fmt: distinguish empty vs nil slice/map in %#v

Also update Scanf tests to cope with DeepEqual
distinguishing empty vs nil slice.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5375091

src/pkg/fmt/fmt_test.go

@@ -357,6 +357,10 @@ var fmttests = []struct {
 	{"%#v", map[string]B{"a": {1, 2}}, `map[string] fmt_test.B{"a":fmt_test.B{I:1, j:2}}`},
 	{"%#v", []string{"a", "b"}, `[]string{"a", "b"}`},
 	{"%#v", SI{}, `fmt_test.SI{I:interface {}(nil)}`},
+	{"%#v", []int(nil), `[]int(nil)`},
+	{"%#v", []int{}, `[]int{}`},
+	{"%#v", map[int]byte(nil), `map[int] uint8(nil)`},
+	{"%#v", map[int]byte{}, `map[int] uint8{}`},
 
 	// slices with other formats
 	{"%#x", []int{1, 2, 15}, `[0x1 0x2 0xf]`},

src/pkg/fmt/print.go

@@ -795,6 +795,10 @@ BigSwitch:
 	case reflect.Map:
 		if goSyntax {
 			p.buf.WriteString(f.Type().String())
+			if f.IsNil() {
+				p.buf.WriteString("(nil)")
+				break
+			}
 			p.buf.WriteByte('{')
 		} else {
 			p.buf.Write(mapBytes)
@@ -873,6 +877,10 @@ BigSwitch:
 		}
 		if goSyntax {
 			p.buf.WriteString(value.Type().String())
+			if f.IsNil() {
+				p.buf.WriteString("(nil)")
+				break
+			}
 			p.buf.WriteByte('{')
 		} else {
 			p.buf.WriteByte('[')

src/pkg/fmt/scan_test.go

@@ -324,7 +324,7 @@ var x, y Xs
 var z IntString
 
 var multiTests = []ScanfMultiTest{
-	{"", "", nil, nil, ""},
+	{"", "", []interface{}{}, []interface{}{}, ""},
 	{"%d", "23", args(&i), args(23), ""},
 	{"%2s%3s", "22333", args(&s, &t), args("22", "333"), ""},
 	{"%2d%3d", "44555", args(&i, &j), args(44, 555), ""},
@@ -378,7 +378,7 @@ func testScan(name string, t *testing.T, scan func(r io.Reader, a ...interface{}
 		}
 		val := v.Interface()
 		if !reflect.DeepEqual(val, test.out) {
-			t.Errorf("%s scanning %q: expected %v got %v, type %T", name, test.text, test.out, val, val)
+			t.Errorf("%s scanning %q: expected %#v got %#v, type %T", name, test.text, test.out, val, val)
 		}
 	}
 }
@@ -417,7 +417,7 @@ func TestScanf(t *testing.T) {
 		}
 		val := v.Interface()
 		if !reflect.DeepEqual(val, test.out) {
-			t.Errorf("scanning (%q, %q): expected %v got %v, type %T", test.format, test.text, test.out, val, val)
+			t.Errorf("scanning (%q, %q): expected %#v got %#v, type %T", test.format, test.text, test.out, val, val)
 		}
 	}
 }
@@ -520,7 +520,7 @@ func testScanfMulti(name string, t *testing.T) {
 		}
 		result := resultVal.Interface()
 		if !reflect.DeepEqual(result, test.out) {
-			t.Errorf("scanning (%q, %q): expected %v got %v", test.format, test.text, test.out, result)
+			t.Errorf("scanning (%q, %q): expected %#v got %#v", test.format, test.text, test.out, result)
 		}
 	}
 }