Allow a comment in code to explicitly configure the page width.

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/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 +403,8 @@ class PieceWriter {
 
     var cache = SolutionCache();
     var solver = Solver(cache,
-        pageWidth: _formatter.pageWidth, leadingIndent: _formatter.indent);
+        pageWidth: pageWidthFromComment ?? _formatter.pageWidth,
+        leadingIndent: _formatter.indent);
     var solution = solver.format(rootPiece);
     var output = solution.code.build(source, _formatter.lineEnding);
 

test/tall/other/format_off.unit

@@ -193,4 +193,134 @@ main() {
   unformatted  +  code  +  // dart format on
       after +
       region;
+}
+>>> "dart format off" whitespace must match exactly.
+main() {
+  //dart format off
+  unformatted  +  code;
+  // dart format on
+
+  //   dart format off
+  unformatted  +  code;
+  // dart format on
+
+  // dart   format off
+  unformatted  +  code;
+  // dart format on
+
+  // dart format   off
+  unformatted  +  code;
+  // dart format on
+}
+<<<
+main() {
+  //dart format off
+  unformatted + code;
+  // dart format on
+
+  //   dart format off
+  unformatted + code;
+  // dart format on
+
+  // dart   format off
+  unformatted + code;
+  // dart format on
+
+  // dart format   off
+  unformatted + code;
+  // dart format on
+}
+>>> "dart format on" whitespace must match exactly.
+main() {
+  // dart format off
+  // Doesn't actually turn back on:
+  //dart format on
+  unformatted  +  code;
+
+  // Does now:
+  // dart format on
+  unformatted  +  code;
+
+  // dart format off
+  // Doesn't actually turn back on:
+  //   dart format on
+  unformatted  +  code;
+
+  // Does now:
+  // dart format on
+  unformatted  +  code;
+
+  // dart format off
+  // Doesn't actually turn back on:
+  // dart   format on
+  unformatted  +  code;
+
+  // Does now:
+  // dart format on
+  unformatted  +  code;
+
+  // dart format off
+  // Doesn't actually turn back on:
+  // dart format   on
+  unformatted  +  code;
+
+  // Does now:
+  // dart format on
+  unformatted  +  code;
+}
+<<<
+main() {
+  // dart format off
+  // Doesn't actually turn back on:
+  //dart format on
+  unformatted  +  code;
+
+  // Does now:
+  // dart format on
+  unformatted + code;
+
+  // dart format off
+  // Doesn't actually turn back on:
+  //   dart format on
+  unformatted  +  code;
+
+  // Does now:
+  // dart format on
+  unformatted + code;
+
+  // dart format off
+  // Doesn't actually turn back on:
+  // dart   format on
+  unformatted  +  code;
+
+  // Does now:
+  // dart format on
+  unformatted + code;
+
+  // dart format off
+  // Doesn't actually turn back on:
+  // dart format   on
+  unformatted  +  code;
+
+  // Does now:
+  // dart format on
+  unformatted + code;
+}
+>>> Can't be doc comment.
+main() {
+  here   +   gets   +   formatted    ;
+  /// dart format off
+  here   +   does    +    too   ;
+  /// dart format on
+  and   +   here   +    does   ;
+}
+<<<
+main() {
+  here + gets + formatted;
+
+  /// dart format off
+  here + does + too;
+
+  /// dart format on
+  and + here + does;
 }

test/tall/other/format_width.unit

@@ -0,0 +1,116 @@
+40 columns                              |
+### Tests for the comment to set formatting width.
+>>> Comment sets page width.
+// dart format width=30
+main() {
+  fitsUnsplitAt40   +   butNotAt30;
+}
+<<<
+// dart format width=30
+main() {
+  fitsUnsplitAt40 +
+      butNotAt30;
+}
+>>> Comment only takes effect if it appears before code.
+main() {
+  // dart format width=30
+  fitsUnsplitAt40   +   butNotAt30;
+}
+<<<
+main() {
+  // dart format width=30
+  fitsUnsplitAt40 + butNotAt30;
+}
+>>> If there are multiple comments, first one wins.
+// dart format width=30
+// dart format width=60
+main() {
+  fitsUnsplitAt40   +   butNotAt30;
+}
+<<<
+// dart format width=30
+// dart format width=60
+main() {
+  fitsUnsplitAt40 +
+      butNotAt30;
+}
+>>> Does nothing if width is not an integer.
+// dart format width=wat
+main() {
+  fitsUnsplitAt40   +   butNotAt30;
+}
+<<<
+// dart format width=wat
+main() {
+  fitsUnsplitAt40 + butNotAt30;
+}
+>>> Can't have trailing text.
+// dart format width=30 some more text
+main() {
+  fitsUnsplitAt40   +   butNotAt30;
+}
+<<<
+// dart format width=30 some more text
+main() {
+  fitsUnsplitAt40 + butNotAt30;
+}
+>>> Whitespace must match exactly.
+//dart format width=30
+main() {
+  fitsUnsplitAt40   +   butNotAt30;
+}
+<<<
+//dart format width=30
+main() {
+  fitsUnsplitAt40 + butNotAt30;
+}
+>>> Whitespace must match exactly.
+// dart   format width=30
+main() {
+  fitsUnsplitAt40   +   butNotAt30;
+}
+<<<
+// dart   format width=30
+main() {
+  fitsUnsplitAt40 + butNotAt30;
+}
+>>> Whitespace must match exactly.
+// dart format width = 30
+main() {
+  fitsUnsplitAt40   +   butNotAt30;
+}
+<<<
+// dart format width = 30
+main() {
+  fitsUnsplitAt40 + butNotAt30;
+}
+>>> Can't be a doc comment.
+/// dart format width=30
+main() {
+  fitsUnsplitAt40 + butNotAt30;
+}
+<<<
+/// dart format width=30
+main() {
+  fitsUnsplitAt40 + butNotAt30;
+}
+>>> Can't be nested inside another comment.
+/* // dart format width=30 */
+main() {
+  fitsUnsplitAt40   +   butNotAt30;
+}
+<<<
+/* // dart format width=30 */
+main() {
+  fitsUnsplitAt40 + butNotAt30;
+}
+>>> Can't be inside a string literal.
+var c = '// dart format width=30';
+main() {
+  fitsUnsplitAt40   +   butNotAt30;
+}
+<<<
+var c = '// dart format width=30';
+main() {
+  fitsUnsplitAt40 + butNotAt30;
+}