[Fix #9734] Add new Style/TopLevelMethodDefinitions cop

[tejasbubane]

Closes #9734

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• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
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• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
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[bbatsov]

I think you can also use define_method at the top-level, which such a cop should probably detect.

[tejasbubane]

@bbatsov @koic Changed as per your comments.

[dvandersluis]

I probably wouldn't want these to be allowed anyways, but I could see an argument for allowing a defs node where the receiver isn't self, since in that case, the method will end up scoped to an object and not be global.

ie.

def MyClass.foo
  # ...
end

I'm fine with this being disallowed but anyone have an opinion there?

[koic]

I think it can be allowed if a receiver is explicitly specified. I think top-level methods that are implicitly declared directly to Object without specifying a receiver should be prevented.

[bbatsov]

Let's use singular here (Definition) to be aligned with how most of the cops are named.

[bbatsov]

You forgot to update the commit message itself.

Btw, I'm thinking that it would be hard to add some check whether the file being analyzed is a script (if it has a shebang) and disable to the cop just for scripts, but probably that's an overkill at this point.

[bbatsov]

I guess we should also document this in the Ruby style guide. I'm surprised we haven't done so already.

[tejasbubane]

@bbatsov Changed to singular and created PR for styleguide.

[bbatsov]

Thanks!