Terraform 0.12 - For each subnet-id in array, get cidr_block
data "aws_subnet" "subnets" {
  for_each = toset(var.aws_subnet_ids)
  id       = each.value
}

locals {
  cidr_blocks = [
    for subnet in data.aws_subnet.subnets :
      subnet.cidr_block
    ]

  cidr_blocks_rendered = [
    for cidr_block in local.cidr_blocks :
     {
       from_port   = 6443,
       to_port     = 6443,
       protocol    = "tcp",
       cidr_blocks = cidr_block
     }
  ]
}

output "output1-1" {
  value = local.cidr_blocks_rendered
}


This is what ultimately I needed.