Fix sass#417 preserve location of trailing loud comments.

lib/src/ast/css/comment.dart

@@ -16,5 +16,9 @@ abstract class CssComment extends CssNode {
   /// compressed mode.
   bool get isPreserved;
 
+  /// Whether this comment follows non-comment text on a line and should remain
+  /// attached to that non-comment text when being serialized.
+  bool get isTrailing;
+
   T accept<T>(CssVisitor<T> visitor) => visitor.visitCssComment(this);
 }

lib/src/ast/css/modifiable/comment.dart

@@ -13,10 +13,12 @@ import 'node.dart';
 class ModifiableCssComment extends ModifiableCssNode implements CssComment {
   final String text;
   final FileSpan span;
+  final bool _isTrailing;
 
   bool get isPreserved => text.codeUnitAt(2) == $exclamation;
+  bool get isTrailing => _isTrailing;
 
-  ModifiableCssComment(this.text, this.span);
+  ModifiableCssComment(this.text, this.span, this._isTrailing);
 
   T accept<T>(ModifiableCssVisitor<T> visitor) => visitor.visitCssComment(this);
 }

lib/src/ast/sass/statement/loud_comment.dart

@@ -15,7 +15,11 @@ class LoudComment implements Statement {
 
   FileSpan get span => text.span;
 
-  LoudComment(this.text);
+  /// Whether this comment follows non-comment text on a line and should remain
+  /// attached to that non-comment text when being serialized.
+  final bool isTrailing;
+
+  LoudComment(this.text, this.isTrailing);
 
   T accept<T>(StatementVisitor<T> visitor) => visitor.visitLoudComment(this);
 

lib/src/parse/parser.dart

@@ -79,11 +79,17 @@ class Parser {
   }
 
   /// Consumes whitespace, but not comments.
+  ///
+  /// Returns whether any newlines were seen in the consumed whitespace.
   @protected
-  void whitespaceWithoutComments() {
+  bool whitespaceWithoutComments() {
+    bool sawAnyNewlines = false;
     while (!scanner.isDone && isWhitespace(scanner.peekChar())) {
-      scanner.readChar();
+      if (isNewline(scanner.readChar())) {
+        sawAnyNewlines = true;
+      }
     }
+    return sawAnyNewlines;
   }
 
   /// Consumes spaces and tabs.

lib/src/parse/sass.dart

@@ -306,7 +306,8 @@ class SassParser extends StylesheetParser {
     }
     if (!buffer.trailingString.trimRight().endsWith("*/")) buffer.write(" */");
 
-    return LoudComment(buffer.interpolation(scanner.spanFrom(start)));
+    // The sass syntax does not support trailing loud comments ==> isTrailing = false.
+    return LoudComment(buffer.interpolation(scanner.spanFrom(start)), false);
   }
 
   void whitespace() {

lib/src/parse/scss.dart

@@ -61,8 +61,8 @@ class ScssParser extends StylesheetParser {
 
   List<Statement> children(Statement child()) {
     scanner.expectChar($lbrace);
-    whitespaceWithoutComments();
     var children = <Statement>[];
+    _whitespaceAndOptionalTrailingLoudComment(children);
     while (true) {
       switch (scanner.peekChar()) {
         case $dollar:
@@ -76,7 +76,7 @@ class ScssParser extends StylesheetParser {
               whitespaceWithoutComments();
               break;
             case $asterisk:
-              children.add(_loudComment());
+              children.add(_loudComment(/* isTrailing= */ false));
               whitespaceWithoutComments();
               break;
             default:
@@ -87,7 +87,7 @@ class ScssParser extends StylesheetParser {
 
         case $semicolon:
           scanner.readChar();
-          whitespaceWithoutComments();
+          _whitespaceAndOptionalTrailingLoudComment(children);
           break;
 
         case $rbrace:
@@ -103,7 +103,7 @@ class ScssParser extends StylesheetParser {
 
   List<Statement> statements(Statement statement()) {
     var statements = <Statement>[];
-    whitespaceWithoutComments();
+    _whitespaceAndOptionalTrailingLoudComment(statements);
     while (!scanner.isDone) {
       switch (scanner.peekChar()) {
         case $dollar:
@@ -117,7 +117,7 @@ class ScssParser extends StylesheetParser {
               whitespaceWithoutComments();
               break;
             case $asterisk:
-              statements.add(_loudComment());
+              statements.add(_loudComment(/* isTrailing= */ false));
               whitespaceWithoutComments();
               break;
             default:
@@ -129,7 +129,7 @@ class ScssParser extends StylesheetParser {
 
         case $semicolon:
           scanner.readChar();
-          whitespaceWithoutComments();
+          _whitespaceAndOptionalTrailingLoudComment(statements);
           break;
 
         default:
@@ -163,7 +163,7 @@ class ScssParser extends StylesheetParser {
   }
 
   /// Consumes a statement-level loud comment block.
-  LoudComment _loudComment() {
+  LoudComment _loudComment(bool isTrailing) {
     var start = scanner.state;
     scanner.expect("/*");
     var buffer = InterpolationBuffer()..write("/*");
@@ -182,7 +182,8 @@ class ScssParser extends StylesheetParser {
           if (scanner.peekChar() != $slash) break;
 
           buffer.writeCharCode(scanner.readChar());
-          return LoudComment(buffer.interpolation(scanner.spanFrom(start)));
+          return LoudComment(
+              buffer.interpolation(scanner.spanFrom(start)), isTrailing);
 
         case $cr:
           scanner.readChar();
@@ -200,4 +201,26 @@ class ScssParser extends StylesheetParser {
       }
     }
   }
+
+  /// Consumes whitespace and optionally a trailing loud comment, adding the
+  /// trailing loud comment to [statements] if consumed.
+  ///
+  /// A trailing loud comment is only possible if the initially consumed
+  /// whitespace lacks any newlines.  If that's the case and a loud comment
+  /// immediately follows, the loud comment will also be consumed.
+  void _whitespaceAndOptionalTrailingLoudComment(List<Statement> statements) {
+    bool sawAnyNewlines = whitespaceWithoutComments();
+    if (_peekForStartOfLoudComment()) {
+      bool isTrailing = !sawAnyNewlines;
+      statements.add(_loudComment(isTrailing));
+      whitespaceWithoutComments();
+    }
+  }
+
+  /// Returns whether the scanner is pointing at the start of a loud comment.
+  bool _peekForStartOfLoudComment() {
+    return !scanner.isDone &&
+        scanner.peekChar() == $slash &&
+        scanner.peekChar(1) == $asterisk;
+  }
 }

lib/src/visitor/async_evaluate.dart

@@ -1500,7 +1500,7 @@ class _EvaluateVisitor
     }
 
     _parent.addChild(ModifiableCssComment(
-        await _performInterpolation(node.text), node.span));
+        await _performInterpolation(node.text), node.span, node.isTrailing));
     return null;
   }
 
@@ -2424,7 +2424,8 @@ class _EvaluateVisitor
       _endOfImports++;
     }
 
-    _parent.addChild(ModifiableCssComment(node.text, node.span));
+    _parent
+        .addChild(ModifiableCssComment(node.text, node.span, node.isTrailing));
   }
 
   Future<void> visitCssDeclaration(CssDeclaration node) async {

lib/src/visitor/clone_css.dart

@@ -37,7 +37,7 @@ class _CloneCssVisitor implements CssVisitor<ModifiableCssNode> {
   }
 
   ModifiableCssComment visitCssComment(CssComment node) =>
-      ModifiableCssComment(node.text, node.span);
+      ModifiableCssComment(node.text, node.span, node.isTrailing);
 
   ModifiableCssDeclaration visitCssDeclaration(CssDeclaration node) =>
       ModifiableCssDeclaration(node.name, node.value, node.span,

lib/src/visitor/evaluate.dart

@@ -5,7 +5,7 @@
 // DO NOT EDIT. This file was generated from async_evaluate.dart.
 // See tool/grind/synchronize.dart for details.
 //
-// Checksum: f4f4c5d1cbc9894d14b6d8ce7c1a3c09146db9ba
+// Checksum: 56067ad7a31ee7adf5bb4e9d484fe95f38d3762c
 //
 // ignore_for_file: unused_import
 
@@ -1498,8 +1498,8 @@ class _EvaluateVisitor
       _endOfImports++;
     }
 
-    _parent.addChild(
-        ModifiableCssComment(_performInterpolation(node.text), node.span));
+    _parent.addChild(ModifiableCssComment(
+        _performInterpolation(node.text), node.span, node.isTrailing));
     return null;
   }
 
@@ -2409,7 +2409,8 @@ class _EvaluateVisitor
       _endOfImports++;
     }
 
-    _parent.addChild(ModifiableCssComment(node.text, node.span));
+    _parent
+        .addChild(ModifiableCssComment(node.text, node.span, node.isTrailing));
   }
 
   void visitCssDeclaration(CssDeclaration node) {

lib/src/visitor/serialize.dart

@@ -162,7 +162,12 @@ class _SerializeVisitor
 
       if (previous != null) {
         if (_requiresSemicolon(previous)) _buffer.writeCharCode($semicolon);
-        _writeLineFeed();
+        if (_shouldWriteLineFeedBeforeChild(child)) {
+          _writeLineFeed();
+        } else {
+          _writeOptionalSpace();
+        }
+
         if (previous.isGroupEnd) _writeLineFeed();
       }
       previous = child;
@@ -1085,23 +1090,41 @@ class _SerializeVisitor
       return;
     }
 
-    _writeLineFeed();
+    bool indentNextChild;
+    if (_shouldWriteLineFeedBeforeChildren(children)) {
+      _writeLineFeed();
+      indentNextChild = true;
+    } else {
+      _writeOptionalSpace();
+      indentNextChild = false;
+    }
+
     CssNode previous;
-    _indent(() {
-      for (var i = 0; i < children.length; i++) {
-        var child = children[i];
-        if (_isInvisible(child)) continue;
+    for (var i = 0; i < children.length; i++) {
+      var child = children[i];
+      if (_isInvisible(child)) continue;
 
-        if (previous != null) {
-          if (_requiresSemicolon(previous)) _buffer.writeCharCode($semicolon);
+      if (previous != null) {
+        if (_requiresSemicolon(previous)) _buffer.writeCharCode($semicolon);
+        if (_shouldWriteLineFeedBeforeChild(child)) {
           _writeLineFeed();
-          if (previous.isGroupEnd) _writeLineFeed();
+          indentNextChild = true;
+        } else {
+          _writeOptionalSpace();
+          indentNextChild = false;
         }
-        previous = child;
+        if (previous.isGroupEnd) _writeLineFeed();
+      }
+      previous = child;
 
-        child.accept(this);
+      if (indentNextChild) {
+        _indentation++;
       }
-    });
+      child.accept(this);
+      if (indentNextChild) {
+        _indentation--;
+      }
+    }
 
     if (_requiresSemicolon(previous) && !_isCompressed) {
       _buffer.writeCharCode($semicolon);
@@ -1115,6 +1138,17 @@ class _SerializeVisitor
   bool _requiresSemicolon(CssNode node) =>
       node is CssParentNode ? node.isChildless : node is! CssComment;
 
+  /// Whether [child] should have a line feed written before the child is serialized.
+  bool _shouldWriteLineFeedBeforeChild(CssNode child) =>
+      child is! CssComment || !(child as CssComment).isTrailing;
+
+  /// Whether [children] should have a line feed written before the children are serialized.
+  bool _shouldWriteLineFeedBeforeChildren(List<CssNode> children) {
+    return children.isEmpty ||
+        !(children.first is CssComment) ||
+        !((children.first as CssComment).isTrailing);
+  }
+
   /// Writes a line feed, unless this emitting compressed CSS.
   void _writeLineFeed() {
     if (!_isCompressed) _buffer.write(_lineFeed.text);

test/output_test.dart

@@ -59,4 +59,34 @@ void main() {
       });
     });
   });
+
+  // Regression test for sass/dart-sass#417.
+  group("preserve trailing loud comments", () {
+    // No need for "in Sass" cases as it's not possible to have
+    // trailing loud comments in the Sass syntax.
+    group("in SCSS", () {
+      test("after open block", () {
+        expect(compileString("""
+selector { /* please don't move me */
+  name: value;
+}"""), equals("""
+selector { /* please don't move me */
+  name: value;
+}"""));
+      });
+      test("after declaration", () {
+        expect(compileString("""
+selector {
+  name: value; /* please don't move me */
+}"""), equals("""
+selector {
+  name: value; /* please don't move me */
+}"""));
+      });
+      test("after top-level statement", () {
+        expect(compileString("@rule; /* please don't move me */"),
+            equals("@rule; /* please don't move me */"));
+      });
+    });
+  });
 }