1 files changed, 72 insertions, 3 deletions
diff --git a/packages/markdown/remark/src/rehype-collect-headings.ts b/packages/markdown/remark/src/rehype-collect-headings.ts
index 97fe30401..a1083f609 100644
--- a/packages/markdown/remark/src/rehype-collect-headings.ts
+++ b/packages/markdown/remark/src/rehype-collect-headings.ts
@@ -1,7 +1,10 @@
+import { type Expression, type Super } from 'estree';
 import Slugger from 'github-slugger';
-import { visit } from 'unist-util-visit';
+import { type MdxTextExpression } from 'mdast-util-mdx-expression';
+import { visit, type Node } from 'unist-util-visit';
 
-import type { MarkdownHeading, MarkdownVFile, RehypePlugin } from './types.js';
+import { InvalidAstroDataError, safelyGetAstroData } from './frontmatter-injection.js';
+import type { MarkdownAstroData, MarkdownHeading, MarkdownVFile, RehypePlugin } from './types.js';
 
 const rawNodeTypes = new Set(['text', 'raw', 'mdxTextExpression']);
 const codeTagNames = new Set(['code', 'pre']);
@@ -11,6 +14,7 @@ export function rehypeHeadingIds(): ReturnType<RehypePlugin> {
 		const headings: MarkdownHeading[] = [];
 		const slugger = new Slugger();
 		const isMDX = isMDXFile(file);
+		const astroData = safelyGetAstroData(file.data);
 		visit(tree, (node) => {
 			if (node.type !== 'element') return;
 			const { tagName } = node;
@@ -31,7 +35,17 @@ export function rehypeHeadingIds(): ReturnType<RehypePlugin> {
 				}
 				if (rawNodeTypes.has(child.type)) {
 					if (isMDX || codeTagNames.has(parent.tagName)) {
-						text += child.value;
+						let value = child.value;
+						if (isMdxTextExpression(child) && !(astroData instanceof InvalidAstroDataError)) {
+							const frontmatterPath = getMdxFrontmatterVariablePath(child);
+							if (Array.isArray(frontmatterPath) && frontmatterPath.length > 0) {
+								const frontmatterValue = getMdxFrontmatterVariableValue(astroData, frontmatterPath);
+								if (typeof frontmatterValue === 'string') {
+									value = frontmatterValue;
+								}
+							}
+						}
+						text += value;
 					} else {
 						text += child.value.replace(/\{/g, '${');
 					}
@@ -57,3 +71,58 @@ export function rehypeHeadingIds(): ReturnType<RehypePlugin> {
 function isMDXFile(file: MarkdownVFile) {
 	return Boolean(file.history[0]?.endsWith('.mdx'));
 }
+
+/**
+ * Check if an ESTree entry is `frontmatter.*.VARIABLE`.
+ * If it is, return the variable path (i.e. `["*", ..., "VARIABLE"]`) minus the `frontmatter` prefix.
+ */
+function getMdxFrontmatterVariablePath(node: MdxTextExpression): string[] | Error {
+	if (!node.data?.estree || node.data.estree.body.length !== 1) return new Error();
+
+	const statement = node.data.estree.body[0];
+
+	// Check for "[ANYTHING].[ANYTHING]".
+	if (statement?.type !== 'ExpressionStatement' || statement.expression.type !== 'MemberExpression')
+		return new Error();
+
+	let expression: Expression | Super = statement.expression;
+	const expressionPath: string[] = [];
+
+	// Traverse the expression, collecting the variable path.
+	while (
+		expression.type === 'MemberExpression' &&
+		expression.property.type === (expression.computed ? 'Literal' : 'Identifier')
+	) {
+		expressionPath.push(
+			expression.property.type === 'Literal'
+				? String(expression.property.value)
+				: expression.property.name
+		);
+
+		expression = expression.object;
+	}
+
+	// Check for "frontmatter.[ANYTHING]".
+	if (expression.type !== 'Identifier' || expression.name !== 'frontmatter') return new Error();
+
+	return expressionPath.reverse();
+}
+
+function getMdxFrontmatterVariableValue(astroData: MarkdownAstroData, path: string[]) {
+	let value: MdxFrontmatterVariableValue = astroData.frontmatter;
+
+	for (const key of path) {
+		if (!value[key]) return undefined;
+
+		value = value[key];
+	}
+
+	return value;
+}
+
+function isMdxTextExpression(node: Node): node is MdxTextExpression {
+	return node.type === 'mdxTextExpression';
+}
+
+type MdxFrontmatterVariableValue =
+	MarkdownAstroData['frontmatter'][keyof MarkdownAstroData['frontmatter']];