Fix MDX heading IDs generation when using a frontmatter reference (#5978)

* Fix MDX heading IDs generation when using a frontmatter reference

* Hoist safelyGetAstroData() call and add statement null check

6 files changed, 172 insertions, 3 deletions

diff --git a/.changeset/eighty-knives-remain.md b/.changeset/eighty-knives-remain.md
new file mode 100644
index 000000000..72cd5c76f
--- /dev/null
+++ b/.changeset/eighty-knives-remain.md
@@ -0,0 +1,6 @@
+---
+'@astrojs/mdx': patch
+'@astrojs/markdown-remark': patch
+---
+
+Fix MDX heading IDs generation when using a frontmatter reference
diff --git a/packages/integrations/mdx/test/fixtures/mdx-get-headings/src/pages/test-with-frontmatter.mdx b/packages/integrations/mdx/test/fixtures/mdx-get-headings/src/pages/test-with-frontmatter.mdx
new file mode 100644
index 000000000..d40537eb8
--- /dev/null
+++ b/packages/integrations/mdx/test/fixtures/mdx-get-headings/src/pages/test-with-frontmatter.mdx
@@ -0,0 +1,45 @@
+---
+title: The Frontmatter Title
+keywords: [Keyword 1, Keyword 2, Keyword 3]
+tags:
+  - Tag 1
+  - Tag 2
+  - Tag 3
+items:
+  - value: Item 1
+  - value: Item 2
+  - value: Item 3
+nested_items:
+  nested:
+    - value: Nested Item 1
+    - value: Nested Item 2
+    - value: Nested Item 3
+---
+
+# {frontmatter.title}
+
+This ID should be the frontmatter title.
+
+## frontmatter.title
+
+The ID should not be the frontmatter title.
+
+### {frontmatter.keywords[1]}
+
+The ID should be the frontmatter keyword #2.
+
+### {frontmatter.tags[0]}
+
+The ID should be the frontmatter tag #1.
+
+#### {frontmatter.items[1].value}
+
+The ID should be the frontmatter item #2.
+
+##### {frontmatter.nested_items.nested[2].value}
+
+The ID should be the frontmatter nested item #3.
+
+###### {frontmatter.unknown}
+
+This ID should not reference the frontmatter.
diff --git a/packages/integrations/mdx/test/mdx-get-headings.test.js b/packages/integrations/mdx/test/mdx-get-headings.test.js
index 03290abc5..1b1987e77 100644
--- a/packages/integrations/mdx/test/mdx-get-headings.test.js
+++ b/packages/integrations/mdx/test/mdx-get-headings.test.js
@@ -149,3 +149,46 @@ describe('MDX heading IDs can be injected before user plugins', () => {
 		expect(h1?.id).to.equal('heading-test');
 	});
 });
+
+describe('MDX headings with frontmatter', () => {
+	let fixture;
+
+	before(async () => {
+		fixture = await loadFixture({
+			root: new URL('./fixtures/mdx-get-headings/', import.meta.url),
+			integrations: [mdx()],
+		});
+
+		await fixture.build();
+	});
+
+	it('adds anchor IDs to headings', async () => {
+		const html = await fixture.readFile('/test-with-frontmatter/index.html');
+		const { document } = parseHTML(html);
+
+		const h3Ids = document.querySelectorAll('h3').map((el) => el?.id);
+
+		expect(document.querySelector('h1').id).to.equal('the-frontmatter-title');
+		expect(document.querySelector('h2').id).to.equal('frontmattertitle');
+		expect(h3Ids).to.contain('keyword-2');
+		expect(h3Ids).to.contain('tag-1');
+		expect(document.querySelector('h4').id).to.equal('item-2');
+		expect(document.querySelector('h5').id).to.equal('nested-item-3');
+		expect(document.querySelector('h6').id).to.equal('frontmatterunknown');
+	});
+
+	it('generates correct getHeadings() export', async () => {
+		const { headingsByPage } = JSON.parse(await fixture.readFile('/pages.json'));
+		expect(JSON.stringify(headingsByPage['./test-with-frontmatter.mdx'])).to.equal(
+			JSON.stringify([
+				{ depth: 1, slug: 'the-frontmatter-title', text: 'The Frontmatter Title' },
+				{ depth: 2, slug: 'frontmattertitle', text: 'frontmatter.title' },
+				{ depth: 3, slug: 'keyword-2', text: 'Keyword 2' },
+				{ depth: 3, slug: 'tag-1', text: 'Tag 1' },
+				{ depth: 4, slug: 'item-2', text: 'Item 2' },
+				{ depth: 5, slug: 'nested-item-3', text: 'Nested Item 3' },
+				{ depth: 6, slug: 'frontmatterunknown', text: 'frontmatter.unknown' },
+			])
+		);
+	});
+});
diff --git a/packages/markdown/remark/package.json b/packages/markdown/remark/package.json
index 9d4641a36..9ed26a5dc 100644
--- a/packages/markdown/remark/package.json
+++ b/packages/markdown/remark/package.json
@@ -44,6 +44,7 @@
   },
   "devDependencies": {
     "@types/chai": "^4.3.1",
+    "@types/estree": "^1.0.0",
     "@types/github-slugger": "^1.3.0",
     "@types/hast": "^2.3.4",
     "@types/mdast": "^3.0.10",
@@ -51,6 +52,7 @@
     "@types/unist": "^2.0.6",
     "astro-scripts": "workspace:*",
     "chai": "^4.3.6",
+    "mdast-util-mdx-expression": "^1.3.1",
     "mocha": "^9.2.2"
   }
 }
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']];
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index c3b3920a8..9a4d0f645 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -3508,6 +3508,7 @@ importers:
     specifiers:
       '@astrojs/prism': ^2.0.0
       '@types/chai': ^4.3.1
+      '@types/estree': ^1.0.0
       '@types/github-slugger': ^1.3.0
       '@types/hast': ^2.3.4
       '@types/mdast': ^3.0.10
@@ -3517,6 +3518,7 @@ importers:
       chai: ^4.3.6
       github-slugger: ^1.4.0
       import-meta-resolve: ^2.1.0
+      mdast-util-mdx-expression: ^1.3.1
       mocha: ^9.2.2
       rehype-raw: ^6.1.1
       rehype-stringify: ^9.0.3
@@ -3544,6 +3546,7 @@ importers:
       vfile: 5.3.6
     devDependencies:
       '@types/chai': 4.3.4
+      '@types/estree': 1.0.0
       '@types/github-slugger': 1.3.0
       '@types/hast': 2.3.4
       '@types/mdast': 3.0.10
@@ -3551,6 +3554,7 @@ importers:
       '@types/unist': 2.0.6
       astro-scripts: link:../../../scripts
       chai: 4.3.7
+      mdast-util-mdx-expression: 1.3.1
       mocha: 9.2.2
 
   packages/telemetry: