lib/src/front_end/piece_factory.dart - dart_style - Git at Google

 // Copyright (c) 2023, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';

 import '../ast_extensions.dart';
 import '../piece/adjacent.dart';
 import '../piece/assign.dart';
 import '../piece/block.dart';
 import '../piece/clause.dart';
 import '../piece/function.dart';
 import '../piece/if.dart';
 import '../piece/infix.dart';
 import '../piece/list.dart';
 import '../piece/piece.dart';
 import '../piece/postfix.dart';
 import '../piece/type.dart';
 import 'ast_node_visitor.dart';
 import 'comment_writer.dart';
 import 'delimited_list_builder.dart';
 import 'piece_writer.dart';
 import 'sequence_builder.dart';

 /// Record type for a destructured binary operator-like syntactic construct.
 typedef BinaryOperation = (AstNode left, Token operator, AstNode right);

 /// Utility methods for creating pieces that share formatting logic across
 /// multiple parts of the language.
 ///
 /// Many AST nodes are structurally similar and receive similar formatting. For
 /// example, imports and exports are mostly the same, with exports a subset of
 /// imports. Likewise, assert statements are formatted like function calls and
 /// argument lists.
 ///
 /// This mixin defines functions that represent a general construct that is
 /// formatted a certain way. The function builds up an appropriate set of
 /// [Piece]s given the various AST subcomponents passed in as parameters. The
 /// main [AstNodeVisitor] class then calls those for all of the AST nodes that
 /// should receive that similar formatting.
 ///
 /// These are all void functions because they generally push their result into
 /// the [PieceWriter].
 ///
 /// Naming these functions can be hard. For example, there isn't an obvious
 /// word for "import or export directive" or "named thing with argument list".
 /// To avoid that, we pick one concrete construct formatted by the function,
 /// usually the most common, and name it after that, as in [createImport()].
 mixin PieceFactory implements CommentWriter {
   void visit(AstNode? node, {void Function()? before, void Function()? after});

   /// Creates a [ListPiece] for an argument list.
   void createArgumentList(
       Token leftBracket, Iterable<AstNode> elements, Token rightBracket) {
     return createList(
         leftBracket: leftBracket,
         elements,
         rightBracket: rightBracket,
         style: const ListStyle(allowBlockElement: true));
   }

   /// Creates a [BlockPiece] for a given bracket-delimited block or declaration
   /// body.
   ///
   /// If [forceSplit] is `true`, then the block will split even if empty. This
   /// is used, for example, with empty blocks in `if` statements followed by
   /// `else` clauses:
   ///
   /// ```
   /// if (condition) {
   /// } else {}
   /// ```
   void createBody(Token leftBracket, List<AstNode> contents, Token rightBracket,
       {bool forceSplit = false}) {
     token(leftBracket);
     var leftBracketPiece = pieces.split();

     var sequence = SequenceBuilder(this);
     for (var node in contents) {
       sequence.visit(node);

       // If the node has a non-empty braced body, then require a blank line
       // between it and the next node.
       if (node.hasNonEmptyBody) sequence.addBlank();
     }

     // Place any comments before the "}" inside the block.
     sequence.addCommentsBefore(rightBracket);

     token(rightBracket);
     var rightBracketPiece = pieces.take();

     pieces.give(BlockPiece(
         leftBracketPiece, sequence.build(), rightBracketPiece,
         alwaysSplit: forceSplit || contents.isNotEmpty));
   }

   /// Creates a [BlockPiece] for a given [Block].
   ///
   /// If [forceSplit] is `true`, then the block will split even if empty. This
   /// is used, for example, with empty blocks in `if` statements followed by
   /// `else` clauses:
   ///
   /// ```
   /// if (condition) {
   /// } else {}
   /// ```
   void createBlock(Block block, {bool forceSplit = false}) {
     createBody(block.leftBracket, block.statements, block.rightBracket,
         forceSplit: forceSplit);
   }

   /// Creates a piece for a `break` or `continue` statement.
   void createBreak(Token keyword, SimpleIdentifier? label, Token semicolon) {
     token(keyword);
     if (label != null) {
       space();
       visit(label);
     }
     token(semicolon);
   }

   /// Creates a [ListPiece] for a collection literal.
   void createCollection(Token? constKeyword, Token leftBracket,
       List<AstNode> elements, Token rightBracket,
       {TypeArgumentList? typeArguments, ListStyle style = const ListStyle()}) {
     modifier(constKeyword);
     visit(typeArguments);

     // TODO(tall): Support a line comment inside a collection literal as a
     // signal to preserve internal newlines. So if you have:
     //
     // ```
     // var list = [
     //   1, 2, 3, // comment
     //   4, 5, 6,
     // ];
     // ```
     //
     // The formatter will preserve the newline after element 3 and the lack of
     // them after the other elements.

     createList(
       leftBracket: leftBracket,
       elements,
       rightBracket: rightBracket,
       style: style,
     );
   }

   /// Visits the leading keyword and parenthesized expression at the beginning
   /// of an `if`, `while`, or `switch` expression or statement.
   void startControlFlow(Token keyword, Token leftParenthesis, Expression value,
       Token rightParenthesis) {
     // Attach the keyword to the `(`.
     token(keyword);
     space();
     token(leftParenthesis);
     visit(value);
     token(rightParenthesis);
   }

   /// Creates metadata annotations for a directive.
   ///
   /// Always forces the annotations to be on a previous line.
   void createDirectiveMetadata(Directive directive) {
     // TODO(tall): Implement. See SourceVisitor._visitDirectiveMetadata().
     if (directive.metadata.isNotEmpty) throw UnimplementedError();
   }

   /// Creates a dotted or qualified identifier.
   void createDotted(NodeList<SimpleIdentifier> components) {
     for (var component in components) {
       // Write the preceding ".".
       if (component != components.first) {
         token(component.beginToken.previous);
       }

       visit(component);
     }
   }

   /// Creates a function, method, getter, or setter declaration.
   ///
   /// If [modifierKeyword] is given, it should be the `static` or `abstract`
   /// modifier on a method declaration. If [operatorKeyword] is given, it
   /// should be the `operator` keyword on an operator declaration. If
   /// [propertyKeyword] is given, it should be the `get` or `set` keyword on a
   /// getter or setter declaration.
   void createFunction(
       {Token? externalKeyword,
       Token? modifierKeyword,
       AstNode? returnType,
       Token? operatorKeyword,
       Token? propertyKeyword,
       required Token name,
       TypeParameterList? typeParameters,
       FormalParameterList? parameters,
       required FunctionBody body}) {
     modifier(externalKeyword);
     modifier(modifierKeyword);

     Piece? returnTypePiece;
     if (returnType != null) {
       visit(returnType);
       returnTypePiece = pieces.split();
     }

     modifier(operatorKeyword);
     modifier(propertyKeyword);
     token(name);

     finishFunction(returnTypePiece, typeParameters, parameters, body);
   }

   /// Creates a function type or function-typed formal.
   void createFunctionType(
       TypeAnnotation? returnType,
       Token? functionKeywordOrName,
       TypeParameterList? typeParameters,
       FormalParameterList parameters,
       Token? question) {
     Piece? returnTypePiece;
     if (returnType != null) {
       visit(returnType);
       returnTypePiece = pieces.split();
     }

     token(functionKeywordOrName);
     visit(typeParameters);
     visit(parameters);
     token(question);
     var parametersPiece = pieces.take();

     pieces.give(FunctionPiece(returnTypePiece, parametersPiece));
   }

   // TODO(tall): Generalize this to work with if elements too.
   /// Creates a piece for a chain of if-else-if... statements.
   void createIf(IfStatement ifStatement) {
     var piece = IfPiece();

     // Recurses through the else branches to flatten them into a linear if-else
     // chain handled by a single [IfPiece].
     void traverse(IfStatement node) {
       startControlFlow(node.ifKeyword, node.leftParenthesis, node.expression,
           node.rightParenthesis);
       var condition = pieces.split();

       // Edge case: When the then branch is a block and there is an else clause
       // after it, we want to force the block to split even if empty, like:
       //
       // ```
       // if (condition) {
       // } else {
       //   body;
       // }
       // ```
       if (node.thenStatement case Block thenBlock
           when node.elseStatement != null) {
         createBlock(thenBlock, forceSplit: true);
       } else {
         visit(node.thenStatement);
       }

       var thenStatement = pieces.split();
       piece.add(condition, thenStatement, isBlock: node.thenStatement is Block);

       switch (node.elseStatement) {
         case IfStatement elseIf:
           // Hit an else-if, so flatten it into the chain with the `else`
           // becoming part of the next section's header.
           token(node.elseKeyword);
           space();
           traverse(elseIf);

         case var elseStatement?:
           // Any other kind of else body ends the chain, with the header for
           // the last section just being the `else` keyword.
           token(node.elseKeyword);
           var header = pieces.split();

           visit(elseStatement);
           var statement = pieces.take();
           piece.add(header, statement, isBlock: elseStatement is Block);
       }
     }

     traverse(ifStatement);

     pieces.give(piece);
   }

   /// Creates an [ImportPiece] for an import or export directive.
   void createImport(NamespaceDirective directive, Token keyword,
       {Token? deferredKeyword, Token? asKeyword, SimpleIdentifier? prefix}) {
     createDirectiveMetadata(directive);
     token(keyword);
     space();
     visit(directive.uri);
     var importPieces = [pieces.take()];

     if (directive.configurations.isNotEmpty) {
       var configurations = <Piece>[];
       for (var configuration in directive.configurations) {
         pieces.split();
         visit(configuration);
         configurations.add(pieces.take());
       }

       importPieces.add(PostfixPiece(configurations));
     }

     if (asKeyword != null) {
       pieces.split();
       token(deferredKeyword, after: space);
       token(asKeyword);
       space();
       visit(prefix);
       importPieces.add(PostfixPiece([pieces.take()]));
     }

     if (directive.combinators.isNotEmpty) {
       var combinators = <ClausePiece>[];
       for (var combinatorNode in directive.combinators) {
         pieces.split();
         token(combinatorNode.keyword);
         var combinatorKeyword = pieces.split();

         switch (combinatorNode) {
           case HideCombinator(hiddenNames: var names):
           case ShowCombinator(shownNames: var names):
             var parts = <Piece>[];
             for (var name in names) {
               pieces.split();
               token(name.token);
               commaAfter(name);
               parts.add(pieces.take());
             }

             var combinator = ClausePiece(combinatorKeyword, parts);
             combinators.add(combinator);
           default:
             throw StateError('Unknown combinator type $combinatorNode.');
         }
       }

       importPieces.add(ClausesPiece(combinators));
     }

     token(directive.semicolon);

     pieces.give(AdjacentPiece(importPieces));
   }

   /// Creates a single infix operation.
   ///
   /// If [hanging] is `true` then the operator goes at the end of the first
   /// line, like `+`. Otherwise, it goes at the beginning of the second, like
   /// `as`.
   ///
   /// The [operator2] parameter may be passed if the "operator" is actually two
   /// separate tokens, as in `foo is! Bar`.
   void createInfix(AstNode left, Token operator, AstNode right,
       {bool hanging = false, Token? operator2}) {
     var operands = <Piece>[];

     visit(left);

     if (hanging) {
       space();
       token(operator);
       token(operator2);
       operands.add(pieces.split());
     } else {
       operands.add(pieces.split());
       token(operator);
       token(operator2);
       space();
     }

     visit(right);
     operands.add(pieces.take());
     pieces.give(InfixPiece(operands));
   }

   /// Creates a chained infix operation: a binary operator expression, or
   /// binary pattern.
   ///
   /// In a tree of binary AST nodes, all operators at the same precedence are
   /// treated as a single chain of operators that either all split or none do.
   /// Operands within those (which may themselves be chains of higher
   /// precedence binary operators) are then formatted independently.
   ///
   /// [T] is the type of node being visited and [destructure] is a callback
   /// that takes one of those and yields the operands and operator. We need
   /// this since there's no interface shared by the various binary operator
   /// AST nodes.
   ///
   /// If [precedence] is given, then this only flattens binary nodes with that
   /// same precedence.
   void createInfixChain<T extends AstNode>(
       T node, BinaryOperation Function(T node) destructure,
       {int? precedence}) {
     var operands = <Piece>[];

     void traverse(AstNode e) {
       // If the node is one if our infix operators, then recurse into the
       // operands.
       if (e is T) {
         var (left, operator, right) = destructure(e);
         if (precedence == null || operator.type.precedence == precedence) {
           traverse(left);
           space();
           token(operator);
           pieces.split();
           traverse(right);
           return;
         }
       }

       // Otherwise, just write the node itself.
       visit(e);
       operands.add(pieces.take());
     }

     traverse(node);

     pieces.give(InfixPiece(operands));
   }

   /// Creates a [ListPiece] for the given bracket-delimited set of elements.
   void createList(Iterable<AstNode> elements,
       {Token? leftBracket,
       Token? rightBracket,
       ListStyle style = const ListStyle()}) {
     var builder = DelimitedListBuilder(this, style);
     if (leftBracket != null) builder.leftBracket(leftBracket);
     elements.forEach(builder.visit);
     if (rightBracket != null) builder.rightBracket(rightBracket);
     pieces.give(builder.build());
   }

   /// Creates a class, enum, extension, mixin, or mixin application class
   /// declaration.
   ///
   /// For all but a mixin application class, [body] should a record containing
   /// the bracket delimiters and the list of member declarations for the type's
   /// body.
   ///
   /// For mixin application classes, [body] is `null` and instead [equals],
   /// [superclass], and [semicolon] are provided.
   ///
   /// If the type is an extension, then [onType] is a record containing the
   /// `on` keyword and the on type.
   void createType(NodeList<Annotation> metadata, List<Token?> modifiers,
       Token keyword, Token? name,
       {TypeParameterList? typeParameters,
       Token? equals,
       NamedType? superclass,
       ExtendsClause? extendsClause,
       OnClause? onClause,
       WithClause? withClause,
       ImplementsClause? implementsClause,
       NativeClause? nativeClause,
       (Token, TypeAnnotation)? onType,
       ({Token leftBracket, List<AstNode> members, Token rightBracket})? body,
       Token? semicolon}) {
     if (metadata.isNotEmpty) throw UnimplementedError('Type metadata.');

     modifiers.forEach(modifier);
     token(keyword);
     token(name, before: space);
     visit(typeParameters);

     // Mixin application classes have ` = Superclass` after the declaration
     // name.
     if (equals != null) {
       space();
       token(equals);
       space();
       visit(superclass);
     }

     var header = pieces.split();

     var clauses = <ClausePiece>[];

     void typeClause(Token keyword, List<AstNode> types) {
       token(keyword);
       var keywordPiece = pieces.split();

       var typePieces = <Piece>[];
       for (var type in types) {
         visit(type);
         commaAfter(type);
         typePieces.add(pieces.split());
       }

       clauses.add(ClausePiece(keywordPiece, typePieces));
     }

     if (extendsClause != null) {
       typeClause(extendsClause.extendsKeyword, [extendsClause.superclass]);
     }

     if (onClause != null) {
       typeClause(onClause.onKeyword, onClause.superclassConstraints);
     }

     if (withClause != null) {
       typeClause(withClause.withKeyword, withClause.mixinTypes);
     }

     if (implementsClause != null) {
       typeClause(
           implementsClause.implementsKeyword, implementsClause.interfaces);
     }

     if (onType case (var onKeyword, var onType)?) {
       typeClause(onKeyword, [onType]);
     }

     ClausesPiece? clausesPiece;
     if (clauses.isNotEmpty) {
       clausesPiece = ClausesPiece(clauses,
           allowLeadingClause: extendsClause != null || onClause != null);
     }

     visit(nativeClause);
     space();

     if (body != null) {
       createBody(body.leftBracket, body.members, body.rightBracket);
     } else {
       token(semicolon);
     }
     var bodyPiece = pieces.take();

     pieces.give(
         TypePiece(header, clausesPiece, bodyPiece, hasBody: body != null));
   }

   /// Creates a [ListPiece] for a type argument or type parameter list.
   void createTypeList(
       Token leftBracket, Iterable<AstNode> elements, Token rightBracket) {
     return createList(
         leftBracket: leftBracket,
         elements,
         rightBracket: rightBracket,
         style: const ListStyle(commas: Commas.nonTrailing, splitCost: 3));
   }

   /// Writes the parts of a formal parameter shared by all formal parameter
   /// types: metadata, `covariant`, etc.
   void startFormalParameter(FormalParameter parameter) {
     if (parameter.metadata.isNotEmpty) throw UnimplementedError();

     modifier(parameter.requiredKeyword);
     modifier(parameter.covariantKeyword);
   }

   /// Handles the `async`, `sync*`, or `async*` modifiers on a function body.
   void functionBodyModifiers(FunctionBody body) {
     // The `async` or `sync` keyword.
     token(body.keyword);
     token(body.star);
     if (body.keyword != null) space();
   }

   /// Creates a [Piece] with "assignment-like" splitting.
   ///
   /// This is used, obviously, for assignments and variable declarations to
   /// handle splitting after the `=`, but is also used in any context where an
   /// expression follows something that it "defines" or "initializes":
   ///
   /// * Assignment
   /// * Variable declaration
   /// * Constructor initializer
   /// * Expression (`=>`) function body
   /// * Named argument or named record field (`:`)
   /// * Map entry (`:`)
   /// * For-in loop iterator (`in`)
   ///
   /// This method assumes the code to the left of the operator has already
   /// been visited.
   ///
   /// If [splitBeforeOperator] is `true`, then puts [operator] at the beginning
   /// of the next line when it splits. Otherwise, puts the operator at the end
   /// of the preceding line.
   void finishAssignment(Token operator, Expression rightHandSide,
       {bool splitBeforeOperator = false}) {
     Piece target;
     if (splitBeforeOperator) {
       target = pieces.split();
       token(operator);
       space();
     } else {
       if (operator.type == TokenType.EQ) space();
       token(operator);
       target = pieces.split();
     }

     visit(rightHandSide);

     var initializer = pieces.take();
     pieces.give(AssignPiece(target, initializer,
         isValueDelimited: rightHandSide.canBlockSplit));
   }

   /// Finishes writing a named function declaration or anonymous function
   /// expression after the return type (if any) and name (if any) has been
   /// written.
   void finishFunction(Piece? returnType, TypeParameterList? typeParameters,
       FormalParameterList? parameters, FunctionBody body) {
     visit(typeParameters);
     visit(parameters);

     Piece parametersPiece;
     Piece? bodyPiece;
     if (body is EmptyFunctionBody) {
       // If the body is just `;`, then don't allow a space or split before the
       // semicolon by making it part of the parameters piece.
       token(body.semicolon);
       parametersPiece = pieces.split();
     } else {
       parametersPiece = pieces.split();
       visit(body);
       bodyPiece = pieces.take();
     }

     pieces.give(FunctionPiece(returnType, parametersPiece, bodyPiece));
   }

   /// Writes an optional modifier that precedes other code.
   void modifier(Token? keyword) {
     token(keyword, after: space);
   }

   /// Write a single space.
   void space() {
     pieces.writeSpace();
   }

   /// Emit [token], along with any comments and formatted whitespace that comes
   /// before it.
   ///
   /// Does nothing if [token] is `null`. If [before] is given, it will be
   /// executed before the token is outout. Likewise, [after] will be called
   /// after the token is output.
   void token(Token? token, {void Function()? before, void Function()? after}) {
     if (token == null) return;

     writeCommentsBefore(token);

     if (before != null) before();
     pieces.writeToken(token);
     if (after != null) after();
   }

   /// Writes a comma after [node], if there is one.
   void commaAfter(AstNode node, {bool trailing = false}) {
     var nextToken = node.endToken.next!;
     if (nextToken.lexeme == ',') {
       token(nextToken);
     } else if (trailing) {
       // If there isn't a comma there, it must be a place where a trailing
       // comma can appear, so synthesize it. During formatting, we will decide
       // whether to include it.
       pieces.writeText(',');
     }
   }
 }
	// Copyright (c) 2023, the Dart project authors. Please see the AUTHORS file
	// for details. All rights reserved. Use of this source code is governed by a
	// BSD-style license that can be found in the LICENSE file.
	import 'package:analyzer/dart/ast/ast.dart';
	import 'package:analyzer/dart/ast/token.dart';

	import '../ast_extensions.dart';
	import '../piece/adjacent.dart';
	import '../piece/assign.dart';
	import '../piece/block.dart';
	import '../piece/clause.dart';
	import '../piece/function.dart';
	import '../piece/if.dart';
	import '../piece/infix.dart';
	import '../piece/list.dart';
	import '../piece/piece.dart';
	import '../piece/postfix.dart';
	import '../piece/type.dart';
	import 'ast_node_visitor.dart';
	import 'comment_writer.dart';
	import 'delimited_list_builder.dart';
	import 'piece_writer.dart';
	import 'sequence_builder.dart';

	/// Record type for a destructured binary operator-like syntactic construct.
	typedef BinaryOperation = (AstNode left, Token operator, AstNode right);

	/// Utility methods for creating pieces that share formatting logic across
	/// multiple parts of the language.
	///
	/// Many AST nodes are structurally similar and receive similar formatting. For
	/// example, imports and exports are mostly the same, with exports a subset of
	/// imports. Likewise, assert statements are formatted like function calls and
	/// argument lists.
	///
	/// This mixin defines functions that represent a general construct that is
	/// formatted a certain way. The function builds up an appropriate set of
	/// [Piece]s given the various AST subcomponents passed in as parameters. The
	/// main [AstNodeVisitor] class then calls those for all of the AST nodes that
	/// should receive that similar formatting.
	///
	/// These are all void functions because they generally push their result into
	/// the [PieceWriter].
	///
	/// Naming these functions can be hard. For example, there isn't an obvious
	/// word for "import or export directive" or "named thing with argument list".
	/// To avoid that, we pick one concrete construct formatted by the function,
	/// usually the most common, and name it after that, as in [createImport()].
	mixin PieceFactory implements CommentWriter {
	void visit(AstNode? node, {void Function()? before, void Function()? after});

	/// Creates a [ListPiece] for an argument list.
	void createArgumentList(
	Token leftBracket, Iterable<AstNode> elements, Token rightBracket) {
	return createList(
	leftBracket: leftBracket,
	elements,
	rightBracket: rightBracket,
	style: const ListStyle(allowBlockElement: true));
	}

	/// Creates a [BlockPiece] for a given bracket-delimited block or declaration
	/// body.
	///
	/// If [forceSplit] is `true`, then the block will split even if empty. This
	/// is used, for example, with empty blocks in `if` statements followed by
	/// `else` clauses:
	///
	/// ```
	/// if (condition) {
	/// } else {}
	/// ```
	void createBody(Token leftBracket, List<AstNode> contents, Token rightBracket,
	{bool forceSplit = false}) {
	token(leftBracket);
	var leftBracketPiece = pieces.split();

	var sequence = SequenceBuilder(this);
	for (var node in contents) {
	sequence.visit(node);

	// If the node has a non-empty braced body, then require a blank line
	// between it and the next node.
	if (node.hasNonEmptyBody) sequence.addBlank();
	}

	// Place any comments before the "}" inside the block.
	sequence.addCommentsBefore(rightBracket);

	token(rightBracket);
	var rightBracketPiece = pieces.take();

	pieces.give(BlockPiece(
	leftBracketPiece, sequence.build(), rightBracketPiece,
	alwaysSplit: forceSplit \|\| contents.isNotEmpty));
	}

	/// Creates a [BlockPiece] for a given [Block].
	///
	/// If [forceSplit] is `true`, then the block will split even if empty. This
	/// is used, for example, with empty blocks in `if` statements followed by
	/// `else` clauses:
	///
	/// ```
	/// if (condition) {
	/// } else {}
	/// ```
	void createBlock(Block block, {bool forceSplit = false}) {
	createBody(block.leftBracket, block.statements, block.rightBracket,
	forceSplit: forceSplit);
	}

	/// Creates a piece for a `break` or `continue` statement.
	void createBreak(Token keyword, SimpleIdentifier? label, Token semicolon) {
	token(keyword);
	if (label != null) {
	space();
	visit(label);
	}
	token(semicolon);
	}

	/// Creates a [ListPiece] for a collection literal.
	void createCollection(Token? constKeyword, Token leftBracket,
	List<AstNode> elements, Token rightBracket,
	{TypeArgumentList? typeArguments, ListStyle style = const ListStyle()}) {
	modifier(constKeyword);
	visit(typeArguments);

	// TODO(tall): Support a line comment inside a collection literal as a
	// signal to preserve internal newlines. So if you have:
	//
	// ```
	// var list = [
	// 1, 2, 3, // comment
	// 4, 5, 6,
	// ];
	// ```
	//
	// The formatter will preserve the newline after element 3 and the lack of
	// them after the other elements.

	createList(
	leftBracket: leftBracket,
	elements,
	rightBracket: rightBracket,
	style: style,
	);
	}

	/// Visits the leading keyword and parenthesized expression at the beginning
	/// of an `if`, `while`, or `switch` expression or statement.
	void startControlFlow(Token keyword, Token leftParenthesis, Expression value,
	Token rightParenthesis) {
	// Attach the keyword to the `(`.
	token(keyword);
	space();
	token(leftParenthesis);
	visit(value);
	token(rightParenthesis);
	}

	/// Creates metadata annotations for a directive.
	///
	/// Always forces the annotations to be on a previous line.
	void createDirectiveMetadata(Directive directive) {
	// TODO(tall): Implement. See SourceVisitor._visitDirectiveMetadata().
	if (directive.metadata.isNotEmpty) throw UnimplementedError();
	}

	/// Creates a dotted or qualified identifier.
	void createDotted(NodeList<SimpleIdentifier> components) {
	for (var component in components) {
	// Write the preceding ".".
	if (component != components.first) {
	token(component.beginToken.previous);
	}

	visit(component);
	}
	}

	/// Creates a function, method, getter, or setter declaration.
	///
	/// If [modifierKeyword] is given, it should be the `static` or `abstract`
	/// modifier on a method declaration. If [operatorKeyword] is given, it
	/// should be the `operator` keyword on an operator declaration. If
	/// [propertyKeyword] is given, it should be the `get` or `set` keyword on a
	/// getter or setter declaration.
	void createFunction(
	{Token? externalKeyword,
	Token? modifierKeyword,
	AstNode? returnType,
	Token? operatorKeyword,
	Token? propertyKeyword,
	required Token name,
	TypeParameterList? typeParameters,
	FormalParameterList? parameters,
	required FunctionBody body}) {
	modifier(externalKeyword);
	modifier(modifierKeyword);

	Piece? returnTypePiece;
	if (returnType != null) {
	visit(returnType);
	returnTypePiece = pieces.split();
	}

	modifier(operatorKeyword);
	modifier(propertyKeyword);
	token(name);

	finishFunction(returnTypePiece, typeParameters, parameters, body);
	}

	/// Creates a function type or function-typed formal.
	void createFunctionType(
	TypeAnnotation? returnType,
	Token? functionKeywordOrName,
	TypeParameterList? typeParameters,
	FormalParameterList parameters,
	Token? question) {
	Piece? returnTypePiece;
	if (returnType != null) {
	visit(returnType);
	returnTypePiece = pieces.split();
	}

	token(functionKeywordOrName);
	visit(typeParameters);
	visit(parameters);
	token(question);
	var parametersPiece = pieces.take();

	pieces.give(FunctionPiece(returnTypePiece, parametersPiece));
	}

	// TODO(tall): Generalize this to work with if elements too.
	/// Creates a piece for a chain of if-else-if... statements.
	void createIf(IfStatement ifStatement) {
	var piece = IfPiece();

	// Recurses through the else branches to flatten them into a linear if-else
	// chain handled by a single [IfPiece].
	void traverse(IfStatement node) {
	startControlFlow(node.ifKeyword, node.leftParenthesis, node.expression,
	node.rightParenthesis);
	var condition = pieces.split();

	// Edge case: When the then branch is a block and there is an else clause
	// after it, we want to force the block to split even if empty, like:
	//
	// ```
	// if (condition) {
	// } else {
	// body;
	// }
	// ```
	if (node.thenStatement case Block thenBlock
	when node.elseStatement != null) {
	createBlock(thenBlock, forceSplit: true);
	} else {
	visit(node.thenStatement);
	}

	var thenStatement = pieces.split();
	piece.add(condition, thenStatement, isBlock: node.thenStatement is Block);

	switch (node.elseStatement) {
	case IfStatement elseIf:
	// Hit an else-if, so flatten it into the chain with the `else`
	// becoming part of the next section's header.
	token(node.elseKeyword);
	space();
	traverse(elseIf);

	case var elseStatement?:
	// Any other kind of else body ends the chain, with the header for
	// the last section just being the `else` keyword.
	token(node.elseKeyword);
	var header = pieces.split();

	visit(elseStatement);
	var statement = pieces.take();
	piece.add(header, statement, isBlock: elseStatement is Block);
	}
	}

	traverse(ifStatement);

	pieces.give(piece);
	}

	/// Creates an [ImportPiece] for an import or export directive.
	void createImport(NamespaceDirective directive, Token keyword,
	{Token? deferredKeyword, Token? asKeyword, SimpleIdentifier? prefix}) {
	createDirectiveMetadata(directive);
	token(keyword);
	space();
	visit(directive.uri);
	var importPieces = [pieces.take()];

	if (directive.configurations.isNotEmpty) {
	var configurations = <Piece>[];
	for (var configuration in directive.configurations) {
	pieces.split();
	visit(configuration);
	configurations.add(pieces.take());
	}

	importPieces.add(PostfixPiece(configurations));
	}

	if (asKeyword != null) {
	pieces.split();
	token(deferredKeyword, after: space);
	token(asKeyword);
	space();
	visit(prefix);
	importPieces.add(PostfixPiece([pieces.take()]));
	}

	if (directive.combinators.isNotEmpty) {
	var combinators = <ClausePiece>[];
	for (var combinatorNode in directive.combinators) {
	pieces.split();
	token(combinatorNode.keyword);
	var combinatorKeyword = pieces.split();

	switch (combinatorNode) {
	case HideCombinator(hiddenNames: var names):
	case ShowCombinator(shownNames: var names):
	var parts = <Piece>[];
	for (var name in names) {
	pieces.split();
	token(name.token);
	commaAfter(name);
	parts.add(pieces.take());
	}

	var combinator = ClausePiece(combinatorKeyword, parts);
	combinators.add(combinator);
	default:
	throw StateError('Unknown combinator type $combinatorNode.');
	}
	}

	importPieces.add(ClausesPiece(combinators));
	}

	token(directive.semicolon);

	pieces.give(AdjacentPiece(importPieces));
	}

	/// Creates a single infix operation.
	///
	/// If [hanging] is `true` then the operator goes at the end of the first
	/// line, like `+`. Otherwise, it goes at the beginning of the second, like
	/// `as`.
	///
	/// The [operator2] parameter may be passed if the "operator" is actually two
	/// separate tokens, as in `foo is! Bar`.
	void createInfix(AstNode left, Token operator, AstNode right,
	{bool hanging = false, Token? operator2}) {
	var operands = <Piece>[];

	visit(left);

	if (hanging) {
	space();
	token(operator);
	token(operator2);
	operands.add(pieces.split());
	} else {
	operands.add(pieces.split());
	token(operator);
	token(operator2);
	space();
	}

	visit(right);
	operands.add(pieces.take());
	pieces.give(InfixPiece(operands));
	}

	/// Creates a chained infix operation: a binary operator expression, or
	/// binary pattern.
	///
	/// In a tree of binary AST nodes, all operators at the same precedence are
	/// treated as a single chain of operators that either all split or none do.
	/// Operands within those (which may themselves be chains of higher
	/// precedence binary operators) are then formatted independently.
	///
	/// [T] is the type of node being visited and [destructure] is a callback
	/// that takes one of those and yields the operands and operator. We need
	/// this since there's no interface shared by the various binary operator
	/// AST nodes.
	///
	/// If [precedence] is given, then this only flattens binary nodes with that
	/// same precedence.
	void createInfixChain<T extends AstNode>(
	T node, BinaryOperation Function(T node) destructure,
	{int? precedence}) {
	var operands = <Piece>[];

	void traverse(AstNode e) {
	// If the node is one if our infix operators, then recurse into the
	// operands.
	if (e is T) {
	var (left, operator, right) = destructure(e);
	if (precedence == null \|\| operator.type.precedence == precedence) {
	traverse(left);
	space();
	token(operator);
	pieces.split();
	traverse(right);
	return;
	}
	}

	// Otherwise, just write the node itself.
	visit(e);
	operands.add(pieces.take());
	}

	traverse(node);

	pieces.give(InfixPiece(operands));
	}

	/// Creates a [ListPiece] for the given bracket-delimited set of elements.
	void createList(Iterable<AstNode> elements,
	{Token? leftBracket,
	Token? rightBracket,
	ListStyle style = const ListStyle()}) {
	var builder = DelimitedListBuilder(this, style);
	if (leftBracket != null) builder.leftBracket(leftBracket);
	elements.forEach(builder.visit);
	if (rightBracket != null) builder.rightBracket(rightBracket);
	pieces.give(builder.build());
	}

	/// Creates a class, enum, extension, mixin, or mixin application class
	/// declaration.
	///
	/// For all but a mixin application class, [body] should a record containing
	/// the bracket delimiters and the list of member declarations for the type's
	/// body.
	///
	/// For mixin application classes, [body] is `null` and instead [equals],
	/// [superclass], and [semicolon] are provided.
	///
	/// If the type is an extension, then [onType] is a record containing the
	/// `on` keyword and the on type.
	void createType(NodeList<Annotation> metadata, List<Token?> modifiers,
	Token keyword, Token? name,
	{TypeParameterList? typeParameters,
	Token? equals,
	NamedType? superclass,
	ExtendsClause? extendsClause,
	OnClause? onClause,
	WithClause? withClause,
	ImplementsClause? implementsClause,
	NativeClause? nativeClause,
	(Token, TypeAnnotation)? onType,
	({Token leftBracket, List<AstNode> members, Token rightBracket})? body,
	Token? semicolon}) {
	if (metadata.isNotEmpty) throw UnimplementedError('Type metadata.');

	modifiers.forEach(modifier);
	token(keyword);
	token(name, before: space);
	visit(typeParameters);

	// Mixin application classes have ` = Superclass` after the declaration
	// name.
	if (equals != null) {
	space();
	token(equals);
	space();
	visit(superclass);
	}

	var header = pieces.split();

	var clauses = <ClausePiece>[];

	void typeClause(Token keyword, List<AstNode> types) {
	token(keyword);
	var keywordPiece = pieces.split();

	var typePieces = <Piece>[];
	for (var type in types) {
	visit(type);
	commaAfter(type);
	typePieces.add(pieces.split());
	}

	clauses.add(ClausePiece(keywordPiece, typePieces));
	}

	if (extendsClause != null) {
	typeClause(extendsClause.extendsKeyword, [extendsClause.superclass]);
	}

	if (onClause != null) {
	typeClause(onClause.onKeyword, onClause.superclassConstraints);
	}

	if (withClause != null) {
	typeClause(withClause.withKeyword, withClause.mixinTypes);
	}

	if (implementsClause != null) {
	typeClause(
	implementsClause.implementsKeyword, implementsClause.interfaces);
	}

	if (onType case (var onKeyword, var onType)?) {
	typeClause(onKeyword, [onType]);
	}

	ClausesPiece? clausesPiece;
	if (clauses.isNotEmpty) {
	clausesPiece = ClausesPiece(clauses,
	allowLeadingClause: extendsClause != null \|\| onClause != null);
	}

	visit(nativeClause);
	space();

	if (body != null) {
	createBody(body.leftBracket, body.members, body.rightBracket);
	} else {
	token(semicolon);
	}
	var bodyPiece = pieces.take();

	pieces.give(
	TypePiece(header, clausesPiece, bodyPiece, hasBody: body != null));
	}

	/// Creates a [ListPiece] for a type argument or type parameter list.
	void createTypeList(
	Token leftBracket, Iterable<AstNode> elements, Token rightBracket) {
	return createList(
	leftBracket: leftBracket,
	elements,
	rightBracket: rightBracket,
	style: const ListStyle(commas: Commas.nonTrailing, splitCost: 3));
	}

	/// Writes the parts of a formal parameter shared by all formal parameter
	/// types: metadata, `covariant`, etc.
	void startFormalParameter(FormalParameter parameter) {
	if (parameter.metadata.isNotEmpty) throw UnimplementedError();

	modifier(parameter.requiredKeyword);
	modifier(parameter.covariantKeyword);
	}

	/// Handles the `async`, `sync`, or `async` modifiers on a function body.
	void functionBodyModifiers(FunctionBody body) {
	// The `async` or `sync` keyword.
	token(body.keyword);
	token(body.star);
	if (body.keyword != null) space();
	}

	/// Creates a [Piece] with "assignment-like" splitting.
	///
	/// This is used, obviously, for assignments and variable declarations to
	/// handle splitting after the `=`, but is also used in any context where an
	/// expression follows something that it "defines" or "initializes":
	///
	/// * Assignment
	/// * Variable declaration
	/// * Constructor initializer
	/// * Expression (`=>`) function body
	/// * Named argument or named record field (`:`)
	/// * Map entry (`:`)
	/// * For-in loop iterator (`in`)
	///
	/// This method assumes the code to the left of the operator has already
	/// been visited.
	///
	/// If [splitBeforeOperator] is `true`, then puts [operator] at the beginning
	/// of the next line when it splits. Otherwise, puts the operator at the end
	/// of the preceding line.
	void finishAssignment(Token operator, Expression rightHandSide,
	{bool splitBeforeOperator = false}) {
	Piece target;
	if (splitBeforeOperator) {
	target = pieces.split();
	token(operator);
	space();
	} else {
	if (operator.type == TokenType.EQ) space();
	token(operator);
	target = pieces.split();
	}

	visit(rightHandSide);

	var initializer = pieces.take();
	pieces.give(AssignPiece(target, initializer,
	isValueDelimited: rightHandSide.canBlockSplit));
	}

	/// Finishes writing a named function declaration or anonymous function
	/// expression after the return type (if any) and name (if any) has been
	/// written.
	void finishFunction(Piece? returnType, TypeParameterList? typeParameters,
	FormalParameterList? parameters, FunctionBody body) {
	visit(typeParameters);
	visit(parameters);

	Piece parametersPiece;
	Piece? bodyPiece;
	if (body is EmptyFunctionBody) {
	// If the body is just `;`, then don't allow a space or split before the
	// semicolon by making it part of the parameters piece.
	token(body.semicolon);
	parametersPiece = pieces.split();
	} else {
	parametersPiece = pieces.split();
	visit(body);
	bodyPiece = pieces.take();
	}

	pieces.give(FunctionPiece(returnType, parametersPiece, bodyPiece));
	}

	/// Writes an optional modifier that precedes other code.
	void modifier(Token? keyword) {
	token(keyword, after: space);
	}

	/// Write a single space.
	void space() {
	pieces.writeSpace();
	}

	/// Emit [token], along with any comments and formatted whitespace that comes
	/// before it.
	///
	/// Does nothing if [token] is `null`. If [before] is given, it will be
	/// executed before the token is outout. Likewise, [after] will be called
	/// after the token is output.
	void token(Token? token, {void Function()? before, void Function()? after}) {
	if (token == null) return;

	writeCommentsBefore(token);

	if (before != null) before();
	pieces.writeToken(token);
	if (after != null) after();
	}

	/// Writes a comma after [node], if there is one.
	void commaAfter(AstNode node, {bool trailing = false}) {
	var nextToken = node.endToken.next!;
	if (nextToken.lexeme == ',') {
	token(nextToken);
	} else if (trailing) {
	// If there isn't a comma there, it must be a place where a trailing
	// comma can appear, so synthesize it. During formatting, we will decide
	// whether to include it.
	pieces.writeText(',');
	}
	}
	}