`assert_equal` shows no diff output on failure, when the diff is a single trailing newline

[z1lk]

test "string equality" do
  assert_equal "hello", "hello\n"
end

produces

Expected objects to be equal (compared with `actual == expected`):

Actual                       Expected
No changes.

Oddly it is only this exact case. assert_equal "hello", "hello\n\n" and assert_equal "hello\n", "hello\n\n" both show a diff.

[joeldrapper]

Thanks, this looks like an issue with Difftastic. Difftastic is amazing when it works. It’s so good at showing diffs between two similar Ruby or HTML strings for example. But it’s not integrated very well into Quickdraw. It makes sense for Difftastic to ignore this diff.

I would like to replace Difftastic with my own tree-sitter diffing thing which would give us better APIs and more options, but it’s a lot of additional work.

[z1lk]

Yeah, I saw your wrapper at https://github.com/joeldrapper/difftastic-ruby. Definitely something to do with Difftastic, but I didn't spend very long investigating how the Ruby wrapper works and if there is maybe a way to get Difftastic to correctly handle this case.

[z1lk]

Spent some time looking into this.

This particular case is definitely an upstream Difftastic issue, logged here.

Ideas below!

One thing to do is introduce a fallback when actual === expected is tested but Difftastic reports "No changes".

diff --git a/lib/quickdraw/assertions.rb b/lib/quickdraw/assertions.rb
index ac8205f..b0c2ba4 100644
--- a/lib/quickdraw/assertions.rb
+++ b/lib/quickdraw/assertions.rb
@@ -12,6 +12,9 @@ module Quickdraw::Assertions
                assert(actual == expected) do
                        if String === actual && String === expected
                                diff = DIFFER.diff_strings(actual, expected)
+                               if diff.end_with?("No changes.")
+                                       diff = DIFFER.diff_strings(actual.inspect, expected.inspect)
+                               end
                        else
                                diff = DIFFER.diff_objects(actual, expected)
                        end

This produces a sensible message for diffing "hi" and "hi\n".

Expected objects to be equal (compared with `actual == expected`):

Actual                        Expected
1 "hi"                        1 "hi\n"

String#dump could be used instead of String#inspect, which would show non-printing characters like the zero-width space.

Expected objects to be equal (compared with `actual == expected`):

Actual                        Expected
1 "hi"                        1 "hi\u200B"

Unfortunately this trailing newline case that Difftastic reports as "No changes" is a special case due to this bit of source. If the trailing character is not a newline but another non-printing character (tab, zero-width space, etc.), Difftastic reports there there is a diff, but its output does not make it obvious what it is. And since it reports a diff, the fallback in the snippet above is not executed.

To get Quickdraw to show obvious diff output in these cases, checks could be made that would go straight to diffing String#dump to avoid a potentially confusing diff. For instance, this checks if any of a string's lines end with a control or formatting character: str.lines.any?(/[\p{C}&&[^\n]]\n?$/). Or, use a similar regexp to sub in a printed version of control and formatting characters with str.gsub!(/[\p{C}&&[^\n]]/) { |ch| "\\u#{ch.ord.to_s(16).rjust(4, '0')}" } and send that to Difftastic:

Expected objects to be equal (compared with `actual == expected`):

Actual                        Expected
1 hi                          1 hi\u200B
2 hello                       2 hello