IMO, `diffable?` is an implementation detail.  I'd rather see a spec that asserts it actually got a diff.  Something like:

``` ruby
it "provides a diff on failure" do
  expect {
    expect("foo").to match(/bar/)
  }.to fail_with(failure_message_that_includes_diff)
end
```

(I'll leave it to you to figure out the exact diff string).

This gives me much higher confidence that the diff is working.