provider/aws: Specify that aws_network_acl_rule requires a cidr block (#13013)

Fixes: #13011

```
% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSNetworkAclRule_'           2 ↵ ✚
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/03/23 17:45:25 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSNetworkAclRule_ -timeout 120m
=== RUN   TestAccAWSNetworkAclRule_basic
--- PASS: TestAccAWSNetworkAclRule_basic (41.10s)
=== RUN   TestAccAWSNetworkAclRule_missingParam
--- PASS: TestAccAWSNetworkAclRule_missingParam (21.21s)
=== RUN   TestAccAWSNetworkAclRule_ipv6
--- PASS: TestAccAWSNetworkAclRule_ipv6 (53.00s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	115.333s
```

builtin/providers/aws/resource_aws_network_acl_rule.go

@@ -109,12 +109,19 @@ func resourceAwsNetworkAclRuleCreate(d *schema.ResourceData, meta interface{}) e
 		},
 	}
 
-	if v, ok := d.GetOk("cidr_block"); ok {
-		params.CidrBlock = aws.String(v.(string))
+	cidr, hasCidr := d.GetOk("cidr_block")
+	ipv6Cidr, hasIpv6Cidr := d.GetOk("ipv6_cidr_block")
+
+	if hasCidr == false && hasIpv6Cidr == false {
+		return fmt.Errorf("Either `cidr_block` or `ipv6_cidr_block` must be defined")
 	}
 
-	if v, ok := d.GetOk("ipv6_cidr_block"); ok {
-		params.Ipv6CidrBlock = aws.String(v.(string))
+	if hasCidr {
+		params.CidrBlock = aws.String(cidr.(string))
+	}
+
+	if hasIpv6Cidr {
+		params.Ipv6CidrBlock = aws.String(ipv6Cidr.(string))
 	}
 
 	// Specify additional required fields for ICMP. For the list

builtin/providers/aws/resource_aws_network_acl_rule_test.go

@@ -2,6 +2,7 @@ package aws
 
 import (
 	"fmt"
+	"regexp"
 	"strconv"
 	"testing"
 
@@ -32,6 +33,21 @@ func TestAccAWSNetworkAclRule_basic(t *testing.T) {
 	})
 }
 
+func TestAccAWSNetworkAclRule_missingParam(t *testing.T) {
+
+	resource.Test(t, resource.TestCase{
+		PreCheck:     func() { testAccPreCheck(t) },
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckAWSNetworkAclRuleDestroy,
+		Steps: []resource.TestStep{
+			{
+				Config:      testAccAWSNetworkAclRuleMissingParam,
+				ExpectError: regexp.MustCompile("Either `cidr_block` or `ipv6_cidr_block` must be defined"),
+			},
+		},
+	})
+}
+
 func TestAccAWSNetworkAclRule_ipv6(t *testing.T) {
 	var networkAcl ec2.NetworkAcl
 
@@ -214,6 +230,27 @@ resource "aws_network_acl_rule" "wibble" {
 }
 `
 
+const testAccAWSNetworkAclRuleMissingParam = `
+provider "aws" {
+  region = "us-east-1"
+}
+resource "aws_vpc" "foo" {
+	cidr_block = "10.3.0.0/16"
+}
+resource "aws_network_acl" "bar" {
+	vpc_id = "${aws_vpc.foo.id}"
+}
+resource "aws_network_acl_rule" "baz" {
+	network_acl_id = "${aws_network_acl.bar.id}"
+	rule_number = 200
+	egress = false
+	protocol = "tcp"
+	rule_action = "allow"
+	from_port = 22
+	to_port = 22
+}
+`
+
 const testAccAWSNetworkAclRuleIpv6Config = `
 resource "aws_vpc" "foo" {
 	cidr_block = "10.3.0.0/16"

website/source/docs/providers/aws/r/network_acl_rule.html.markdown

@@ -29,6 +29,8 @@ resource "aws_network_acl_rule" "bar" {
 }
 ```
 
+~> **Note:** One of either `cidr_block` or `ipv6_cidr_block` is required.
+
 ## Argument Reference
 
 The following arguments are supported: