Better debug output for piece trees. (#1324)

* Better debug output for piece trees.

The old formatter has pretty nice debug output, but the new one is still
pretty rudimentary. This improves that situation somewhat from:

```
Sequence(Type(`class Foo` Block(`{` Sequence(Var(`static late final int` `i;`) Var(`static late int` `i;`) Var(`static late var` `i;`) Var(`covariant late var` `i;`) Var(`covariant late int` `i;`) Var(`late final int` `i;`) Var(`late int` `i;`) Var(`late var` `i;`)) `}`)))
```

To:

```
Seq(
  Type(
    `class Foo`
    Block(
      `{`
      Seq(
        Var(`static late final int` `i;`)
        Var(`static late int` `i;`)
        Var(`static late var` `i;`)
        Var(`covariant late var` `i;`)
        Var(`covariant late int` `i;`)
        Var(`late final int` `i;`)
        Var(`late int` `i;`)
        Var(`late var` `i;`)
      )
      `}`
    )
  )
)
```

It also eliminates a bunch of pointless `toString()` implementations
that are only used for debugging and can be inferred from runtimeType.

* Update doc comment.

lib/src/debug.dart

@@ -268,25 +268,73 @@ void dumpLines(List<Chunk> chunks, SplitSet splits) {
 /// Build a string representation of the [piece] tree.
 String pieceTree(Piece piece) {
   var buffer = StringBuffer();
+  _PieceDebugTree(piece).write(buffer, 0);
+  return buffer.toString();
+}
 
-  void traverse(Piece piece) {
-    buffer.write(piece);
+/// A stringified representation of a tree of pieces for debug output.
+class _PieceDebugTree {
+  final String label;
+  final List<_PieceDebugTree> children = [];
 
-    if (piece is! TextPiece) {
-      var children = <Piece>[];
-      piece.forEachChild(children.add);
+  _PieceDebugTree(Piece piece) : label = piece.toString() {
+    piece.forEachChild((child) {
+      children.add(_PieceDebugTree(child));
+    });
+  }
 
-      buffer.write('(');
-      for (var child in children) {
-        if (child != children.first) buffer.write(' ');
-        traverse(child);
-      }
-      buffer.write(')');
+  /// The approximate number of characters of output needed to print this tree
+  /// on a single line.
+  ///
+  /// Used to determine when to show a tree's children inline or split. Note
+  /// that this is O(n^2), but we don't really care since it's only used for
+  /// debug output.
+  int get width {
+    var result = label.length;
+    for (var child in children) {
+      result += child.width;
     }
+    return result;
   }
 
-  traverse(piece);
-  return buffer.toString();
+  void write(StringBuffer buffer, int indent) {
+    buffer.write(label);
+    if (children.isEmpty) return;
+
+    buffer.write('(');
+
+    // Split the tree if it is too long.
+    var isSplit = indent * 2 + width > 80;
+    if (isSplit) {
+      indent++;
+      buffer.writeln();
+      buffer.write('  ' * indent);
+    }
+
+    var first = true;
+    for (var child in children) {
+      if (!first) {
+        if (isSplit) {
+          buffer.writeln();
+          buffer.write('  ' * indent);
+        } else {
+          buffer.write(' ');
+        }
+      }
+
+      child.write(buffer, indent);
+
+      first = false;
+    }
+
+    if (isSplit) {
+      indent--;
+      buffer.writeln();
+      buffer.write('  ' * indent);
+    }
+
+    buffer.write(')');
+  }
 }
 
 String _color(String ansiEscape) => useAnsiColors ? ansiEscape : '';

lib/src/piece/adjacent.dart

@@ -21,7 +21,4 @@ class AdjacentPiece extends Piece {
   void forEachChild(void Function(Piece piece) callback) {
     _pieces.forEach(callback);
   }
-
-  @override
-  String toString() => 'Adjacent';
 }

lib/src/piece/assign.dart

@@ -129,7 +129,4 @@ class AssignPiece extends Piece {
     callback(target);
     callback(value);
   }
-
-  @override
-  String toString() => 'Assign';
 }

lib/src/piece/block.dart

@@ -55,7 +55,4 @@ class BlockPiece extends Piece {
     callback(contents);
     callback(rightBracket);
   }
-
-  @override
-  String toString() => 'Block';
 }

lib/src/piece/chain.dart

@@ -46,7 +46,4 @@ class ChainPiece extends Piece {
   void forEachChild(void Function(Piece piece) callback) {
     _operations.forEach(callback);
   }
-
-  @override
-  String toString() => 'Chain';
 }

lib/src/piece/clause.dart

@@ -121,9 +121,6 @@ class ClausesPiece extends Piece {
   void forEachChild(void Function(Piece piece) callback) {
     _clauses.forEach(callback);
   }
-
-  @override
-  String toString() => 'Clauses';
 }
 
 /// A keyword followed by a comma-separated list of items described by that
@@ -156,7 +153,4 @@ class ClausePiece extends Piece {
     callback(_keyword);
     _parts.forEach(callback);
   }
-
-  @override
-  String toString() => 'Clause';
 }

lib/src/piece/do_while.dart

@@ -25,7 +25,4 @@ class DoWhilePiece extends Piece {
     callback(_body);
     callback(_condition);
   }
-
-  @override
-  String toString() => 'DoWhile';
 }

lib/src/piece/for.dart

@@ -59,7 +59,4 @@ class ForPiece extends Piece {
     callback(_forParts);
     callback(_body);
   }
-
-  @override
-  String toString() => 'For';
 }

lib/src/piece/function.dart

@@ -56,5 +56,5 @@ class FunctionPiece extends Piece {
   }
 
   @override
-  String toString() => 'Fn';
+  String get debugName => 'Fn';
 }

lib/src/piece/if.dart

@@ -75,9 +75,6 @@ class IfPiece extends Piece {
       callback(section.statement);
     }
   }
-
-  @override
-  String toString() => 'If';
 }
 
 /// A single section in a chain of if-elses.

lib/src/piece/infix.dart

@@ -42,7 +42,4 @@ class InfixPiece extends Piece {
   void forEachChild(void Function(Piece piece) callback) {
     operands.forEach(callback);
   }
-
-  @override
-  String toString() => 'Infix';
 }

lib/src/piece/list.dart

@@ -123,9 +123,6 @@ class ListPiece extends Piece {
 
     if (_after case var after?) callback(after);
   }
-
-  @override
-  String toString() => 'List';
 }
 
 /// An element in a [ListPiece].

lib/src/piece/piece.dart

@@ -59,6 +59,14 @@ abstract class Piece {
   void pin(State state) {
     _pinnedState = state;
   }
+
+  /// The name of this piece as it appears in debug output.
+  ///
+  /// By default, this is the class's name with `Piece` removed.
+  String get debugName => runtimeType.toString().replaceAll('Piece', '');
+
+  @override
+  String toString() => debugName;
 }
 
 /// A simple atomic piece of code.

lib/src/piece/postfix.dart

@@ -46,7 +46,4 @@ class PostfixPiece extends Piece {
   void forEachChild(void Function(Piece piece) callback) {
     pieces.forEach(callback);
   }
-
-  @override
-  String toString() => 'Post';
 }

lib/src/piece/sequence.dart

@@ -39,7 +39,7 @@ class SequencePiece extends Piece {
   }
 
   @override
-  String toString() => 'Sequence';
+  String get debugName => 'Seq';
 }
 
 /// An element inside a [SequencePiece].

lib/src/piece/type.dart

@@ -39,7 +39,4 @@ class TypePiece extends Piece {
     if (_clauses case var clauses?) callback(clauses);
     callback(_body);
   }
-
-  @override
-  String toString() => 'Type';
 }

lib/src/piece/variable.dart

@@ -95,5 +95,5 @@ class VariablePiece extends Piece {
   }
 
   @override
-  String toString() => 'Var';
+  String get debugName => 'Var';
 }