feat: add stable route for all warning/error codes

This establishes a new route `e/[code]` which serves as a stable reference for all of our warning/error codes. All links to warnings/errors from the Svelte compiler/runtime should go through this route in order to have stable references. That way we can move codes between warnings/errors/different pages or even adjust codes without breaking links.

Part of sveltejs/svelte#11305

Author: dummdidumm

apps/svelte.dev/src/lib/server/content.ts

@@ -60,16 +60,16 @@ export const blog_posts = index.blog.children
 		return a.date < b.date ? 1 : -1;
 	});
 
+export function remove_section(slug: string) {
+	return slug.replace(/\/[^/]+(\/[^/]+)$/g, '$1');
+}
+
 /**
  * Create docs index, which is basically the same structure as the original index
  * but with adjusted slugs (the section part is omitted for cleaner URLs), separated
  * topics/pages and next/prev adjusted so that they don't point to different topics.
  */
 function create_docs() {
-	function remove_section(slug: string) {
-		return slug.replace(/\/[^/]+(\/[^/]+)$/g, '$1');
-	}
-
 	let docs: {
 		/** The top level entries/packages: svelte/kit/etc. Key is the topic */
 		topics: Record<string, Document>;

apps/svelte.dev/src/routes/e/[code]/+page.server.ts

@@ -0,0 +1,37 @@
+import { index, remove_section } from '$lib/server/content';
+import { error, redirect } from '@sveltejs/kit';
+
+// All links to warnings/errors from the Svelte compiler/runtime go through this page in order to have stable references
+// (i.e. we can move codes between warnings/errors/different pages or even adjust codes without breaking links).
+const reference = index['docs/svelte/reference'].children.filter(
+	(child) => child.slug.endsWith('-errors') || child.slug.endsWith('-warnings')
+);
+
+export const prerender = true;
+
+export function load({ params }) {
+	// Since codes are not top level section we gotta jump through some hoops to get the right hash
+	for (const page of reference) {
+		const idx = page.body.indexOf(`### ${params.code}`);
+		if (idx === -1) continue;
+
+		const slug_idx = page.body.lastIndexOf('\n## ', idx);
+
+		if (slug_idx > 0 || page.body.startsWith(`## `)) {
+			const title = page.body.slice(slug_idx + 4, page.body.indexOf('\n', slug_idx + 4)).trim();
+			const slug = page.sections.find((s) => s.title === title)?.slug;
+
+			if (!slug) {
+				throw new Error(
+					`Internal Error: Could not find previous title for code ${params.code} on ${page.slug}`
+				);
+			}
+
+			redirect(307, `/${remove_section(page.slug)}#${slug}-${params.code}`);
+		} else {
+			redirect(307, `/${remove_section(page.slug)}#${params.code}`);
+		}
+	}
+
+	error(404, 'Not found');
+}