Apply class modifiers to all the classes. (#1575)

Now that we've bumped the major version and can make breaking changes,
I went ahead and locked down all of the classes. It mostly matters for
the public ones, but I went ahead and applied it to the internal ones
too just to see if it makes things easier to maintain.

CHANGELOG.md

@@ -67,6 +67,12 @@
   If `--stdin-name` and `--language-version` are both omitted, then parses
   stdin using the latest supported language version.
 
+* **Apply class modifiers to API classes.** The dart_style package exposes only
+  a few classes in its public API: `DartFormatter`, `SourceCode`,
+  `FormatterException`, and `UnexpectedOutputException`. None were ever
+  intended to be extended or implemented. They are now all marked `final` to
+  make that intention explicit.
+
 ## 2.3.7
 
 * Allow passing a language version to `DartFomatter()`. Formatted code will be

lib/src/back_end/code.dart

@@ -177,7 +177,7 @@ enum _Marker { start, end }
 
 /// Traverses a [Code] tree and produces the final string of output code and
 /// the selection markers, if any.
-class _StringBuilder {
+final class _StringBuilder {
   /// Pre-calculated whitespace strings for various common levels of
   /// indentation.
   ///

lib/src/back_end/code_writer.dart

@@ -20,7 +20,7 @@ import 'solution_cache.dart';
 /// an instance of this class. It has methods that the piece can call to add
 /// output text to the resulting code, recursively format child pieces, insert
 /// whitespace, etc.
-class CodeWriter {
+final class CodeWriter {
   final int _pageWidth;
 
   /// Previously cached formatted subtrees.
@@ -415,7 +415,7 @@ enum Whitespace {
 }
 
 /// A level of indentation in the indentation stack.
-class _Indent {
+final class _Indent {
   /// The total number of spaces of indentation.
   final int indent;
 

lib/src/back_end/solution.dart

@@ -17,7 +17,7 @@ import 'solution_cache.dart';
 /// of the pieces in the tree so they can format themselves. That in turn
 /// yields a total number of overflow characters, cost, and formatted output,
 /// which are all stored here.
-class Solution implements Comparable<Solution> {
+final class Solution implements Comparable<Solution> {
   /// The states that pieces have been bound to.
   ///
   /// Note that order that keys are inserted into this map is significant. When

lib/src/back_end/solution_cache.dart

@@ -22,7 +22,7 @@ import 'solver.dart';
 /// the same child Piece and wanting to format it separately with the same
 /// indentation. When that happens, sharing this cache allows us to reuse that
 /// cached subtree Solution.
-class SolutionCache {
+final class SolutionCache {
   final _cache = <_Key, Solution>{};
 
   /// Returns a previously cached solution for formatting [root] with leading

lib/src/back_end/solver.dart

@@ -42,7 +42,7 @@ const _maxAttempts = 10000;
 ///     use it across different Solutions. This enables us to both divide and
 ///     conquer the Piece tree and solve portions separately, while also
 ///     reusing work across different solutions.
-class Solver {
+final class Solver {
   final SolutionCache _cache;
 
   final int _pageWidth;

lib/src/cli/format_command.dart

@@ -14,7 +14,7 @@ import 'output.dart';
 import 'show.dart';
 import 'summary.dart';
 
-class FormatCommand extends Command<int> {
+final class FormatCommand extends Command<int> {
   @override
   String get name => 'format';
 

lib/src/cli/formatter_options.dart

@@ -15,7 +15,7 @@ import 'summary.dart';
 const dartStyleVersion = '2.3.7';
 
 /// Global options that affect how the formatter produces and uses its outputs.
-class FormatterOptions {
+final class FormatterOptions {
   /// The language version formatted code should be parsed at or `null` if not
   /// specified.
   final Version? languageVersion;

lib/src/cli/summary.dart

@@ -8,7 +8,7 @@ import '../source_code.dart';
 import 'formatter_options.dart';
 
 /// The kind of summary shown after all formatting is complete.
-class Summary {
+final class Summary {
   static const Summary none = Summary._();
 
   /// Creates a Summary that tracks how many files were formatted and the total
@@ -41,7 +41,7 @@ class Summary {
 }
 
 /// Tracks how many files were formatted and the total time.
-class _LineSummary extends Summary {
+final class _LineSummary extends Summary {
   final DateTime _start = DateTime.now();
 
   /// The number of processed files.
@@ -81,7 +81,7 @@ class _LineSummary extends Summary {
 }
 
 /// Reports how long it took for format each file.
-class _ProfileSummary implements Summary {
+final class _ProfileSummary implements Summary {
   /// The files that have been started but have not completed yet.
   ///
   /// Maps a file label to the time that it started being formatted.

lib/src/constants.dart

@@ -13,7 +13,7 @@ const tallStyleExperimentFlag = 'tall-style';
 
 /// Constants for the cost heuristics used to determine which set of splits is
 /// most desirable.
-class Cost {
+final class Cost {
   /// The cost of splitting after the `=>` in a lambda or arrow-bodied member.
   ///
   /// We make this zero because there is already a span around the entire body
@@ -64,7 +64,7 @@ class Cost {
 }
 
 /// Constants for the number of spaces for various kinds of indentation.
-class Indent {
+final class Indent {
   /// Reset back to no indentation.
   static const none = 0;
 

lib/src/dart_formatter.dart

@@ -22,7 +22,7 @@ import 'source_code.dart';
 import 'string_compare.dart' as string_compare;
 
 /// Dart source code formatter.
-class DartFormatter {
+final class DartFormatter {
   /// The latest Dart language version that can be parsed and formatted by this
   /// version of the formatter.
   static final latestLanguageVersion = Version(3, 3, 0);

lib/src/debug.dart

@@ -277,7 +277,7 @@ String pieceTree(Piece piece) {
 }
 
 /// A stringified representation of a tree of pieces for debug output.
-class _PieceDebugTree {
+final class _PieceDebugTree {
   final String label;
   final List<_PieceDebugTree> children = [];
 

lib/src/exceptions.dart

@@ -7,7 +7,7 @@ import 'package:source_span/source_span.dart';
 
 /// Thrown when one or more errors occurs while parsing the code to be
 /// formatted.
-class FormatterException implements Exception {
+final class FormatterException implements Exception {
   /// The [AnalysisError]s that occurred.
   final List<AnalysisError> errors;
 
@@ -53,7 +53,7 @@ class FormatterException implements Exception {
 
 /// Exception thrown when the internal sanity check that only whitespace
 /// changes are made fails.
-class UnexpectedOutputException implements Exception {
+final class UnexpectedOutputException implements Exception {
   /// The source being formatted.
   final String _input;
 

lib/src/front_end/ast_node_visitor.dart

@@ -34,7 +34,7 @@ import 'sequence_builder.dart';
 /// To avoid this class becoming a monolith, functionality is divided into a
 /// couple of mixins, one for each area of functionality. This class then
 /// contains only shared state and the visitor methods for the AST.
-class AstNodeVisitor extends ThrowingAstVisitor<void> with PieceFactory {
+final class AstNodeVisitor extends ThrowingAstVisitor<void> with PieceFactory {
   @override
   final PieceWriter pieces;
 

lib/src/front_end/chain_builder.dart

@@ -41,7 +41,7 @@ import 'piece_factory.dart';
 ///
 /// This lets us create a single [ChainPiece] for the entire series of dotted
 /// operations, so that we can control splitting them or not as a unit.
-class ChainBuilder {
+final class ChainBuilder {
   final PieceFactory _visitor;
 
   /// The outermost expression being converted to a chain.

lib/src/front_end/comment_writer.dart

@@ -41,7 +41,7 @@ import '../comment_type.dart';
 /// construct. These get directly embedded in the [TextPiece] of the code being
 /// written. When that [TextPiece] is output later, it will include the comments
 /// as well.
-class CommentWriter {
+final class CommentWriter {
   final LineInfo _lineInfo;
 
   /// The tokens whose preceding comments have already been taken by calls to
@@ -147,7 +147,7 @@ class CommentWriter {
 
 /// A comment in the source, with a bit of information about the surrounding
 /// whitespace.
-class SourceComment {
+final class SourceComment {
   /// The text of the comment, including `//`, `/*`, and `*/`.
   final String text;
 
@@ -210,7 +210,7 @@ class SourceComment {
 /// * 2 newlines between `/* c2 */` and `/* c3 */`
 /// * Comment `/* c3 */`
 /// * 3 newlines between `/* c3 */` and `b`
-class CommentSequence extends ListBase<SourceComment> {
+final class CommentSequence extends ListBase<SourceComment> {
   static const CommentSequence empty = CommentSequence._([0], []);
 
   /// The number of newlines between a pair of comments or the preceding or

lib/src/front_end/delimited_list_builder.dart

@@ -18,7 +18,7 @@ import 'piece_factory.dart';
 /// delimiter token. Then call [add()] for each [AstNode] that is inside the
 /// delimiters. The [rightBracket()] with the closing delimiter and finally
 /// [build()] to get the resulting [ListPiece].
-class DelimitedListBuilder {
+final class DelimitedListBuilder {
   final PieceFactory _visitor;
 
   /// The opening bracket before the elements, if any.

lib/src/front_end/piece_writer.dart

@@ -24,7 +24,7 @@ import 'sequence_builder.dart';
 ///
 /// Handles updating selection markers and attaching comments to the tokens
 /// before and after the comments.
-class PieceWriter {
+final class PieceWriter {
   final DartFormatter _formatter;
 
   final SourceCode _source;

lib/src/front_end/sequence_builder.dart

@@ -22,7 +22,7 @@ import 'piece_factory.dart';
 /// separate statements inside the sequence. This lets us gracefully handle
 /// indenting them and supporting blank lines around them the same way we handle
 /// other statements or members in a sequence.
-class SequenceBuilder {
+final class SequenceBuilder {
   final PieceFactory _visitor;
 
   /// The opening bracket before the elements, if any.

lib/src/language_version_cache.dart

@@ -23,7 +23,7 @@ import 'profile.dart';
 /// looking for package configs for each file as it did formatting if we don't
 /// cache. Caching makes it ~10x faster to find the language version for each
 /// file.)
-class LanguageVersionCache {
+final class LanguageVersionCache {
   /// The previously cached default language version for all files immediately
   /// within a given directory.
   ///

lib/src/piece/adjacent.dart

@@ -5,7 +5,7 @@ import '../back_end/code_writer.dart';
 import 'piece.dart';
 
 /// A simple piece that just writes its child pieces one after the other.
-class AdjacentPiece extends Piece {
+final class AdjacentPiece extends Piece {
   final List<Piece> pieces;
 
   AdjacentPiece(this.pieces);

lib/src/piece/assign.dart

@@ -67,7 +67,7 @@ import 'piece.dart';
 ///     var [unsplitBlock] =
 ///         longOperand +
 ///             anotherOperand;
-class AssignPiece extends Piece {
+final class AssignPiece extends Piece {
   /// Force the block left-hand side to split and allow the right-hand side to
   /// split.
   static const State _blockSplitLeft = State(1);

lib/src/piece/case.dart

@@ -7,7 +7,7 @@ import '../constants.dart';
 import 'piece.dart';
 
 /// Piece for a case pattern, guard, and body in a switch expression.
-class CaseExpressionPiece extends Piece {
+final class CaseExpressionPiece extends Piece {
   /// Split after the `=>` before the body.
   static const State _beforeBody = State(1);
 

lib/src/piece/chain.dart

@@ -49,7 +49,7 @@ import 'piece.dart';
 ///           argument,
 ///           argument,
 ///         );
-class ChainPiece extends Piece {
+final class ChainPiece extends Piece {
   /// Allow newlines in the last (or next-to-last) call but nowhere else.
   static const State _blockFormatTrailingCall = State(1, cost: 0);
 
@@ -248,7 +248,7 @@ class ChainPiece extends Piece {
 
 /// A method or getter call in a call chain, along with any postfix operations
 /// applies to it.
-class ChainCall {
+final class ChainCall {
   /// Piece for the call.
   Piece _call;
 

lib/src/piece/clause.dart

@@ -61,7 +61,7 @@ import 'piece.dart';
 ///
 /// This ensures that when any wrapping occurs, the keywords are always at the
 /// beginning of the line.
-class ClausePiece extends Piece {
+final class ClausePiece extends Piece {
   /// State where we split between the clauses but not before the first one.
   static const State _betweenClauses = State(1);
 

lib/src/piece/constructor.dart

@@ -48,7 +48,7 @@ import 'piece.dart';
 ///     ]) : firstInitializer = 1,
 ///          second = 2;
 ///     //  ^ Five spaces of indentation.
-class ConstructorPiece extends Piece {
+final class ConstructorPiece extends Piece {
   static const _splitBeforeInitializers = State(1, cost: 1);
 
   static const _splitBetweenInitializers = State(2, cost: 2);

lib/src/piece/control_flow.dart

@@ -7,7 +7,7 @@ import 'piece.dart';
 
 /// A piece for an if statement or element, while statement, or for statement
 /// without a block body.
-class ControlFlowPiece extends Piece {
+final class ControlFlowPiece extends Piece {
   /// Whether this is an if statement versus if collection element.
   ///
   /// It's not meaningful for while and for statements/elements.
@@ -97,7 +97,7 @@ class ControlFlowPiece extends Piece {
 /// is the `else if (condition)` and the statement is the subsequent then
 /// branch. For the final `else` branch, if there is one, the [header] is just
 /// `else` and the statement is the else branch.
-class _Section {
+final class _Section {
   final Piece header;
   final Piece statement;
 

lib/src/piece/for.dart

@@ -6,7 +6,7 @@ import '../constants.dart';
 import 'piece.dart';
 
 /// A piece for the `for (...)` part of a for statement or element.
-class ForPiece extends Piece {
+final class ForPiece extends Piece {
   /// The `for` keyword.
   final Piece _forKeyword;
 
@@ -73,7 +73,7 @@ class ForPiece extends Piece {
 ///             anotherOperand) {
 ///       ...
 ///     }
-class ForInPiece extends Piece {
+final class ForInPiece extends Piece {
   /// The variable or pattern initialized with each loop iteration.
   final Piece _variable;
 

lib/src/piece/if_case.dart

@@ -36,7 +36,7 @@ import 'piece.dart';
 ///     if (obj
 ///         case pattern
 ///         when cond) ...
-class IfCasePiece extends Piece {
+final class IfCasePiece extends Piece {
   /// Split before the `when` guard clause.
   static const State _beforeWhen = State(1);
 

lib/src/piece/infix.dart

@@ -8,7 +8,7 @@ import 'piece.dart';
 /// A piece for a series of binary expressions at the same precedence, like:
 ///
 ///     a + b + c
-class InfixPiece extends Piece {
+final class InfixPiece extends Piece {
   /// The series of operands.
   ///
   /// Since we don't split on both sides of the operator, the operators will be

lib/src/piece/leading_comment.dart

@@ -23,7 +23,7 @@ import 'piece.dart';
 ///         // comment
 ///         a +
 ///             b;
-class LeadingCommentPiece extends Piece {
+final class LeadingCommentPiece extends Piece {
   final List<Piece> _comments;
   final Piece _piece;
 

lib/src/piece/list.dart

@@ -38,7 +38,7 @@ import 'piece.dart';
 ///
 /// ListPieces are usually constructed using [createList()] or
 /// [DelimitedListBuilder].
-class ListPiece extends Piece {
+final class ListPiece extends Piece {
   /// The opening bracket before the elements, if any.
   final Piece? _before;
 
@@ -435,7 +435,7 @@ enum BlockFormat {
 /// they vary in whether or not a trailing comma is allowed, whether there
 /// should be spaces inside the delimiters when the elements aren't split, etc.
 /// This class captures those options.
-class ListStyle {
+final class ListStyle {
   /// How commas should be handled by the list.
   ///
   /// Most lists use [Commas.trailing]. Type parameters and type arguments use