Only allow the comment at the top of the formatted code.

lib/src/dart_formatter.dart

@@ -21,6 +21,11 @@ import 'short/source_visitor.dart';
 import 'source_code.dart';
 import 'string_compare.dart' as string_compare;
 
+/// Regular expression that matches a format width comment like:
+///
+///     // dart format width=123
+final RegExp _widthCommentPattern = RegExp(r'^// dart format width=(\d+)$');
+
 /// Dart source code formatter.
 class DartFormatter {
   /// The latest Dart language version that can be parsed and formatted by this
@@ -178,8 +183,21 @@ class DartFormatter {
 
     SourceCode output;
     if (experimentFlags.contains(tallStyleExperimentFlag)) {
+      // Look for a page width comment before the code.
+      int? pageWidthFromComment;
+      for (Token? comment = node.beginToken.precedingComments;
+          comment != null;
+          comment = comment.next) {
+        if (_widthCommentPattern.firstMatch(comment.lexeme) case var match?) {
+          // If integer parsing fails for some reason, the returned `null`
+          // means we correctly ignore the comment.
+          pageWidthFromComment = int.tryParse(match[1]!);
+          break;
+        }
+      }
+
       var visitor = AstNodeVisitor(this, lineInfo, unitSourceCode);
-      output = visitor.run(unitSourceCode, node);
+      output = visitor.run(unitSourceCode, node, pageWidthFromComment);
     } else {
       var visitor = SourceVisitor(this, lineInfo, unitSourceCode);
       output = visitor.run(node);

lib/src/front_end/ast_node_visitor.dart

@@ -66,7 +66,10 @@ class AstNodeVisitor extends ThrowingAstVisitor<void> with PieceFactory {
   ///
   /// This is the only method that should be called externally. Everything else
   /// is effectively private.
-  SourceCode run(SourceCode source, AstNode node) {
+  ///
+  /// If there is a `// dart format width=123` comment before the formatted
+  /// code, then [pageWidthFromComment] is that width.
+  SourceCode run(SourceCode source, AstNode node, int? pageWidthFromComment) {
     Profile.begin('AstNodeVisitor.run()');
 
     Profile.begin('AstNodeVisitor build Piece tree');
@@ -123,7 +126,7 @@ class AstNodeVisitor extends ThrowingAstVisitor<void> with PieceFactory {
     Profile.end('AstNodeVisitor build Piece tree');
 
     // Finish writing and return the complete result.
-    var result = pieces.finish(source, unitPiece);
+    var result = pieces.finish(source, unitPiece, pageWidthFromComment);
 
     Profile.end('AstNodeVisitor.run()');
 

lib/src/front_end/comment_writer.dart

@@ -42,29 +42,12 @@ import '../comment_type.dart';
 /// written. When that [TextPiece] is output later, it will include the comments
 /// as well.
 class CommentWriter {
-  /// Regular expression that matches a format width comment like:
-  ///
-  ///     // dart format width=123
-  static final RegExp _widthCommentPattern =
-      RegExp(r'^// dart format width=(\d+)$');
-
   final LineInfo _lineInfo;
 
   /// The tokens whose preceding comments have already been taken by calls to
   /// [takeCommentsBefore()].
   final Set<Token> _takenTokens = {};
 
-  /// If we have encountered a comment that sets an explicit page width, that
-  /// width, otherwise `null`.
-  ///
-  /// The comment looks like:
-  ///
-  ///     // dart format width=123
-  ///
-  /// If there are multiple of these comments, the first one wins.
-  int? get pageWidthFromComment => _pageWidthFromComment;
-  int? _pageWidthFromComment;
-
   CommentWriter(this._lineInfo);
 
   /// Returns the comments that appear before [token].
@@ -123,14 +106,6 @@ class CommentWriter {
         if (comment == token.precedingComments) linesBefore = 2;
       }
 
-      // If this comment configures the page width, remember it.
-      if (_widthCommentPattern.firstMatch(text) case var match?
-          when _pageWidthFromComment == null) {
-        // If integer parsing fails for some reason, the returned `null`
-        // means we correctly ignore the comment.
-        _pageWidthFromComment = int.tryParse(match[1]!);
-      }
-
       CommentType type;
       if (text.startsWith('///') && !text.startsWith('////') ||
           text.startsWith('/**') && text != '/**/') {

lib/src/front_end/piece_writer.dart

@@ -390,7 +390,11 @@ class PieceWriter {
 
   /// Finishes writing and returns a [SourceCode] containing the final output
   /// and updated selection, if any.
-  SourceCode finish(SourceCode source, Piece rootPiece) {
+  ///
+  /// If there is a `// dart format width=123` comment before the formatted
+  /// code, then [pageWidthFromComment] is that width.
+  SourceCode finish(
+      SourceCode source, Piece rootPiece, int? pageWidthFromComment) {
     if (debug.tracePieceBuilder) {
       debug.log(debug.pieceTree(rootPiece));
     }
@@ -399,7 +403,7 @@ class PieceWriter {
 
     var cache = SolutionCache();
     var solver = Solver(cache,
-        pageWidth: _comments.pageWidthFromComment ?? _formatter.pageWidth,
+        pageWidth: pageWidthFromComment ?? _formatter.pageWidth,
         leadingIndent: _formatter.indent);
     var solution = solver.format(rootPiece);
     var output = solution.code.build(source, _formatter.lineEnding);

test/tall/other/format_width.unit

@@ -1,118 +1,116 @@
 40 columns                              |
 ### Tests for the comment to set formatting width.
 >>> Comment sets page width.
+// dart format width=30
 main() {
-  // dart format width=30
   fitsUnsplitAt40   +   butNotAt30;
 }
 <<<
+// dart format width=30
 main() {
-  // dart format width=30
   fitsUnsplitAt40 +
       butNotAt30;
 }
->>> Comment anywhere affects all code.
+>>> Comment only takes effect if it appears before code.
 main() {
+  // dart format width=30
   fitsUnsplitAt40   +   butNotAt30;
 }
-// dart format width=30
 <<<
 main() {
-  fitsUnsplitAt40 +
-      butNotAt30;
+  // dart format width=30
+  fitsUnsplitAt40 + butNotAt30;
 }
-
+>>> If there are multiple comments, first one wins.
 // dart format width=30
->>> If multiple comments, first one wins.
+// dart format width=60
 main() {
-  // dart format width=30
-  // dart format width=60
   fitsUnsplitAt40   +   butNotAt30;
 }
 <<<
+// dart format width=30
+// dart format width=60
 main() {
-  // dart format width=30
-  // dart format width=60
   fitsUnsplitAt40 +
       butNotAt30;
 }
 >>> Does nothing if width is not an integer.
+// dart format width=wat
 main() {
-  // dart format width=wat
   fitsUnsplitAt40   +   butNotAt30;
 }
 <<<
+// dart format width=wat
 main() {
-  // dart format width=wat
   fitsUnsplitAt40 + butNotAt30;
 }
 >>> Can't have trailing text.
+// dart format width=30 some more text
 main() {
-  // dart format width=30 some more text
   fitsUnsplitAt40   +   butNotAt30;
 }
 <<<
+// dart format width=30 some more text
 main() {
-  // dart format width=30 some more text
   fitsUnsplitAt40 + butNotAt30;
 }
 >>> Whitespace must match exactly.
+//dart format width=30
 main() {
-  //dart format width=30
   fitsUnsplitAt40   +   butNotAt30;
 }
 <<<
+//dart format width=30
 main() {
-  //dart format width=30
   fitsUnsplitAt40 + butNotAt30;
 }
 >>> Whitespace must match exactly.
+// dart   format width=30
 main() {
-  // dart   format width=30
   fitsUnsplitAt40   +   butNotAt30;
 }
 <<<
+// dart   format width=30
 main() {
-  // dart   format width=30
   fitsUnsplitAt40 + butNotAt30;
 }
 >>> Whitespace must match exactly.
+// dart format width = 30
 main() {
-  // dart format width = 30
   fitsUnsplitAt40   +   butNotAt30;
 }
 <<<
+// dart format width = 30
 main() {
-  // dart format width = 30
   fitsUnsplitAt40 + butNotAt30;
 }
->>> Can't be doc comment.
+>>> Can't be a doc comment.
+/// dart format width=30
 main() {
-  /// dart format width=30
   fitsUnsplitAt40 + butNotAt30;
 }
 <<<
+/// dart format width=30
 main() {
-  /// dart format width=30
   fitsUnsplitAt40 + butNotAt30;
 }
->>> Can't be nested inside other comment.
+>>> Can't be nested inside another comment.
+/* // dart format width=30 */
 main() {
-  /* // dart format width=30 */
   fitsUnsplitAt40   +   butNotAt30;
 }
 <<<
+/* // dart format width=30 */
 main() {
-  /* // dart format width=30 */
   fitsUnsplitAt40 + butNotAt30;
 }
->>> Can't be inside string literal.
+>>> Can't be inside a string literal.
+var c = '// dart format width=30';
 main() {
-  var c = '// dart format width=30';
   fitsUnsplitAt40   +   butNotAt30;
 }
 <<<
+var c = '// dart format width=30';
 main() {
-  var c = '// dart format width=30';
   fitsUnsplitAt40 + butNotAt30;
 }