decoder: Ensure partially unknown dependent body is handled

[radeksimko]

As demonstrated by the attached test, we can have a situation of "nested dependent modules" which was previously handled incorrectly. This is best explained on an example.

variable "terraform_rs" {
  default = {
    backend              = "azurerm"
    workspace            = "***"
    storage_account_name = "***"
    container_name       = "***"
    key                  = "***"
    access_key           = "***"
  }
}

data "terraform_remote_state" "this" {
  backend   = var.terraform_rs.backend
  workspace = var.terraform_rs.workspace
  config = {
    storage_account_name = var.terraform_rs.storage_account_name
    container_name       = var.terraform_rs.container_name
    key                  = var.terraform_rs.key
    access_key           = var.terraform_rs.access_key
  }
}

Above, we have multiple dependent body schemas for the data block:

• The first one is dependent on 1st label matching terraform_remote_state - that brings us backend, workspace and other attributes generally relevant for that particular data source
• The second one is then dependent on the value of backend (i.e. attribute which was brought in as dependent from the earlier merging).

In the example above we would succeed in the first lookup, but fail in the second one - because we don't evaluate the expression in this context and cannot tell what var.terraform_rs.backend actually is. This is basically equivalent to any other invalid value, such as backend = "foobar" but the example above is a little more realistic. Now we report a more granular state reflecting the reality more accurately.

This is perhaps not ideal in the sense that although we suppress irrelevant diagnostics, we also suppress some helpful ones, e.g. invalid backend

data "terraform_remote_state" "this" {
  backend   = "foobar"
  workspace = "noot"
}

Also we could run the validation in the top example with interpolated var.terraform_rs.backend, i.e. validate config in the context of backend = "azurerm" but interpolation is whole another problem we haven't really tackled yet.

We can treat this as an opportunity for future enhancements while reducing false negatives in the meantime.

UX Before

UX After

[dbanck]

👍 Appreciate the detailed examples

[radeksimko]

I created follow-up issues as mentioned:

• Report mismatching dependent keys as diagnostics terraform-ls#1484
• Introduce attribute (interpolated) value based validation terraform-ls#1483