Ensure that line comments always get newlines after them.

lib/src/back_end/code_writer.dart

@@ -28,6 +28,19 @@ class CodeWriter {
   /// Buffer for the code being written.
   final StringBuffer _buffer = StringBuffer();
 
+  /// What whitespace should be written before the next non-whitespace text.
+  ///
+  /// When whitespace is written, instead of immediately writing it, we queue
+  /// it as pending. This ensures that we don't write trailing whitespace,
+  /// avoids writing spaces at the beginning of lines, and allows collapsing
+  /// multiple redundant newlines.
+  _Whitespace _pendingWhitespace = _Whitespace.none;
+
+  /// The number of spaces of indentation that should be begin the next line
+  /// when [_pendingWhitespace] is [_Whitespace.newline] or
+  /// [_Whitespace.blankLine].
+  int _pendingIndent = 0;
+
   /// The cost of the currently chosen line splits.
   int _cost = 0;
 
@@ -143,6 +156,7 @@ class CodeWriter {
     // TextPieces because that will preserve selection markers. Consider doing
     // something smarter for commas in lists and semicolons in for loops.
 
+    _flushWhitespace();
     _buffer.write(text);
     _column += text.length;
 
@@ -180,7 +194,10 @@ class CodeWriter {
 
   /// Writes a single space to the output.
   void space() {
-    write(' ');
+    // If a newline is already pending, then ignore the space.
+    if (_pendingWhitespace == _Whitespace.none) {
+      _pendingWhitespace = _Whitespace.space;
+    }
   }
 
   /// Inserts a line split in the output.
@@ -193,12 +210,15 @@ class CodeWriter {
     if (indent != null) setIndent(indent);
 
     handleNewline();
-    _finishLine();
-    _buffer.writeln();
-    if (blank) _buffer.writeln();
 
-    _column = _options.indent;
-    _buffer.write(' ' * _column);
+    // Collapse redundant newlines.
+    if (blank) {
+      _pendingWhitespace = _Whitespace.blankLine;
+    } else if (_pendingWhitespace != _Whitespace.blankLine) {
+      _pendingWhitespace = _Whitespace.newline;
+    }
+
+    _pendingIndent = _options.indent;
   }
 
   /// Sets whether newlines are allowed to occur from this point on for the
@@ -246,15 +266,46 @@ class CodeWriter {
   /// Sets [selectionStart] to be [start] code units into the output.
   void startSelection(int start) {
     assert(_selectionStart == null);
+
+    _flushWhitespace();
     _selectionStart = _buffer.length + start;
   }
 
   /// Sets [selectionEnd] to be [end] code units into the output.
   void endSelection(int end) {
     assert(_selectionEnd == null);
+
+    _flushWhitespace();
     _selectionEnd = _buffer.length + end;
   }
 
+  /// Write any pending whitespace.
+  ///
+  /// This is called before non-whitespace text is about to be written, or
+  /// before the selection is updated since the latter requires an accurate
+  /// count of the written text, including whitespace.
+  void _flushWhitespace() {
+    switch (_pendingWhitespace) {
+      case _Whitespace.none:
+        break; // Nothing to do.
+
+      case _Whitespace.newline:
+      case _Whitespace.blankLine:
+        _finishLine();
+        _buffer.writeln();
+        if (_pendingWhitespace == _Whitespace.blankLine) _buffer.writeln();
+
+        _column = _pendingIndent;
+        _buffer.write(' ' * _column);
+
+      case _Whitespace.space:
+        _buffer.write(' ');
+        _column++;
+    }
+
+    _pendingWhitespace = _Whitespace.none;
+  }
+
   void _finishLine() {
     // If the completed line is too long, track the overflow.
     if (_column >= _pageWidth) {
@@ -276,6 +327,21 @@ class CodeWriter {
   }
 }
 
+/// Different kinds of pending whitespace that have been requested.
+enum _Whitespace {
+  /// No pending whitespace.
+  none,
+
+  /// A single newline.
+  newline,
+
+  /// Two newlines.
+  blankLine,
+
+  /// A single space.
+  space
+}
+
 /// The mutable state local to a single piece being formatted.
 class _PieceOptions {
   /// The absolute number of spaces of leading indentation coming from

lib/src/front_end/comment_writer.dart

@@ -78,7 +78,6 @@ mixin CommentWriter {
 
       if (comments.isHanging(i)) {
         // Attach the comment to the previous token.
-        pieces.writeSpace();
         pieces.writeComment(comment, hanging: true);
       } else {
         pieces.writeNewline();
@@ -202,8 +201,7 @@ class SourceComment {
       {required this.flushLeft, required this.offset});
 
   /// Whether this comment contains a mandatory newline, either because it's a
-  /// comment that should be on its own line or a lexeme with a newline inside
-  /// it (i.e. multi-line block comment, multi-line string).
+  /// comment that should be on its own line or is a multi-line block comment.
   bool get containsNewline =>
       type != CommentType.inlineBlock || text.contains('\n');
 

lib/src/front_end/piece_writer.dart

@@ -124,10 +124,6 @@ class PieceWriter {
   /// Whether we should write a space before the next text that is written.
   bool _pendingSpace = false;
 
-  /// Whether we should write a newline in the current [TextPiece] before the
-  /// next text that is written.
-  bool _pendingNewline = false;
-
   /// Whether we should create a new [TextPiece] the next time text is written.
   bool _pendingSplit = false;
 
@@ -221,14 +217,14 @@ class PieceWriter {
 
   /// Writes a mandatory newline from a comment to the current [TextPiece].
   void writeNewline() {
-    _pendingNewline = true;
+    _currentText!.newline();
   }
 
   /// Write the contents of [comment] to the current innermost [TextPiece],
   /// handling any newlines that may appear in it.
   ///
   /// If [hanging] is `true`, then the comment is appended to the current line
-  /// even if call to [split()] has just happened. This is used for writing a
+  /// even if a call to [split()] has happened. This is used for writing a
   /// comment that should be on the end of a line.
   void writeComment(SourceComment comment, {bool hanging = false}) {
     _write(comment.text,
@@ -246,10 +242,9 @@ class PieceWriter {
     // current text.
     if (textPiece == null || _pendingSplit && !hanging) {
       textPiece = _currentText = TextPiece();
-    } else if (_pendingNewline) {
-      textPiece.newline();
-    } else if (_pendingSpace) {
-      textPiece.append(' ');
+    } else if (_pendingSpace || hanging) {
+      // Always write a space before hanging comments.
+      textPiece.appendSpace();
     }
 
     if (offset != null) {
@@ -267,7 +262,6 @@ class PieceWriter {
     textPiece.append(text, containsNewline: containsNewline);
 
     _pendingSpace = false;
-    _pendingNewline = false;
     if (!hanging) _pendingSplit = false;
   }
 

lib/src/piece/piece.dart

@@ -80,14 +80,19 @@ class TextPiece extends Piece {
   /// multiline strings, etc.
   bool _containsNewline = false;
 
+  /// Whether this piece should have a newline written at the end of it.
+  ///
+  /// This is true during piece construction while lines are still being
+  /// written. It may also be true once a piece is fully complete if it ends in
+  /// a line comment.
+  bool _trailingNewline = false;
+
   /// The offset from the beginning of [text] where the selection starts, or
   /// `null` if the selection does not start within this chunk.
-  int? get selectionStart => _selectionStart;
   int? _selectionStart;
 
   /// The offset from the beginning of [text] where the selection ends, or
   /// `null` if the selection does not start within this chunk.
-  int? get selectionEnd => _selectionEnd;
   int? _selectionEnd;
 
   /// Whether the last line of this piece's text ends with [text].
@@ -98,16 +103,25 @@ class TextPiece extends Piece {
   /// If [text] internally contains a newline, then [containsNewline] should
   /// be `true`.
   void append(String text, {bool containsNewline = false}) {
-    if (_lines.isEmpty) _lines.add('');
+    if (_lines.isEmpty || _trailingNewline) _lines.add('');
 
     // TODO(perf): Consider a faster way of accumulating text.
     _lines.last = _lines.last + text;
 
     if (containsNewline) _containsNewline = true;
+
+    _trailingNewline = false;
+  }
+
+  void appendSpace() {
+    // Don't write an unnecessary space at the beginning of a line.
+    if (_trailingNewline) return;
+
+    append(' ');
   }
 
   void newline() {
-    _lines.add('');
+    _trailingNewline = true;
   }
 
   @override
@@ -128,6 +142,8 @@ class TextPiece extends Piece {
       if (i > 0) writer.newline();
       writer.write(_lines[i]);
     }
+
+    if (_trailingNewline) writer.newline();
   }
 
   @override
@@ -141,7 +157,6 @@ class TextPiece extends Piece {
       start += line.length;
     }
 
-    // print('TextPiece start $start (absolute = $abs)');
     _selectionStart = start;
   }
 
@@ -153,7 +168,6 @@ class TextPiece extends Piece {
       end += line.length;
     }
 
-    // print('TextPiece end $end (absolute = $abs)');
     _selectionEnd = end;
   }