Fix to prefer flow over definitions, setext headings

An undocumented part of CommonMark is how to deal with things in definition
labels or definition titles (which both can span multiple lines).
Can flow (or containers?) interrupt them?
They can according to the `cmark` reference parser, so this was implemented here.

This adds a new `Content` content type, which houses zero or more definitions,
and then zero-or-one paragraphs.
Content can be followed by a setext heading underline, which either turns
into a setext heading when the content ends in a paragraph, or turns into
the start of the following paragraph when it is followed by content that
starts with a paragraph, or turns into a stray paragraph.

3 files changed, 43 insertions, 1 deletions

diff --git a/tests/definition.rs b/tests/definition.rs
index 11f783d..6f680ff 100644
--- a/tests/definition.rs
+++ b/tests/definition.rs
@@ -441,6 +441,42 @@ fn definition() -> Result<(), String> {
     );
 
     assert_eq!(
+        micromark("[\na\n=\n]: b"),
+        "<h1>[\na</h1>\n<p>]: b</p>",
+        "should prefer setext headings over definition labels"
+    );
+
+    assert_eq!(
+        micromark("[a]: b '\nc\n=\n'"),
+        "<h1>[a]: b '\nc</h1>\n<p>'</p>",
+        "should prefer setext headings over definition titles"
+    );
+
+    assert_eq!(
+        micromark("[\n***\n]: b"),
+        "<p>[</p>\n<hr />\n<p>]: b</p>",
+        "should prefer thematic breaks over definition labels"
+    );
+
+    assert_eq!(
+        micromark("[a]: b '\n***\n'"),
+        "<p>[a]: b '</p>\n<hr />\n<p>'</p>",
+        "should prefer thematic breaks over definition titles"
+    );
+
+    assert_eq!(
+        micromark("[\n```\n]: b"),
+        "<p>[</p>\n<pre><code>]: b\n</code></pre>\n",
+        "should prefer code (fenced) over definition labels"
+    );
+
+    assert_eq!(
+        micromark("[a]: b '\n```\n'"),
+        "<p>[a]: b '</p>\n<pre><code>'\n</code></pre>\n",
+        "should prefer code (fenced) over definition titles"
+    );
+
+    assert_eq!(
         micromark_with_options(
             "[foo]: /url \"title\"",
             &Options {
diff --git a/tests/fuzz.rs b/tests/fuzz.rs
index 146ff24..47dbea5 100644
--- a/tests/fuzz.rs
+++ b/tests/fuzz.rs
@@ -6,7 +6,7 @@ use pretty_assertions::assert_eq;
 fn fuzz() -> Result<(), String> {
     assert_eq!(
         micromark("[\n~\na\n-\n\n"),
-        "<p>[\n~\na</p>\n<ul>\n<li></li>\n</ul>\n",
+        "<h2>[\n~\na</h2>\n",
         "1: label, blank lines, and code"
     );
 
diff --git a/tests/gfm_table.rs b/tests/gfm_table.rs
index 619bf2a..b7f884a 100644
--- a/tests/gfm_table.rs
+++ b/tests/gfm_table.rs
@@ -338,6 +338,12 @@ fn gfm_table() -> Result<(), String> {
     );
 
     assert_eq!(
+        micromark_with_options("[\na\n:-\n]: b", &gfm)?,
+        "<p>[</p>\n<table>\n<thead>\n<tr>\n<th align=\"left\">a</th>\n</tr>\n</thead>\n<tbody>\n<tr>\n<td align=\"left\">]: b</td>\n</tr>\n</tbody>\n</table>",
+        "should prefer GFM tables over definitions"
+    );
+
+    assert_eq!(
         micromark_with_options(
             r###"# Align