fix: ensure top level snippets are defined when binding to component prop (#11104)

...by hoisting top level snippets out of the binding loop in ssr mode
fixes #11086

.changeset/gold-tools-nail.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: ensure top level snippets are defined when binding to component prop

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

@@ -1614,14 +1614,15 @@ const template_visitors = {
 		state.template.push(block_close);
 	},
 	SnippetBlock(node, context) {
-		// TODO hoist where possible
-		context.state.init.push(
-			b.function_declaration(
-				node.expression,
-				[b.id('$$payload'), ...node.parameters],
-				/** @type {import('estree').BlockStatement} */ (context.visit(node.body))
-			)
+		const fn = b.function_declaration(
+			node.expression,
+			[b.id('$$payload'), ...node.parameters],
+			/** @type {import('estree').BlockStatement} */ (context.visit(node.body))
 		);
+		// @ts-expect-error - TODO remove this hack once $$render_inner for legacy bindings is gone
+		fn.___snippet = true;
+		// TODO hoist where possible
+		context.state.init.push(fn);
 
 		if (context.state.options.dev) {
 			context.state.init.push(b.stmt(b.call('$.add_snippet_symbol', node.expression)));
@@ -2221,14 +2222,27 @@ export function server_component(analysis, options) {
 	// If the component binds to a child, we need to put the template in a loop and repeat until legacy bindings are stable.
 	// We can remove this once the legacy syntax is gone.
 	if (analysis.uses_component_bindings) {
+		const snippets = template.body.filter(
+			(node) =>
+				node.type === 'FunctionDeclaration' &&
+				// @ts-expect-error
+				node.___snippet
+		);
+		const rest = template.body.filter(
+			(node) =>
+				node.type !== 'FunctionDeclaration' ||
+				// @ts-expect-error
+				!node.___snippet
+		);
 		template.body = [
+			...snippets,
 			b.let('$$settled', b.true),
 			b.let('$$inner_payload'),
 			b.stmt(
 				b.function(
 					b.id('$$render_inner'),
 					[b.id('$$payload')],
-					b.block(/** @type {import('estree').Statement[]} */ (template.body))
+					b.block(/** @type {import('estree').Statement[]} */ (rest))
 				)
 			),
 			b.do_while(

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

@@ -0,0 +1,39 @@
+// index.svelte (Svelte VERSION)
+// Note: compiler output will change before 5.0 is released!
+import "svelte/internal/disclose-version";
+import * as $ from "svelte/internal/client";
+import TextInput from './Child.svelte';
+
+var root_1 = $.template(`Something`, 1);
+var root = $.template(`<!> `, 1);
+
+export default function Bind_component_snippet($$anchor, $$props) {
+	$.push($$props, true);
+
+	let value = $.source('');
+	const _snippet = snippet;
+	var fragment_1 = root();
+
+	function snippet($$anchor) {
+		var fragment = root_1();
+
+		$.append($$anchor, fragment);
+	}
+
+	var node = $.first_child(fragment_1);
+
+	TextInput(node, {
+		get value() {
+			return $.get(value);
+		},
+		set value($$value) {
+			$.set(value, $.proxy($$value));
+		}
+	});
+
+	var text = $.sibling(node, true);
+
+	$.render_effect(() => $.set_text(text, ` value: ${$.stringify($.get(value))}`));
+	$.append($$anchor, fragment_1);
+	$.pop();
+}

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

@@ -0,0 +1,43 @@
+// index.svelte (Svelte VERSION)
+// Note: compiler output will change before 5.0 is released!
+import * as $ from "svelte/internal/server";
+import TextInput from './Child.svelte';
+
+export default function Bind_component_snippet($$payload, $$props) {
+	$.push(true);
+
+	let value = '';
+	const _snippet = snippet;
+
+	function snippet($$payload) {
+		$$payload.out += `Something`;
+	}
+
+	let $$settled = true;
+	let $$inner_payload;
+
+	function $$render_inner($$payload) {
+		$$payload.out += `<!--[-->`;
+
+		TextInput($$payload, {
+			get value() {
+				return value;
+			},
+			set value($$value) {
+				value = $$value;
+				$$settled = false;
+			}
+		});
+
+		$$payload.out += `<!--]--> value: ${$.escape(value)}`;
+	};
+
+	do {
+		$$settled = true;
+		$$inner_payload = $.copy_payload($$payload);
+		$$render_inner($$inner_payload);
+	} while (!$$settled);
+
+	$.assign_payload($$payload, $$inner_payload);
+	$.pop();
+}

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

@@ -0,0 +1,13 @@
+<script>
+	import TextInput from './Child.svelte';
+
+	let value = $state('');
+	const _snippet = snippet;
+</script>
+
+{#snippet snippet()}
+	Something
+{/snippet}
+
+<TextInput bind:value />
+value: {value}