fix, different approach without needing symbol

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

@@ -840,6 +840,9 @@ function serialize_inline_component(node, component_name, context) {
 			push_prop(
 				b.init('children', context.state.options.dev ? b.call('$.wrap_snippet', slot_fn) : slot_fn)
 			);
+			// We additionally add the default slot as a boolean, so that the slot render function on the other
+			// side knows it should get the content to render from $$props.children
+			serialized_slots.push(b.init(slot_name, b.true));
 		} else {
 			serialized_slots.push(b.init(slot_name, slot_fn));
 		}

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

@@ -1084,7 +1084,16 @@ function serialize_inline_component(node, component_name, context) {
 		);
 
 		if (slot_name === 'default' && !has_children_prop) {
-			push_prop(b.prop('init', b.id('children'), b.call('$.add_snippet_symbol', slot_fn)));
+			push_prop(
+				b.prop(
+					'init',
+					b.id('children'),
+					context.state.options.dev ? b.call('$.add_snippet_symbol', slot_fn) : slot_fn
+				)
+			);
+			// We additionally add the default slot as a boolean, so that the slot render function on the other
+			// side knows it should get the content to render from $$props.children
+			serialized_slots.push(b.init('default', b.true));
 		} else {
 			const slot = b.prop('init', b.literal(slot_name), slot_fn);
 			serialized_slots.push(slot);
@@ -1619,7 +1628,9 @@ const template_visitors = {
 		// TODO hoist where possible
 		context.state.init.push(fn);
 
-		context.state.init.push(b.stmt(b.call('$.add_snippet_symbol', node.expression)));
+		if (context.state.options.dev) {
+			context.state.init.push(b.stmt(b.call('$.add_snippet_symbol', node.expression)));
+		}
 	},
 	Component(node, context) {
 		const state = context.state;

packages/svelte/src/internal/client/dom/elements/custom-element.js

@@ -1,6 +1,5 @@
 import { createClassComponent } from '../../../../legacy/legacy-client.js';
 import { destroy_effect, render_effect } from '../../reactivity/effects.js';
-import { add_snippet_symbol } from '../blocks/snippet.js';
 import { append } from '../template.js';
 import { define_property, object_keys } from '../../utils.js';
 
@@ -111,7 +110,8 @@ if (typeof HTMLElement === 'function') {
 				for (const name of this.$$s) {
 					if (name in existing_slots) {
 						if (name === 'default' && !this.$$d.children) {
-							this.$$d.children = add_snippet_symbol(create_slot(name));
+							this.$$d.children = create_slot(name);
+							$$slots.default = true;
 						} else {
 							$$slots[name] = create_slot(name);
 						}

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

@@ -1,4 +1,3 @@
-import { is_snippet } from './dom/blocks/snippet.js';
 import { untrack } from './runtime.js';
 import { get_descriptor, is_array } from './utils.js';
 import * as e from './errors.js';

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

@@ -10,10 +10,6 @@ import {
 } from '../../constants.js';
 import { escape_html } from '../../escaping.js';
 import { DEV } from 'esm-env';
-
-export * from '../client/validate.js';
-export { add_snippet_symbol } from '../client/dom/blocks/snippet.js';
-export { default_slot } from '../client/dom/legacy/misc.js';
 import { current_component, pop, push } from './context.js';
 import { BLOCK_CLOSE, BLOCK_OPEN } from './hydration.js';
 import { validate_store } from '../shared/validate.js';
@@ -280,8 +276,9 @@ export function spread_attributes(attrs, lowercase_attributes, is_html, class_ha
 	for (let i = 0; i < attrs.length; i++) {
 		const obj = attrs[i];
 		for (key in obj) {
-			// omit functions
-			if (typeof obj[key] !== 'function') {
+			// omit functions and internal svelte properties
+			const prefix = key[0] + key[1]; // this is faster than key.slice(0, 2)
+			if (typeof obj[key] !== 'function' && prefix !== '$$') {
 				merged_attrs[key] = obj[key];
 			}
 		}
@@ -626,6 +623,7 @@ export function once(get_value) {
 export { push, pop } from './context.js';
 
 export {
+	add_snippet_symbol,
 	validate_component,
 	validate_dynamic_element_tag,
 	validate_snippet,
@@ -633,3 +631,5 @@ export {
 } from '../shared/validate.js';
 
 export { escape_html as escape };
+
+export { default_slot } from '../client/dom/legacy/misc.js';

packages/svelte/tests/snapshot/samples/function-prop-no-getter/_expected/client/index.svelte.js

@@ -16,12 +16,13 @@ export default function Function_prop_no_getter($$anchor) {
 		onmousedown: () => $.set(count, $.get(count) + 1),
 		onmouseup,
 		onmouseenter: () => $.set(count, $.proxy(plusOne($.get(count)))),
-		children: $.add_snippet_symbol(($$anchor, $$slotProps) => {
+		children: ($$anchor, $$slotProps) => {
 			var text = $.text($$anchor);
 
 			$.template_effect(() => $.set_text(text, `clicks: ${$.stringify($.get(count))}`));
 			$.append($$anchor, text);
-		})
+		},
+		$$slots: { default: true }
 	});
 
 	$.append($$anchor, fragment);

packages/svelte/tests/snapshot/samples/function-prop-no-getter/_expected/server/index.svelte.js

@@ -17,9 +17,10 @@ export default function Function_prop_no_getter($$payload, $$props) {
 		onmousedown: () => count += 1,
 		onmouseup,
 		onmouseenter: () => count = plusOne(count),
-		children: $.add_snippet_symbol(($$payload, $$slotProps) => {
+		children: ($$payload, $$slotProps) => {
 			$$payload.out += `clicks: ${$.escape(count)}`;
-		})
+		},
+		$$slots: { default: true }
 	});
 
 	$$payload.out += `<!--]-->`;