[Fix #7586] Fix infinite loop in FrozenStringLiteralComment

It was detecting frozen_string_literal in the first three lines of the
file, while there's no guideline to separate magic comments with
newlines. If shebang and encoding were separated by a newline, that was
three lines already, and frozen_string_literal was added on line 4, and
not detected as such after auto-correction, triggering an infinite loop.

CHANGELOG.md

@@ -13,6 +13,7 @@
 * [#7569](https://github.com/rubocop-hq/rubocop/issues/7569): Make `Style/YodaCondition` accept `__FILE__ == $0`. ([@koic][])
 * [#7576](https://github.com/rubocop-hq/rubocop/issues/7576): Fix an error for `Gemspec/OrderedDependencies` when using a local variable in an argument of dependent gem. ([@koic][])
 * [#7595](https://github.com/rubocop-hq/rubocop/issues/7595): Make `Style/NumericPredicate` aware of ignored methods when specifying ignored methods. ([@koic][])
+* [#7607](https://github.com/rubocop-hq/rubocop/issues/7607): Fix `Style/FrozenStringLiteralComment` infinite loop when magic comments are newline-separated. ([@pirj][])
 
 ## 0.78.0 (2019-12-18)
 

lib/rubocop/cop/mixin/frozen_string_literal.rb

@@ -40,13 +40,7 @@ def frozen_string_literals_enabled?
       end
 
       def leading_comment_lines
-        comments = processed_source.comments
-
-        comments.each_with_object([]) do |comment, leading_comments|
-          next if comment.loc.line > 3
-
-          leading_comments << comment.text
-        end
+        processed_source.comments.first(3).map(&:text)
       end
     end
   end

spec/rubocop/cop/style/frozen_string_literal_comment_spec.rb

@@ -117,6 +117,18 @@
       RUBY
     end
 
+    it 'accepts a frozen string literal comment below newline-separated ' \
+       'magic comments' do
+      expect_no_offenses(<<~RUBY)
+        #!/usr/bin/env ruby
+
+        # encoding: utf-8
+
+        # frozen_string_literal: true
+        puts 1
+      RUBY
+    end
+
     it 'accepts a disabled frozen string literal comment below shebang and ' \
        'encoding comments' do
       expect_no_offenses(<<~RUBY)