[Fix #49] Add new Rails/RescueFromExceptionsVariableName cop

Ensures that rescued exception variables are named as expected.

The `PreferredName` config option specifies the required name of the variable.
Its default is `e`, as referenced from `Naming/RescuedExceptionsVariableName`.

Author: ydakuka

changelog/new_rails_rescue_from_exceptions_variable_name_cop.md

@@ -0,0 +1 @@
+* [#49](https://github.com/rubocop/rubocop-rails/issues/49): Add new `Rails/RescueFromExceptionsVariableName` cop. ([@anthony-robin][], [@ydakuka][])

config/default.yml

@@ -926,6 +926,12 @@ Rails/RequireDependency:
   Enabled: false
   VersionAdded: '2.10'
 
+Rails/RescueFromExceptionsVariableName:
+  Description: 'Use consistent rescued exceptions variables naming.'
+  Enabled: 'pending'
+  VersionAdded: '<<next>>'
+  PreferredName: e
+
 Rails/ResponseParsedBody:
   Description: Prefer `response.parsed_body` to custom parsing logic for `response.body`.
   Enabled: pending

lib/rubocop/cop/rails/rescue_from_exceptions_variable_name.rb

@@ -0,0 +1,192 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Rails
+      # Ensures that rescued exception variables are named as expected.
+      #
+      # The `PreferredName` config option specifies the required name of the variable.
+      # Its default is `e`, as referenced from `Naming/RescuedExceptionsVariableName`.
+      #
+      # @example PreferredName: e (default)
+      #   # bad
+      #   rescue_from MyException do |exception|
+      #     # do something
+      #   end
+      #
+      #   # bad
+      #   rescue_from MyException do |_exception|
+      #     # do something
+      #   end
+      #
+      #   # bad
+      #   rescue_from MyException { |exception| do_something(exception) }
+      #
+      #   # bad
+      #   rescue_from MyException, with: ->(exception) do
+      #     do_something(exception)
+      #   end
+      #
+      #   # bad
+      #   rescue_from MyException, with: ->(exception) { do_something(exception) }
+      #
+      #
+      #   # good
+      #   rescue_from MyException do |e|
+      #     # do something
+      #   end
+      #
+      #   # good
+      #   rescue_from MyException do |_e|
+      #     # do something
+      #   end
+      #
+      #   # good
+      #   rescue_from MyException do |exception, context|
+      #     # do something
+      #   end
+      #
+      #   # good
+      #   rescue_from MyException { |e| do_something(e) }
+      #
+      #   # good
+      #   rescue_from MyException, with: ->(e) do
+      #     do_something(e)
+      #   end
+      #
+      #   # good
+      #   rescue_from MyException, with: ->(e) { do_something(e) }
+      #
+      # @example PreferredName: exception
+      #   # bad
+      #   rescue_from MyException do |e|
+      #     # do something
+      #   end
+      #
+      #   # bad
+      #   rescue_from MyException do |_e|
+      #     # do something
+      #   end
+      #
+      #   # bad
+      #   rescue_from MyException do |exception, context|
+      #     # do something
+      #   end
+      #
+      #   # bad
+      #   rescue_from MyException { |e| do_something(e) }
+      #
+      #   # bad
+      #   rescue_from MyException, with: ->(e) do
+      #     do_something(e)
+      #   end
+      #
+      #   # bad
+      #   rescue_from MyException, with: ->(e) { do_something(e) }
+      #
+      #
+      #   # good
+      #   rescue_from MyException do |exception|
+      #     # do something
+      #   end
+      #
+      #   # good
+      #   rescue_from MyException do |_exception|
+      #     # do something
+      #   end
+      #
+      #   # good
+      #   rescue_from MyException { |exception| do_something(exception) }
+      #
+      #   # good
+      #   rescue_from MyException, with: ->(exception) do
+      #     do_something(exception)
+      #   end
+      #
+      #   # good
+      #   rescue_from MyException, with: ->(exception) { do_something(exception) }
+      #
+      class RescueFromExceptionsVariableName < Base
+        include ConfigurableEnforcedStyle
+        extend AutoCorrector
+
+        MSG = 'Use `%<preferred>s` instead of `%<current>s`.'
+
+        def_node_matcher :rescue_from_block_argument_variable?, <<~PATTERN
+          (block (send nil? :rescue_from ...) (args (arg $_)) _)
+        PATTERN
+
+        def_node_matcher :rescue_from_with_lambda_variable?, <<~PATTERN
+          (send nil? :rescue_from ... (hash <(pair (sym :with) (block _ (args (arg $_)) _))>))
+        PATTERN
+
+        def_node_matcher :rescue_from_with_block_variable?, <<~PATTERN
+          (send nil? :rescue_from ... {(block _ (args (arg $_)) _) (splat (block _ (args (arg $_)) _))})
+        PATTERN
+
+        def on_block(node)
+          rescue_from_block_argument_variable?(node) do |arg_name|
+            check_offense(node.first_argument, arg_name)
+          end
+        end
+        alias on_numblock on_block
+
+        def on_send(node)
+          check_rescue_from_variable(node, :rescue_from_with_lambda_variable?)
+          check_rescue_from_variable(node, :rescue_from_with_block_variable?)
+        end
+
+        private
+
+        def check_rescue_from_variable(node, matcher)
+          send(matcher, node) do |arg_name|
+            arg_node = node.each_descendant(:args).first.children.first
+            check_offense(arg_node, arg_name)
+          end
+        end
+
+        def check_offense(arg_node, arg_name)
+          preferred_name = preferred_name(arg_name)
+          return if arg_name.to_s == preferred_name
+
+          range = adjusted_range(arg_node)
+          preferred, current = format_names(arg_node, arg_name, preferred_name)
+          message = format(MSG, preferred: preferred, current: current)
+
+          add_offense(range, message: message) do |corrector|
+            autocorrect(corrector, range, preferred, arg_node, preferred_name)
+          end
+        end
+
+        def preferred_name(name)
+          config_name = cop_config.fetch('PreferredName', 'e')
+          name.start_with?('_') ? "_#{config_name}" : config_name
+        end
+
+        def adjusted_range(arg_node)
+          arg_node.source_range.with(
+            begin_pos: arg_node.source_range.begin_pos - 1,
+            end_pos: arg_node.source_range.end_pos + 1
+          )
+        end
+
+        def format_names(arg_node, arg_name, preferred_name)
+          if arg_node.parent.parent.lambda?
+            ["(#{preferred_name})", "(#{arg_name})"]
+          else
+            ["|#{preferred_name}|", "|#{arg_name}|"]
+          end
+        end
+
+        def autocorrect(corrector, range, preferred, arg_node, preferred_name)
+          corrector.replace(range, preferred)
+          parent_block = arg_node.ancestors.find(&:block_type?)
+
+          parent_block.each_descendant(:lvar).each do |lvar_node|
+            corrector.replace(lvar_node, preferred_name)
+          end
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/rails_cops.rb

@@ -107,6 +107,7 @@
 require_relative 'rails/render_plain_text'
 require_relative 'rails/request_referer'
 require_relative 'rails/require_dependency'
+require_relative 'rails/rescue_from_exceptions_variable_name'
 require_relative 'rails/response_parsed_body'
 require_relative 'rails/reversible_migration'
 require_relative 'rails/reversible_migration_method_definition'