core: Properly handle no-op changes in plan

Previously we just left these out of the plan altogether, but in the new
plan types we intentionally include change information for every resource
instance, even if no changes are actually planned, to allow alternative
plan file viewers to show what isn't changing as well as what is.

command/format/plan.go

@@ -91,6 +91,8 @@ func NewPlan(changes *plans.Changes) *Plan {
 		}
 
 		switch rc.Action {
+		case plans.NoOp:
+			continue
 		case plans.Create:
 			if dataSource {
 				// Use "refresh" as the action for display, but core

helper/plugin/grpc_provider.go

@@ -386,6 +386,15 @@ func (s *GRPCProviderServer) PlanResourceChange(_ context.Context, req *proto.Pl
 		return resp, nil
 	}
 
+	if diff == nil {
+		// schema.Provider.Diff returns nil if it ends up making a diff with
+		// no changes, but our new interface wants us to return an actual
+		// change description that _shows_ there are no changes, so we need
+		// to synthesize one here.
+		resp.PlannedState = req.PriorState
+		return resp, nil
+	}
+
 	// now we need to apply the diff to the prior state, so get the planned state
 	plannedStateVal, err := schema.ApplyDiff(priorStateVal, diff, block)
 	if err != nil {

plugin/grpc_provider.go

@@ -6,6 +6,8 @@ import (
 	"log"
 	"sync"
 
+	"github.com/zclconf/go-cty/cty"
+
 	plugin "github.com/hashicorp/go-plugin"
 	"github.com/hashicorp/terraform/plugin/convert"
 	"github.com/hashicorp/terraform/plugin/proto"
@@ -398,14 +400,17 @@ func (p *GRPCProvider) ApplyResourceChange(r providers.ApplyResourceChangeReques
 
 	resp.Private = protoResp.Private
 
-	state, err := msgpack.Unmarshal(protoResp.NewState.Msgpack, resSchema.Block.ImpliedType())
-	if err != nil {
-		resp.Diagnostics = resp.Diagnostics.Append(err)
-		return resp
+	if protoResp.NewState != nil {
+		state, err := msgpack.Unmarshal(protoResp.NewState.Msgpack, resSchema.Block.ImpliedType())
+		if err != nil {
+			resp.Diagnostics = resp.Diagnostics.Append(err)
+			return resp
+		}
+		resp.NewState = state
+	} else {
+		resp.NewState = cty.NullVal(resSchema.Block.ImpliedType())
 	}
 
-	resp.NewState = state
-
 	return resp
 }
 

terraform/transform_diff.go

@@ -34,6 +34,8 @@ func (t *DiffTransformer) Transform(g *Graph) error {
 		// other actions.
 		var update, delete bool
 		switch rc.Action {
+		case plans.NoOp:
+			continue
 		case plans.Delete:
 			delete = true
 		case plans.Replace: