Introduce resource attributes defined only on resource creation to avoid unnecessary updates in depending resources

[AlexPykavy]

Terraform Version

N/A

Use Cases

This is an attempt to overcome the known limitation that data sources are being recreated/repulled if they depends_on the resource with pending updates. Let's take a look at the following example:

resource "aws_sns_topic" "my_topic" {
  name = "my-topic"
}

resource "aws_iam_role" "sns_access_role" {
  name = "sns-access-role"

  assume_role_policy = jsonencode({
    Version = "2012-10-17"
    Statement = [
      {
        Effect    = "Allow"
        Action    = "sts:AssumeRole"
        Principal = {
          Service = "ec2.amazonaws.com"
        }
      }
    ]
  })
}

data "aws_iam_policy_document" "sns_access_policy" {
  statement {
    actions   = ["sns:Publish", "sns:Subscribe", "sns:ListSubscriptionsByTopic"]
    resources = [aws_sns_topic.my_topic.arn]
  }
}

resource "aws_iam_role_policy" "sns_access_inline_policy" {
  name = "sns-access-inline-policy"
  role = aws_iam_role.sns_access_role.name
  policy = data.aws_iam_policy_document.sns_access_policy.json
}

If I update aws_sns_topic, the aws_iam_policy_document will be repulled leading to aws_iam_role_policy updates.

Attempted Solutions

Calculate topic ARN in locals and use it in data source:

locals {
   topic_arn = "arn:aws:sns:${data.aws_region.current.name}:${data.aws_caller_identity.current.account_id}:my-topic"
}

...

data "aws_iam_policy_document" "sns_access_policy" {
  statement {
    actions   = ["sns:Publish", "sns:Subscribe", "sns:ListSubscriptionsByTopic"]
    resources = [local.topic_arn]
  }
}

Proposal

Usually resources have some attributes, such as identifiers (arn in AWS or id in Azure) that are defined on the resource creation and remain unchanged on in-place (~) modification. However, currently Terraform doesn't take it into account (at least in data sources case) and it leads to a complex Terraform plan.

References

There are many similar GitHub issues but I really don't know which one to link.