Should Rails/TimeZone be marked as unsafe cop?

[vfonic]

I just spent some time debugging my app trying to figure out why, after the last set of changes, my timezones are all messed up.

I ran git bisect which led me to this:

9be16555a4a3d4e64de2606cfdaac07fcf4d6805 is the first bad commit
commit 9be16555a4a3d4e64de2606cfdaac07fcf4d6805
Author: Viktor Fonic <viktor.fonic@gmail.com>
Date:   Wed Apr 3 23:56:17 2019 +0900

    Run `rubocop -DRES --safe-auto-correct`

So, as the commit message states, I only ran rubocop -DRES --safe-auto-correct. This introduced a bug in my app that I still didn't track down (due to this command having changed 89 files (with 389 additions and 232 deletions). Most of these changes are adding # frozen_string_literal: true, but...I digress.

This is just really frustrating. I ran the above command and never looked back. However, safe autocorrect broke my app. What's the point of having both safe autocorrect and "just autocorrect" if both are unsafe?

Please, please, let's mark this cop as unsafe until we figure out how to make it safe (if possible). I'd also suggest this for all cops. --safe-auto-correct in my opinion should NEVER break the code. Ok, ok, ruby is dynamic language. But, in most normal cases, it shouldn't break.

I'll edit this issue and report back once I figure out which exact change introduced the issue with timezones.

Sorry for the rant. ❤️

EDIT:
Here's what happened:

In my app, I show a timeline graph to the user. I request the graph via AJAX after the page loads. The backend controller gets params[:now] and uses that (user's browser-detected timezone), to calculate beginning and end of day:

# code simplified for demo purposes
def current_day_span
  # params[:now] currently returns "2019-04-20T14:29:19+07:00"
  now = Time.parse(params[:now])
  [now.beginning_of_day, now.end_of_day]
end

After running rubocop --safe-auto-correct, this has been changed to:

def current_day_span
- now = Time.parse(params[:now])
+ now = Time.parse(params[:now]).in_time_zone
  [now.beginning_of_day, now.end_of_day]
end

This now returns:
[Sat, 20 Apr 2019 00:00:00 UTC +00:00, Sat, 20 Apr 2019 23:59:59 UTC +00:00]
as opposed to:
[2019-04-20 00:00:00 +0700, 2019-04-20 23:59:59 +0700]

I understand, I could make the code work by extracting the timezone from the :now param, but what's the point? The code I initially wrote, worked just fine until the safe cop broke it. 🤷‍♂️

Halp!

What are you thoughts?

[matkoniecz]

Should Rails/TimeZone be marked as unsafe cop?

Given that this cop may (will?) change what data returned by a normal code it certainly should be marked as am unsafe.

[bbatsov]

Let's mark it as unsafe.

[vfonic]

I created a PR that marks it as unsafe: #6966