1 files changed, 113 insertions, 0 deletions
diff --git a/packages/integrations/mdx/src/rehype-apply-frontmatter-export.ts b/packages/integrations/mdx/src/rehype-apply-frontmatter-export.ts
new file mode 100644
index 000000000..5880c30b3
--- /dev/null
+++ b/packages/integrations/mdx/src/rehype-apply-frontmatter-export.ts
@@ -0,0 +1,113 @@
+import path from 'node:path';
+import { fileURLToPath } from 'node:url';
+import { isFrontmatterValid } from '@astrojs/markdown-remark';
+import type { Root, RootContent } from 'hast';
+import type { VFile } from 'vfile';
+import { jsToTreeNode } from './utils.js';
+
+// Passed metadata to help determine adding charset utf8 by default
+declare module 'vfile' {
+	interface DataMap {
+		applyFrontmatterExport?: {
+			srcDir?: URL;
+		};
+	}
+}
+
+const exportConstPartialTrueRe = /export\s+const\s+partial\s*=\s*true/;
+
+export function rehypeApplyFrontmatterExport() {
+	return function (tree: Root, vfile: VFile) {
+		const frontmatter = vfile.data.astro?.frontmatter;
+		if (!frontmatter || !isFrontmatterValid(frontmatter))
+			throw new Error(
+				// Copied from Astro core `errors-data`
+				// TODO: find way to import error data from core
+				'[MDX] A remark or rehype plugin attempted to inject invalid frontmatter. Ensure "astro.frontmatter" is set to a valid JSON object that is not `null` or `undefined`.',
+			);
+		const extraChildren: RootContent[] = [
+			jsToTreeNode(`export const frontmatter = ${JSON.stringify(frontmatter)};`),
+		];
+		if (frontmatter.layout) {
+			extraChildren.unshift(
+				jsToTreeNode(
+					// NOTE: Use `__astro_*` import names to prevent conflicts with user code
+					/** @see 'vite-plugin-markdown' for layout props reference */
+					`\
+import { jsx as __astro_layout_jsx__ } from 'astro/jsx-runtime';
+import __astro_layout_component__ from ${JSON.stringify(frontmatter.layout)};
+
+export default function ({ children }) {
+	const { layout, ...content } = frontmatter;
+	content.file = file;
+	content.url = url;
+	return __astro_layout_jsx__(__astro_layout_component__, {
+		file,
+		url,
+		content,
+		frontmatter: content,
+		headings: getHeadings(),
+		'server:root': true,
+		children,
+	});
+};`,
+				),
+			);
+		} else if (shouldAddCharset(tree, vfile)) {
+			extraChildren.unshift({
+				type: 'mdxJsxFlowElement',
+				name: 'meta',
+				attributes: [
+					{
+						type: 'mdxJsxAttribute',
+						name: 'charset',
+						value: 'utf-8',
+					},
+				],
+				children: [],
+			});
+		}
+		tree.children = extraChildren.concat(tree.children);
+	};
+}
+
+/**
+ * If this is a page (e.g. in src/pages), has no layout frontmatter (handled before calling this function),
+ * has no leading component that looks like a wrapping layout, and `partial` isn't set to true, we default to
+ * adding charset=utf-8 like markdown so that users don't have to worry about it for MDX pages without layouts.
+ */
+function shouldAddCharset(tree: Root, vfile: VFile) {
+	const srcDirUrl = vfile.data.applyFrontmatterExport?.srcDir;
+	if (!srcDirUrl) return false;
+
+	const hasConstPartialTrue = tree.children.some(
+		(node) => node.type === 'mdxjsEsm' && exportConstPartialTrueRe.test(node.value),
+	);
+	if (hasConstPartialTrue) return false;
+
+	// NOTE: the pages directory is a non-configurable Astro behaviour
+	const pagesDir = path.join(fileURLToPath(srcDirUrl), 'pages').replace(/\\/g, '/');
+	// `vfile.path` comes from Vite, which is a normalized path (no backslashes)
+	const filePath = vfile.path;
+	if (!filePath.startsWith(pagesDir)) return false;
+
+	const hasLeadingUnderscoreInPath = filePath
+		.slice(pagesDir.length)
+		.replace(/\\/g, '/')
+		.split('/')
+		.some((part) => part.startsWith('_'));
+	if (hasLeadingUnderscoreInPath) return false;
+
+	// Bail if the first content found is a wrapping layout component
+	for (const child of tree.children) {
+		if (child.type === 'element') break;
+		if (child.type === 'mdxJsxFlowElement') {
+			// If is fragment or lowercase tag name (html tags), skip and assume there's no layout
+			if (child.name == null) break;
+			if (child.name[0] === child.name[0].toLowerCase()) break;
+			return false;
+		}
+	}
+
+	return true;
+}