Use string formatting for slice of bytes

If a slice of bytes is mostly text, format them as text
instead of as []byte literal with hexadecimal digits.

Avoid always printing the type. This is technically invalid Go code,
but is unnecessary in many cases since the type is inferred
from the parent concrete type.

Fixes #272

cmp/compare_test.go

@@ -1351,6 +1351,24 @@ using the AllowUnexported option.`, "\n"),
 	"bar": true,
 }`,
 		reason: "short multiline JSON should prefer triple-quoted string diff as it is more readable",
+	}, {
+		label: label + "/SliceOfBytesText",
+		x: [][]byte{
+			[]byte("hello"), []byte("foo"), []byte("barbaz"), []byte("blahdieblah"),
+		},
+		y: [][]byte{
+			[]byte("foo"), []byte("foo"), []byte("barbaz"), []byte("added"), []byte("here"), []byte("hrmph"),
+		},
+		reason: "should print text byte slices as strings",
+	}, {
+		label: label + "/SliceOfBytesBinary",
+		x: [][]byte{
+			[]byte("\xde\xad\xbe\xef"), []byte("\xffoo"), []byte("barbaz"), []byte("blahdieblah"),
+		},
+		y: [][]byte{
+			[]byte("\xffoo"), []byte("foo"), []byte("barbaz"), []byte("added"), []byte("here"), []byte("hrmph\xff"),
+		},
+		reason: "should print text byte slices as strings except those with binary",
 	}}
 }
 

cmp/example_test.go

@@ -37,7 +37,7 @@ func ExampleDiff_testing() {
 	//   	SSID:      "CoffeeShopWiFi",
 	// - 	IPAddress: s"192.168.0.2",
 	// + 	IPAddress: s"192.168.0.1",
-	//   	NetMask:   {0xff, 0xff, 0x00, 0x00},
+	//   	NetMask:   s"ffff0000",
 	//   	Clients: []cmp_test.Client{
 	//   		... // 2 identical elements
 	//   		{Hostname: "macchiato", IPAddress: s"192.168.0.153", LastSeen: s"2009-11-10 23:39:43 +0000 UTC"},

cmp/report_compare.go

@@ -115,8 +115,10 @@ func (opts formatOptions) FormatDiff(v *valueNode, ptrs *pointerReferences) (out
 		parentKind = v.parent.Type.Kind()
 	}
 
-	// For leaf nodes, format the value based on the reflect.Values alone.
-	if v.MaxDepth == 0 {
+	// For leaf nodes, format the value based on the reflect.Values alone.+	// As a special case, treat equal []byte as a leaf nodes.
+	isBytes := v.Type.Kind() == reflect.Slice && v.Type.Elem() == reflect.TypeOf(byte(0))
+	isEqualBytes := isBytes && v.NumDiff+v.NumIgnored+v.NumTransformed == 0
+	if v.MaxDepth == 0 || isEqualBytes {
 		switch opts.DiffMode {
 		case diffUnknown, diffIdentical:
 			// Format Equal.

cmp/report_reflect.go

@@ -211,7 +211,7 @@ func (opts formatOptions) FormatValue(v reflect.Value, parentKind reflect.Kind,
 			if len(b) > 0 && utf8.Valid(b) && len(bytes.TrimFunc(b, isPrintSpace)) == 0 {
 				out = opts.formatString("", string(b))
 				skipType = true
-				return opts.WithTypeMode(emitType).FormatType(t, out)
+				return opts.FormatType(t, out)
 			}
 		}
 

cmp/testdata/diffs

@@ -241,9 +241,9 @@
   		"string",
   	}),
   	Bytes: []uint8(Inverse(SplitBytes, [][]uint8{
-  		{0x73, 0x6f, 0x6d, 0x65},
-  		{0x6d, 0x75, 0x6c, 0x74, ...},
-  		{0x6c, 0x69, 0x6e, 0x65},
+  		"some",
+  		"multi",
+  		"line",
   		{
 - 			0x62,
 + 			0x42,
@@ -1134,6 +1134,30 @@
   	"""
   )
 >>> TestDiff/Reporter/ShortJSON
+<<< TestDiff/Reporter/SliceOfBytesText
+  [][]uint8{
+- 	"hello",
+  	"foo",
++ 	"foo",
+  	"barbaz",
++ 	"added",
++ 	"here",
+- 	"blahdieblah",
++ 	"hrmph",
+  }
+>>> TestDiff/Reporter/SliceOfBytesText
+<<< TestDiff/Reporter/SliceOfBytesBinary
+  [][]uint8{
+- 	{0xde, 0xad, 0xbe, 0xef},
+  	{0xff, 0x6f, 0x6f},
++ 	"foo",
+  	"barbaz",
++ 	"added",
++ 	"here",
+- 	"blahdieblah",
++ 	{0x68, 0x72, 0x6d, 0x70, 0x68, 0xff},
+  }
+>>> TestDiff/Reporter/SliceOfBytesBinary
 <<< TestDiff/EmbeddedStruct/ParentStructA/Inequal
   teststructs.ParentStructA{
   	privateStruct: teststructs.privateStruct{