Add new Lint/BigDecimalNew cop

[koic]

Feature

BigDecimal.new() is deprecated since BigDecimal 1.3.3 for Ruby 2.5.
This cop identifies places where BigDecimal.new() can be replaced by BigDecimal().

% cat /tmp/example.rb
# frozen_string_literal: true

BigDecimal.new(123.456, 3)

% rubocop /tmp/example.rb
Inspecting 1 file
W

Offenses:

/tmp/example.rb:3:12: W: Lint/BigDecimalNew: BigDecimal.new() is deprecated. Use BigDecimal() instead.
BigDecimal.new(123.456, 3)
           ^^^

1 file inspected, 1 offense detected

I have verified that these methods can be replaced using the lowest
Ruby version 2.1 supported by RuboCop.

% ruby -v
ruby 2.1.10p492 (2016-04-01 revision 54464) [x86_64-darwin13.0]
% gem list | grep bigdecimal
bigdecimal (default: 1.2.4)
% ruby -rbigdecimal -e 'p BigDecimal(123.456, 3) == BigDecimal.new(123.456, 3)'
true

Other Information

The following is a change of BigDecimal 1.3.3 for Ruby 2.5 related to this PR.

• ruby/bigdecimal@5337373
• ruby/bigdecimal@16738ad

---

Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Run rake default or rake parallel. It executes all tests and RuboCop for itself, and generates the documentation.

[koic]

I saw this change at rails/rails#31435.

[pocke]

BTW, I think we should ship v0.52.1, that is a bug fix release. And the release should not contain any new features. So we need to wait merging this pull-request until we ship the release.
What do you think? @bbatsov

[garettarrowood]

This looks like it'd be a good check to add to Lint/DeprecatedClassMethods, perhaps instead of adding a new cop.

[koic]

Thanks for the review. I have worries. BigDecimal is not a standard library, it is a default gems managed by ruby/bigdecimal repo.

% gem list | grep default
bigdecimal (default: 1.3.2)
(snip)

I don't know whether it is preferable to change configuration in the same lifecycle as Lint/DeprecatedClassMethods cop.

Of course, as a practical use case, I think that there is no need to make an independent configuration change, so adding it to Lint/DeprecatedClassMethods is an approach.

[bbatsov]

BTW, I think we should ship v0.52.1, that is a bug fix release. And the release should not contain any new features. So we need to wait merging this pull-request until we ship the release.
What do you think? @bbatsov

I agree. That was my plan as well - do a bugfix release over the weekend or early next week. Unfortunately I'll be super busy until Tuesday.

[bbatsov]

Please, rebase on top of the current master.

[koic]

Thanks. I did rebase.