net/packet: remove NewIP, offer only a netaddr constructor.

Signed-off-by: David Anderson <danderson@tailscale.com>

net/packet/ip4.go

@@ -7,7 +7,6 @@ package packet
 import (
 	"encoding/binary"
 	"fmt"
-	"net"
 
 	"inet.af/netaddr"
 )
@@ -15,17 +14,7 @@ import (
 // IP4 is an IPv4 address.
 type IP4 uint32
 
-// NewIP converts a standard library IP address into an IP.
-// It panics if b is not an IPv4 address.
-func NewIP4(b net.IP) IP4 {
-	b4 := b.To4()
-	if b4 == nil {
-		panic(fmt.Sprintf("To4(%v) failed", b))
-	}
-	return IP4(binary.BigEndian.Uint32(b4))
-}
-
-// IPFromNetaddr converts a netaddr.IP to an IP.
+// IPFromNetaddr converts a netaddr.IP to an IP. Panics if !ip.Is4.
 func IP4FromNetaddr(ip netaddr.IP) IP4 {
 	ipbytes := ip.As4()
 	return IP4(binary.BigEndian.Uint32(ipbytes[:]))

net/packet/packet_test.go

@@ -6,7 +6,6 @@ package packet
 
 import (
 	"bytes"
-	"net"
 	"reflect"
 	"testing"
 
@@ -31,7 +30,7 @@ func mustIP6(s string) IP6 {
 
 func TestIP4String(t *testing.T) {
 	const str = "1.2.3.4"
-	ip := NewIP4(net.ParseIP(str))
+	ip := mustIP4(str)
 
 	var got string
 	allocs := testing.AllocsPerRun(1000, func() {