net/netip: make AddrPort.MarshalText format 4-in-6 IPs consistently

Thanks again to @capnspacehook.

Fixes #50110

Change-Id: I1973bdea68eac9842b45f9524f62152e4f5342cf
Reviewed-on: https://go-review.googlesource.com/c/go/+/371114
Run-TryBot: Brad Fitzpatrick <[email protected]>
Reviewed-by: Matt Layher <[email protected]>
Trust: Matt Layher <[email protected]>
Trust: Brad Fitzpatrick <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: bradfitz

src/net/netip/netip.go

@@ -1157,8 +1157,17 @@ func (p AddrPort) AppendTo(b []byte) []byte {
 	case z4:
 		b = p.ip.appendTo4(b)
 	default:
-		b = append(b, '[')
-		b = p.ip.appendTo6(b)
+		if p.ip.Is4In6() {
+			b = append(b, "[::ffff:"...)
+			b = p.ip.Unmap().appendTo4(b)
+			if z := p.ip.Zone(); z != "" {
+				b = append(b, '%')
+				b = append(b, z...)
+			}
+		} else {
+			b = append(b, '[')
+			b = p.ip.appendTo6(b)
+		}
 		b = append(b, ']')
 	}
 	b = append(b, ':')

src/net/netip/netip_test.go

@@ -347,6 +347,32 @@ func TestAddrMarshalUnmarshalBinary(t *testing.T) {
 	}
 }
 
+func TestAddrPortMarshalTextString(t *testing.T) {
+	tests := []struct {
+		in   AddrPort
+		want string
+	}{
+		{mustIPPort("1.2.3.4:80"), "1.2.3.4:80"},
+		{mustIPPort("[1::CAFE]:80"), "[1::cafe]:80"},
+		{mustIPPort("[1::CAFE%en0]:80"), "[1::cafe%en0]:80"},
+		{mustIPPort("[::FFFF:192.168.140.255]:80"), "[::ffff:192.168.140.255]:80"},
+		{mustIPPort("[::FFFF:192.168.140.255%en0]:80"), "[::ffff:192.168.140.255%en0]:80"},
+	}
+	for i, tt := range tests {
+		if got := tt.in.String(); got != tt.want {
+			t.Errorf("%d. for (%v, %v) String = %q; want %q", i, tt.in.Addr(), tt.in.Port(), got, tt.want)
+		}
+		mt, err := tt.in.MarshalText()
+		if err != nil {
+			t.Errorf("%d. for (%v, %v) MarshalText error: %v", i, tt.in.Addr(), tt.in.Port(), err)
+			continue
+		}
+		if string(mt) != tt.want {
+			t.Errorf("%d. for (%v, %v) MarshalText = %q; want %q", i, tt.in.Addr(), tt.in.Port(), mt, tt.want)
+		}
+	}
+}
+
 func TestAddrPortMarshalUnmarshalBinary(t *testing.T) {
 	tests := []struct {
 		ipport   string