fix: renderToString function not render properly nested slots when they are components (#13432)

* Fix/render to string slots (#1)

* add force HTMLString with local tests

* remove local test & logs

* remove test counter in basic exemple

* re-add test & create SlotString instead of HTMLString

* add test of a nested rendered component inside renderToString slots function

* add changeset on astro package

* Apply suggestions from code review

---------

Co-authored-by: Emanuele Stoppa <my.burning@gmail.com>

Co-authored-by: ematipico <602478+ematipico@users.noreply.github.com>

4 files changed, 41 insertions, 0 deletions

diff --git a/.changeset/wide-carrots-dream.md b/.changeset/wide-carrots-dream.md
new file mode 100644
index 000000000..b0e30081e
--- /dev/null
+++ b/.changeset/wide-carrots-dream.md
@@ -0,0 +1,5 @@
+---
+'astro': patch
+---
+
+Fix an issue in the Container API, where the `renderToString` function doesn't render adequately nested slots when they are components.
diff --git a/examples/container-with-vitest/src/components/CounterLight.astro b/examples/container-with-vitest/src/components/CounterLight.astro
new file mode 100644
index 000000000..7cee23bbe
--- /dev/null
+++ b/examples/container-with-vitest/src/components/CounterLight.astro
@@ -0,0 +1,9 @@
+---
+interface Props {
+	count?: number;
+}
+
+let { count = 0 } = Astro.props;
+---
+
+<p id="counter">{count}</p>
diff --git a/examples/container-with-vitest/test/Card.test.ts b/examples/container-with-vitest/test/Card.test.ts
index 26d766d1a..119087a36 100644
--- a/examples/container-with-vitest/test/Card.test.ts
+++ b/examples/container-with-vitest/test/Card.test.ts
@@ -1,6 +1,7 @@
 import { experimental_AstroContainer as AstroContainer } from 'astro/container';
 import { expect, test } from 'vitest';
 import Card from '../src/components/Card.astro';
+import CounterLight from '../src/components/CounterLight.astro';
 
 test('Card with slots', async () => {
 	const container = await AstroContainer.create();
@@ -13,3 +14,16 @@ test('Card with slots', async () => {
 	expect(result).toContain('This is a card');
 	expect(result).toContain('Card content');
 });
+
+test('Card with nested CounterLight', async () => {
+	const container = await AstroContainer.create();
+	const counterLight = await container.renderToString(CounterLight, { props: { count: 1 } });
+	const result = await container.renderToString(Card, {
+		slots: {
+			default: counterLight,
+		},
+	});
+
+	expect(result).toContain('This is a card');
+	expect(result).toContain(counterLight);
+});
diff --git a/packages/astro/src/container/index.ts b/packages/astro/src/container/index.ts
index 0a1b854d1..3943ad57b 100644
--- a/packages/astro/src/container/index.ts
+++ b/packages/astro/src/container/index.ts
@@ -26,6 +26,7 @@ import type {
 	SSRResult,
 } from '../types/public/internal.js';
 import { ContainerPipeline } from './pipeline.js';
+import { SlotString } from '../runtime/server/render/slot.js';
 
 /** Public type, used for integrations to define a renderer for the container API */
 export type ContainerRenderer = {
@@ -474,6 +475,10 @@ export class experimental_AstroContainer {
 		component: AstroComponentFactory,
 		options: ContainerRenderOptions = {},
 	): Promise<string> {
+		if (options.slots) {
+			options.slots = markAllSlotsAsSlotString(options.slots);
+		}
+
 		const response = await this.renderToResponse(component, options);
 		return await response.text();
 	}
@@ -588,3 +593,11 @@ export class experimental_AstroContainer {
 function isNamedRenderer(renderer: any): renderer is NamedSSRLoadedRendererValue {
 	return !!renderer?.name;
 }
+
+function markAllSlotsAsSlotString(slots: Record<string, any>): Record<string, any> {
+	const markedSlots: Record<string, any> = {};
+	for (const slotName in slots) {
+		markedSlots[slotName] = new SlotString(slots[slotName], null);
+	}
+	return markedSlots;
+}