[FEATURE] Do not evaluate lets block if condition is false in generate_file and generate_hcl

[g13013]

Is your feature request related to a problem? Please describe.
The current behavior of the lets block is that it's evaluated all the time regardless of the condition result, this leads errors in some scenarios when lets block depends on whether the condition is true or false, the problem becomes more annoying when the lets bloc have many variables that depend on the same condition

example

generate_hcl 'some_file.tf' {
  condition = length(global.data.vnets) > 0 && global.someOtherCondition

  lets: { // Repeated condition
    data   = global.someOtherCondition ?  existantValue ? nonExistantValue
    data2 = global.someOtherCondition ? existantValue2 ? nonExistantValue2
  }
}

Describe the solution you'd like
The lets block or any other block should not be evaluated if the condition is false

generate_hcl 'some_file.tf' {
  condition = length(global.data.vnets) > 0 && global.someOtherCondition

  lets: {
    data   = existantValue
    data2 = existantValue2
  }
}

[i4ki]

Hi @g13013

We are aware of this issue. About your proposed solution, what if the condition depends on any let variable? Would it makes sense to only evaluate the condition and any dependent variable?

In our experience, usually the condition can get complex then being able to create temporary let variables would be good.

[mariux]

We are planning to improve the evaluation of variables, so we only evaluate needed parts.

[g13013]

@i4ki

We are aware of this issue. About your proposed solution, what if the condition depends on any let variable? Would it makes sense to only evaluate the condition and any dependent variable?

Even better, it makes a lot of sense and it would awesome to have this !

[bart-braidwell]

First, it should evaluate the condition, then if all conditions are met, it should start evaluating/generating lets variables.
In other cases, I end up with all stacks trying to generate lets for the same template, even stack is not using it.

for example, if I am using an external terraform state file as a data source inside the lets variables:

- /stacks/level_0/security/iam
	error: /terramate/templates/level_1/eks/config/eks_config.tm.hcl:8,40-109: lets eval: unknown variable namespace: data
	error: /terramate/templates/level_1/eks/config/eks_config.tm.hcl:8,111-180: lets eval: unknown variable namespace: data
	error: /terramate/templates/level_1/eks/config/eks_config.tm.hcl:9,26-95: lets eval: unknown variable namespace: data
	error: /terramate/templates/level_1/eks/config/eks_config.tm.hcl:9,97-166: lets eval: unknown variable namespace: data
- /stacks/level_1/rds
	error: /terramate/templates/level_1/eks/config/eks_config.tm.hcl:8,40-109: lets eval: unknown variable namespace: data
	error: /terramate/templates/level_1/eks/config/eks_config.tm.hcl:8,111-180: lets eval: unknown variable namespace: data
	error: /terramate/templates/level_1/eks/config/eks_config.tm.hcl:9,26-95: lets eval: unknown variable namespace: data
	error: /terramate/templates/level_1/eks/config/eks_config.tm.hcl:9,97-166: lets eval: unknown variable namespace: data