substr(var.small_string, -4, -1) crashes if the string is not long enough

[marcosdiez]

Terraform Version

Terraform v0.11.1
+ provider.aws v1.6.0

Terraform Configuration Files

variable "small_string" {
  default = "mmm"
}

# Here we have a basic string handling error
# substr will crash because it can't get the last 4 charcters of a 3 caracter string

resource "aws_security_group" "whatever" {
  name        = "this_will_crash"
  description = "This will crash because var.small_string has less than 4 characters ${ substr(var.small_string, -4, -1) }"
}

Debug Output

https://gist.github.com/marcosdiez/e3fed4bf49274706be3c656b03cc0498

Crash Output

https://gist.github.com/marcosdiez/c648c31911b1e5a3155fba4ddb4598ba

Expected Behavior

terraform validate (and terraform plan and terraform apply should complain that string ( var.small_string ) is too small and therefore it can't proceed

Actual Behavior

it crashes

Steps to Reproduce

Additional Context

The issue is quite simple: substr should check if the string has the appropriate size.

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