fix: check boundary `pending` attribute at runtime on server (#16855)

* fix: check boundary `pending` attribute at runtime on server

* fix

---------

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

.changeset/lazy-cooks-return.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: check boundary `pending` attribute at runtime on server

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

@@ -7,6 +7,7 @@ import {
 	block_open,
 	block_open_else,
 	build_attribute_value,
+	build_template,
 	create_async_block
 } from './shared/utils.js';
 
@@ -19,6 +20,11 @@ export function SvelteBoundary(node, context) {
 	const pending_attribute = /** @type {AST.Attribute} */ (
 		node.attributes.find((node) => node.type === 'Attribute' && node.name === 'pending')
 	);
+	const is_pending_attr_nullish =
+		pending_attribute &&
+		typeof pending_attribute.value === 'object' &&
+		!Array.isArray(pending_attribute.value) &&
+		!context.state.scope.evaluate(pending_attribute.value.expression).is_defined;
 
 	const pending_snippet = /** @type {AST.SnippetBlock} */ (
 		node.fragment.nodes.find(
@@ -27,6 +33,24 @@ export function SvelteBoundary(node, context) {
 	);
 
 	if (pending_attribute || pending_snippet) {
+		if (pending_attribute && is_pending_attr_nullish && !pending_snippet) {
+			const callee = build_attribute_value(
+				pending_attribute.value,
+				context,
+				(expression) => expression,
+				false,
+				true
+			);
+			const pending = b.call(callee, b.id('$$renderer'));
+			const block = /** @type {BlockStatement} */ (context.visit(node.fragment));
+			context.state.template.push(
+				b.if(
+					callee,
+					b.block(build_template([block_open_else, pending, block_close])),
+					b.block(build_template([block_open, block, block_close]))
+				)
+			);
+		} else {
 			const pending = pending_attribute
 				? b.call(
 						build_attribute_value(
@@ -39,8 +63,8 @@ export function SvelteBoundary(node, context) {
 						b.id('$$renderer')
 					)
 				: /** @type {BlockStatement} */ (context.visit(pending_snippet.body));
-
 			context.state.template.push(block_open_else, pending, block_close);
+		}
 	} else {
 		const block = /** @type {BlockStatement} */ (context.visit(node.fragment));
 		const statement = node.fragment.metadata.has_await

packages/svelte/src/compiler/phases/scope.js

@@ -260,6 +260,12 @@ class Evaluation {
 						break;
 					}
 
+					if (binding.initial?.type === 'SnippetBlock') {
+						this.is_defined = true;
+						this.is_known = false;
+						break;
+					}
+
 					if (!binding.updated && binding.initial !== null && !is_prop) {
 						binding.scope.evaluate(/** @type {Expression} */ (binding.initial), this.values);
 						break;

packages/svelte/tests/server-side-rendering/samples/async-nullish-pending-snippet/_expected.html

@@ -0,0 +1 @@
+<!--[--><!--[--><!---->awaited<!--]--><!--]-->

packages/svelte/tests/server-side-rendering/samples/async-nullish-pending-snippet/main.svelte

@@ -0,0 +1,6 @@
+<script>
+    let pending = null;
+</script>
+<svelte:boundary {pending}>
+    {await 'awaited'}
+</svelte:boundary>