Make have_file_content diffable

[cllns]

Summary

This makes the have_file_content matcher diffable in cases when it's useful, without breaking existing behavior.

Details

Custom RSpec matchers aren't diffable by default. You have to manually specify that a matcher should be diffable. This change makes this have_file_content matcher diffable. In particular, it's important for files to be diffable, since they're likely going to be several lines, and RSpec will abbreviate the expectation / actual with a darned ....

Motivation and Context

I'm a core team member for Hanami, and we used to use MiniTest for everything. We have since changed all our internal test suites to RSpec, but the generated projects still use MiniTest. I'm working on changing that, and there are a number of changes to generated files.

A big part of the Hanami ethos is to be good Ruby ecosystem citizens. We want the whole Ruby community to work well together, since it's in all of our interests. This PR will help us (it's already helped me solve my problem locally) and I'm sure it'll help others too. 🌸

How Has This Been Tested?

I did not add a test for this, since I wasn't sure how file generation worked. I did make sure that the tests did not fail (that's why I added .is_a?(String). Another approach I played with, which worked, was only showing the diff if the file was longer than one line.)

I'm happy to add a test for the diffable behavior, I just need a little direction at this point :)

Screenshots:

Before:

After:

"Better... much better"

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactoring (cleanup of codebase without changing any existing functionality)
• Update documentation

Checklist:

• I've added tests for my code
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[cllns]

Another example I just came across, which is more significant, in that that the diff is so much longer that it doesn't it in a single terminal window.

Before:

After (top of screenshot):

[maxmeyer]

As far as I know we don't have any tests for diff-functionality of our matchers. LGTM.

[xtrasimplicity]

The AppVeyor build is failing due to ffi incompatibilities with ruby < 2.0, and byebug incompatibilities with ruby < 2.2.

Perhaps we should remove the debuggers from CI?

Edit: this is unrelated to this PR, so I'll merge this anyway.

[aslakhellesoy]

Hi @cllns,

Thanks for your making your first contribution to Cucumber, and welcome to the Cucumber committers team! You can now push directly to this repo and all other repos under the cucumber organization! 🍾

In return for this generous offer we hope you will:

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[olleolleolle]

(@xtrasimplicity - removing debuggers from CI is a Good Thing.)

[xtrasimplicity]

Agreed. I'll submit a PR when I get back to my computer, if it hasn't already been resolved in master.

Edit: There's a PR already, so I might jump on that.

[cllns]

@aslakhellesoy Thanks, I just joined!

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