Make assignments to undeclared variables an error

[wata727]

Fixes #1877
See also https://github.com/hashicorp/terraform/blob/v1.5.7/internal/backend/unparsed_value.go

Previously assignments to undeclared variables was not an error, but this PR makes it an error:

// Before
$ tflint --var foo=1
// No errors

// After
$ tflint --var foo=1
Failed to parse variables; <value for var.foo>:1,1-1: Value for undeclared variable; A variable named "foo" was assigned, but the root module does not declare a variable of that name.:

Error: Value for undeclared variable

  on <value for var.foo> line 1:
  (source code not available)

A variable named "foo" was assigned, but the root module does not declare a variable of that name.

In most cases, the previous behavior will lead you to overlook misconfigurations, so this change will help you find problems sooner.

Note that the environment variables TF_VAR_* are not subject to this check. This is the same behavior as Terraform.

[dhoppe]

@wata727 Is it possible to make this behavior optional?

I use Terragrunt and execute TFLint via a before_hook.

terraform {
  before_hook "tflint" {
    commands = ["apply", "plan", "validate"]
    execute  = ["tflint", "--terragrunt-external-tflint"]
  }
}

A merge function will expand a variable if it exists.

inputs = merge(
  {
    tags = {
      Owner       = "dhoppeIT"
      Contact     = "Dennis Hoppe"
      Project     = "Atlassian"
      Environment = local.env_name
    }
  }
)

In case a variable does not exist, TFLint will complain that it could not parse the variable.

❯ terragrunt validate
INFO[0000] Terragrunt Cache server is listening on 127.0.0.1:63878
INFO[0000] Start Terragrunt Cache server
INFO[0002] Executing hook: tflint                        prefix=[/Users/dhoppe/Code/dhoppeIT/terragrunt/terragrunt-aws-atlassian/stacks/dev/eu-west-1/aws-data]
Plugin "aws" is already installed
Plugin "terraform" is already installed
Failed to parse variables; <value for var.tags>:1,1-1: Value for undeclared variable; A variable named "tags" was assigned, but the root module does not declare a variable of that name.:

Error: Value for undeclared variable

  on <value for var.tags> line 1:
  (source code not available)

A variable named "tags" was assigned, but the root module does not declare a variable of that name.

ERRO[0002] Error running hook tflint with message: Error while running tflint with args: [tflint --config /Users/dhoppe/Code/dhoppeIT/terragrunt/terragrunt-aws-atlassian/.tflint.hcl --chdir /Users/dhoppe/Code/dhoppeIT/terragrunt/terragrunt-aws-atlassian/stacks/dev/eu-west-1/aws-data/.terragrunt-cache/JHGvRr6O57V65qPwdIrZEaHB9dg/Lb2YuBCTFbsGXgB18I374bFq0XU --var=tags={"Contact":"Dennis Hoppe","Environment":"dev","Owner":"dhoppeIT","Project":"Atlassian"} --var=env_name=dev]  prefix=[/Users/dhoppe/Code/dhoppeIT/terragrunt/terragrunt-aws-atlassian/stacks/dev/eu-west-1/aws-data]
ERRO[0002] Errors encountered running before_hooks. Not running 'terraform'.  prefix=[/Users/dhoppe/Code/dhoppeIT/terragrunt/terragrunt-aws-atlassian/stacks/dev/eu-west-1/aws-data]
INFO[0002] Shutting down Terragrunt Cache server...
INFO[0002] Terragrunt Cache server stopped
ERRO[0003] 1 error occurred:
    * Error while running tflint with args: [tflint --config /Users/dhoppe/Code/dhoppeIT/terragrunt/terragrunt-aws-atlassian/.tflint.hcl --chdir /Users/dhoppe/Code/dhoppeIT/terragrunt/terragrunt-aws-atlassian/stacks/dev/eu-west-1/aws-data/.terragrunt-cache/JHGvRr6O57V65qPwdIrZEaHB9dg/Lb2YuBCTFbsGXgB18I374bFq0XU --var=tags={"Contact":"Dennis Hoppe","Environment":"dev","Owner":"dhoppeIT","Project":"Atlassian"} --var=env_name=dev]

ERRO[0003] Unable to determine underlying exit code, so Terragrunt will exit with error code 1

Edit: Never mind, it is working just fine when I remove the argument --terragrunt-external-tflint.

[velkovb]

@dhoppe it worked because Terragrunt had an older version of the TFLint library. Now it is the same.

This is rather a regression than a feature for and would be great if it can be made optional like a rule.