Bug in Style/FrozenStringLiteralComment where comment can exist anywhere in file

[jeffcarbs]

Looking at this history of the Style/FrozenStringLiteralComment cop it looks like it:

• originally only checked the first three lines of source: https://github.com/rubocop-hq/rubocop/blob/58f2f8d37b733436bbc6cfa49594828699825de2/lib/rubocop/cop/mixin/frozen_string_literal.rb#L37
• was updated to check the first three comments but still looked at the LOC to see it was within the first three: https://github.com/rubocop-hq/rubocop/blob/75eb10738e356121bcf5dc6ce54d0ba45cd1e595/lib/rubocop/cop/mixin/frozen_string_literal.rb#L42-L50
• then was updated to simply check the first three comments (to handle cases where comments were separated by newlines): 8c792d1

frozen_string_literal actually must appear before any code. Here's an example that should raise an exception at runtime but doesn't:

1 + 2
# frozen_string_literal: true

K = "foo"
K.slice!(4)

However, since the cop is checking the first three comments they can appear anywhere in the source. For example, according to the cop these are OK:

class Something
end

# frozen_string_literal: true

# One

class Something
  # Two
end

# frozen_string_literal: true

This is actually invalid but "accidentally" because the cop has logic about this needing to be one of the first three comments:

# One

class Something
  # Two
end

# Three
# frozen_string_literal: true

There's actually nothing special about the first three lines/comments either. This should be valid as well:

# something else that's important
# I love comments
# comments commenty comments
# Oh boy, more comments

# frozen_string_literal: true
q = 1 + 2
K = "foo"
K.slice!(4)

The reason this cop cares about "first three" is because it has special logic dealing with the shebang and encoding comments.

---

Expected behavior

This cop would enforce the string literal comment is somewhere in the leading comments (i.e. comments appearing before any ruby code), not necessarily just the first three.

Actual behavior

The cop checks the first three comments in source, regardless of their location in the file

Steps to reproduce the problem

See test cases above.

RuboCop version

$ [bundle exec] rubocop -V
0.82.0 (using Parser 2.5.1.2, running on ruby 2.5.1 x86_64-linux)

[zhustec]

Consider a executable ruby file with Unix shebang

would it be

#!/usr/bin/env ruby
# frozen_string_literal: true

or

# frozen_string_literal: true
#!/usr/bin/env ruby

[koic]

Shebang would always have to be on the first line.

% cat example1.rb
#!/usr/bin/env ruby
# frozen_string_literal: true
p 'str'.frozen?

% example1.rb
true

% cat example2.rb
# frozen_string_literal: true
#!/usr/bin/env ruby
p 'str'.frozen?

% example2.rb
example2.rb: line 3: p: command not found

See also: https://docs.rubocop.org/en/stable/cops_lint/#lintorderedmagiccomments

[jeffcarbs]

Have a fix out in #7939 🙂