Don't accidentally drop comments when rewriting to old syntax

Previously, a comment between a condition and a `then` or a `do` would
be dropped when rewriting to old syntax with parentheses.

Author: odersky

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -575,7 +575,8 @@ object Parsers {
 
     /** Drop (...) or { ... }, replacing the closing element with `endStr` */
     def dropParensOrBraces(start: Offset, endStr: String): Unit = {
-      patch(source, Span(start, start + 1), "")
+      patch(source, Span(start, start + 1),
+        if (testChar(start - 1, Chars.isIdentifierPart)) " " else "")
       val closingStartsLine = testChar(skipBlanks(in.lastOffset - 2, -1), Chars.LF)
       val preFill = if (closingStartsLine || endStr.isEmpty) "" else " "
       val postFill = if (in.lastOffset == in.offset) " " else ""
@@ -585,11 +586,25 @@ object Parsers {
       patch(source, Span(startClosing, endClosing), s"$preFill$endStr$postFill")
     }
 
+    /** Drop current token, which is assumed to be `then` or `do`. */
+    def dropTerminator(): Unit = {
+      var startOffset = in.offset
+      var endOffset = in.lastCharOffset
+      if (in.isAfterLineEnd()) {
+        if (testChar(endOffset, ' ')) endOffset += 1
+      }
+      else {
+        if (testChar(startOffset - 1, ' ')) startOffset -= 1
+      }
+      patch(source, Span(startOffset, endOffset), "")
+    }
+
     /** rewrite code with (...) around the source code of `t` */
     def revertToParens(t: Tree): Unit =
       if (t.span.exists) {
         patch(source, t.span.startPos, "(")
-        patch(source, Span(t.span.end, in.lastOffset), ")")
+        patch(source, t.span.endPos, ")")
+        dropTerminator()
       }
 
     /** In the tokens following the current one, does `query` precede any of the tokens that
@@ -1352,16 +1367,16 @@ object Parsers {
             expr1Rest(postfixExprRest(simpleExprRest(t)), Location.ElseWhere)
           }
         if (in.token == altToken) {
-          in.nextToken()
           if (in.rewriteOldSyntax) revertToParens(t)
+          in.nextToken()
         }
         else if (in.rewriteNewSyntax)
           dropParensOrBraces(t.span.start, s"${tokenString(altToken)}")
         t
       } else {
         val t = inSepRegion(LPAREN, RPAREN)(expr())
-        accept(altToken)
         if (in.rewriteOldSyntax) revertToParens(t)
+        accept(altToken)
         t
       }
     }
@@ -2027,17 +2042,7 @@ object Parsers {
         ForYield(enums, expr())
       }
       else if (in.token == DO) {
-        if (in.rewriteOldSyntax) {
-          var startOffset = in.offset
-          var endOffset = in.lastCharOffset
-          if (in.isAfterLineEnd()) {
-            if (testChar(endOffset, ' ')) endOffset += 1
-          }
-          else {
-            if (testChar(startOffset - 1, ' ')) startOffset -= 1
-          }
-          patch(source, Span(startOffset, endOffset), "")
-        }
+        if (in.rewriteOldSyntax) dropTerminator()
         in.nextToken()
         ForDo(enums, expr())
       }