issue with referencing ipv6 cidr block in subnet

[anilkaliya123]

Expected Behavior

• I am trying to create a vpc and subnet and assign amazon-provided ipv6 to vpc.

• The second resource is subnet which uses the ipv6 cidr_block of vpc but has smaller subnet ("/64").

• I assumed this should have worked, since subnet is dependent on vpc,

• it should be able to read the ipv6 cidr block of vpc and truncate to /64.

  test_vpc = Vpc(
             self,
             "cmc-vpc",
             cidr_block="10.10.0.0/16",
             enable_dns_hostnames=True,
             tags={**DEFAULT_TAGS, "Name": f"{stack_id}-test"},
             assign_generated_ipv6_cidr_block=true
         )
               test_subnet = Subnet(
             self,
             "test-subnet-external",
             tags={**DEFAULT_TAGS_CMC, "Name": f"{stack_id}-test-external"},
             cidr_block="10.10.0.0/16",
             availability_zone=REGION_AZ_MAPPING[region][0],
             vpc_id=test_vpc.id,
             map_public_ip_on_launch=False,
             depends_on =[cmc_vpc],
             ipv6_cidr_block = test_vpc.ipv6_cidr_block.split("/")[0] + "/64"
         )
  

### Actual Behavior

> The ipv6_cidr_block of vpc is /56 (amazon provided ipv6), 
> Subnet is not able to get the /64 cidr_block and it throws an error like 
> invalid cidr block  2600:1f18:7fb:9600::/56/64
> seems like  ipv6 of subnet is getting appended with /64 , and truncation is not happening . 
> Any suggestion here, how can i achieve the same

### Steps to Reproduce

1. create stack 

class DbVpcStack(TerraformStack):
def init(
self,
scope: Construct,
stack_id: str,
region: str,
dbvpc_cidr_block: str,
):
super().init(scope, stack_id)
test_vpc = Vpc(
self,
"cmc-vpc",
cidr_block="10.10.0.0/16",
enable_dns_hostnames=True,
tags={**DEFAULT_TAGS, "Name": f"{stack_id}-test"},
assign_generated_ipv6_cidr_block=true
)
test_subnet = Subnet(
self,
"test-subnet-external",
tags={**DEFAULT_TAGS_CMC, "Name": f"{stack_id}-test-external"},
cidr_block="10.10.0.0/16",
availability_zone=REGION_AZ_MAPPING[region][0],
vpc_id=test_vpc.id,
map_public_ip_on_launch=False,
depends_on =[cmc_vpc],
ipv6_cidr_block = test_vpc.ipv6_cidr_block.split("/")[0] + "/64"
)

2. run terrform init and terraform apply

### Versions

0.16.3

### Providers

_No response_

### Gist

_No response_

### Possible Solutions

_No response_

### Workarounds

_No response_

### Anything Else?

_No response_

### References

_No response_

### Help Wanted

- [ ] I'm interested in contributing a fix myself

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[nbaju1]

Pythonic string manipulation only happens when you synthesize the stack. The ipv6 block is resolved on deploy. You need to use the objects that corresponds with hcl string manipulation, which is resolved on deploy:

from cdktf import StringConcat, FnGenerated

str_concat = StringConcat()
test_vpc = Vpc(self, "cmc-vpc", cidr_block="10.10.0.0/16", enable_dns_hostnames=True, assign_generated_ipv6_cidr_block=True)
test_subnet = Subnet(
    self,
    "test-subnet-external",
    cidr_block="10.10.0.0/16",
    vpc_id=test_vpc.id,
    map_public_ip_on_launch=False,
    ipv6_cidr_block=str_concat.join(FnGenerated.element(FnGenerated.split("/", test_vpc.ipv6_cidr_block), 0), "/64"),
)

Converted into JSON:

  "resource": {
    "aws_subnet": {
      "test-subnet-external": {
        "//": {
          "metadata": {
            "path": "example/test-subnet-external",
            "uniqueId": "test-subnet-external"
          }
        },
        "cidr_block": "10.10.0.0/16",
        "ipv6_cidr_block": "${element(split(\"/\", aws_vpc.cmc-vpc.ipv6_cidr_block), 0)}/64",
        "map_public_ip_on_launch": false,
        "vpc_id": "${aws_vpc.cmc-vpc.id}"
      }
    },
    "aws_vpc": {
      "cmc-vpc": {
        "//": {
          "metadata": {
            "path": "example/cmc-vpc",
            "uniqueId": "cmc-vpc"
          }
        },
        "assign_generated_ipv6_cidr_block": true,
        "cidr_block": "10.10.0.0/16",
        "enable_dns_hostnames": true
      }
    }
  },