[Fix #2758] Fix camel case variables not allowing underscore prefixes

CHANGELOG.md

@@ -42,6 +42,7 @@
 * [#2721](https://github.com/bbatsov/rubocop/issues/2721): Do not register an offense for constants wrapped in parentheses passed to `rescue` in `Style/RedundantParentheses`. ([@rrosenblum][])
 * [#2742](https://github.com/bbatsov/rubocop/issues/2742): Fix `Style/TrailingCommaInArguments` & `Style/TrailingCommaInLiteral` for inline single element arrays. ([@annih][])
 * [#2768](https://github.com/bbatsov/rubocop/issues/2768): Allow parentheses after keyword `not` in `Style/MethodCallParentheses`. ([@lumeet][])
+* [#2758](https://github.com/bbatsov/rubocop/issues/2758): Allow leading underscores in camel case variable names.([@mmcguinn][])
 
 ### Changes
 
@@ -1934,3 +1935,4 @@
 [@seikichi]: https://github.com/seikichi
 [@madwort]: https://github.com/madwort
 [@annih]: https://github.com/annih
+[@mmcguinn]: https://github.com/mmcguinn

lib/rubocop/cop/mixin/configurable_naming.rb

@@ -9,7 +9,7 @@ module ConfigurableNaming
       include ConfigurableEnforcedStyle
 
       SNAKE_CASE = /^@{0,2}[\da-z_]+[!?=]?$/
-      CAMEL_CASE = /^@{0,2}[a-z][\da-zA-Z]+[!?=]?$/
+      CAMEL_CASE = /^@{0,2}_?[a-z][\da-zA-Z]+[!?=]?$/
 
       def check_name(node, name, name_range)
         return if operator?(name)

spec/rubocop/cop/style/variable_name_spec.rb

@@ -64,6 +64,12 @@
       expect(cop.highlights).to eq(['funnyArg'])
     end
 
+    it 'registers an offense for camel case local variables marked as unused' do
+      inspect_source(cop, '_myLocal = 1')
+      expect(cop.offenses.size).to eq(1)
+      expect(cop.highlights).to eq(['_myLocal'])
+    end
+
     include_examples 'always accepted'
   end
 
@@ -106,6 +112,11 @@
       expect(cop.highlights).to eq(['funny_arg'])
     end
 
+    it 'accepts camel case local variables marked as unused' do
+      inspect_source(cop, '_myLocal = 1')
+      expect(cop.offenses).to be_empty
+    end
+
     include_examples 'always accepted'
   end