Add Lint/ConstantResolution cop

[robotdana]

Ruby constant resolution sometimes gets things wrong.

I think generally gems (that are meant for including as a library, not
like rubocop itself) should fully qualify all constants to avoid
stepping on any toes inadvertently. I would run this on my gems without
using Only/Ignore

Large projects will probably end up with one or two constant names that
are problematic because of a conflict with a library or just internally
having sensible names for directories that end up being used by e.g.
rails as namespaces in multiple different places. I would run these with
Only: [The, Constant, Names, Causing Issues]

Sometimes a method refers to a constant that you want to refer to a
different constant at different calls, e.g. the method is defined on a
superclass and the constant differs between subclasses. (e.g. MSG in
rubocop add_offense). I would use
Ignore: [The, Intentionally, Abiguous, Constant] in these cases.
(or self.class::MSG now that I've looked up how add_offense works)

I've left this Enabled: false because I'm 50-50 on whether it should
be Enabled by default.

I didn't bother creating an autocorrect because I see there being two
options and thought I'd get feedback.

1. carelessly assume the written constant is fully qualified and just
   missing a :: prefix. This has the advantage of probably looking correct
   more often, but also being wrong a lot.
2. look at the current namespace and prefix that. This has the advantage
   of not necessarily changing behaviour for classes. For modules leaving
   it unchanged or carelessly assuming, or checking if the constant is
   defined in the module or not before prefixing
3. just don't autocorrect

• 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 bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[Drenmi]

Code looks great to me. 🚀

My only wonder is if this cop doesn't deserve an upgrade to the Lint namespace, since it deals with code that can lead to unexpected behaviour.

[bbatsov]

Btw, won't this cause a lot noise for references to "naked" constants in namespaced modules/classes? I like the general idea, but we have to careful not to generate a ton of offenses with the new cop. I'm wondering how we can improve the situation, though. That's a tough nut to crack on a single-file basis.

[robotdana]

The noise is why i have Enabled: false.

[bbatsov]

Ah, sorry. I totally missed that!

Now I'm wondering if that's truly a lint check instead a style check, as lint checks are typically enabled by default and they typically target more serious problems in the code. Anyways, let's keep it under Lint for now. I see you wrote some great explanations in the description of the PR (which I should have read more carefully) and I feel it's a good idea to move a lot of this to the rationale of the cop, so it's easier for people to understand how to use it and why it's disabled by default.

[bbatsov]

Thanks!