Load configs with Vite when loading with Proload fails (#4112)

* 4078 breaking test

* Use Vite for loading the config

* Try it

* Fallback to loading with Vite only when needed

* Remove console.error

* Remove extra console.log

* Add a changeset

* Use middlewareMode

7 files changed, 80 insertions, 33 deletions

diff --git a/.changeset/dull-radios-sparkle.md b/.changeset/dull-radios-sparkle.md
new file mode 100644
index 000000000..76f207898
--- /dev/null
+++ b/.changeset/dull-radios-sparkle.md
@@ -0,0 +1,6 @@
+---
+'astro': patch
+'@astrojs/mdx': patch
+---
+
+Fix MDX working with a ts config file
diff --git a/packages/astro/src/core/config.ts b/packages/astro/src/core/config.ts
index d5525cbf8..b8057df99 100644
--- a/packages/astro/src/core/config.ts
+++ b/packages/astro/src/core/config.ts
@@ -16,6 +16,7 @@ import { z } from 'zod';
 import { LogOptions } from './logger/core.js';
 import { appendForwardSlash, prependForwardSlash, trimSlashes } from './path.js';
 import { arraify, isObject } from './util.js';
+import * as vite from 'vite';
 
 load.use([loadTypeScript]);
 
@@ -409,10 +410,13 @@ export async function resolveConfigURL(
 	const flags = resolveFlags(configOptions.flags || {});
 	let userConfigPath: string | undefined;
 
+	
+
 	if (flags?.config) {
 		userConfigPath = /^\.*\//.test(flags.config) ? flags.config : `./${flags.config}`;
 		userConfigPath = fileURLToPath(new URL(userConfigPath, `file://${root}/`));
 	}
+
 	// Resolve config file path using Proload
 	// If `userConfigPath` is `undefined`, Proload will search for `astro.config.[cm]?[jt]s`
 	const configPath = await resolve('astro', {
@@ -447,21 +451,7 @@ export async function openConfig(configOptions: LoadConfigOptions): Promise<Open
 		);
 	}
 
-	// Automatically load config file using Proload
-	// If `userConfigPath` is `undefined`, Proload will search for `astro.config.[cm]?[jt]s`
-	let config;
-	try {
-		config = await load('astro', {
-			mustExist: !!userConfigPath,
-			cwd: root,
-			filePath: userConfigPath,
-		});
-	} catch (err) {
-		if (err instanceof ProloadError && flags.config) {
-			throw new Error(`Unable to resolve --config "${flags.config}"! Does the file exist?`);
-		}
-		throw err;
-	}
+	const config = await tryLoadConfig(configOptions, flags, userConfigPath, root);
 	if (config) {
 		userConfig = config.value;
 		userConfigPath = config.filePath;
@@ -483,6 +473,52 @@ export async function openConfig(configOptions: LoadConfigOptions): Promise<Open
 	};
 }
 
+interface TryLoadConfigResult {
+	value: Record<string, any>;
+	filePath?: string;
+}
+
+async function tryLoadConfig(configOptions: LoadConfigOptions, flags: CLIFlags, userConfigPath: string | undefined, root: string): Promise<TryLoadConfigResult | undefined> {
+	try {
+		// Automatically load config file using Proload
+		// If `userConfigPath` is `undefined`, Proload will search for `astro.config.[cm]?[jt]s`
+		const config = await load('astro', {
+			mustExist: !!userConfigPath,
+			cwd: root,
+			filePath: userConfigPath,
+		});
+
+		return config as TryLoadConfigResult;
+	} catch(e) {
+		if (e instanceof ProloadError && flags.config) {
+			throw new Error(`Unable to resolve --config "${flags.config}"! Does the file exist?`);
+		}
+
+		const configURL = await resolveConfigURL(configOptions);
+		if(!configURL) {
+			throw e;
+		}
+
+		// Fallback to use Vite DevServer
+		const viteServer = await vite.createServer({
+			server: { middlewareMode: true, hmr: false },
+			appType: 'custom'
+		});
+		try {
+			const mod = await viteServer.ssrLoadModule(fileURLToPath(configURL));
+
+			if(mod?.default) {
+				return {
+					value: mod.default,
+					filePath: fileURLToPath(configURL),
+				};
+			};
+		} finally {
+			await viteServer.close();
+		}
+	}
+}
+
 /**
  * Attempt to load an `astro.config.mjs` file
  * @deprecated
@@ -500,21 +536,7 @@ export async function loadConfig(configOptions: LoadConfigOptions): Promise<Astr
 		);
 	}
 
-	// Automatically load config file using Proload
-	// If `userConfigPath` is `undefined`, Proload will search for `astro.config.[cm]?[jt]s`
-	let config;
-	try {
-		config = await load('astro', {
-			mustExist: !!userConfigPath,
-			cwd: root,
-			filePath: userConfigPath,
-		});
-	} catch (err) {
-		if (err instanceof ProloadError && flags.config) {
-			throw new Error(`Unable to resolve --config "${flags.config}"! Does the file exist?`);
-		}
-		throw err;
-	}
+	const config = await tryLoadConfig(configOptions, flags, userConfigPath, root);
 	if (config) {
 		userConfig = config.value;
 	}
diff --git a/packages/integrations/mdx/src/index.ts b/packages/integrations/mdx/src/index.ts
index 139e08515..81187cc60 100644
--- a/packages/integrations/mdx/src/index.ts
+++ b/packages/integrations/mdx/src/index.ts
@@ -52,7 +52,7 @@ export default function mdx(mdxOptions: MdxOptions = {}): AstroIntegration {
 						// Workarounds tried:
 						// - "import * as remarkShikiTwoslash"
 						// - "import { default as remarkShikiTwoslash }"
-						(remarkShikiTwoslash as any).default,
+						(remarkShikiTwoslash as any).default ?? remarkShikiTwoslash,
 						config.markdown.shikiConfig,
 					]);
 					rehypePlugins.push([rehypeRaw, { passThrough: nodeTypes }]);
diff --git a/packages/integrations/mdx/test/fixtures/mdx-page/astro.config.ts b/packages/integrations/mdx/test/fixtures/mdx-page/astro.config.ts
new file mode 100644
index 000000000..f1d5e8bd7
--- /dev/null
+++ b/packages/integrations/mdx/test/fixtures/mdx-page/astro.config.ts
@@ -0,0 +1,5 @@
+import mdx from '@astrojs/mdx';
+
+export default {
+	integrations: [mdx()]
+}
diff --git a/packages/integrations/mdx/test/fixtures/mdx-page/package.json b/packages/integrations/mdx/test/fixtures/mdx-page/package.json
new file mode 100644
index 000000000..c8f3217b3
--- /dev/null
+++ b/packages/integrations/mdx/test/fixtures/mdx-page/package.json
@@ -0,0 +1,7 @@
+{
+  "name": "@test/mdx-page",
+  "dependencies": {
+    "astro": "workspace:*",
+    "@astrojs/mdx": "workspace:*"
+  }
+}
diff --git a/packages/integrations/mdx/test/mdx-page.test.js b/packages/integrations/mdx/test/mdx-page.test.js
index e375a9f17..487923d42 100644
--- a/packages/integrations/mdx/test/mdx-page.test.js
+++ b/packages/integrations/mdx/test/mdx-page.test.js
@@ -9,8 +9,7 @@ describe('MDX Page', () => {
 
 	before(async () => {
 		fixture = await loadFixture({
-			root: new URL('./fixtures/mdx-page/', import.meta.url),
-			integrations: [mdx()],
+			root: new URL('./fixtures/mdx-page/', import.meta.url)
 		});
 	});
 
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index b198726e9..8dc64c9fe 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -2231,6 +2231,14 @@ importers:
       reading-time: 1.5.0
       remark-toc: 8.0.1
 
+  packages/integrations/mdx/test/fixtures/mdx-page:
+    specifiers:
+      '@astrojs/mdx': workspace:*
+      astro: workspace:*
+    dependencies:
+      '@astrojs/mdx': link:../../..
+      astro: link:../../../../../astro
+
   packages/integrations/netlify:
     specifiers:
       '@astrojs/webapi': ^0.12.0