Allow batched diffing of slices with a custom comparer

For correctness, cmp checks applicability of the options to every
element in a slice. For large []byte, this is a significant performance
detriment. The workaround is to specify Comparer(bytes.Equal).
However, we would still like to have the batched diffing if the
slices are different. Specialize for this situation.

cmp/compare_test.go

@@ -833,6 +833,15 @@ func reporterTests() []test {
 		y:         MyComposite{IntsA: []int8{12, 29, 13, 27, 22, 23, 17, 18, 19, 20, 21, 10, 26, 16, 25, 28, 11, 15, 24, 14}},
 		wantEqual: false,
 		reason:    "batched diffing desired since many elements differ",
+	}, {
+		label:     label + "/BatchedWithComparer",
+		x:         MyComposite{BytesA: []byte{10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29}},
+		y:         MyComposite{BytesA: []byte{12, 29, 13, 27, 22, 23, 17, 18, 19, 20, 21, 10, 26, 16, 25, 28, 11, 15, 24, 14}},
+		wantEqual: false,
+		opts: []cmp.Option{
+			cmp.Comparer(bytes.Equal),
+		},
+		reason: "batched diffing desired since many elements differ",
 	}, {
 		label: label,
 		x: MyComposite{

cmp/report_slices.go

@@ -23,13 +23,25 @@ func (opts formatOptions) CanFormatDiffSlice(v *valueNode) bool {
 		return false // Must be formatting in diff mode
 	case v.NumDiff == 0:
 		return false // No differences detected
-	case v.NumIgnored+v.NumCompared+v.NumTransformed > 0:
-		// TODO: Handle the case where someone uses bytes.Equal on a large slice.
-		return false // Some custom option was used to determined equality
 	case !v.ValueX.IsValid() || !v.ValueY.IsValid():
 		return false // Both values must be valid
 	case v.Type.Kind() == reflect.Slice && (v.ValueX.IsNil() || v.ValueY.IsNil()):
 		return false // Both of values have to be non-nil
+	case v.NumIgnored > 0:
+		return false // Some ignore option was used
+	case v.NumTransformed > 0:
+		return false // Some transform option was used
+	case v.NumCompared > 1:
+		return false // More than one comparison was used
+	case v.NumCompared == 1 && v.Type.Name() != "":
+		// The need for cmp to check applicability of options on every element
+		// in a slice is a significant performance detriment for large []byte.
+		// The workaround is to specify Comparer(bytes.Equal),
+		// which enables cmp to compare []byte more efficiently.
+		// If they differ, we still want to provide batched diffing.
+		// The logic disallows named types since they tend to have their own
+		// String method, with nicer formatting than what this provides.
+		return false
 	}
 
 	switch t := v.Type; t.Kind() {

cmp/testdata/diffs

@@ -294,6 +294,22 @@
   	... // 6 identical fields
   }
 >>> TestDiff/Reporter#01
+<<< TestDiff/Reporter/BatchedWithComparer
+  cmp_test.MyComposite{
+  	StringA: "",
+  	StringB: "",
+  	BytesA: []uint8{
+- 		0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f, 0x10,             // -|.......|
++ 		0x0c, 0x1d, 0x0d, 0x1b, 0x16, 0x17,                   // +|......|
+  		0x11, 0x12, 0x13, 0x14, 0x15,                         //  |.....|
+- 		0x16, 0x17, 0x18, 0x19, 0x1a, 0x1b, 0x1c, 0x1d,       // -|........|
++ 		0x0a, 0x1a, 0x10, 0x19, 0x1c, 0x0b, 0x0f, 0x18, 0x0e, // +|.........|
+  	},
+  	BytesB: nil,
+  	BytesC: nil,
+  	... // 9 identical fields
+  }
+>>> TestDiff/Reporter/BatchedWithComparer
 <<< TestDiff/Reporter#02
   cmp_test.MyComposite{
   	StringA: "",