[Fix #6903] Handle variables with _ for `Naming/RescuedExceptionsVa…

…riableName` cop (#6906)

Currently, this cop do not play nicely with variables that are prefixed by an `_`.

The message should take into consideration if the variable is going to be
used or not when showing the error.

CHANGELOG.md

@@ -6,6 +6,7 @@
 
 * [#5977](https://github.com/rubocop-hq/rubocop/issues/5977): Remove Performance cops. ([@koic][])
 * Add auto-correction to `Naming/RescuedExceptionsVariableName`. ([@anthony-robin][])
+* [#6903](https://github.com/rubocop-hq/rubocop/issues/6903): Handle variables prefixed with `_` in `Naming/RescuedExceptionsVariableName` cop. ([@anthony-robin][])
 
 ## 0.67.2 (2019-04-05)
 

lib/rubocop/cop/naming/rescued_exceptions_variable_name.rb

@@ -24,6 +24,13 @@ module Naming
       #     # do something
       #   end
       #
+      #   # good
+      #   begin
+      #     # do something
+      #   rescue MyException => _e
+      #     # do something
+      #   end
+      #
       # @example PreferredName: exception
       #   # bad
       #   begin
@@ -39,6 +46,13 @@ module Naming
       #     # do something
       #   end
       #
+      #   # good
+      #   begin
+      #     # do something
+      #   rescue MyException => _exception
+      #     # do something
+      #   end
+      #
       class RescuedExceptionsVariableName < Cop
         MSG = 'Use `%<preferred>s` instead of `%<bad>s`.'.freeze
 
@@ -62,7 +76,11 @@ def autocorrect(_node)
         private
 
         def preferred_name
-          @preferred_name ||= cop_config.fetch('PreferredName', 'e')
+          @preferred_name ||= begin
+            name = cop_config.fetch('PreferredName', 'e')
+            name = "_#{name}" if variable_name.to_s.start_with?('_')
+            name
+          end
         end
 
         def variable_name

manual/cops_naming.md

@@ -507,6 +507,13 @@ begin
 rescue MyException => e
   # do something
 end
+
+# good
+begin
+  # do something
+rescue MyException => _e
+  # do something
+end
 ```
 #### PreferredName: exception
 
@@ -524,6 +531,13 @@ begin
 rescue MyException => exception
   # do something
 end
+
+# good
+begin
+  # do something
+rescue MyException => _exception
+  # do something
+end
 ```
 
 ### Configurable attributes

spec/rubocop/cop/naming/rescued_exceptions_variable_name_spec.rb

@@ -25,6 +25,25 @@
           RUBY
         end
 
+        it 'registers an offense when using `_exc`' do
+          expect_offense(<<-RUBY.strip_indent)
+            begin
+              something
+            rescue MyException => _exc
+                                  ^^^^ Use `_e` instead of `_exc`.
+              # do something
+            end
+          RUBY
+
+          expect_correction(<<-RUBY.strip_indent)
+            begin
+              something
+            rescue MyException => _e
+              # do something
+            end
+          RUBY
+        end
+
         it 'does not register an offense when using `e`' do
           expect_no_offenses(<<-RUBY.strip_indent)
             begin
@@ -34,6 +53,16 @@
             end
           RUBY
         end
+
+        it 'does not register an offense when using `_e`' do
+          expect_no_offenses(<<-RUBY.strip_indent)
+            begin
+              something
+            rescue MyException => _e
+              # do something
+            end
+          RUBY
+        end
       end
 
       context 'without `Exception` variable' do
@@ -70,6 +99,25 @@
           RUBY
         end
 
+        it 'registers an offense when using `_exc`' do
+          expect_offense(<<-RUBY.strip_indent)
+            begin
+              something
+            rescue _exc
+                   ^^^^ Use `_e` instead of `_exc`.
+              # do something
+            end
+          RUBY
+
+          expect_correction(<<-RUBY.strip_indent)
+            begin
+              something
+            rescue _e
+              # do something
+            end
+          RUBY
+        end
+
         it 'does not register an offense when using `e`' do
           expect_no_offenses(<<-RUBY.strip_indent)
             begin
@@ -79,6 +127,16 @@
             end
           RUBY
         end
+
+        it 'does not register an offense when using `_e`' do
+          expect_no_offenses(<<-RUBY.strip_indent)
+            begin
+              something
+            rescue _e
+              # do something
+            end
+          RUBY
+        end
       end
 
       context 'without `Exception` variable' do
@@ -121,6 +179,25 @@
       RUBY
     end
 
+    it 'registers an offense when using `_e`' do
+      expect_offense(<<-RUBY.strip_indent)
+        begin
+          something
+        rescue MyException => _e
+                              ^^ Use `_exception` instead of `_e`.
+          # do something
+        end
+      RUBY
+
+      expect_correction(<<-RUBY.strip_indent)
+        begin
+          something
+        rescue MyException => _exception
+          # do something
+        end
+      RUBY
+    end
+
     it 'does not register an offense when using `exception`' do
       expect_no_offenses(<<-RUBY.strip_indent)
         begin
@@ -130,5 +207,15 @@
         end
       RUBY
     end
+
+    it 'does not register an offense when using `_exception`' do
+      expect_no_offenses(<<-RUBY.strip_indent)
+        begin
+          something
+        rescue MyException => _exception
+          # do something
+        end
+      RUBY
+    end
   end
 end