validator: Provide validation for mismatching types

[radeksimko]

Context

One of the sources of misunderstanding and friction in the Terraform language is type (constraint) mismatch, which is in part because types were added in a later version of HCL/Terraform and - relatedly - because Terraform is still relatively permissive when comparing and converting types.

Wrong Type

resource "aws_instance" "name" {
  ami = compact(["ami-1", "ami-2"])
}

Here ami has constraint of cty.String and we're supplying a function with return type of cty.List(cty.String).

resource "aws_instance" "name" {
  ami = compact({ foo = "ami-1", bar = "ami-2" })
}

Here the compact function expects a cty.List(cty.String) but we're supplying a cty.Object().

Convertible Types (bad practice)

resource "aws_instance" "name" {
  count                       = "0"
  associate_public_ip_address = "false"
}

Here count expects cty.Number and we're passing cty.String, then associate_public_ip_address expects cty.Bool and we're passing cty.String.

In either case the value happens to be convertible to the constraint type (i.e. "0" converts to 0 and "false" converts to false). So this is accepted but does not represent good practice and users should avoid it.

Proposal

• (pre-requisite) meta: Support all Terraform expression types vscode-terraform#641
• Introduce validator to raise errors for unconvertible types
• Introduce validator to raise warnings for convertible but mismatching types