Format || patterns like fallthrough cases in switch expressions. (#1620)

Switch statements allow multiple cases to share a body like:

```dart
switch (obj) {
  case pattern1:
  case pattern2:
    body;
}
```

Switch expressions don't support that, but `||` patterns are the idiomatic way to accomplish the same thing. Because of that, the formatter has some special formatting when the outermost pattern in a switch expression case is `||`:

```dart
x = switch (obj) {
  pattern1 ||
  pattern2 =>
    body,
};
```

Note how the `pattern2` operand isn't indented.

This PR extends that special handling to allow the `=>` on the same line as the `=>` even if the pattern is a split `||` pattern, like:

```dart
x = switch (obj) {
  pattern1 ||
  pattern2 => body,
};
```

And it prefers to split the `||` over the body when the body is block formatted:

```dart
// Prefer:
x = switch (obj) {
  pattern1 ||
  pattern2 => function(argument),
};

// Over:
x = switch (obj) {
  pattern1 || pattern2 => function(
    argument,
  ),
};
```

This is one of those rules that's mostly a matter of taste, but I ran this on a large corpus and most of the diffs look better to me. Here are a few examples:

```dart
// Before:
      typeName = switch (targetType) {
        DriftSqlType.int || DriftSqlType.bigInt || DriftSqlType.bool =>
          'INTEGER',
        DriftSqlType.string => 'CHAR',
        DriftSqlType.double => 'DOUBLE',
        DriftSqlType.blob => 'BINARY',
        DriftSqlType.dateTime => 'DATETIME',
        DriftSqlType.any => '',
        CustomSqlType() || DialectAwareSqlType() => targetType.sqlTypeName(
          context,
        ),
      };

// After:
      typeName = switch (targetType) {
        DriftSqlType.int ||
        DriftSqlType.bigInt ||
        DriftSqlType.bool => 'INTEGER',
        DriftSqlType.string => 'CHAR',
        DriftSqlType.double => 'DOUBLE',
        DriftSqlType.blob => 'BINARY',
        DriftSqlType.dateTime => 'DATETIME',
        DriftSqlType.any => '',
        CustomSqlType() ||
        DialectAwareSqlType() => targetType.sqlTypeName(context),
      };

// Before:
    return switch (side) {
      AxisSide.right || AxisSide.left =>
        titlesPadding.vertical + borderPadding.vertical,
      AxisSide.top || AxisSide.bottom =>
        titlesPadding.horizontal + borderPadding.horizontal,
    };

// After:
    return switch (side) {
      AxisSide.right ||
      AxisSide.left => titlesPadding.vertical + borderPadding.vertical,
      AxisSide.top ||
      AxisSide.bottom => titlesPadding.horizontal + borderPadding.horizontal,
    };

// Before:
    final defaultConstraints = switch (side) {
      ShadSheetSide.top || ShadSheetSide.bottom => BoxConstraints(
        minWidth: mSize.width,
      ),
      ShadSheetSide.left || ShadSheetSide.right => BoxConstraints(
        minHeight: mSize.height,
      ),
    };

    final defaultConstraints = switch (side) {
      ShadSheetSide.top ||
      ShadSheetSide.bottom => BoxConstraints(minWidth: mSize.width),
      ShadSheetSide.left ||
      ShadSheetSide.right => BoxConstraints(minHeight: mSize.height),
    };
```

Fix #1602.

Author: munificent

CHANGELOG.md

@@ -1,6 +1,7 @@
 ## 3.0.1-wip
 
 * Handle trailing commas in for-loop updaters (#1354).
+* Format `||` patterns like fallthrough cases in switch expressions (#1602).
 * Handle comments and metadata before variables more gracefully (#1604).
 * Ensure comment formatting is idempotent (#1606).
 * Better indentation of leading comments on property accesses in binary operator

lib/src/piece/case.dart

@@ -8,13 +8,20 @@ import 'piece.dart';
 
 /// Piece for a case pattern, guard, and body in a switch expression.
 final class CaseExpressionPiece extends Piece {
+  /// Split inside the body, which must be block formattable, like:
+  ///
+  ///     pattern => function(
+  ///       argument,
+  ///     ),
+  static const State _blockSplitBody = State(1, cost: 0);
+
   /// Split after the `=>` before the body.
-  static const State _beforeBody = State(1);
+  static const State _beforeBody = State(2);
 
   /// Split before the `when` guard clause and after the `=>`.
-  static const State _beforeWhenAndBody = State(2);
+  static const State _beforeWhenAndBody = State(3);
 
-  /// The pattern the value is matched against along with the leading `case`.
+  /// The pattern the value is matched against.
   final Piece _pattern;
 
   /// If there is a `when` clause, that clause.
@@ -54,17 +61,23 @@ final class CaseExpressionPiece extends Piece {
 
   @override
   List<State> get additionalStates => [
+        if (_canBlockSplitBody) _blockSplitBody,
         _beforeBody,
         if (_guard != null) ...[_beforeWhenAndBody],
       ];
 
   @override
   bool allowNewlineInChild(State state, Piece child) {
     return switch (state) {
+      // If the outermost pattern is `||`, then always let it split even while
+      // allowing the body on the same line as `=>`.
+      _ when child == _pattern && _patternIsLogicalOr => true,
+
+      // There are almost never splits in the arrow piece. It requires a comment
+      // in a funny location, but if it happens, allow it.
       _ when child == _arrow => true,
-      State.unsplit when child == _body && _canBlockSplitBody => true,
-      _beforeBody when child == _pattern =>
-        _guard == null || _patternIsLogicalOr,
+      _blockSplitBody when child == _body => true,
+      _beforeBody when child == _pattern => _guard == null,
       _beforeBody when child == _body => true,
       _beforeWhenAndBody => true,
       _ => false,
@@ -94,12 +107,13 @@ final class CaseExpressionPiece extends Piece {
     writer.space();
     writer.format(_arrow);
 
-    if (state != State.unsplit) writer.pushIndent(Indent.block);
+    var indentBody = state != State.unsplit && state != _blockSplitBody;
+    if (indentBody) writer.pushIndent(Indent.block);
 
     writer.splitIf(state == _beforeBody || state == _beforeWhenAndBody);
     writer.format(_body);
 
-    if (state != State.unsplit) writer.popIndent();
+    if (indentBody) writer.popIndent();
   }
 
   @override

test/tall/expression/switch.stmt

@@ -120,8 +120,7 @@ e = switch (obj) {
 e = switch (obj) {
   oneConstant ||
   twoConstant ||
-  threeConstant =>
-    body,
+  threeConstant => body,
 };
 >>> Nested logic-or operands are indented.
 e = switch (obj) {
@@ -175,4 +174,15 @@ e = switch (obj) {
       veryLongElement,
       veryLongElement,
     ),
-};
+};
+>>> Prefer to split `||` pattern instead of case body.
+e = switch (obj) {
+  pattern || another => function(
+    argument,
+  ),
+};
+<<<
+e = switch (obj) {
+  pattern ||
+  another => function(argument),
+};

test/tall/expression/switch_guard.stmt

@@ -96,8 +96,7 @@ e = switch (obj) {
 <<<
 e = switch (obj) {
   veryVeryLongPattern ||
-  reallyMustSplitHere when true =>
-    body,
+  reallyMustSplitHere when true => body,
 };
 >>> Outermost logic-or split in pattern, expression split in guard.
 e = switch (obj) {

test/tall/regression/1100/1197.unit

@@ -25,6 +25,9 @@ main() {
   }
 }
 <<<
+### TODO(1466): Ideally, the first case would also split at the `||` instead of
+### of before `.`, but the formatter can't distinguish that case without fixing
+### #1466.
 main() {
   {
     return TextFieldTapRegion(
@@ -38,14 +41,13 @@ main() {
           TargetPlatform.android ||
           TargetPlatform.fuchsia ||
           TargetPlatform.linux ||
-          TargetPlatform.windows =>
-            _renderEditable.selectWordsInRange(
-              from:
-                  longPressMoveUpdateDetails.globalPosition -
-                  longPressMoveUpdateDetails.offsetFromOrigin,
-              to: longPressMoveUpdateDetails.globalPosition,
-              cause: SelectionChangedCause.longPress,
-            ),
+          TargetPlatform.windows => _renderEditable.selectWordsInRange(
+            from:
+                longPressMoveUpdateDetails.globalPosition -
+                longPressMoveUpdateDetails.offsetFromOrigin,
+            to: longPressMoveUpdateDetails.globalPosition,
+            cause: SelectionChangedCause.longPress,
+          ),
         });
       },
     );
@@ -77,6 +79,9 @@ main() {
   }
 }
 <<<
+### TODO(1466): Ideally, the first case would also split at the `||` instead of
+### of before `.`, but the formatter can't distinguish that case without fixing
+### #1466.
 main() {
   {
     return TextFieldTapRegion(
@@ -92,14 +97,13 @@ main() {
             TargetPlatform.android ||
             TargetPlatform.fuchsia ||
             TargetPlatform.linux ||
-            TargetPlatform.windows =>
-              _renderEditable.selectWordsInRange(
-                from:
-                    longPressMoveUpdateDetails.globalPosition -
-                    longPressMoveUpdateDetails.offsetFromOrigin,
-                to: longPressMoveUpdateDetails.globalPosition,
-                cause: SelectionChangedCause.longPress,
-              ),
+            TargetPlatform.windows => _renderEditable.selectWordsInRange(
+              from:
+                  longPressMoveUpdateDetails.globalPosition -
+                  longPressMoveUpdateDetails.offsetFromOrigin,
+              to: longPressMoveUpdateDetails.globalPosition,
+              cause: SelectionChangedCause.longPress,
+            ),
           },
     );
   }

test/tall/regression/1600/1602.stmt

@@ -0,0 +1,12 @@
+>>> (indent 4)
+    return switch (_advance().type) {
+      TokenType.float || TokenType.int || TokenType.string => LiteralExpression(
+        _previous.value!,
+      ),
+    };
+<<<
+    return switch (_advance().type) {
+      TokenType.float ||
+      TokenType.int ||
+      TokenType.string => LiteralExpression(_previous.value!),
+    };