fix: don't spread away `$$slots` from `$props` is it's used (#15849)

* fix: don't spread away `$$slots` from `$props` is it's used

* beef up test

* fix

* Update .changeset/ten-plants-carry.md

---------

Co-authored-by: Rich Harris <rich.harris@vercel.com>

.changeset/ten-plants-carry.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: don't redeclare `$$slots`

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

@@ -1,6 +1,7 @@
 /** @import { VariableDeclaration, VariableDeclarator, Expression, CallExpression, Pattern, Identifier } from 'estree' */
 /** @import { Binding } from '#compiler' */
 /** @import { Context } from '../types.js' */
+/** @import { ComponentAnalysis } from '../../../types.js' */
 /** @import { Scope } from '../../../scope.js' */
 import { build_fallback, extract_paths } from '../../../../utils/ast.js';
 import * as b from '#compiler/builders';
@@ -50,20 +51,26 @@ export function VariableDeclaration(node, context) {
 						}
 					}
 				});
+
+				// if `$$slots` is declared separately, deconflict
+				const slots_name = /** @type {ComponentAnalysis} */ (context.state.analysis).uses_slots
+					? b.id('$$slots_')
+					: b.id('$$slots');
+
 				if (id.type === 'ObjectPattern' && has_rest) {
 					// If a rest pattern is used within an object pattern, we need to ensure we don't expose $$slots or $$events
 					id.properties.splice(
 						id.properties.length - 1,
 						0,
 						// @ts-ignore
-						b.prop('init', b.id('$$slots'), b.id('$$slots')),
+						b.prop('init', b.id('$$slots'), slots_name),
 						b.prop('init', b.id('$$events'), b.id('$$events'))
 					);
 				} else if (id.type === 'Identifier') {
 					// If $props is referenced as an identifier, we need to ensure we don't expose $$slots or $$events as properties
 					// on the identifier reference
 					id = b.object_pattern([
-						b.prop('init', b.id('$$slots'), b.id('$$slots')),
+						b.prop('init', b.id('$$slots'), slots_name),
 						b.prop('init', b.id('$$events'), b.id('$$events')),
 						b.rest(b.id(id.name))
 					]);

packages/svelte/tests/runtime-runes/samples/props-and-slots/Child.svelte

@@ -0,0 +1,9 @@
+<script>
+	const props = $props();
+</script>
+
+<p>{Object.keys(props)}</p>
+
+{#if $$slots.foo}
+	<p>foo exists</p>
+{/if}

packages/svelte/tests/runtime-runes/samples/props-and-slots/_config.js

@@ -0,0 +1,8 @@
+import { test } from '../../test';
+
+export default test({
+	html: `
+		<p>a</p>
+		<p>foo exists</p>
+	`
+});

packages/svelte/tests/runtime-runes/samples/props-and-slots/main.svelte

@@ -0,0 +1,7 @@
+<script>
+	import Child from './Child.svelte';
+</script>
+
+<Child a="b">
+	<div slot="foo">foo</div>
+</Child>