Allow a comment in code to explicitly configure the page width. (#1574)

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

If the code being formatted begins with a comment like:

```
// dart format width=123
```

Then the code is formatted at that width instead of the default width.

The intent is that tools that generate code and then format it will put
this comment in the generated code. That way, the tool doesn't have to
handle the complex logic to find a surrounding analysis_options.yaml
file and get the user's project-wide page width configuration.

End users may also want to use this comment in rare cases. For example,
maybe they have a library that contains a large value of data that looks
better as a big wide table but they don't want their entire project to
have a wider 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.
 final class DartFormatter {
   /// The latest Dart language version that can be parsed and formatted by this
@@ -178,8 +183,21 @@ final 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 @@ final 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 @@ final 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 @@ final 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 @@ final 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;
+}