How to get available AZs in terraform from data and subnet list.

available_azs.tf

data "aws_availability_zones" "available" {}

# Assuming we vars for list of private_subnet_ids and or public_subnet_ids
# we take a slice of available AZs.
# 
# Add or remove private/public vars when necessary.
locals {
  available_azs = "${
    slice(
      data.aws_availability_zones.available.names,
      0,
      max(length(var.private_subnet_ids), length(var.public_subnet_ids)))
    }"
}