Add a rule prefer_guard_clause for reversing nested if statements

lib/analysis_options.yaml

@@ -78,6 +78,7 @@ custom_lint:
     - newline_before_return
     - no_empty_block
     - no_equal_then_else
+    - prefer_early_return
 
     - no_magic_number:
       allowed_in_widget_params: true

lib/solid_lints.dart

@@ -23,6 +23,7 @@ import 'package:solid_lints/src/lints/no_equal_then_else/no_equal_then_else_rule
 import 'package:solid_lints/src/lints/no_magic_number/no_magic_number_rule.dart';
 import 'package:solid_lints/src/lints/number_of_parameters/number_of_parameters_metric.dart';
 import 'package:solid_lints/src/lints/prefer_conditional_expressions/prefer_conditional_expressions_rule.dart';
+import 'package:solid_lints/src/lints/prefer_early_return/prefer_early_return_rule.dart';
 import 'package:solid_lints/src/lints/prefer_first/prefer_first_rule.dart';
 import 'package:solid_lints/src/lints/prefer_last/prefer_last_rule.dart';
 import 'package:solid_lints/src/lints/prefer_match_file_name/prefer_match_file_name_rule.dart';
@@ -62,6 +63,7 @@ class _SolidLints extends PluginBase {
       PreferMatchFileNameRule.createRule(configs),
       ProperSuperCallsRule.createRule(configs),
       AvoidDebugPrint.createRule(configs),
+      PreferEarlyReturnRule.createRule(configs),
       AvoidFinalWithGetterRule.createRule(configs),
     ];
 

lib/src/lints/prefer_early_return/prefer_early_return_rule.dart

@@ -0,0 +1,68 @@
+import 'package:analyzer/error/listener.dart';
+import 'package:custom_lint_builder/custom_lint_builder.dart';
+import 'package:solid_lints/src/lints/prefer_early_return/visitors/prefer_early_return_visitor.dart';
+import 'package:solid_lints/src/models/rule_config.dart';
+import 'package:solid_lints/src/models/solid_lint_rule.dart';
+
+/// A rule which highlights `if` statements that span the entire body,
+/// and suggests replacing them with a reversed boolean check
+/// with an early return.
+///
+/// ### Example
+///
+/// #### BAD:
+///
+/// ```dart
+/// void func() {
+///   if (a) { //LINT
+///     if (b) { //LINT
+///       c;
+///     }
+///   }
+/// }
+/// ```
+///
+/// #### GOOD:
+///
+/// ```dart
+/// void func() {
+///   if (!a) return;
+///   if (!b) return;
+///   c;
+/// }
+/// ```
+class PreferEarlyReturnRule extends SolidLintRule {
+  /// The [LintCode] of this lint rule that represents the error if
+  /// 'if' statements should be reversed
+  static const String lintName = 'prefer_early_return';
+
+  PreferEarlyReturnRule._(super.config);
+
+  /// Creates a new instance of [PreferEarlyReturnRule]
+  /// based on the lint configuration.
+  factory PreferEarlyReturnRule.createRule(CustomLintConfigs configs) {
+    final rule = RuleConfig(
+      configs: configs,
+      name: lintName,
+      problemMessage: (_) => "Use reverse if to reduce nesting",
+    );
+
+    return PreferEarlyReturnRule._(rule);
+  }
+
+  @override
+  void run(
+    CustomLintResolver resolver,
+    ErrorReporter reporter,
+    CustomLintContext context,
+  ) {
+    context.registry.addBlockFunctionBody((node) {
+      final visitor = PreferEarlyReturnVisitor();
+      node.accept(visitor);
+
+      for (final element in visitor.nodes) {
+        reporter.reportErrorForNode(code, element);
+      }
+    });
+  }
+}

lib/src/lints/prefer_early_return/visitors/prefer_early_return_visitor.dart

@@ -0,0 +1,73 @@
+import 'package:analyzer/dart/ast/ast.dart';
+import 'package:analyzer/dart/ast/visitor.dart';
+import 'package:solid_lints/src/lints/prefer_early_return/visitors/return_statement_visitor.dart';
+
+/// The AST visitor that will collect all unnecessary if statements
+class PreferEarlyReturnVisitor extends RecursiveAstVisitor<void> {
+  final _nodes = <AstNode>[];
+
+  /// All unnecessary if statements and conditional expressions.
+  Iterable<AstNode> get nodes => _nodes;
+
+  @override
+  void visitBlockFunctionBody(BlockFunctionBody node) {
+    super.visitBlockFunctionBody(node);
+
+    if (node.block.statements.isEmpty) return;
+
+    final (ifStatements, nextStatement) = _getStartIfStatements(node);
+    if (ifStatements.isEmpty) return;
+
+    // limit visitor to only work with functions
+    // that don't have a return statement or the return statement is empty
+    final nextStatementIsEmptyReturn =
+        nextStatement is ReturnStatement && nextStatement.expression == null;
+    final nextStatementIsNull = nextStatement == null;
+
+    if (!nextStatementIsEmptyReturn && !nextStatementIsNull) return;
+
+    _handleIfStatement(ifStatements.last);
+  }
+
+  void _handleIfStatement(IfStatement node) {
+    if (_isElseIfStatement(node)) return;
+    if (_hasElseStatement(node)) return;
+    if (_hasReturnStatement(node)) return;
+
+    _nodes.add(node);
+  }
+
+// returns a list of if statements at the start of the function
+// and the next statement after it
+// examples:
+// [if, if, if, return] -> ([if, if, if], return)
+// [if, if, if, _doSomething, return] -> ([if, if, if], _doSomething)
+// [if, if, if] -> ([if, if, if], null)
+  (List<IfStatement>, Statement?) _getStartIfStatements(
+    BlockFunctionBody body,
+  ) {
+    final List<IfStatement> ifStatements = [];
+    for (final statement in body.block.statements) {
+      if (statement is IfStatement) {
+        ifStatements.add(statement);
+      } else {
+        return (ifStatements, statement);
+      }
+    }
+    return (ifStatements, null);
+  }
+
+  bool _hasElseStatement(IfStatement node) {
+    return node.elseStatement != null;
+  }
+
+  bool _isElseIfStatement(IfStatement node) {
+    return node.elseStatement != null && node.elseStatement is IfStatement;
+  }
+
+  bool _hasReturnStatement(Statement node) {
+    final visitor = ReturnStatementVisitor();
+    node.accept(visitor);
+    return visitor.nodes.isNotEmpty;
+  }
+}

lib/src/lints/prefer_early_return/visitors/return_statement_visitor.dart

@@ -0,0 +1,16 @@
+import 'package:analyzer/dart/ast/ast.dart';
+import 'package:analyzer/dart/ast/visitor.dart';
+
+/// The AST visitor that will collect every Return statement
+class ReturnStatementVisitor extends RecursiveAstVisitor<void> {
+  final _nodes = <ReturnStatement>[];
+
+  /// All unnecessary return statements
+  Iterable<ReturnStatement> get nodes => _nodes;
+
+  @override
+  void visitReturnStatement(ReturnStatement node) {
+    super.visitReturnStatement(node);
+    _nodes.add(node);
+  }
+}

lint_test/analysis_options.yaml

@@ -25,6 +25,7 @@ custom_lint:
     - no_empty_block
     - no_equal_then_else
     - avoid_debug_print
+    - prefer_early_return
     - member_ordering:
       alphabetize: true
       order:

lint_test/cyclomatic_complexity_test.dart

@@ -1,4 +1,4 @@
-// ignore_for_file: literal_only_boolean_expressions
+// ignore_for_file: literal_only_boolean_expressions, prefer_early_return
 // ignore_for_file: no_empty_block, prefer_match_file_name
 
 /// Check complexity fail

lint_test/no_empty_block_test.dart

@@ -1,4 +1,4 @@
-// ignore_for_file: prefer_const_declarations, prefer_match_file_name
+// ignore_for_file: prefer_const_declarations, prefer_match_file_name, prefer_early_return
 // ignore_for_file: unused_local_variable
 // ignore_for_file: cyclomatic_complexity
 // ignore_for_file: avoid_unused_parameters