Add 404 error page and update book configuration

book.toml

@@ -72,6 +72,7 @@ additional-css = [
 site-url = "/comprehensive-rust/"
 git-repository-url = "https://github.com/google/comprehensive-rust"
 edit-url-template = "https://github.com/google/comprehensive-rust/edit/main/{path}"
+input-404 = "404.md"
 
 [output.html.fold]
 enable = true

src/404.md

@@ -0,0 +1,39 @@
+# Document not found (404)
+
+This URL is invalid, sorry. Please use the navigation bar or search to continue.
+
+<script>
+(function () {
+  const baseEl = document.querySelector("base");
+  const baseHref = baseEl ? baseEl.getAttribute("href") || "/" : "/";
+  const knownLangs = new Set(
+    Array.from(document.querySelectorAll("#language-list a[id]"), (link) => link.id),
+  );
+
+  const basePath = new URL(baseHref, window.location.origin).pathname;
+
+  const fullPath = window.location.pathname;
+
+  let relPath = fullPath.startsWith(basePath)
+    ? fullPath.slice(basePath.length)
+    : fullPath.replace(/^\/+/, "");
+
+  if (!relPath) {
+    return;
+  }
+
+  const segments = relPath.split("/");
+  const maybeLang = segments[0];
+
+  if (!knownLangs.has(maybeLang) || maybeLang === "en") {
+    return;
+  }
+
+  if (relPath === maybeLang + "/404.html" || relPath === "404.html") {
+    return;
+  }
+
+  const targetUrl = new URL(maybeLang + "/404.html", baseHref).toString();
+  window.location.replace(targetUrl);
+})();
+</script>