Access provider configuration within a validateFunc

Terraform Providers Plugin Development
6

I recently found an limitation with the CustomizeDiff approach in that if i Get a field which is interpolated its empty, if you look at the simplistic example:

resource "foo" "bar" {
  configuration = <<EOF
  {
     "a": "foo",
     "b": "bar
  }
  EOF
}

resource "bar" "foo" {
  configuration = <<EOF
  {
     "a": "foo",
     "b": "${foo.bar.id}"
  }
  EOF
}

Resource foo.bar can be validated, bar.foo cannot. I can’t see any methods on *schema.ResourceDiff which could get me the un-interpolated values (i’m only doing key presence validation). Is it possible to handle this sort of use case?