Remove some unnecessary Piece classes. (#1491)

Remove some unnecessary Piece classes.

In exploring various optimizations, it's clear that I'll probably need to make
some changes to how all of the Piece classes work. The more of those classes
there are, the harder that work is.

Fortunately, there are a few Piece classes that don't really need to exist. Most
were created before we had AdjacentPiece or the ability to insert spaces between
pieces.

This change:

* Removes ClausePiece which it turns out behaves exactly like InfixPiece (and
  then renames ClausesPiece to ClausePiece).

* Removes TryPiece. All it did was write its children with some spaces between
  them.

* Removes PostfixPiece. It was just like InfixPiece but without a first operand.
  But it's only used by imports, where we can use the import header as the first
  operand and just use InfixPiece.

* Remove AdjacentStringsPiece. It's just an InfixPiece that always splits.

* Get rid of CaseStatementPiece. Once you hoist out the optionality of the
  guard, it's just an InfixPiece.

* Reorganize ForPiece. Make that Piece just for the header part of a for
  statement or element. (We can almost get rid of this entirely and use
  AdjacentPiece, but it does some indentation that isn't otherwise supported.)
  
  Then to handle the body, we either write it inline if it's a block (the most
  common), or we use IfPiece, which has the same logic. Rename IfPiece to
  ControlFlowPiece to emphasize its more general use. It was already used for
  while statements, so this makes sense.

Author: munificent

lib/src/front_end/ast_node_visitor.dart

@@ -9,11 +9,10 @@ import 'package:analyzer/source/line_info.dart';
 import '../ast_extensions.dart';
 import '../constants.dart';
 import '../dart_formatter.dart';
-import '../piece/adjacent_strings.dart';
 import '../piece/assign.dart';
 import '../piece/case.dart';
 import '../piece/constructor.dart';
-import '../piece/if.dart';
+import '../piece/control_flow.dart';
 import '../piece/infix.dart';
 import '../piece/list.dart';
 import '../piece/piece.dart';
@@ -133,8 +132,13 @@ class AstNodeVisitor extends ThrowingAstVisitor<void> with PieceFactory {
 
   @override
   void visitAdjacentStrings(AdjacentStrings node) {
-    pieces.add(AdjacentStringsPiece(node.strings.map(nodePiece).toList(),
-        indent: node.indentStrings));
+    var piece = InfixPiece(const [], node.strings.map(nodePiece).toList(),
+        indent: node.indentStrings);
+
+    // Adjacent strings always split.
+    piece.pin(State.split);
+
+    pieces.add(piece);
   }
 
   @override
@@ -873,7 +877,7 @@ class AstNodeVisitor extends ThrowingAstVisitor<void> with PieceFactory {
 
   @override
   void visitIfElement(IfElement node) {
-    var piece = IfPiece(isStatement: false);
+    var piece = ControlFlowPiece(isStatement: false);
 
     // Recurses through the else branches to flatten them into a linear if-else
     // chain handled by a single [IfPiece].
@@ -960,7 +964,7 @@ class AstNodeVisitor extends ThrowingAstVisitor<void> with PieceFactory {
 
   @override
   void visitIfStatement(IfStatement node) {
-    var piece = IfPiece(isStatement: true);
+    var piece = ControlFlowPiece();
 
     // Recurses through the else branches to flatten them into a linear if-else
     // chain handled by a single [IfPiece].
@@ -1759,10 +1763,13 @@ class AstNodeVisitor extends ThrowingAstVisitor<void> with PieceFactory {
             pieces.space();
 
             var patternPiece = nodePiece(member.guardedPattern.pattern);
-            var guardPiece =
-                optionalNodePiece(member.guardedPattern.whenClause);
 
-            pieces.add(CaseStatementPiece(patternPiece, guardPiece));
+            if (member.guardedPattern.whenClause case var whenClause?) {
+              pieces.add(
+                  InfixPiece(const [], [patternPiece, nodePiece(whenClause)]));
+            } else {
+              pieces.add(patternPiece);
+            }
 
           case SwitchDefault():
             break; // Nothing to do.
@@ -1912,7 +1919,7 @@ class AstNodeVisitor extends ThrowingAstVisitor<void> with PieceFactory {
 
     var body = nodePiece(node.body);
 
-    var piece = IfPiece(isStatement: true);
+    var piece = ControlFlowPiece();
     piece.add(condition, body, isBlock: node.body is Block);
     pieces.add(piece);
   }

lib/src/front_end/piece_factory.dart

@@ -8,15 +8,14 @@ import '../ast_extensions.dart';
 import '../piece/adjacent.dart';
 import '../piece/assign.dart';
 import '../piece/clause.dart';
+import '../piece/control_flow.dart';
 import '../piece/for.dart';
 import '../piece/function.dart';
 import '../piece/if_case.dart';
 import '../piece/infix.dart';
 import '../piece/list.dart';
 import '../piece/piece.dart';
-import '../piece/postfix.dart';
 import '../piece/sequence.dart';
-import '../piece/try.dart';
 import '../piece/type.dart';
 import '../piece/variable.dart';
 import 'ast_node_visitor.dart';
@@ -506,12 +505,21 @@ mixin PieceFactory {
       _ => false,
     };
 
-    var forPiece = ForPiece(forKeywordPiece, forPartsPiece, bodyPiece,
-        indentForParts: indentHeader, hasBlockBody: hasBlockBody);
-
-    if (forceSplitBody) forPiece.pin(State.split);
-
-    pieces.add(forPiece);
+    if (hasBlockBody) {
+      pieces
+          .add(ForPiece(forKeywordPiece, forPartsPiece, indent: indentHeader));
+      pieces.space();
+      pieces.add(bodyPiece);
+    } else {
+      var forPiece = ControlFlowPiece();
+      forPiece.add(
+          ForPiece(forKeywordPiece, forPartsPiece, indent: indentHeader),
+          bodyPiece,
+          isBlock: false);
+
+      if (forceSplitBody) forPiece.pin(State.split);
+      pieces.add(forPiece);
+    }
   }
 
   /// Writes a normal (not function-typed) formal parameter with a name and/or
@@ -738,43 +746,40 @@ mixin PieceFactory {
 
   /// Writes a [TryPiece] for try statement.
   void writeTry(TryStatement tryStatement) {
-    var piece = TryPiece();
-
-    var tryHeader = tokenPiece(tryStatement.tryKeyword);
-    var tryBlock = pieces.build(() {
-      writeBlock(tryStatement.body);
-    });
-    piece.add(tryHeader, tryBlock);
+    pieces.token(tryStatement.tryKeyword);
+    pieces.space();
+    writeBlock(tryStatement.body);
 
     for (var i = 0; i < tryStatement.catchClauses.length; i++) {
       var catchClause = tryStatement.catchClauses[i];
 
-      var catchClauseHeader = pieces.build(() {
-        if (catchClause.onKeyword case var onKeyword?) {
-          pieces.token(onKeyword, spaceAfter: true);
-          pieces.visit(catchClause.exceptionType);
-        }
+      pieces.space();
+      if (catchClause.onKeyword case var onKeyword?) {
+        pieces.token(onKeyword, spaceAfter: true);
+        pieces.visit(catchClause.exceptionType);
+      }
 
-        if (catchClause.onKeyword != null && catchClause.catchKeyword != null) {
-          pieces.space();
-        }
+      if (catchClause.onKeyword != null && catchClause.catchKeyword != null) {
+        pieces.space();
+      }
 
-        if (catchClause.catchKeyword case var catchKeyword?) {
-          pieces.token(catchKeyword);
-          pieces.space();
+      if (catchClause.catchKeyword case var catchKeyword?) {
+        pieces.token(catchKeyword);
+        pieces.space();
 
-          var parameters = DelimitedListBuilder(this);
-          parameters.leftBracket(catchClause.leftParenthesis!);
-          if (catchClause.exceptionParameter case var exceptionParameter?) {
-            parameters.visit(exceptionParameter);
-          }
-          if (catchClause.stackTraceParameter case var stackTraceParameter?) {
-            parameters.visit(stackTraceParameter);
-          }
-          parameters.rightBracket(catchClause.rightParenthesis!);
-          pieces.add(parameters.build());
+        var parameters = DelimitedListBuilder(this);
+        parameters.leftBracket(catchClause.leftParenthesis!);
+        if (catchClause.exceptionParameter case var exceptionParameter?) {
+          parameters.visit(exceptionParameter);
         }
-      });
+        if (catchClause.stackTraceParameter case var stackTraceParameter?) {
+          parameters.visit(stackTraceParameter);
+        }
+        parameters.rightBracket(catchClause.rightParenthesis!);
+        pieces.add(parameters.build());
+      }
+
+      pieces.space();
 
       // Edge case: When there's another catch/on/finally after this one, we
       // want to force the block to split even if it's empty.
@@ -787,72 +792,70 @@ mixin PieceFactory {
       //     }
       var forceSplit = i < tryStatement.catchClauses.length - 1 ||
           tryStatement.finallyBlock != null;
-      var catchClauseBody = pieces.build(() {
-        writeBlock(catchClause.body, forceSplit: forceSplit);
-      });
-      piece.add(catchClauseHeader, catchClauseBody);
+      writeBlock(catchClause.body, forceSplit: forceSplit);
     }
 
     if (tryStatement.finallyBlock case var finallyBlock?) {
-      var finallyHeader = tokenPiece(tryStatement.finallyKeyword!);
-      var finallyBody = pieces.build(() {
-        writeBlock(finallyBlock);
-      });
-      piece.add(finallyHeader, finallyBody);
+      pieces.space();
+      pieces.token(tryStatement.finallyKeyword!);
+      pieces.space();
+      writeBlock(finallyBlock);
     }
-
-    pieces.add(piece);
   }
 
   /// Writes an [ImportPiece] for an import or export directive.
   void writeImport(NamespaceDirective directive, Token keyword,
       {Token? deferredKeyword, Token? asKeyword, SimpleIdentifier? prefix}) {
     pieces.withMetadata(directive.metadata, () {
-      pieces.token(keyword);
-      pieces.space();
-      pieces.visit(directive.uri);
+      if (directive.configurations.isEmpty && asKeyword == null) {
+        // If there are no configurations or prefix (the common case), just
+        // write the import directly inline.
+        pieces.token(keyword);
+        pieces.space();
+        pieces.visit(directive.uri);
+      } else {
+        // Otherwise, allow splitting between the configurations and prefix.
+        var sections = [
+          pieces.build(() {
+            pieces.token(keyword);
+            pieces.space();
+            pieces.visit(directive.uri);
+          })
+        ];
 
-      if (directive.configurations.isNotEmpty) {
-        var configurations = <Piece>[];
         for (var configuration in directive.configurations) {
-          configurations.add(nodePiece(configuration));
+          sections.add(nodePiece(configuration));
         }
 
-        pieces.add(PostfixPiece(configurations));
-      }
-
-      if (asKeyword != null) {
-        pieces.add(PostfixPiece([
-          pieces.build(() {
+        if (asKeyword != null) {
+          sections.add(pieces.build(() {
             pieces.token(deferredKeyword, spaceAfter: true);
             pieces.token(asKeyword);
             pieces.space();
             pieces.visit(prefix!);
-          })
-        ]));
+          }));
+        }
+
+        pieces.add(InfixPiece(const [], sections));
       }
 
       if (directive.combinators.isNotEmpty) {
-        var combinators = <ClausePiece>[];
+        var combinators = <Piece>[];
         for (var combinatorNode in directive.combinators) {
-          var combinatorKeyword = tokenPiece(combinatorNode.keyword);
-
           switch (combinatorNode) {
             case HideCombinator(hiddenNames: var names):
             case ShowCombinator(shownNames: var names):
-              var parts = <Piece>[];
-              for (var name in names) {
-                parts.add(tokenPiece(name.token, commaAfter: true));
-              }
-
-              var combinator = ClausePiece(combinatorKeyword, parts);
-              combinators.add(combinator);
+              combinators.add(InfixPiece(const [], [
+                tokenPiece(combinatorNode.keyword),
+                for (var name in names)
+                  tokenPiece(name.token, commaAfter: true),
+              ]));
             default:
               throw StateError('Unknown combinator type $combinatorNode.');
           }
         }
 
-        pieces.add(ClausesPiece(combinators));
+        pieces.add(ClausePiece(combinators));
       }
 
       pieces.token(directive.semicolon);
@@ -1209,17 +1212,13 @@ mixin PieceFactory {
         }
       });
 
-      var clauses = <ClausePiece>[];
+      var clauses = <Piece>[];
 
       void typeClause(Token keyword, List<AstNode> types) {
-        var keywordPiece = tokenPiece(keyword);
-
-        var typePieces = <Piece>[];
-        for (var type in types) {
-          typePieces.add(nodePiece(type, commaAfter: true));
-        }
-
-        clauses.add(ClausePiece(keywordPiece, typePieces));
+        clauses.add(InfixPiece(const [], [
+          tokenPiece(keyword),
+          for (var type in types) nodePiece(type, commaAfter: true),
+        ]));
       }
 
       if (extendsClause != null) {
@@ -1248,9 +1247,9 @@ mixin PieceFactory {
             [if (nativeClause.name case var name?) name]);
       }
 
-      ClausesPiece? clausesPiece;
+      ClausePiece? clausesPiece;
       if (clauses.isNotEmpty) {
-        clausesPiece = ClausesPiece(clauses,
+        clausesPiece = ClausePiece(clauses,
             allowLeadingClause: extendsClause != null || onClause != null);
       }