Show a diff for comparisons of arrays of one-line strings

lib/rspec/support/differ.rb

@@ -13,11 +13,9 @@ class Differ
       def diff(actual, expected)
         diff = ""
 
-        unless actual.nil? || expected.nil?
+        unless actual.nil? || expected.nil? || pointless_diff?(actual, expected)
           if all_strings?(actual, expected)
-            if any_multiline_strings?(actual, expected)
-              diff = diff_as_string(coerce_to_string(actual), coerce_to_string(expected))
-            end
+            diff = diff_as_string(coerce_to_string(actual), coerce_to_string(expected))
           elsif no_procs?(actual, expected) && no_numbers?(actual, expected)
             diff = diff_as_object(actual, expected)
           end
@@ -81,8 +79,8 @@ def all_strings?(*args)
         safely_flatten(args).all? { |a| String === a }
       end
 
-      def any_multiline_strings?(*args)
-        all_strings?(*args) && safely_flatten(args).any? { |a| multiline?(a) }
+      def pointless_diff?(actual, expected)
+        String === actual && String === expected && !multiline?(actual) && !multiline?(expected)
       end
 
       def no_numbers?(*args)

spec/rspec/support/differ_spec.rb

@@ -116,6 +116,36 @@ module Support
           expect { differ.diff(actual, expected) }.not_to change { differ_ivars }
         end
 
+        it 'returns the expected diff for arrays of single-line strings' do
+          expected = ["foo"]
+          actual   = ["foo", "bar"]
+
+          expected_diff = dedent(<<-'EOS')
+            |
+            |@@ -1,2 +1,3 @@
+            | foo
+            |+bar
+            |
+          EOS
+          diff = differ.diff(actual, expected)
+          expect(diff).to eq(expected_diff)
+        end
+
+        it 'does not give an empty string for single-element arrays of single-line strings' do
+          expected = ["foo"]
+          actual   = ["bar"]
+
+          expected_diff = dedent(<<-'EOS')
+            |
+            |@@ -1 +1 @@
+            |-foo
+            |+bar
+            |
+          EOS
+          diff = differ.diff(actual, expected)
+          expect(diff).to eq(expected_diff)
+        end
+
         def differ_ivars
           Hash[ differ.instance_variables.map do |ivar|
             [ivar, differ.instance_variable_get(ivar)]