data "aws_iam_policy_document" "allow_describe_regions" {
  statement {
    effect = "Allow"
    actions = ["ec2:DescribeRegions"]
    resources = ["*"]
  }
}

resource "aws_iam_policy" "example" {
  name = "example"
  policy = data.aws_iam_policy_document.allow_describe_regions.json
}

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": ["ec2:DescribeRegions"],
            "Resource": ["*"]
        }
    ]
}

...
resource "aws_iam_policy" "example" {
  name = "example"
  policy = file("./policy.json")
}

------------------------------------------------------------------------

No changes. Infrastructure is up-to-date.

This means that Terraform did not detect any differences between your
configuration and real physical resources that exist. As a result, no
actions need to be performed.

resource "aws_iam_role" "default" {
  name = var.name
  assume_role_policy = data.aws_iam_policy_document.assume_role.json
}

...

resource "aws_iam_policy" "default" {
  name = var.name
  policy = var.policy
}

resource "aws_iam_role_policy_attachment" "default" {
  role = aws_iam_role.default.name
  policy_arn = aws_iam_policy.default.arn
}