Output used availability zones

I have a VPC module.
I have an RDS module.

In the RDS module I want to specify the ‘availability_zones’ option.
In my VPC module I decide which availability zones to use by:

  1. Get all AZ’s
  2. Blacklist missbehaving AZ’s
  3. Use only as many AZ’s as I’ve requested subents (via variable)

In this case, I can’t seem to find documentation on using count in either output, or locals, to send back JUST the availability zones I’m in. Additionally, the availability zone isn’t an output on subnets. Thoughts on how I can work around this?

For more detail, here’s a gist from the VPC module: https://gist.github.com/throwaway8787/bae0d03c5e214b8dd178744aae624eca