[Fix rubocop#3037] StringLiterals understands that '#' is the same as…

… "#"

CHANGELOG.md

@@ -16,6 +16,7 @@
 
 ### Bug fixes
 
+* [#3037](https://github.com/bbatsov/rubocop/issues/3037): `Style/StringLiterals` understands that a bare '#', not '#@variable' or '#{interpolation}', does not require double quotes. ([@alexdowad][])
 * [#2722](https://github.com/bbatsov/rubocop/issues/2722): `Style/ExtraSpacing` does not attempt to align an equals sign in an argument list with one in an assignment statement. ([@alexdowad][])
 * [#3133](https://github.com/bbatsov/rubocop/issues/3133): `Style/MultilineMethodCallBraceLayout` does not register offenses for single-line calls. ([@alexdowad][])
 * [#3170](https://github.com/bbatsov/rubocop/issues/3170): `Style/MutableConstant` does not infinite-loop when correcting an array with no brackets. ([@alexdowad][])

lib/rubocop/cop/mixin/string_literals_help.rb

@@ -13,7 +13,7 @@ def wrong_quotes?(node)
         if style == :single_quotes
           src !~ /'/ && !double_quotes_acceptable?(node.str_content)
         else
-          src !~ /" | \\ | \#/x
+          src !~ /" | \\ | \#(@|\{)/x
         end
       end
 

spec/rubocop/cop/style/string_literals_spec.rb

@@ -212,6 +212,11 @@
       expect(cop.offenses).to be_empty
     end
 
+    it 'flags single quotes with plain # (not #@var or #{interpolation}' do
+      inspect_source(cop, "a = 'blah #'")
+      expect(cop.offenses.size).to be 1
+    end
+
     it 'accepts single quotes at the start of regexp literals' do
       inspect_source(cop, "s = /'((?:[^\\']|\\.)*)'/")
       expect(cop.offenses).to be_empty