Fix comment undecoration when ddoc code fences are present

src/dparse/trivia.d

@@ -142,7 +142,12 @@ unittest
         "/**b1\n*b2\n*b3*/",
         "/**c1\n    *c2\n    *c3*/",
         "/**d1\n    *d2\n    *d3\n*/",
-        "///a\fbc\n///def"
+        "///a\fbc\n///def",
+        // code fence directly before the closing delimiter (Phobos writeln
+        // style): the fence lines and their content are not decorated
+        "/**\n * a\n * Example:\n *   c.\n---\ncode\n---\n */",
+        "/**\n * a\n---\ncode\n---\n * b\n */",
+        "/++\n + a\n + Example:\n +   c.\n---\ncode\n---\n +/"
     ];
     string[] outputs = [
         "",
@@ -167,7 +172,10 @@ unittest
         "b1\nb2\nb3",
         "c1\nc2\nc3",
         "d1\nd2\nd3",
-        "a\fbc\ndef"
+        "a\fbc\ndef",
+        "a\nExample:\n  c.\n---\ncode\n---",
+        "a\n---\ncode\n---\nb",
+        "a\nExample:\n  c.\n---\ncode\n---"
     ];
 
     // tests where * and + are not interchangeable
@@ -331,33 +339,100 @@ private:
         const size_t hi = (lines[$-1].length > 1 &&
             (lines[$-1][0] == commentChar || lines[$-1][0..2] == pattern[patternIndex]))
             ?  lines.length : lines.length-1;
-        // deco with a leading white
-        foreach (const i; lo .. hi)
+        // DDoc code fences (lines of `---`) and their content are not
+        // decorated even in an otherwise fully decorated comment (e.g.
+        // Phobos writeln). Lines inside a fence must not participate in
+        // the allDecorated detection, and must not have decoration
+        // stripped (there is none to strip).
+        // Returns true when every line in [lo, hi) is decorated (with the
+        // given leading pattern), a fence line, or content inside a fence.
+        // `pattern2` is the two-char leading pattern (" *" / "++"), while
+        // `pattern1` is the single-char decoration ("*" / "+").
+        static bool allLinesDecorated(in char[][] lines, size_t lo, size_t hi,
+            const char[2] pattern2, char pattern1) @safe @nogc pure nothrow
         {
-            if (lines[i].length < 2)
-                break;
-            else if (lines[i][0..2] != pattern[patternIndex])
-                break;
-            else if (i == hi-1)
-                allDecorated = true;
-        }
-        // deco w/o leading white
-        if (!allDecorated)
+            bool inFence;
             foreach (const i; lo .. hi)
-        {
-            if (lines[i].length == 0)
-                break;
-            if (lines[i][0] != commentChar)
-                break;
-            else if (i == hi-1)
-                allDecorated = true;
+            {
+                if (lines[i].length == 0)
+                {
+                    // empty lines are valid inside a fence; outside one
+                    // they end the decoration scan (like the original
+                    // break-on-empty behavior)
+                    if (inFence)
+                        continue;
+                    return false;
+                }
+                else if (lines[i][0] == pattern1
+                    || lines[i].length >= 2 && lines[i][0..2] == pattern2)
+                    continue;
+                else if (isFenceLine(lines[i]))
+                    inFence = !inFence;
+                else if (!inFence)
+                    return false;
+                // decorated content inside a fence: keep scanning
+            }
+            return !inFence;
         }
+
+        // deco with a leading white
+        allDecorated = allLinesDecorated(lines, lo, hi,
+            pattern[patternIndex], commentChar);
         if (!allDecorated)
             return;
 
         const size_t indexToChange = (lines[lo][0] == commentChar) ? 0 : 1;
+        // Does the comment contain a code fence? Fence lines and their
+        // content are undecorated, which changes how the decoration must
+        // be removed (see below).
+        bool hasFence;
         foreach (ref line; lines[lo .. hi])
-            line[indexToChange] = ' ';
+            if (isFenceLine(line))
+                hasFence = true;
+        bool inFence;
+        foreach (ref line; lines[lo .. hi])
+        {
+            if (isFenceLine(line))
+                inFence = !inFence;
+            else if (!inFence)
+            {
+                if (!hasFence)
+                {
+                    line[indexToChange] = ' ';
+                    continue;
+                }
+                // The comment mixes decorated lines with undecorated
+                // fence content, so stripLeft cannot remove the decoration
+                // column (the fence lines have no leading whitespace).
+                // Remove the leading whitespace, the decoration marker and
+                // ONE separator whitespace by shifting the content left in
+                // place; relative indentation beyond that is preserved.
+                size_t start;
+                while (start < line.length && line[start].isWhite)
+                    start++;
+                if (start < line.length && line[start] == commentChar)
+                    start++;
+                if (start < line.length && line[start].isWhite)
+                    start++;
+                foreach (const j; start .. line.length)
+                    line[j - start] = line[j];
+                line = line[0 .. $ - start];
+            }
+        }
+    }
+
+    /// A DDoc code fence line: `---` possibly indented, with optional
+    /// trailing whitespace.
+    static bool isFenceLine(const char[] line) @safe @nogc pure nothrow
+    {
+        size_t i;
+        while (i < line.length && line[i].isWhite)
+            i++;
+        size_t dashes;
+        while (i + dashes < line.length && line[i + dashes] == '-')
+            dashes++;
+        return dashes >= 3
+            && i + dashes == line.length;
     }
 
     void stripLeft() @safe @nogc pure nothrow