider using optional chaining \`{@render snippet?.()}\``);
+
+		error.name = 'Svelte error';
+		throw error;
+	} else {
+		// TODO print a link to the documentation
+		throw new Error("invalid_snippet");
+	}
+}
+
 /**
  * `%name%(...)` cannot be used in runes mode
  * @param {string} name

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

@@ -0,0 +1,13 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	test({ assert, target }) {
+		const btn = target.querySelector('button');
+
+		assert.throws(() => {
+			btn?.click();
+			flushSync();
+		}, /invalid_snippet/);
+	}
+});

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

@@ -0,0 +1,13 @@
+<script>
+  let state = $state({
+		value: counter
+	});
+</script>
+
+{#snippet counter()}
+  Test
+{/snippet}
+
+{@render state.value()}
+
+<button onclick={() => state.value = undefined}>change</button>