terraform-config-inspect | helper library for shallow inspection | Infrastructure Automation library

Input Variables in Terraform are a planning option and so to resolve them fully requires creating a Terraform plan. If you are able to create a Terraform plan then you can find the resolved values in the JSON serialization of the plan, using steps like the following:

• terraform plan -out=tfplan (optionally include -var=... and -var-file=... if you need to set particular values for those variables.
• terraform show -json tfplan to get a JSON representation of the plan.

Alternatively, if you've already applied the configuration you want to analyse then you can get similar information from the JSON representation of the latest state snapshot:

• terraform show -json to get a JSON representation of the latest state snapshot.

As you've seen, terraform-config-inspect is only for static analysis of the top-level declarations and so it contains no functionality for evaluating expressions.

In order to properly evaluate expressions here without creating a Terraform plan or reading from a Terraform state snapshot would require reimplementing the Terraform Core runtime, at least to some extent. However, for this particular expression (which only relies on input variable values) you could potentially use the HCL API directly with some hard-coded placeholder values for those variables in order to get a value for that argument, derived from whatever you happen to have set var.deployment and var.service_name to in the hcl.EvalContext you construct yourself.