[Fix #1463] Mark Rails/IndexWith as unsafe autocorrect

Fixes #1463
Closes #1470

Author: tejasbubane

changelog/change_mark_rails_index_with_as_unsafe.md

@@ -0,0 +1 @@
+* [#1463](https://github.com/rubocop/rubocop-rails/issues/1463): Mark `Rails/IndexWith` as unsafe autocorrect. ([@tejasbubane][])

config/default.yml

@@ -653,8 +653,9 @@ Rails/IndexBy:
 Rails/IndexWith:
   Description: 'Prefer `index_with` over `each_with_object`, `to_h`, or `map`.'
   Enabled: true
+  SafeAutoCorrect: false
   VersionAdded: '2.5'
-  VersionChanged: '2.8'
+  VersionChanged: '<<next>>'
 
 Rails/Inquiry:
   Description: "Prefer Ruby's comparison operators over Active Support's `Array#inquiry` and `String#inquiry`."

lib/rubocop/cop/rails/index_with.rb

@@ -8,6 +8,11 @@ module Rails
       # an enumerable into a hash where the keys are the original elements.
       # Rails provides the `index_with` method for this purpose.
       #
+      # @safety
+      #   This cop is marked as unsafe autocorrection, because `nil.to_h` returns {}
+      #   but `nil.with_index` throws `NoMethodError`. Therefore, autocorrection is not
+      #   compatible if the receiver is nil.
+      #
       # @example
       #   # bad
       #   [1, 2, 3].each_with_object({}) { |el, h| h[el] = foo(el) }