Feature request. Interpolations in terraform {} configuration block.

[DenisBY]

It would be very useful to have support of variables in terraform {} configuration block.
In my case I use one state to manage many ECS Services. They are mostly the same but with few differences like name(engine), priority, etc. and of course I have these services on different environments, like DEV or PROD.
I know that I can use empty terraform { backend “s3” {} } and then use -backend-config in command line to provide values, but It would be nice to provide values for variable only once.

Terraform Version

Terraform v0.9.2

Affected Resource(s)

terraform {backend "s3" {}}

Terraform Configuration Files

terraform {
    backend "s3" {
        bucket = "tag-terraform-states"
        key = "ecs-service/${var.env_name}/${var.engine}/${var.prio}/terraform.tfstate"
        region = "${var.region}"
    }
}

Expected Behavior

Variables are interpolated to values

Actual Behavior

terraform.backend: configuration cannot contain interpolations

Steps to Reproduce

terraform plan

[jbardin]

@DenisBY

Thanks for opening the issue. We are tracking this in a duplicate here: #13022.

[ghost]

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