Adds dataLanguage property to the replacement <pre> element. (#10538)

* Update highlight.ts

* Create cold-snakes-train.md

* Update Code.astro

Solution for use-case described in withastro/roadmap#276 (withastro/roadmap#276)

* roll-back initial fix

* new fix

* update changeset

* Update packages/markdown/remark/src/rehype-prism.ts

* Update .changeset/cold-snakes-train.md

Co-authored-by: Sarah Rainsberger <sarah@rainsberger.ca>

* Update .changeset/cold-snakes-train.md

Co-authored-by: Sarah Rainsberger <sarah@rainsberger.ca>

* Update .changeset/cold-snakes-train.md

Co-authored-by: Sarah Rainsberger <sarah@rainsberger.ca>

* Update .changeset/cold-snakes-train.md

Co-authored-by: Sarah Rainsberger <sarah@rainsberger.ca>

---------

Co-authored-by: Bjorn Lu <bjornlu.dev@gmail.com>
Co-authored-by: Matthew Phillips <matthew@matthewphillips.info>
Co-authored-by: Sarah Rainsberger <sarah@rainsberger.ca>

.changeset/cold-snakes-train.md

@@ -0,0 +1,42 @@
+---
+"@astrojs/markdown-remark": minor
+---
+
+Adds a `data-language` attribute on the rendered `pre` elements to expose the highlighted syntax language.
+
+For example, the following Markdown code block will expose `data-language="python"`:
+```
+\```python
+def func():
+    print('Hello Astro!')
+\```
+```
+
+This allows retrieving the language in a rehype plugin from `node.properties.dataLanguage` by accessing the `<pre>` element using `{ tagName: "pre" }`:
+```js
+// myRehypePre.js
+import { visit } from "unist-util-visit";
+export default function myRehypePre() {
+  return (tree) => {
+    visit(tree, { tagName: "pre" }, (node) => {
+      const lang = node.properties.dataLanguage;
+      [...]
+    });
+  };
+}
+```
+
+Note: The `<pre>` element is not exposed when using Astro's `<Code />` component which outputs flattened HTML.
+
+
+The `data-language` attribute may also be used in css rules:
+```css
+pre::before {
+    content: attr(data-language);
+}
+
+pre[data-language="javascript"] {
+  font-size: 2rem;
+}
+```
+

packages/markdown/remark/src/rehype-prism.ts

@@ -9,7 +9,7 @@ export const rehypePrism: Plugin<[], Root> = () => {
 			let { html, classLanguage } = runHighlighterWithAstro(language, code);
 
 			return Promise.resolve(
-				`<pre class="${classLanguage}"><code is:raw class="${classLanguage}">${html}</code></pre>`
+				`<pre class="${classLanguage}" data-language="${language}"><code is:raw class="${classLanguage}">${html}</code></pre>`
 			);
 		});
 	};

packages/markdown/remark/src/shiki.ts

@@ -102,7 +102,10 @@ export async function createShikiHighlighter({
 
 							// Replace "shiki" class naming with "astro-code"
 							node.properties.class = classValue.replace(/shiki/g, 'astro-code');
-
+
+							// Add data-language attribute
+							node.properties.dataLanguage = lang;
+
 							// Handle code wrapping
 							// if wrap=null, do nothing.
 							if (wrap === false) {