Reworked the \n vs \\n mu_pp_for_diff situation.

Don't munge the output if there is both \n and \\n (nerd mode), but
also munge single vs double differently to improve readability in
different ways.

I think this actually works well. The last one? Not quite right.

Fixes #791.

[git-p4: depot-paths = "//src/minitest/dev/": change = 12259]

lib/minitest/assertions.rb

@@ -61,12 +61,15 @@ def diff exp, act
       butwas = mu_pp_for_diff act
       result = nil
 
+      e1, e2 = expect.include?("\n"), expect.include?("\\n")
+      b1, b2 = butwas.include?("\n"), butwas.include?("\\n")
+
       need_to_diff =
-        (expect.include?("\n")    ||
-         butwas.include?("\n")    ||
-         expect.size > 30         ||
-         butwas.size > 30         ||
-         expect == butwas)        &&
+        (e1 ^ e2           ||
+         b1 ^ b2           ||
+         expect.size > 30  ||
+         butwas.size > 30  ||
+         expect == butwas) &&
         Minitest::Assertions.diff
 
       return "Expected: #{mu_pp exp}\n  Actual: #{mu_pp act}" unless
@@ -133,14 +136,18 @@ def mu_pp_for_diff obj
       str = mu_pp obj
 
       # both '\n' & '\\n' (_after_ mu_pp (aka inspect))
-      nerd = (str.index(/(?<=\\|^)\\n/) &&
-              str.index(/(?<!\\|^)\\n/))
+      single = str.match?(/(?<!\\|^)\\n/)
+      double = str.match?(/(?<=\\|^)\\n/)
 
       process =
-        if nerd then                                   # nerd view:
-          lambda { |s| "#{s}#{ "\n" if s == "\\n" }" } # keep escapes, add nl for "\n"
-        else                                           # normal view:
-          lambda { |s| s == "\\n" ? "\n" :  "\\n\n" }  # de-escape a bit, add nls
+        if single ^ double then
+          if single then
+            lambda { |s| s == "\\n"   ? "\n"    : s } # unescape
+          else
+            lambda { |s| s == "\\\\n" ? "\\n\n" : s } # unescape a bit, add nls
+          end
+        else
+          :itself                                     # leave it alone
         end
 
       str.

test/minitest/test_minitest_assertions.rb

@@ -302,19 +302,34 @@ def test_assert_equal_does_not_allow_lhs_nil_triggered
     end
   end
 
+  def test_assert_equal_string_bug791
+    exp = <<-'EOF'.gsub(/^ {10}/, "") # note single quotes
+          --- expected
+          +++ actual
+          @@ -1,2 +1 @@
+          -"\\n
+          -"
+          +"\\\"
+        EOF
+
+    exp = "Expected: \"\\\\n\"\n  Actual: \"\\\\\""
+    assert_triggered exp do
+      @tc.assert_equal "\\n", "\\"
+    end
+  end
+
   def test_assert_equal_string_both_escaped_unescaped_newlines
-    msg = <<-'EOM'.gsub(/^ {10}/, "") # NOTE single quotes on heredoc
+    msg = <<-EOM.gsub(/^ {10}/, "")
           --- expected
           +++ actual
-          @@ -1,2 +1,2 @@
-          -"A\\n\n
-          -B"
-          +"A\n
-          +\\nB"
+          @@ -1,2 +1 @@
+          -\"A\\n
+          -B\"
+          +\"A\\n\\\\nB\"
           EOM
 
     assert_triggered msg do
-      exp = "A\\n\nB"
+      exp = "A\\nB"
       act = "A\n\\nB"
 
       @tc.assert_equal exp, act
@@ -1219,11 +1234,9 @@ def test_diff_str_mixed
     msg = <<-'EOM'.gsub(/^ {10}/, "") # NOTE single quotes on heredoc
           --- expected
           +++ actual
-          @@ -1,2 +1,2 @@
-          -"A\\n\n
-          -B"
-          +"A\n
-          +\\nB"
+          @@ -1 +1 @@
+          -"A\\n\nB"
+          +"A\n\\nB"
           EOM
 
     exp = "A\\n\nB"
@@ -1309,7 +1322,7 @@ def test_mu_pp_for_diff_str_bad_encoding
 
   def test_mu_pp_for_diff_str_bad_encoding_both
     str = "\666A\\n\nB".force_encoding Encoding::UTF_8
-    exp = "# encoding: UTF-8\n#    valid: false\n\"\\xB6A\\\\n\\n\nB\""
+    exp = "# encoding: UTF-8\n#    valid: false\n\"\\xB6A\\\\n\\nB\""
 
     assert_mu_pp_for_diff exp, str, :raw
   end
@@ -1323,23 +1336,23 @@ def test_mu_pp_for_diff_str_encoding
 
   def test_mu_pp_for_diff_str_encoding_both
     str = "A\\n\nB".b
-    exp = "# encoding: ASCII-8BIT\n#    valid: true\n\"A\\\\n\\n\nB\""
+    exp = "# encoding: ASCII-8BIT\n#    valid: true\n\"A\\\\n\\nB\""
 
     assert_mu_pp_for_diff exp, str, :raw
   end
 
   def test_mu_pp_for_diff_str_nerd
-    assert_mu_pp_for_diff "A\\n\nB\\\\nC", "A\nB\\nC"
-    assert_mu_pp_for_diff "\\n\nB\\\\nC",  "\nB\\nC"
-    assert_mu_pp_for_diff "\\n\nB\\\\n",   "\nB\\n"
-    assert_mu_pp_for_diff "\\n\n\\\\n",    "\n\\n"
-    assert_mu_pp_for_diff "\\\\n\\n\n",    "\\n\n"
-    assert_mu_pp_for_diff "\\\\nB\\n\n",   "\\nB\n"
-    assert_mu_pp_for_diff "\\\\nB\\n\nC",  "\\nB\nC"
-    assert_mu_pp_for_diff "A\\\\n\\n\nB",  "A\\n\nB"
-    assert_mu_pp_for_diff "A\\n\n\\\\nB",  "A\n\\nB"
-    assert_mu_pp_for_diff "\\\\n\\n\n",    "\\n\n"
-    assert_mu_pp_for_diff "\\n\n\\\\n",    "\n\\n"
+    assert_mu_pp_for_diff "A\\nB\\\\nC", "A\nB\\nC"
+    assert_mu_pp_for_diff "\\nB\\\\nC",  "\nB\\nC"
+    assert_mu_pp_for_diff "\\nB\\\\n",   "\nB\\n"
+    assert_mu_pp_for_diff "\\n\\\\n",    "\n\\n"
+    assert_mu_pp_for_diff "\\\\n\\n",    "\\n\n"
+    assert_mu_pp_for_diff "\\\\nB\\n",   "\\nB\n"
+    assert_mu_pp_for_diff "\\\\nB\\nC",  "\\nB\nC"
+    assert_mu_pp_for_diff "A\\\\n\\nB",  "A\\n\nB"
+    assert_mu_pp_for_diff "A\\n\\\\nB",  "A\n\\nB"
+    assert_mu_pp_for_diff "\\\\n\\n",    "\\n\n"
+    assert_mu_pp_for_diff "\\n\\\\n",    "\n\\n"
   end
 
   def test_mu_pp_for_diff_str_normal