lix-doc: don't chomp bold headings off

There are a few places in nixpkgs lib where `**Foo**:` is used as a heading instead of the usual markdown `# Foo` ones. I think this is intentional with how it gets rendered in the manual, e.g. [`lib.lists.sortOn`][1].

[1]: https://nixos.org/manual/nixpkgs/stable/#function-library-lib.lists.sortOn

`nix-doc` prints this as
```
   *Laws**:
       ```nix
       sortOn f == sort (p: q: f p < f q)
       ```
```
chomping off the first asterisk as part of `cleanup_single_line` that's meant to deal with `/** \n * \n * \n */` style doc comments. This also means the usage in lix ends up funny-looking with a trailing asterisk as if there's a footnote to pay attention to (which is how I first noticed it, heh)

The fix:

When cleaning up a single line and removing a prefix comment character,
ensure it's followed by whitespace (or the last character of the line).

Upstream-PR: https://github.com/lf-/nix-doc/pull/26
Change-Id: If2870c53a632f6bbbcca98a4bfbd72f5bef37879

lix-doc/src/lib.rs

@@ -84,9 +84,13 @@ fn indented(s: &str, indent: usize) -> String {
 /// Cleans up a single line, erasing prefix single line comments but preserving indentation
 fn cleanup_single_line<'a>(s: &'a str) -> &'a str {
     let mut cmt_new_start = 0;
-    for (idx, ch) in s.char_indices() {
+    let mut iter = s.char_indices().peekable();
+    while let Some((idx, ch)) = iter.next() {
+        // peek at the next character, with an explicit '\n' as "next character" at end of line
+        let (_, next_ch) = iter.peek().unwrap_or(&(0, '\n'));
+
         // if we find a character, save the byte position after it as our new string start
-        if ch == '#' || ch == '*' {
+        if ch == '#' || (ch == '*' && next_ch.is_whitespace()) {
             cmt_new_start = idx + 1;
             break;
         }
@@ -206,7 +210,7 @@ fn visit_lambda(name: String, lambda: &Lambda) -> SearchResult {
     SearchResult {
         identifier: name,
         doc: comment,
-        param_block
+        param_block,
     }
 }
 
@@ -319,8 +323,16 @@ mod tests {
         let ex1 = "    * a";
         let ex2 = "    # a";
         let ex3 = "   a";
+        let ex4 = "   *";
         assert_eq!(cleanup_single_line(ex1), " a");
         assert_eq!(cleanup_single_line(ex2), " a");
         assert_eq!(cleanup_single_line(ex3), ex3);
+        assert_eq!(cleanup_single_line(ex4), "");
+    }
+
+    #[test]
+    fn test_single_line_retains_bold_headings() {
+        let ex1 = "   **Foo**:";
+        assert_eq!(cleanup_single_line(ex1), ex1);
     }
 }