FormatWriter: replace single-line mlc with slc

docs/configuration.md

@@ -3200,6 +3200,27 @@ comments.wrapStandaloneSlcAsSlc = true
 val b = 2 // long singleline comment
 ```
 
+### `comments.wrapSingleLineMlcAsSlc`
+
+> Since v3.3.1.
+
+If comment wrapping is enabled ([`comments.wrap != no`](#commentswrap)), this parameter
+allows formatting a trailing or standalone multi-line comment (i.e., `/* ... */`) as a
+single-line comment (`//`) if it occupies a single line.
+
+```scala mdoc:defaults
+comments.wrapSingleLineMlcAsSlc
+```
+
+```scala mdoc:scalafmt
+maxColumn = 50
+comments.wrap = trailing
+comments.wrapSingleLineMlcAsSlc = true
+---
+/* standalone multi-line comment */
+val b = 2 /* mlc */ /* trailing mlc */
+```
+
 ### `docstrings.style`
 
 > Since v2.6.0.

scalafmt-core/shared/src/main/scala/org/scalafmt/config/Comments.scala

@@ -16,6 +16,7 @@ import metaconfig._
   */
 case class Comments(
     wrap: Comments.Wrap = Comments.Wrap.no,
+    wrapSingleLineMlcAsSlc: Boolean = false,
     wrapStandaloneSlcAsSlc: Boolean = false
 ) {
   @inline def willWrap: Boolean = wrap ne Comments.Wrap.no

scalafmt-core/shared/src/main/scala/org/scalafmt/internal/FormatWriter.scala

@@ -663,13 +663,15 @@ class FormatWriter(formatOps: FormatOps) {
             iter: WordIter,
             appendLineBreak: () => Unit,
             lineLength: Int = 0,
-            extraMargin: String = " "
-        ): Unit =
+            extraMargin: String = " ",
+            linesSoFar: Int = 0
+        ): Int =
           if (iter.hasNext) {
             val word = iter.next()
             val length = word.length
             val maybeNextLineLength = 1 + length +
               (if (lineLength == 0) leadingMargin else lineLength)
+            var lines = linesSoFar
             val nextLineLength =
               if (
                 lineLength < extraMargin.length ||
@@ -679,12 +681,13 @@ class FormatWriter(formatOps: FormatOps) {
                 maybeNextLineLength
               } else {
                 appendLineBreak()
+                lines += 1
                 sb.append(extraMargin)
                 length + extraMargin.length
               }
             sb.append(word)
-            iterWords(iter, appendLineBreak, nextLineLength, extraMargin)
-          }
+            iterWords(iter, appendLineBreak, nextLineLength, extraMargin, lines)
+          } else linesSoFar
       }
 
       private class FormatSlc(text: String)(implicit sb: StringBuilder)
@@ -720,8 +723,12 @@ class FormatWriter(formatOps: FormatOps) {
           val contents = text.substring(2).trim
           val wordIter = splitAsIterator(slcDelim)(contents)
           sb.append(if (useSlc) "//" else "/*")
-          iterWords(wordIter, appendLineBreak, getFirstLineLength)
-          if (!useSlc) sb.append(" */")
+          val curlen = sb.length
+          val lines = iterWords(wordIter, appendLineBreak, getFirstLineLength)
+          if (!useSlc)
+            if (lines == 0 && style.comments.wrapSingleLineMlcAsSlc)
+              sb.setCharAt(curlen - 1, '/')
+            else sb.append(" */")
         }
       }
 
@@ -770,8 +777,11 @@ class FormatWriter(formatOps: FormatOps) {
               }
             }
             sb.append("/*")
-            iterSections(sectionIter, getFirstLineLength)
-            sb.append(" */")
+            val curlen = sb.length
+            val lines = iterSections(sectionIter, getFirstLineLength)
+            if (lines == 0 && style.comments.wrapSingleLineMlcAsSlc)
+              sb.setCharAt(curlen - 1, '/')
+            else sb.append(" */")
           } else {
             val trimmed = removeTrailingWhiteSpace(text)
             sb.append(leadingAsteriskSpace.matcher(trimmed).replaceAll(spaces))
@@ -783,22 +793,24 @@ class FormatWriter(formatOps: FormatOps) {
         }
 
         private type ParaIter = Iterator[WordIter]
-        private def iterParagraphs(iter: ParaIter, firstLineLen: Int): Unit = {
-          iterWords(iter.next(), appendLineBreak, firstLineLen)
+        private def iterParagraphs(iter: ParaIter, firstLineLen: Int): Int = {
+          var lines = iterWords(iter.next(), appendLineBreak, firstLineLen)
           while (iter.hasNext) {
             appendLineBreak()
-            iterWords(iter.next(), appendLineBreak)
+            lines += 1 + iterWords(iter.next(), appendLineBreak)
           }
+          lines
         }
 
         private type SectIter = Iterator[ParaIter]
-        private def iterSections(iter: SectIter, firstLineLen: Int): Unit = {
-          iterParagraphs(iter.next(), firstLineLen)
+        private def iterSections(iter: SectIter, firstLineLen: Int): Int = {
+          var lines = iterParagraphs(iter.next(), firstLineLen)
           while (iter.hasNext) {
             appendLineBreak()
             appendLineBreak()
-            iterParagraphs(iter.next(), 0)
+            lines += 2 + iterParagraphs(iter.next(), 0)
           }
+          lines
         }
       }
 

scalafmt-tests/src/test/resources/unit/Comment.stat

@@ -404,6 +404,19 @@ object a {
 object a {
   /* foo bar */
 }
+<<< wrap with empty first line 1, mlc -> slc
+comments.wrap = trailing
+comments.wrapSingleLineMlcAsSlc = true
+===
+object a {
+  /* mlc */ /*
+   * foo bar
+   */
+}
+>>>
+object a {
+  /* mlc */ // foo bar
+}
 <<< wrap with empty first line 2
 comments.wrap = trailing
 ===
@@ -415,6 +428,18 @@ object a {
 object a {
   /* foo bar */
 }
+<<< wrap with empty first line 2, mlc -> slc
+comments.wrap = trailing
+comments.wrapSingleLineMlcAsSlc = true
+===
+object a {
+  /* mlc */ /*
+   * foo bar   */
+}
+>>>
+object a {
+  /* mlc */ // foo bar
+}
 <<< #2043
 object a {
   val