fix: give a heading with no slug-able characters no anchor at all

mdit_py_plugins/anchors/index.py

@@ -32,6 +32,8 @@ def anchors_plugin(
     Explicit heading IDs (e.g. ``{#id}`` from ``attrs_block_plugin``) are preserved,
     at any level, and generated IDs avoid collisions with them.
     Explicit IDs are not de-duplicated, and an empty explicit ID counts as no ID.
+    A generated empty slug (a title with no slug-able characters, e.g. ``## ???``)
+    likewise counts as no ID.
 
     :param min_level: minimum header level to apply anchors
     :param max_level: maximum header level to apply anchors
@@ -93,7 +95,14 @@ def _anchor_func(state: StateCore) -> None:
             )
             slug = token.attrGet("id")
             if not slug:
-                slug = unique_slug(slug_func(title), slugs)
+                generated = slug_func(title)
+                if not generated:
+                    # A title with no slug-able characters slugifies to "".
+                    # An empty id="" is invalid HTML, so treat a generated empty
+                    # slug like an empty explicit ID: the heading gets no anchor
+                    # (and no permalink, which would point at "#").
+                    continue
+                slug = unique_slug(generated, slugs)
                 token.attrSet("id", slug)
 
             if permalink:

tests/test_anchors.py

@@ -115,6 +115,35 @@ def test_explicit_heading_id_plugin_order() -> None:
     assert md.render("{#custom}\n# Heading") == '<h1 id="custom">Heading</h1>\n'
 
 
+@pytest.mark.parametrize(
+    "source, expected_ids",
+    [
+        pytest.param("## ???", [None], id="no-slugable-chars"),
+        pytest.param("## ???\n\n## ???", [None, None], id="no-slugable-chars-twice"),
+        pytest.param(
+            "## ???\n\n## Real", [None, "real"], id="following-heading-unaffected"
+        ),
+    ],
+)
+def test_empty_generated_slug_gets_no_id(source: str, expected_ids: list) -> None:
+    """A generated empty slug is treated like an empty explicit ID: no id at all.
+
+    A heading whose title has no slug-able characters (e.g. ``## ???``)
+    would otherwise get an invalid empty ``id=""`` attribute, and with
+    ``permalink=True`` a link pointing at ``#``.
+    """
+    md = MarkdownIt("commonmark").use(anchors_plugin, permalink=True)
+    tokens = md.parse(source)
+    assert [
+        token.attrGet("id") for token in tokens if token.type == "heading_open"
+    ] == expected_ids
+    rendered = md.renderer.render(tokens, md.options, {})
+    assert 'id=""' not in rendered
+    assert 'href="#"' not in rendered
+    expected_anchors = sum(1 for i in expected_ids if i is not None)
+    assert rendered.count('class="header-anchor"') == expected_anchors
+
+
 @pytest.mark.parametrize(
     "source, expected_id",
     [