fix: detect build-emitted assets in prerenderer fetch (#12201)

* prerender emitted assets

* mend

* simplify

* simplify

* Update .changeset/chilled-humans-roll.md

---------

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

Author: JelmerBot

.changeset/chilled-humans-roll.md

@@ -0,0 +1,5 @@
+---
+"@sveltejs/kit": patch
+---
+
+fix: `fetch` imported assets during prerender

packages/kit/src/core/postbuild/prerender.js

@@ -242,6 +242,11 @@ async function prerender({ hash, out, manifest_path, metadata, verbose, env }) {
 				const filepath = saved.get(file);
 				if (filepath) return readFileSync(filepath);
 
+				// Static assets emitted during build
+				if (file.startsWith(config.appDir)) {
+					return readFileSync(`${out}/server/${file}`);
+				}
+
 				// stuff in `static`
 				return readFileSync(join(config.files.assets, file));
 			},

packages/kit/test/prerendering/paths-base/src/routes/assets/emitted/+page.server.js

@@ -0,0 +1,8 @@
+// @ts-ignore
+import url from './message.csv?url';
+
+export async function load({ fetch }) {
+	const response = await fetch(url);
+	const message = await response.text();
+	return { message };
+}

packages/kit/test/prerendering/paths-base/src/routes/assets/emitted/+page.svelte

@@ -0,0 +1,5 @@
+<script>
+	const { data } = $props();
+</script>
+
+<p>{data.message}</p>

packages/kit/test/prerendering/paths-base/src/routes/assets/emitted/message.csv

@@ -0,0 +1 @@
+hello from message.csv

packages/kit/test/prerendering/paths-base/test/tests.spec.js

@@ -29,3 +29,8 @@ test('prerenders /path-base/assets', () => {
 	const content = read('assets.html');
 	assert.match(content, /<img[^>]+src="\/path-base\//u);
 });
+
+test('prerenders /path-base/assets/emitted', () => {
+	const content = read('assets/emitted.html');
+	assert.ok(content.includes('<p>hello from message.csv'));
+});