Give a warning for unclosed HTML tags

This changes the internal error message to a warning to let the user
know that the HTML tags are unbalanced. In the future this will be a
denyable lint.

This is a very primitive approach of just ignoring the end tag. Ideally
it should recover using the standard HTML parsing algorithm, since there
is a chance that there will be a cascade of errors under certain
unbalanced situations.

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

@@ -19,7 +19,7 @@ use pulldown_cmark::{Alignment, CodeBlockKind, CowStr, Event, LinkType, Tag, Tag
 use std::borrow::Cow;
 use std::collections::{HashMap, HashSet};
 use std::ops::Deref;
-use tracing::{error, trace, warn};
+use tracing::{trace, warn};
 
 /// Helper to create a [`QualName`].
 macro_rules! attr_qual_name {
@@ -664,9 +664,18 @@ where
         *is_raw = false;
         if self.is_html_tag_matching(&tag.name) {
             self.pop();
+        } else {
+            // The proper thing to do here is to recover. However, the HTML
+            // parsing algorithm for that is quite complex. See
+            // https://html.spec.whatwg.org/multipage/parsing.html#parsing-main-inbody
+            // and the adoption agency algorithm.
+            warn!(
+                "unexpected HTML end tag `</{}>` found in `{}`\n\
+                 Check that the HTML tags are properly balanced.",
+                tag.name,
+                self.options.path.display()
+            );
         }
-        // else the stack is corrupt. I'm not really sure
-        // what to do here...
     }
 
     /// This is used to verify HTML parsing keeps the stack of tags in sync.
@@ -675,16 +684,10 @@ where
         if let Node::Element(el) = current
             && el.name() == name
         {
-            return true;
+            true
+        } else {
+            false
         }
-        error!(
-            "internal error: HTML tag stack out of sync.\n
-             path: `{}`\n\
-             current={current:?}\n\
-             pop name: {name}",
-            self.options.path.display()
-        );
-        false
     }
 
     /// Eats all pulldown-cmark events until the next `End` matching the

tests/testsuite/rendering.rs

@@ -274,18 +274,12 @@ fn unbalanced_html_tags() {
             cmd.expect_stderr(str![[r#"
  INFO Book building has started
  INFO Running the html backend
-ERROR internal error: HTML tag stack out of sync.
+ WARN unexpected HTML end tag `</div>` found in `chapter_1.md`
-
+Check that the HTML tags are properly balanced.
-             path: `chapter_1.md`
-current=Element(Element { name: QualName { prefix: None, ns: Atom('http://www.w3.org/1999/xhtml' type=static), local: Atom('span' type=inline) }, attrs: {}, self_closing: false, was_raw: true })
-pop name: div
  WARN unclosed HTML tag `<div>` found in `chapter_1.md`
  INFO HTML book written to `[ROOT]/book`
 
 "#]]);
         })
-        .check_main_file(
+        .check_main_file("book/chapter_1.html", str!["<div>x<span>foo</span></div>"]);
-            "book/chapter_1.html",
-            str!["<div>x<span>foo</span></div>"],
-        );
 }