fix: Throw on unrendered snippets in dev

.changeset/strong-pianos-promise.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: Throw on unrendered snippets in `dev`

documentation/docs/98-reference/.generated/shared-errors.md

@@ -60,6 +60,43 @@ Certain lifecycle methods can only be used during component initialisation. To f
 <button onclick={handleClick}>click me</button>
 ```
 
+### snippet_without_render_tag
+
+```
+Attempted to render a snippet without a `{@render}` block. This would cause the snippet to be rendered directly to the DOM. To fix this, change `{snippet}` to `{@render snippet()}`.
+```
+
+A component throwing this error will look something like this (`children` is not being rendered):
+
+```svelte
+<script>
+    let { children } = $props();
+</script>
+
+{children}
+```
+
+...or like this (a parent component is passing a snippet where a non-snippet value is expected):
+
+```svelte
+<!--- file: Parent.svelte --->
+<ChildComponent>
+  {#slot label()}
+    <span>Hi!</span>
+  {/slot}
+</ChildComponent>
+```
+
+```svelte
+<!--- file: Child.svelte --->
+<script>
+  let { label } = $props();
+</script>
+
+<!-- This component doesn't expect a snippet, but the parent provided one -->
+<p>{label}</p>
+```
+
 ### store_invalid_shape
 
 ```

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

@@ -52,6 +52,41 @@ Certain lifecycle methods can only be used during component initialisation. To f
 <button onclick={handleClick}>click me</button>
 ```
 
+## snippet_without_render_tag
+
+> Attempted to render a snippet without a `{@render}` block. This would cause the snippet to be rendered directly to the DOM. To fix this, change `{snippet}` to `{@render snippet()}`.
+
+A component throwing this error will look something like this (`children` is not being rendered):
+
+```svelte
+<script>
+    let { children } = $props();
+</script>
+
+{children}
+```
+
+...or like this (a parent component is passing a snippet where a non-snippet value is expected):
+
+```svelte
+<!--- file: Parent.svelte --->
+<ChildComponent>
+  {#slot label()}
+    <span>Hi!</span>
+  {/slot}
+</ChildComponent>
+```
+
+```svelte
+<!--- file: Child.svelte --->
+<script>
+  let { label } = $props();
+</script>
+
+<!-- This component doesn't expect a snippet, but the parent provided one -->
+<p>{label}</p>
+```
+
 ## store_invalid_shape
 
 > `%name%` is not a store with a `subscribe` method

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

@@ -1,4 +1,4 @@
-/** @import { BlockStatement } from 'estree' */
+/** @import { ArrowFunctionExpression, BlockStatement, CallExpression } from 'estree' */
 /** @import { AST } from '#compiler' */
 /** @import { ComponentContext } from '../types.js' */
 import { dev } from '../../../../state.js';
@@ -9,20 +9,27 @@ import * as b from '../../../../utils/builders.js';
  * @param {ComponentContext} context
  */
 export function SnippetBlock(node, context) {
-	const fn = b.function_declaration(
-		node.expression,
-		[b.id('$$payload'), ...node.parameters],
-		/** @type {BlockStatement} */ (context.visit(node.body))
-	);
+	const body = /** @type {BlockStatement} */ (context.visit(node.body));
+
 	if (dev) {
-		fn.body.body.unshift(b.stmt(b.call('$.validate_snippet_args', b.id('$$payload'))));
+		body.body.unshift(b.stmt(b.call('$.validate_snippet_args', b.id('$$payload'))));
 	}
+
+	/** @type {ArrowFunctionExpression | CallExpression} */
+	let fn = b.arrow([b.id('$$payload'), ...node.parameters], body);
+
+	if (dev) {
+		fn = b.call('$.prevent_snippet_stringification', fn);
+	}
+
+	const declaration = b.declaration('const', [b.declarator(node.expression, fn)]);
+
 	// @ts-expect-error - TODO remove this hack once $$render_inner for legacy bindings is gone
 	fn.___snippet = true;
 
 	if (node.metadata.can_hoist) {
-		context.state.hoisted.push(fn);
+		context.state.hoisted.push(declaration);
 	} else {
-		context.state.init.push(fn);
+		context.state.init.push(declaration);
 	}
 }

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

@@ -4,6 +4,7 @@
 import { empty_comment, build_attribute_value } from './utils.js';
 import * as b from '../../../../../utils/builders.js';
 import { is_element_node } from '../../../../nodes.js';
+import { dev } from '../../../../../state.js';
 
 /**
  * @param {AST.Component | AST.SvelteComponent | AST.SvelteSelf} node
@@ -238,7 +239,13 @@ export function build_inline_component(node, expression, context) {
 				)
 			) {
 				// create `children` prop...
-				push_prop(b.prop('init', b.id('children'), slot_fn));
+				push_prop(
+					b.prop(
+						'init',
+						b.id('children'),
+						dev ? b.call('$.prevent_snippet_stringification', slot_fn) : slot_fn
+					)
+				);
 
 				// and `$$slots.default: true` so that `<slot>` on the child works
 				serialized_slots.push(b.init(slot_name, b.true));

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

@@ -15,6 +15,7 @@ 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';
+import { prevent_snippet_stringification } from '../../../shared/validate.js';
 
 /**
  * @template {(node: TemplateNode, ...args: any[]) => void} SnippetFn
@@ -60,7 +61,7 @@ export function snippet(node, get_snippet, ...args) {
  * @param {(node: TemplateNode, ...args: any[]) => void} fn
  */
 export function wrap_snippet(component, fn) {
-	return (/** @type {TemplateNode} */ node, /** @type {any[]} */ ...args) => {
+	const snippet = (/** @type {TemplateNode} */ node, /** @type {any[]} */ ...args) => {
 		var previous_component_function = dev_current_component_function;
 		set_dev_current_component_function(component);
 
@@ -70,6 +71,10 @@ export function wrap_snippet(component, fn) {
 			set_dev_current_component_function(previous_component_function);
 		}
 	};
+
+	prevent_snippet_stringification(snippet);
+
+	return snippet;
 }
 
 /**

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

@@ -157,7 +157,8 @@ export {
 	invalid_default_snippet,
 	validate_dynamic_element_tag,
 	validate_store,
-	validate_void_dynamic_element
+	validate_void_dynamic_element,
+	prevent_snippet_stringification
 } from '../shared/validate.js';
 export { strict_equals, equals } from './dev/equality.js';
 export { log_if_contains_state } from './dev/console-log.js';

packages/svelte/src/internal/server/index.js

@@ -509,7 +509,8 @@ export { fallback } from '../shared/utils.js';
 export {
 	invalid_default_snippet,
 	validate_dynamic_element_tag,
-	validate_void_dynamic_element
+	validate_void_dynamic_element,
+	prevent_snippet_stringification
 } from '../shared/validate.js';
 
 export { escape_html as escape };

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

@@ -48,6 +48,21 @@ export function lifecycle_outside_component(name) {
 	}
 }
 
+/**
+ * Attempted to render a snippet without a `{@render}` block. This would cause the snippet to be rendered directly to the DOM. To fix this, change `{snippet}` to `{@render snippet()}`.
+ * @returns {never}
+ */
+export function snippet_without_render_tag() {
+	if (DEV) {
+		const error = new Error(`snippet_without_render_tag\nAttempted to render a snippet without a \`{@render}\` block. This would cause the snippet to be rendered directly to the DOM. To fix this, change \`{snippet}\` to \`{@render snippet()}\`.\nhttps://svelte.dev/e/snippet_without_render_tag`);
+
+		error.name = 'Svelte error';
+		throw error;
+	} else {
+		throw new Error(`https://svelte.dev/e/snippet_without_render_tag`);
+	}
+}
+
 /**
  * `%name%` is not a store with a `subscribe` method
  * @param {string} name

packages/svelte/src/internal/shared/validate.js

@@ -35,3 +35,15 @@ export function validate_store(store, name) {
 		e.store_invalid_shape(name);
 	}
 }
+
+/**
+ * @template {() => unknown} T
+ * @param {T} fn
+ */
+export function prevent_snippet_stringification(fn) {
+	fn.toString = () => {
+		e.snippet_without_render_tag();
+		return '';
+	};
+	return fn;
+}

packages/svelte/tests/runtime-runes/samples/snippet-block-without-render-tag-dev/_config.js

@@ -0,0 +1,8 @@
+import { test } from '../../test';
+
+export default test({
+	compileOptions: {
+		dev: true
+	},
+	runtime_error: 'snippet_without_render_tag'
+});

packages/svelte/tests/runtime-runes/samples/snippet-block-without-render-tag-dev/main.svelte

@@ -0,0 +1,5 @@
+{testSnippet}
+
+{#snippet testSnippet()}
+  <p>hi again</p>
+{/snippet}

packages/svelte/tests/runtime-runes/samples/snippet-block-without-render-tag-prod/_config.js

@@ -0,0 +1,3 @@
+import { test } from '../../test';
+
+export default test({});

packages/svelte/tests/runtime-runes/samples/snippet-block-without-render-tag-prod/main.svelte

@@ -0,0 +1,5 @@
+{testSnippet}
+
+{#snippet testSnippet()}
+  <p>hi again</p>
+{/snippet}

packages/svelte/tests/runtime-runes/samples/snippet-children-without-render-tag-dev-prod/_config.js

@@ -0,0 +1,3 @@
+import { test } from '../../test';
+
+export default test({});

packages/svelte/tests/runtime-runes/samples/snippet-children-without-render-tag-dev-prod/main.svelte

@@ -0,0 +1,5 @@
+<script>
+  import UnrenderedChildren from './unrendered-children.svelte';
+</script>
+
+<UnrenderedChildren>Hi</UnrenderedChildren>

packages/svelte/tests/runtime-runes/samples/snippet-children-without-render-tag-dev-prod/unrendered-children.svelte

@@ -0,0 +1,5 @@
+<script>
+  let { children } = $props();
+</script>
+
+{children}

packages/svelte/tests/runtime-runes/samples/snippet-children-without-render-tag-dev/_config.js

@@ -0,0 +1,8 @@
+import { test } from '../../test';
+
+export default test({
+	compileOptions: {
+		dev: true
+	},
+	runtime_error: 'snippet_without_render_tag'
+});

packages/svelte/tests/runtime-runes/samples/snippet-children-without-render-tag-dev/main.svelte

@@ -0,0 +1,5 @@
+<script>
+  import UnrenderedChildren from './unrendered-children.svelte';
+</script>
+
+<UnrenderedChildren>Hi</UnrenderedChildren>

packages/svelte/tests/runtime-runes/samples/snippet-children-without-render-tag-dev/unrendered-children.svelte

@@ -0,0 +1,5 @@
+<script>
+  let { children } = $props();
+</script>
+
+{children}

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

@@ -1,9 +1,9 @@
 import * as $ from 'svelte/internal/server';
 import TextInput from './Child.svelte';
 
-function snippet($$payload) {
+const snippet = ($$payload) => {
 	$$payload.out += `<!---->Something`;
-}
+};
 
 export default function Bind_component_snippet($$payload) {
 	let value = '';