Allow expect_failure to assert on an error message

[bebold-jhr]

Terraform Version

Terraform v1.13.4
on darwin_arm64

Use Cases

I'm doing a lot of input validation. However I would like to be able to not only check that validation fails by using:

expect_failures = [
    var.input,
  ]

I wan to be able to pinpoint the exact cause.
The reason is that the current expect_failure does not guarantee that my test setup doesn't have a different invalid value. For complex objects with various validations this can lead to an invalid test not verifying the correct validation under test.

Attempted Solutions

I don't see a workaround.

Proposal

For a variable with complex setup and multiple validations:

variable "input" {
  type = object({
    name = string
   // anything else
  })

  validation {
    condition = var.input != null && startswith(var.input.name, " ") ])
    error_message = "Validation error in {input.name}: Must not start with a whitespace"
  }

// further validations

Maybe something like this:

expect_failures = {
    "Validation error in {input.name}: Must not start with a whitespace" = var.input
}

Where the key is the expected error_message and the value is the reference.

You could add multiple entries, but only one per message.

expect_failures = {
    "Validation error in {input.name}: Must not start with a whitespace" = var.input
    "Validation error in {input.id}: is not a valid UUID" = var.input
}

To solve that maybe something like:

expect_failures = {
    "Validation error in {input.name}: Must not start with a whitespace" = [
      var.input,
    ]
    "ID not a valid UUID" = [
      var.input,
      var.target,
    ]
}

References

Other programming languages give you the ability to check for the message of an exception.

[crw]

Thanks for this feature request! If you are viewing this issue and would like to indicate your interest, please use the 👍 reaction on the issue description to upvote this issue. We also welcome additional use case descriptions. Thanks again!