Add -F/--fail-fast option

[jonas054]

This is something that I have found useful when working with correcting offenses (in RuboCop itself). I have an external program that monitors file changes in the directory tree and re-runs ./bin/rubocop when something changes. When I save a file it is the first one checked in the next run, and execution stops at the first file with offenses.

There's some overlap of functionality between guard-rubocop and this option, but I just found that adding this functionality inside RuboCop itself solved my problem. I'd like to hear what @yujinakayama thinks, and @bbatsov of course.

[bbatsov]

Seems like a good idea to me. The code looks fine.

[yujinakayama]

Initially I felt this feature is not efficient since running rubocop on the entire project for each modification of a file is wasteful. But I'm satisfied after reading the implementation that sorts files in modification time order. So I'm 👍 for this feature. I think it would be better if the description includes a note about the file inspection order.

[bbatsov]

@jonas054 Let me know when you've updated the description as suggested by @yujinakayama.

[mikegee]

I don't understand why the description would be improved with a note about the inspection order. Wouldn't this feature work exactly the same without that sorting (perhaps a bit slower)? It seems like an implementation detail to me.

[bbatsov]

If people didn't know the feature is modification time aware, they might not be inclined to use it as often as they would otherwise (or at least I think so).

[jonas054]

@bbatsov I'll try to do the update soon (--help text, README, and also CHANGELOG).

@mikegee I agree with @yujinakayama and @bbatsov that describing inspection order is a good idea, and that people are more likely to try the feature if they get a glimpse into this rather important implementation detail. It's called "fail fast", so failing as quickly as possible is key.

[jonas054]

Oops, made a strange self-referencing comment for this PR. Last one should be OK.

[bbatsov]

👍