fix(html): generate fallback id for punctuation-only headings

crates/mdbook-html/src/html/print.rs

@@ -6,7 +6,7 @@
 
 use super::Node;
 use crate::html::{ChapterTree, Element, serialize};
-use crate::utils::{ToUrlPath, id_from_content, normalize_path, unique_id};
+use crate::utils::{ToUrlPath, id_from_content_or_fallback, normalize_path, unique_id};
 use mdbook_core::static_regex;
 use std::collections::{HashMap, HashSet};
 use std::path::PathBuf;
@@ -103,8 +103,7 @@ fn make_root_id_map(
             // TODO: This might want to be a warning? Chapters generally
             // should start with an h1.
             let mut h1 = Element::new("h1");
-            let id = id_from_content(&chapter.name);
-            let id = unique_id(&id, id_counter);
+            let id = id_from_content_or_fallback(&chapter.name, id_counter);
             h1.insert_attr("id", id.clone().into());
             let mut root = tree.root_mut();
             let mut h1 = root.prepend(Node::Element(h1));

crates/mdbook-html/src/html/tree.rs

@@ -7,7 +7,7 @@
 
 use super::tokenizer::parse_html;
 use super::{HtmlRenderOptions, hide_lines, wrap_rust_main};
-use crate::utils::{id_from_content, unique_id};
+use crate::utils::id_from_content_or_fallback;
 use ego_tree::{NodeId, NodeRef, Tree};
 use html5ever::tendril::StrTendril;
 use html5ever::tokenizer::{TagKind, Token};
@@ -924,8 +924,7 @@ where
                 let node_id = node.id();
                 let node_ref = self.tree.get(node_id).unwrap();
                 text_in_node(node_ref, &mut id);
-                let id = id_from_content(&id);
-                let id = unique_id(&id, &mut id_counter);
+                let id = id_from_content_or_fallback(&id, &mut id_counter);
                 let mut node = self.tree.get_mut(heading).unwrap();
                 let el = node.value().as_element_mut().unwrap();
                 let href = format!("#{id}");

crates/mdbook-html/src/utils.rs

@@ -99,6 +99,19 @@ pub(crate) fn id_from_content(content: &str) -> String {
         .collect()
 }
 
+/// Returns a non-empty HTML id for heading content.
+///
+/// Headings that contain only punctuation (e.g. `## ::`) produce an empty slug via
+/// [`id_from_content`]; this helper falls back to a generic prefix so anchor links work.
+pub(crate) fn id_from_content_or_fallback(content: &str, used: &mut HashSet<String>) -> String {
+    let id = id_from_content(content);
+    if id.is_empty() {
+        unique_id("section", used)
+    } else {
+        unique_id(&id, used)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -129,7 +142,15 @@ mod tests {
         assert_eq!(id_from_content("にほんご"), "にほんご");
         assert_eq!(id_from_content("한국어"), "한국어");
         assert_eq!(id_from_content(""), "");
+        assert_eq!(id_from_content("::"), "");
         assert_eq!(id_from_content("中文標題 CJK title"), "中文標題-cjk-title");
         assert_eq!(id_from_content("Über"), "über");
     }
+
+    #[test]
+    fn empty_heading_gets_fallback_id() {
+        let mut used = HashSet::new();
+        assert_eq!(id_from_content_or_fallback("::", &mut used), "section");
+        assert_eq!(id_from_content_or_fallback("::", &mut used), "section-1");
+    }
 }