[BUG] Terramate panics when using ternary expression with `tm_hcl_expression()` function about terramate

[katcipis]

Thanks for taking the time to explain your issue @ottramst.

This is a known (but not well documented) limitation. Globals today are always about fully evaluated information, there is no partial evaluation on them, given that they can't hold arbitrary un-evaluated expressions as their final value. The problem is not related to ternary, even this simple example will fail with a similar error:

globals {
  data = tm_hcl_expression("expr")
}

generate_hcl "test.tf" {
  content {
    data = global.data
  }
}

We will be reviewing globals behavior soon, but for now I will only document better the contexts where tm_hcl_expression can be used safely. We should also remove the function from the global evaluation context, so you are going to get a much nicer "function doesn't exist" error than the not so friendly panic.

Given the provided example, if it is similar to what you are trying to achieve, this should work:

globals {
  module_rds_enabled = true

  database = global.module_rds_enabled ? "module.rds.db_instance_database_name" : "module.rds_aurora.cluster_database_name"
}

generate_hcl "_test.tf" {
  content {
    database = tm_hcl_expression(global.database)
  }
}

stack {
  name = "test-stack"
}