Cop idea: Set the default value of ENV.fetch by autocorrect

[j-miyake]

Is your feature request related to a problem? Please describe.

From: #10502 (comment)

With Style/FetchEnvVar, code like

value = ENV['EXAMPLE_KEY'] || 'default_value'

gets autocorrected to

value = ENV.fetch('EXAMPLE_KEY', nil) || 'default_value'

Autocorrecting like below helps people reduce their manual changings like above.

value = ENV.fetch('EXAMPLE_KEY', 'default_value')

# This version would be better on performance if the default value is calculated by an expensive computation.
value = ENV.fetch('EXAMPLE_KEY'){'default_value'}

Describe the solution you'd like

• When ENV[] is the left-hand of ||, the autocorrect can move the right-hand of || to the default value of ENV.fetch.
• In some cases, the right hand of || can't be the default value (e.g., next), so we need to carefully handle them.

[bbatsov]

Seems like a good idea to me.

As for the auto-correct - I guess if the default is some literal or a local value we can use the first form and if it's a method call we can go for the block style, although I don't have a strong preference. The performance gains would be notable only if the computation of the default value is expensive.

[ShockwaveNN]

Please note that if you got code like this:

return ENV['foo'] if ENV.key?('foo')

File.read('file_if_foo_not_exists')

Code like this will work fine if ENV['foo'] exists and File file_if_foo_not_exists not exists

But if you by similarity with proposal from this issue change it to

ENV.fetch('foo', File.read('file_if_foo_not_exists'))

If will fail in any case if file_if_foo_not_exists do not exists (even if ENV['foo'] is initialized)

[j-miyake]

Great point. That would be one of the cases we should use the block style.

ENV.fetch('foo') { File.read('file_if_foo_not_exists') }

In my opinion, in most cases always using the block style is safer and reduces the rules. Any thoughts?

[j-miyake]

I created a PR for this.
#10585
I made the default value style configurable so that users can choose their preferred way.
Any help in finding missing test cases or edge cases is very appreciated.