plan: indicate when resource deleted due to move

Add a new ChangeReason, ReasonDeleteBecauseNoMoveTarget, to provide better
information in cases where a planned deletion is due to moving a resource to
a target not in configuration.

Consider a case in which a resource instance exists in state at address A, and
the user adds a moved block to move A to address B. Whether by the user's
intention or not, address B does not exist in configuration.
Terraform combines the move from A to B, and the lack of configuration for B,
into a single delete action for the (previously nonexistent) entity B.
Prior to this commit, the Terraform plan will report only that resource B will
be destroyed because it does not exist in configuration, with no mention of the
move.

This commit provides the user an additional clue as to what has happened, in a
case in which Terraform has elided a user's action and inaction into one
potentially destructive change.

internal/command/format/diff.go

@@ -115,6 +115,8 @@ func ResourceChange(
 		switch change.ActionReason {
 		case plans.ResourceInstanceDeleteBecauseNoResourceConfig:
 			buf.WriteString(fmt.Sprintf("\n  # (because %s is not in configuration)", addr.Resource.Resource))
+		case plans.ResourceInstanceDeleteBecauseNoMoveTarget:
+			buf.WriteString(fmt.Sprintf("\n  # (because %s was moved to %s, which is not in configuration)", change.PrevRunAddr, addr.Resource.Resource))
 		case plans.ResourceInstanceDeleteBecauseNoModule:
 			// FIXME: Ideally we'd truncate addr.Module to reflect the earliest
 			// step that doesn't exist, so it's clearer which call this refers

internal/command/jsonplan/plan.go

@@ -405,6 +405,8 @@ func (p *plan) marshalResourceChanges(resources []*plans.ResourceInstanceChangeS
 			r.ActionReason = "delete_because_each_key"
 		case plans.ResourceInstanceDeleteBecauseNoModule:
 			r.ActionReason = "delete_because_no_module"
+		case plans.ResourceInstanceDeleteBecauseNoMoveTarget:
+			r.ActionReason = "delete_because_no_move_target"
 		case plans.ResourceInstanceReadBecauseConfigUnknown:
 			r.ActionReason = "read_because_config_unknown"
 		case plans.ResourceInstanceReadBecauseDependencyPending:

internal/command/views/json/change.go

@@ -80,6 +80,7 @@ const (
 	ReasonDeleteBecauseCountIndex       ChangeReason = "delete_because_count_index"
 	ReasonDeleteBecauseEachKey          ChangeReason = "delete_because_each_key"
 	ReasonDeleteBecauseNoModule         ChangeReason = "delete_because_no_module"
+	ReasonDeleteBecauseNoMoveTarget     ChangeReason = "delete_because_no_move_target"
 	ReasonReadBecauseConfigUnknown      ChangeReason = "read_because_config_unknown"
 	ReasonReadBecauseDependencyPending  ChangeReason = "read_because_dependency_pending"
 )
@@ -108,6 +109,8 @@ func changeReason(reason plans.ResourceInstanceChangeActionReason) ChangeReason
 		return ReasonDeleteBecauseNoModule
 	case plans.ResourceInstanceReadBecauseConfigUnknown:
 		return ReasonReadBecauseConfigUnknown
+	case plans.ResourceInstanceDeleteBecauseNoMoveTarget:
+		return ReasonDeleteBecauseNoMoveTarget
 	case plans.ResourceInstanceReadBecauseDependencyPending:
 		return ReasonReadBecauseDependencyPending
 	default:

internal/plans/changes.go

@@ -427,6 +427,12 @@ const (
 	// specific reasons for a particular instance to no longer be declared.
 	ResourceInstanceDeleteBecauseNoModule ResourceInstanceChangeActionReason = 'M'
 
+	// ResourceInstanceDeleteBecauseNoMoveTarget indicates that the resource
+	// address appears as the target ("to") in a moved block, but no
+	// configuration exists for that resource. According to our move rules,
+	// this combination evaluates to a deletion of the "new" resource.
+	ResourceInstanceDeleteBecauseNoMoveTarget ResourceInstanceChangeActionReason = 'A'
+
 	// ResourceInstanceReadBecauseConfigUnknown indicates that the resource
 	// must be read during apply (rather than during planning) because its
 	// configuration contains unknown values. This reason applies only to

internal/plans/internal/planproto/planfile.proto

@@ -149,6 +149,7 @@ enum ResourceInstanceActionReason {
     REPLACE_BY_TRIGGERS = 9;
     READ_BECAUSE_CONFIG_UNKNOWN = 10;
     READ_BECAUSE_DEPENDENCY_PENDING = 11;
+    DELETE_BECAUSE_NO_MOVE_TARGET = 12;
 }
 
 message ResourceInstanceChange {

internal/plans/planfile/tfplan.go

@@ -282,6 +282,8 @@ func resourceChangeFromTfplan(rawChange *planproto.ResourceInstanceChange) (*pla
 		ret.ActionReason = plans.ResourceInstanceReadBecauseConfigUnknown
 	case planproto.ResourceInstanceActionReason_READ_BECAUSE_DEPENDENCY_PENDING:
 		ret.ActionReason = plans.ResourceInstanceReadBecauseDependencyPending
+	case planproto.ResourceInstanceActionReason_DELETE_BECAUSE_NO_MOVE_TARGET:
+		ret.ActionReason = plans.ResourceInstanceDeleteBecauseNoMoveTarget
 	default:
 		return nil, fmt.Errorf("resource has invalid action reason %s", rawChange.ActionReason)
 	}
@@ -633,6 +635,8 @@ func resourceChangeToTfplan(change *plans.ResourceInstanceChangeSrc) (*planproto
 		ret.ActionReason = planproto.ResourceInstanceActionReason_READ_BECAUSE_CONFIG_UNKNOWN
 	case plans.ResourceInstanceReadBecauseDependencyPending:
 		ret.ActionReason = planproto.ResourceInstanceActionReason_READ_BECAUSE_DEPENDENCY_PENDING
+	case plans.ResourceInstanceDeleteBecauseNoMoveTarget:
+		ret.ActionReason = planproto.ResourceInstanceActionReason_DELETE_BECAUSE_NO_MOVE_TARGET
 	default:
 		return nil, fmt.Errorf("resource %s has unsupported action reason %s", change.Addr, change.ActionReason)
 	}

internal/terraform/node_resource_plan_orphan.go

@@ -157,6 +157,13 @@ func (n *NodePlannableResourceInstanceOrphan) managedResourceExecute(ctx EvalCon
 func (n *NodePlannableResourceInstanceOrphan) deleteActionReason(ctx EvalContext) plans.ResourceInstanceChangeActionReason {
 	cfg := n.Config
 	if cfg == nil {
+		if !n.Addr.Equal(n.prevRunAddr(ctx)) {
+			// This means the resource was moved - see also
+			// ResourceInstanceChange.Moved() which calculates
+			// this the same way.
+			return plans.ResourceInstanceDeleteBecauseNoMoveTarget
+		}
+
 		return plans.ResourceInstanceDeleteBecauseNoResourceConfig
 	}