Autocorrect Style/HashTransformValues is broken

[mdemare]

Firstly, it transforms some usages of each_with_object where the receiver is an Array.
Secondly, looking up a missing value in the resulting hash will return a Proc object, instead of nil.

This is not a safe transformation.

[koic]

Style/HashTransformValues is marked as unsafe.
https://github.com/rubocop-hq/rubocop/blob/v0.80.1/config/default.yml#L2824

Using rubocop --safe and rubocop --safe-auto-correct command-line options run to only safe cops. So users can omit unsafe cops using this option.

By the way, could you show me the reproduction example code of the issue?

[lucascaton]

Hi @koic! 👋

I have a reproduction example code:

[[1, 2], [3, 4]].map { |k, v| [k, [v, nil]] }.to_h
# => {1=>[2, nil], 3=>[4, nil]}

When I run $ rubocop, this is the output:

code.rb:3:1: C: Style/HashTransformValues: Prefer transform_values over map {...}.to_h.
[[1, 2], [3, 4]].map { |k, v| [k, [v, nil]] }.to_h

And running $ rubocop -a, it auto-fixes it to:

-[[1, 2], [3, 4]].map { |k, v| [k, [v, nil]] }.to_h
+[[1, 2], [3, 4]].transform_values { |v| [v, nil] }

However, the produced (above) code isn't valid:

[[1, 2], [3, 4]].transform_values { |v| [v, nil] }
# NoMethodError: undefined method `transform_values' for [[1, 2], [3, 4]]:Array

[tejasbubane]

@koic I think this cop should skip array literals?

[koic]

Surely Array instance does not have the transform_values method, so I agree to skip for array literals.

[koic]

Also the transform_keys method (Style/HashTransformKeys cop) is the same.

[fsoikin]

This is not limited to literals though. In our codebase I found hit rate of this rule (and other similar hash-related rules) to be about 60% overall. For example, this is also considered a hit:

{ x: 1,  y: 5 }.sort_by(&:first).to_h { |k, v| [k, v * 2] }

Even though sort_by returns an Array, not a Hash.

Just in general, this check reacts to the code pattern, but doesn't always make sure that the receiver object is actually a hash.