feat: enable snippets to fill slots

This allows people to use snippets to fill slots. It is implemented in the same way the default slot interop is already implemented, by passing a boolean to the hidden `$$slots` object, and using that at runtime to determine the correct outcome. The impact on bundle size is neglible.

By enabling this, we can enhance our migration script to always transform slot usages (including `let:x` etc) to snippets. This wasn't possible before because we couldn't be sure if the other side was transformed to using render tags at the same time. This will be part of #13419. This is important because currently the migration script is transforming `<slot />` creations inside components, but since it's not touching its usage points the migration will make your app end up in a broken state which you have to finish by hand.

This is a reduced alternative to, and closes #11619, which was also enabling the other way around, but that is a) not as necessary and b) more likely to confuse people / break, because it only works if your render function has 0-1 arguments.

Author: dummdidumm

.changeset/neat-ways-allow.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+feat: enable snippets to fill slots

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

@@ -1,4 +1,4 @@
-/** @import { BlockStatement, Expression, ExpressionStatement, Property } from 'estree' */
+/** @import { BlockStatement, Expression, ExpressionStatement, Literal, Property } from 'estree' */
 /** @import { AST } from '#compiler' */
 /** @import { ComponentContext } from '../types' */
 import * as b from '../../../../utils/builders.js';
@@ -23,7 +23,7 @@ export function SlotElement(node, context) {
 
 	let is_default = true;
 
-	/** @type {Expression} */
+	/** @type {Literal} */
 	let name = b.literal('default');
 
 	for (const attribute of node.attributes) {
@@ -33,7 +33,7 @@ export function SlotElement(node, context) {
 			const { value } = build_attribute_value(attribute.value, context);
 
 			if (attribute.name === 'name') {
-				name = value;
+				name = /** @type {Literal} */ (value);
 				is_default = false;
 			} else if (attribute.name !== 'slot') {
 				if (attribute.metadata.expression.has_state) {
@@ -58,10 +58,14 @@ export function SlotElement(node, context) {
 			? b.literal(null)
 			: b.arrow([b.id('$$anchor')], /** @type {BlockStatement} */ (context.visit(node.fragment)));
 
-	const expression = is_default
-		? b.call('$.default_slot', b.id('$$props'))
-		: b.member(b.member(b.id('$$props'), '$$slots'), name, true, true);
+	const slot = b.call(
+		'$.slot',
+		context.state.node,
+		b.id('$$props'),
+		name,
+		props_expression,
+		fallback
+	);
 
-	const slot = b.call('$.slot', context.state.node, expression, props_expression, fallback);
 	context.state.init.push(b.stmt(slot));
 }

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

@@ -215,6 +215,8 @@ export function build_component(node, component_name, context, anchor = context.
 
 	/** @type {Statement[]} */
 	const snippet_declarations = [];
+	/** @type {import('estree').Property[]} */
+	const serialized_slots = [];
 
 	// Group children by slot
 	for (const child of node.fragment.nodes) {
@@ -229,6 +231,9 @@ export function build_component(node, component_name, context, anchor = context.
 
 			push_prop(b.prop('init', child.expression, child.expression));
 
+			// Interop: allows people to pass snippets when component still uses slots
+			serialized_slots.push(b.init(child.expression.name, b.true));
+
 			continue;
 		}
 
@@ -238,8 +243,6 @@ export function build_component(node, component_name, context, anchor = context.
 	}
 
 	// Serialize each slot
-	/** @type {Property[]} */
-	const serialized_slots = [];
 	for (const slot_name of Object.keys(children)) {
 		const block = /** @type {BlockStatement} */ (
 			context.visit(

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

@@ -1,4 +1,4 @@
-/** @import { BlockStatement, Expression, Property } from 'estree' */
+/** @import { BlockStatement, Expression, Literal, Property } from 'estree' */
 /** @import { AST } from '#compiler' */
 /** @import { ComponentContext } from '../types.js' */
 import * as b from '../../../../utils/builders.js';
@@ -15,8 +15,8 @@ export function SlotElement(node, context) {
 	/** @type {Expression[]} */
 	const spreads = [];
 
-	/** @type {Expression} */
-	let expression = b.call('$.default_slot', b.id('$$props'));
+	/** @type {Literal} */
+	let name = b.literal('default');
 
 	for (const attribute of node.attributes) {
 		if (attribute.type === 'SpreadAttribute') {
@@ -25,7 +25,7 @@ export function SlotElement(node, context) {
 			const value = build_attribute_value(attribute.value, context, false, true);
 
 			if (attribute.name === 'name') {
-				expression = b.member(b.member_id('$$props.$$slots'), value, true, true);
+				name = /** @type {Literal} */ (value);
 			} else if (attribute.name !== 'slot') {
 				props.push(b.init(attribute.name, value));
 			}
@@ -42,7 +42,14 @@ export function SlotElement(node, context) {
 			? b.literal(null)
 			: b.thunk(/** @type {BlockStatement} */ (context.visit(node.fragment)));
 
-	const slot = b.call('$.slot', b.id('$$payload'), expression, props_expression, fallback);
+	const slot = b.call(
+		'$.slot',
+		b.id('$$payload'),
+		b.id('$$props'),
+		name,
+		props_expression,
+		fallback
+	);
 
 	context.state.template.push(empty_comment, b.stmt(slot), empty_comment);
 }

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

@@ -101,6 +101,8 @@ export function build_inline_component(node, expression, context) {
 
 	/** @type {Statement[]} */
 	const snippet_declarations = [];
+	/** @type {Property[]} */
+	const serialized_slots = [];
 
 	// Group children by slot
 	for (const child of node.fragment.nodes) {
@@ -115,6 +117,9 @@ export function build_inline_component(node, expression, context) {
 
 			push_prop(b.prop('init', child.expression, child.expression));
 
+			// Interop: allows people to pass snippets when component still uses slots
+			serialized_slots.push(b.init(child.expression.name, b.true));
+
 			continue;
 		}
 
@@ -142,9 +147,6 @@ export function build_inline_component(node, expression, context) {
 	}
 
 	// Serialize each slot
-	/** @type {Property[]} */
-	const serialized_slots = [];
-
 	for (const slot_name of Object.keys(children)) {
 		const block = /** @type {BlockStatement} */ (
 			context.visit(

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

@@ -2,21 +2,30 @@ import { hydrate_next, hydrating } from '../hydration.js';
 
 /**
  * @param {Comment} anchor
- * @param {void | ((anchor: Comment, slot_props: Record<string, unknown>) => void)} slot_fn
+ * @param {Record<string, any>} $$props
+ * @param {string} name
  * @param {Record<string, unknown>} slot_props
  * @param {null | ((anchor: Comment) => void)} fallback_fn
  */
-export function slot(anchor, slot_fn, slot_props, fallback_fn) {
+export function slot(anchor, $$props, name, slot_props, fallback_fn) {
 	if (hydrating) {
 		hydrate_next();
 	}
 
+	var slot_fn = $$props.$$slots?.[name];
+	// Interop: Can use snippets to fill slots
+	var is_interop = false;
+	if (slot_fn === true) {
+		slot_fn = $$props[name === 'default' ? 'children' : name];
+		is_interop = true;
+	}
+
 	if (slot_fn === undefined) {
 		if (fallback_fn !== null) {
 			fallback_fn(anchor);
 		}
 	} else {
-		slot_fn(anchor, slot_props);
+		slot_fn(anchor, is_interop ? () => slot_props : slot_props);
 	}
 }
 

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

@@ -66,15 +66,3 @@ export function update_legacy_props($$new_props) {
 		}
 	}
 }
-
-/**
- * @param {Record<string, any>} $$props
- */
-export function default_slot($$props) {
-	var children = $$props.$$slots?.default;
-	if (children === true) {
-		return $$props.children;
-	} else {
-		return children;
-	}
-}

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

@@ -80,8 +80,7 @@ export {
 	add_legacy_event_listener,
 	bubble_event,
 	reactive_import,
-	update_legacy_props,
-	default_slot
+	update_legacy_props
 } from './dom/legacy/misc.js';
 export {
 	append,

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

@@ -405,12 +405,19 @@ export function unsubscribe_stores(store_values) {
 
 /**
  * @param {Payload} payload
- * @param {void | ((payload: Payload, props: Record<string, unknown>) => void)} slot_fn
+ * @param {Record<string, any>} $$props
+ * @param {string} name
  * @param {Record<string, unknown>} slot_props
  * @param {null | (() => void)} fallback_fn
  * @returns {void}
  */
-export function slot(payload, slot_fn, slot_props, fallback_fn) {
+export function slot(payload, $$props, name, slot_props, fallback_fn) {
+	var slot_fn = $$props.$$slots?.[name];
+	// Interop: Can use snippets to fill slots
+	if (slot_fn === true) {
+		slot_fn = $$props[name === 'default' ? 'children' : name];
+	}
+
 	if (slot_fn !== undefined) {
 		slot_fn(payload, slot_props);
 	} else {
@@ -545,5 +552,3 @@ export {
 } from '../shared/validate.js';
 
 export { escape_html as escape };
-
-export { default_slot } from '../client/dom/legacy/misc.js';

packages/svelte/tests/runtime-runes/samples/snippets-as-slots/_config.js

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

packages/svelte/tests/runtime-runes/samples/snippets-as-slots/child.svelte

@@ -0,0 +1,2 @@
+<p><slot /></p>
+<p><slot name="named" foo="foo" /></p>

packages/svelte/tests/runtime-runes/samples/snippets-as-slots/main.svelte

@@ -0,0 +1,10 @@
+<script>
+	import Child from './child.svelte';
+</script>
+
+<Child>
+	Default
+	{#snippet named({ foo })}
+		Named {foo}
+	{/snippet}
+</Child>