Lint/ShadowedException fails for rescue nil

[janraasch]

Test code

begin
  # something
rescue nil
end

Actual behavior

An error occurred while Lint/ShadowedException cop was inspecting /test.rb

Scanning /test.rb
An error occurred while Lint/ShadowedException cop was inspecting /vagrant/sellery/app/test.rb.
undefined method `<' for nil:NilClass
/home/vagrant/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/rubocop-0.41.1/lib/rubocop/cop/lint/shadowed_exception.rb:63:in `block (2 levels) in contains_multiple_levels_of_exceptions?'

RuboCop version

$ rubocop -V
0.41.1 (using Parser 2.3.1.2, running on ruby 2.3.0 x86_64-linux)

[rrosenblum]

I can update the code to safeguard against this, but when I try out an example for rescuing nil, it does not appear to be valid Ruby. @bbatsov and @jonas054 what are your thoughts on this?

[2] pry(main)> begin
[2] pry(main)*   1/0
[2] pry(main)* rescue nil  
[2] pry(main)*   puts 'rescued'
[2] pry(main)* end  
TypeError: class or module required for rescue clause

[janraasch]

What version of ruby are you using?

Have you tried putting that code into a *.rb file instead of pry?

[rrosenblum]

The example that I posted was run against Ruby 2.3.0. I tried it in a .rb file and had the same result. I repeated the same test for multiple versions of Ruby. It appears that rescue nil is valid in Ruby 1.9.3.

# test.rb
begin
  1/0
rescue nil
  puts 'rescued'
end

rrosenblum@C02NV0KSG3QN:~/work/rubocop (master u=)$ rbenv local 1.9.3-p484 
rrosenblum@C02NV0KSG3QN:~/work/rubocop (master u=)$ ruby test.rb 
test.rb:2:in `/': divided by 0 (ZeroDivisionError)
        from test.rb:2:in `<main>'
rrosenblum@C02NV0KSG3QN:~/work/rubocop (master u=)$ rbenv local 2.0.0-p598 
rrosenblum@C02NV0KSG3QN:~/work/rubocop (master u=)$ ruby test.rb          
test.rb:3:in `rescue in <main>': class or module required for rescue clause (TypeError)
        from test.rb:1:in `<main>'
rrosenblum@C02NV0KSG3QN:~/work/rubocop (master u=)$ rbenv local 2.1.6
rrosenblum@C02NV0KSG3QN:~/work/rubocop (master u=)$ ruby test.rb     
test.rb:3:in `rescue in <main>': class or module required for rescue clause (TypeError)
        from test.rb:1:in `<main>'
rrosenblum@C02NV0KSG3QN:~/work/rubocop (master u=)$ rbenv local 2.2.4 
rrosenblum@C02NV0KSG3QN:~/work/rubocop (master u=)$ ruby test.rb 
test.rb:3:in `rescue in <main>': class or module required for rescue clause (TypeError)
        from test.rb:1:in `<main>'
rrosenblum@C02NV0KSG3QN:~/work/rubocop (master u=)$ rbenv local 2.3.0 
rrosenblum@C02NV0KSG3QN:~/work/rubocop (master u=)$ ruby test.rb 
test.rb:3:in `rescue in <main>': class or module required for rescue clause (TypeError)
        from test.rb:1:in `<main>'

According to the output of rubocop -V that you posted, it looks like you are running Ruby 2.3.0. Can you verify what version you are running against?

I can update this cop to account for rescue nil. Looking at the output running of against Ruby 1.9.3, it appears that rescue nil does not actually rescue anything. I would like to do a bit more research on this, and it seems like we should have a cop that recommends against rescue nil. This may prove to be a bit tricky because rescue nil is valid in the modifier form.

[janraasch]

Sorry, I think I misunderstood what you meant by

it does not appear to be valid Ruby.

To clarify, writing

begin
  1/0
rescue nil
  puts 'rescued'
end

is perfectly correct from a syntax perspective.

However, it does raise a TypeError, when the rescue clause is called.

So you are probably right, it is an edge-case. We just ran into this because of a code-smell. Essentially we had something like

begin
  puts('This never fails')
  puts('Thus we never run into the TypeError`)
rescue nil
  puts('rescued')
end

in our code. So we never ran into the TypeError and I only noticed this, when running your Cop against our codebase. So maybe we should just have a cop to guard against rescue nil here. Not sure if something like this is in scope for rubocop.

[rrosenblum]

Thank you for the clarification, I believe that we are on the same page now. Since this does not produce a TypeError in Ruby 1.9.3, we will need to add a safeguard to this cop.

RuboCop can check for rescue nil and recommend against using it. We will need to do some cleanup around how we check for a rescue modifier in cops other than Style/RescueModifier.