Variable validation should allow checking other variables

[alexjurkiewicz]

Sometimes I have variables like shut_down_on_weekends:

variable environment_type {
  type = string
  description = "test or prod"
}

variable shut_down_on_weekends {
  type = bool
  description = "You can't shut production environments down on the weekend"
  default = false
}

locals {
  actually_shut_down = var.shut_down_on_weekends && var.environment_type == "test"
}

Rather than creating an intermediate local to silently fix the issue, I would prefer to use variable validation to raise an error at planning time:

variable shut_down_on_weekends {
  type = bool
  validation {
    condition     = var.shut_down_on_weekends || var.environment_type == "test"
    error_message = "You can only shut test environments down on the weekend."
  }
}

It would be possible to write cyclic dependencies with this functionality, so some care would need to be made to ensure that case is detected.

[colans]

Can we mark this as a duplicate of #25609 given that it has somewhat of a plan?

[crw]

Yes, marking dupe of #25609.

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