Add new Performance/IoReadlines cop

[fatkodima]

IO (and File as a subclass)readlines class (and instance) methods read full content into memory, which can be inefficient and not necessary. This cop helps to identify such places. Specifically, when readlines method is followed by one of Enumerable methods, we can safely replace it with each_line, which reads one line at a time thus saving memory.

We can autocorrect usages of instance level #readlines method, but cannot for class level .readlines method due to its signature. We can only show an offense and user should manually correct it.

# bad
File.readlines('testfile').each { |l| puts l }
IO.readlines('testfile', chomp: true).each { |l| puts l }

conn.readlines(10).map { |l| l.size }
file.readlines.find { |l| l.start_with?('#') }
file.readlines.each { |l| puts l }

# good
File.open('testfile', 'r').each_line { |l| puts l }
IO.open('testfile').each_line(chomp: true) { |l| puts l }

conn.each_line(10).map { |l| l.size }
file.each_line.find { |l| l.start_with?('#') }
file.each_line { |l| puts l }

[fatkodima]

Updated.

[bbatsov]

Btw, are there any references we can link to cops like this one? E.g. from Fast Ruby or something like that? I think that'd be useful to the end users who are going over the docs.

[fatkodima]

Updated:

1. made cop disabled by default
2. added reference (there are no anything related in fast-ruby repo, but found a good section in gitlab docs).

[koic]

Thanks!