Add super_diff extension

Gemfile

@@ -7,7 +7,8 @@ eval_gemfile "Gemfile.devtools"
 gemspec
 
 group :test do
-  gem "dry-monads"
+  gem "dry-monads", github: "dry-rb/dry-monads"
+  gem "super_diff"
 end
 
 group :benchmarks do

lib/dry/struct/extensions.rb

@@ -3,3 +3,7 @@
 Dry::Struct.register_extension(:pretty_print) do
   require "dry/struct/extensions/pretty_print"
 end
+
+Dry::Struct.register_extension(:super_diff) do
+  require "dry/struct/extensions/super_diff"
+end

lib/dry/struct/extensions/super_diff.rb

@@ -0,0 +1,10 @@
+# frozen_string_literal: true
+
+require "super_diff"
+require "super_diff/rspec"
+
+module Dry
+  class Struct
+    def attributes_for_super_diff = attributes
+  end
+end

spec/extensions/super_diff_spec.rb

@@ -0,0 +1,109 @@
+# frozen_string_literal: true
+
+require "tempfile"
+require "spec_helper"
+
+RSpec.describe Dry::Struct do
+  let(:output_start_marker) do
+    /(expected:)|(Expected )/
+  end
+
+  let(:output_end_marker) do
+    /#{output_start_marker.source}|Finished/
+  end
+
+  def run_spec(code)
+    temp_spec = Tempfile.new(["failing_spec", ".rb"])
+    temp_spec.write(<<~RUBY)
+      require "dry/struct"
+
+      RSpec.describe "A failing example" do
+        before(:all) do
+          Dry::Struct.load_extensions(:super_diff)
+        end
+
+        #{code}
+      end
+    RUBY
+    temp_spec.close
+
+    process_output(`rspec #{temp_spec.path}`, temp_spec.path)
+  end
+
+  def process_output(output, path)
+    uncolored = output.gsub(/\e\[([;\d]+)?m/, "")
+    # cut out significant lines
+    lines = extract_diff(uncolored, path)
+    prefix = lines.filter_map { |line|
+      line.match(/^\A(\s+)/).to_s unless line.strip.empty?
+    }.min
+    processed_lines = lines.map { |line| line.gsub(prefix, "") }
+    remove_banner(processed_lines).join.gsub("\n\n\n", "\n\n").gsub(/\n\n\z/, "\n")
+  end
+
+  # remove this part from the output:
+  #
+  # Diff:
+  #
+  # ┌ (Key) ──────────────────────────┐
+  # │ ‹-› in expected, not in actual  │
+  # │ ‹+› in actual, not in expected  │
+  # │ ‹ › in both expected and actual │
+  # └─────────────────────────────────┘
+  #
+  def remove_banner(lines)
+    before_banner = lines.take_while { |line| !line.start_with?("Diff:") }
+    after_banner = lines.drop_while { |line|
+      !line.include?("└")
+    }.drop(1)
+    before_banner + after_banner
+  end
+
+  def extract_diff(output, path)
+    output.lines.drop_while { |line|
+      !line[output_start_marker]
+    }.take_while.with_index { |line, idx|
+      idx.zero? || !(line.include?(path) || line[output_start_marker])
+    }
+  end
+
+  it "produces a nice diff" do
+    output = run_spec(<<~RUBY)
+      let(:user_type) do
+        module Test
+          class User < Dry::Struct
+            attribute :name, 'string'
+            attribute :age, 'integer'
+          end
+        end
+
+        Test::User
+      end
+
+      let(:user) do
+        user_type[name: "Jane", age: 21]
+      end
+
+      let(:other_user) do
+        user_type[name: "Jane", age: 22]
+      end
+
+      example "failing" do
+        expect(user).to eql(other_user)
+      end
+    RUBY
+
+    expect(output).to eql(<<~DIFF)
+      expected: #<Test::User name: "Jane", age: 22>
+           got: #<Test::User name: "Jane", age: 21>
+
+      (compared using eql?)
+
+        #<Test::User {
+          name: "Jane",
+      -   age: 22
+      +   age: 21
+        }>
+    DIFF
+  end
+end

spec/spec_helper.rb

@@ -27,6 +27,11 @@ module DryStructSpec
 end
 
 Dir[Pathname(__dir__).join("shared/*.rb")].each(&method(:require))
+
+Warning.ignore(/rspec-expectations/)
+Warning.ignore(/super_diff/)
+Warning.process { raise _1 }
+
 require "dry/types/spec/types"
 
 RSpec.configure do |config|