Don't show changes outside of Terraform for cross-resource move operations

[jbardin]

Providers may choose to implement MoveResourceState by only transferring the identifying attributes from the source resource state, leaving Terraform to fill in the remainder during the next ReadResource RPC. Functionally this can work fine for providers, and requires less work than mapping the schema-less previous state to the new resource (far less work if there are major structural changes between the resources). Reading the state in during plan however leaves Terraform with the problem of reconciling the minimal prior state against the new state for the CLI output. The changes are always collected, and we only surface them when they contribute to other changes in the plan, but they could still produce significant output for large resources because we must show the entire resource change rather than individual attributes, and that change is effectively the entire resource state.

What we could do is try and see if there is a way to disconnect the contributing changes feature from resources which have been moved between resource types. We don't want to disable the contributing changes output in all move cases however, because a simple move between configuration addresses should be completely transparent, and unexpected changes contributing to the plan would still be desired in the output.

[lantoli]

It can also happen that the old schema doesn't have all the attributes needed in the new schema, and since resource is not initialized when MoveState func is called, provider API can't be called.
So in that case, it's not even possible to fill the destination model using only info from the source.

[jbardin]

Oh yes, that was anticipated. The initial design of that feature expected providers to build as much of the new object as possible during MoveResourceState, which usually would leave a minimal amount of data to be updated. That means there was a much lower chance of it affecting the plan, and when it did, the change presented would only be showing the new data of interest to the user.

That's another thing to consider in making this change, because it would hide any new attributes which we did intend to show, though it's admittedly a small use case where that new data causes changes which can't be inferred from elsewhere.