feat: provide isSnippet type, deduplicate children prop from default slot

fixes #10790
part of #9774

.changeset/cold-cheetahs-judge.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: deduplicate children prop and default slot

.changeset/famous-grapes-refuse.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+feat: provide `isSnippet` function to determine whether a given value is a snippet

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

@@ -774,6 +774,13 @@ function serialize_inline_component(node, component_name, context) {
 	 */
 	let slot_scope_applies_to_itself = false;
 
+	/**
+	 * Components may have a children prop and also have child nodes. In this case, we assume
+	 * that the child component isn't using render tags yet and pass the slot as $$slots.default.
+	 * We're not doing it for spread attributes, as this would result in too many false positives.
+	 */
+	let has_children_prop = false;
+
 	/**
 	 * @param {import('estree').Property} prop
 	 */
@@ -823,6 +830,10 @@ function serialize_inline_component(node, component_name, context) {
 				slot_scope_applies_to_itself = true;
 			}
 
+			if (attribute.name === 'children') {
+				has_children_prop = true;
+			}
+
 			const [, value] = serialize_attribute_value(attribute.value, context);
 
 			if (attribute.metadata.dynamic) {
@@ -944,13 +955,8 @@ function serialize_inline_component(node, component_name, context) {
 			b.block([...(slot_name === 'default' && !slot_scope_applies_to_itself ? lets : []), ...body])
 		);
 
-		if (slot_name === 'default') {
+		if (slot_name === 'default' && !has_children_prop) {
-			push_prop(
+			push_prop(b.init('children', b.call('$.add_snippet_symbol', slot_fn)));
-				b.init(
-					'children',
-					context.state.options.dev ? b.call('$.add_snippet_symbol', slot_fn) : slot_fn
-				)
-			);
 		} else {
 			serialized_slots.push(b.init(slot_name, slot_fn));
 		}
@@ -2685,9 +2691,7 @@ export const template_visitors = {
 		} else {
 			context.state.init.push(b.const(node.expression, b.arrow(args, body)));
 		}
-		if (context.state.options.dev) {
+		context.state.init.push(b.stmt(b.call('$.add_snippet_symbol', node.expression)));
-			context.state.init.push(b.stmt(b.call('$.add_snippet_symbol', node.expression)));
-		}
 	},
 	FunctionExpression: function_visitor,
 	ArrowFunctionExpression: function_visitor,
@@ -3106,7 +3110,7 @@ export const template_visitors = {
 					);
 
 		const expression = is_default
-			? b.member(b.id('$$props'), b.id('children'))
+			? b.call('$.default_slot', b.id('$$props'))
 			: b.member(b.member(b.id('$$props'), b.id('$$slots')), name, true, true);
 
 		const slot = b.call('$.slot', context.state.node, expression, props_expression, fallback);

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

@@ -947,6 +947,13 @@ function serialize_inline_component(node, component_name, context) {
 	 */
 	let slot_scope_applies_to_itself = false;
 
+	/**
+	 * Components may have a children prop and also have child nodes. In this case, we assume
+	 * that the child component isn't using render tags yet and pass the slot as $$slots.default.
+	 * We're not doing it for spread attributes, as this would result in too many false positives.
+	 */
+	let has_children_prop = false;
+
 	/**
 	 * @param {import('estree').Property} prop
 	 */
@@ -975,6 +982,10 @@ function serialize_inline_component(node, component_name, context) {
 				slot_scope_applies_to_itself = true;
 			}
 
+			if (attribute.name === 'children') {
+				has_children_prop = true;
+			}
+
 			const value = serialize_attribute_value(attribute.value, context, false, true);
 			push_prop(b.prop('init', b.key(attribute.name), value));
 		} else if (attribute.type === 'BindDirective' && attribute.name !== 'this') {
@@ -1049,14 +1060,8 @@ function serialize_inline_component(node, component_name, context) {
 			b.block([...(slot_name === 'default' && !slot_scope_applies_to_itself ? lets : []), ...body])
 		);
 
-		if (slot_name === 'default') {
+		if (slot_name === 'default' && !has_children_prop) {
-			push_prop(
+			push_prop(b.prop('init', b.id('children'), b.call('$.add_snippet_symbol', slot_fn)));
-				b.prop(
-					'init',
-					b.id('children'),
-					context.state.options.dev ? b.call('$.add_snippet_symbol', slot_fn) : slot_fn
-				)
-			);
 		} else {
 			const slot = b.prop('init', b.literal(slot_name), slot_fn);
 			serialized_slots.push(slot);
@@ -1614,9 +1619,7 @@ const template_visitors = {
 			)
 		);
 
-		if (context.state.options.dev) {
+		context.state.init.push(b.stmt(b.call('$.add_snippet_symbol', node.expression)));
-			context.state.init.push(b.stmt(b.call('$.add_snippet_symbol', node.expression)));
-		}
 	},
 	Component(node, context) {
 		const state = context.state;
@@ -1744,7 +1747,7 @@ const template_visitors = {
 		const lets = [];
 
 		/** @type {import('estree').Expression} */
-		let expression = b.member_id('$$props.children');
+		let expression = b.call('$.default_slot', b.id('$$props'));
 
 		for (const attribute of node.attributes) {
 			if (attribute.type === 'SpreadAttribute') {

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

@@ -38,3 +38,22 @@ export function snippet(get_snippet, node, ...args) {
 		};
 	}, block);
 }
+
+const snippet_symbol = Symbol.for('svelte.snippet');
+
+/**
+ * @param {any} fn
+ */
+export function add_snippet_symbol(fn) {
+	fn[snippet_symbol] = true;
+	return fn;
+}
+
+/**
+ * Returns true if given parameter is a snippet.
+ * @param {any} maybeSnippet
+ * @returns {maybeSnippet is import('svelte').Snippet}
+ */
+export function isSnippet(maybeSnippet) {
+	return /** @type {any} */ (maybeSnippet)?.[snippet_symbol] === true;
+}

packages/svelte/src/internal/client/dom/legacy/misc.js

@@ -1,6 +1,7 @@
 import { set, source } from '../../reactivity/sources.js';
 import { get } from '../../runtime.js';
 import { is_array } from '../../utils.js';
+import { isSnippet } from '../blocks/snippet.js';
 
 /**
  * Under some circumstances, imports may be reactive in legacy mode. In that case,
@@ -66,3 +67,17 @@ export function update_legacy_props($$new_props) {
 		}
 	}
 }
+
+/**
+ * @param {Record<string, any>} $$props
+ */
+export function default_slot($$props) {
+	var children = $$props.$$slots?.default;
+	if (children) {
+		return children;
+	}
+	children = $$props.children;
+	if (isSnippet(children)) {
+		return children;
+	}
+}

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

@@ -1,3 +1,4 @@
+import { isSnippet } from './dom/blocks/snippet.js';
 import { untrack } from './runtime.js';
 import { is_array } from './utils.js';
 
@@ -103,22 +104,12 @@ export function loop_guard(timeout) {
 	};
 }
 
-const snippet_symbol = Symbol.for('svelte.snippet');
-
-/**
- * @param {any} fn
- */
-export function add_snippet_symbol(fn) {
-	fn[snippet_symbol] = true;
-	return fn;
-}
-
 /**
  * Validate that the function handed to `{@render ...}` is a snippet function, and not some other kind of function.
  * @param {any} snippet_fn
  */
 export function validate_snippet(snippet_fn) {
-	if (snippet_fn && snippet_fn[snippet_symbol] !== true) {
+	if (snippet_fn && !isSnippet(snippet_fn)) {
 		throw new Error(
 			'The argument to `{@render ...}` must be a snippet function, not a component or some other kind of function. ' +
 				'If you want to dynamically render one snippet or another, use `$derived` and pass its result to `{@render ...}`.'
@@ -132,7 +123,7 @@ export function validate_snippet(snippet_fn) {
  * @param {any} component_fn
  */
 export function validate_component(component_fn) {
-	if (component_fn?.[snippet_symbol] === true) {
+	if (isSnippet(component_fn)) {
 		throw new Error('A snippet must be rendered with `{@render ...}`');
 	}
 	return component_fn;

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

@@ -11,6 +11,8 @@ import { DEV } from 'esm-env';
 import { UNINITIALIZED } from '../client/constants.js';
 
 export * from '../client/validate.js';
+export { add_snippet_symbol } from '../client/dom/blocks/snippet.js';
+export { default_slot } from '../client/dom/legacy/misc.js';
 
 /**
  * @typedef {{

packages/svelte/src/legacy/legacy-client.js

@@ -12,7 +12,7 @@ import * as $ from '../internal/index.js';
  * @template {Record<string, any>} Slots
  *
  * @param {import('../main/public.js').ComponentConstructorOptions<Props> & {
- * 	component: import('../main/public.js').SvelteComponent<Props, Events, Slots>;
+ * 	component: typeof import('../main/public.js').SvelteComponent<Props, Events, Slots>;
  * 	immutable?: boolean;
  * 	hydrate?: boolean;
  * 	recover?: boolean;

packages/svelte/src/main/main-client.js

@@ -181,5 +181,6 @@ export {
 	hasContext,
 	getContext,
 	getAllContexts,
-	setContext
+	setContext,
+	isSnippet
 } from '../internal/index.js';

packages/svelte/src/main/main-server.js

@@ -12,7 +12,8 @@ export {
 	tick,
 	unmount,
 	untrack,
-	createRoot
+	createRoot,
+	isSnippet
 } from './main-client.js';
 
 /** @returns {void} */

packages/svelte/tests/runtime-legacy/samples/slot-children-prop/A.svelte

@@ -0,0 +1,6 @@
+<script>
+  export let children;
+</script>
+
+{children}
+<slot />

packages/svelte/tests/runtime-legacy/samples/slot-children-prop/_config.js

@@ -0,0 +1,5 @@
+import { test } from '../../test';
+
+export default test({
+	html: `foo bar`
+});

packages/svelte/tests/runtime-legacy/samples/slot-children-prop/main.svelte

@@ -0,0 +1,7 @@
+<script>
+	import A from "./A.svelte"; 
+</script>
+
+<A children="foo">
+	bar
+</A>

packages/svelte/tests/types/snippet.ts

@@ -1,4 +1,4 @@
-import type { Snippet } from 'svelte';
+import { type Snippet, isSnippet } from 'svelte';
 
 const return_type: ReturnType<Snippet> = null as any;
 
@@ -38,3 +38,8 @@ const h: Snippet<[{ a: true }]> = (a) => {
 const i: Snippet = () => {
 	return return_type;
 };
+
+let j = null as any;
+if (isSnippet(j)) {
+	let x: Snippet = j;
+}

packages/svelte/types/index.d.ts

@@ -291,6 +291,12 @@ declare module 'svelte' {
 	 * Anything except a function
 	 */
 	type NotFunction<T> = T extends Function ? never : T;
+	/**
+	 * Returns true if given parameter is a snippet.
+	 * */
+	export function isSnippet(maybeSnippet: any): maybeSnippet is (this: void) => unique symbol & {
+		_: "functions passed to {@render ...} tags must use the `Snippet` type imported from \"svelte\"";
+	};
 	/**
 	 * @deprecated Use `mount` or `hydrate` instead
 	 */
@@ -1729,7 +1735,17 @@ declare module 'svelte/legacy' {
 	 *
 	 * */
 	export function createClassComponent<Props extends Record<string, any>, Exports extends Record<string, any>, Events extends Record<string, any>, Slots extends Record<string, any>>(options: ComponentConstructorOptions<Props> & {
-		component: SvelteComponent<Props, Events, Slots>;
+		component: {
+			new (options: ComponentConstructorOptions<Props & (Props extends {
+				children?: any;
+			} ? {} : Slots extends {
+				default: any;
+			} ? {
+				children?: ((this: void) => unique symbol & {
+					_: "functions passed to {@render ...} tags must use the `Snippet` type imported from \"svelte\"";
+				}) | undefined;
+			} : {})>): SvelteComponent<Props, Events, Slots>;
+		};
 		immutable?: boolean | undefined;
 		hydrate?: boolean | undefined;
 		recover?: boolean | undefined;