Merge PR #1788: formats: the Markdown code fence outgrows any backtick run in its content, so a round trip keeps it

crates/formats/src/markdown.rs

@@ -1335,7 +1335,9 @@ fn write_blocks(blocks: &[FBlock], out: &mut Vec<String>) {
                     i += 1;
                 }
                 let longest = lines.iter().map(|l| l.chars().filter(|c| *c == '`').count()).max().unwrap_or(0);
-                let fence = "`".repeat(3.max(longest + 1).min(64));
+                // The fence must be longer than the longest run in the content, or a content
+                // line is read back as the closing fence (the `.min(64)` cap did exactly that).
+                let fence = "`".repeat(3.max(longest + 1));
                 out.push(fence.clone());
                 out.extend(lines);
                 out.push(fence);

crates/formats/src/tests.rs

@@ -214,6 +214,20 @@ fn markdown_round_trip() {
     check_round_trip("md", true);
 }
 
+/// A code line of backticks must not be mistaken for the closing fence when the file comes
+/// back: the writer's fence has to be one backtick longer than any content line.
+#[test]
+fn markdown_code_fence_outgrows_its_content() {
+    let line = "`".repeat(100);
+    let doc = import("md", format!("    {line}\n").as_bytes()).unwrap().unwrap();
+    assert_eq!(doc.plain_text(Default::default()).trim(), line);
+    let out = export("md", &doc).unwrap().unwrap();
+    let text = String::from_utf8(out).unwrap();
+    assert!(text.contains(&"`".repeat(101)), "the fence is longer than the content: {text}");
+    let back = import("md", text.as_bytes()).unwrap().unwrap();
+    assert_eq!(back.plain_text(Default::default()).trim(), line, "the code line survives a round trip");
+}
+
 #[test]
 fn html_round_trip() {
     check_round_trip("html", true);