helper/plugin: Discard meaningless differences from provider planning

Due to various inprecisions in the old SDK implementation, applying the
generated diff can potentially make changes to the data structure that
have no real effect, such as replacing an empty list with a null list or
vice-versa.

Although we can't totally eliminate such diff noise, here we attempt to
avoid it in situations where there are _only_ meaningless changes -- where
the prior state and planned state are equivalent -- by just echoing back
the prior state verbatim to ensure that Terraform will treat it as a noop
change.

If there _are_ some legitimate changes then the result may still contain
meaningless changes alongside it, but that is just a cosmetic problem for
the diff renderer, because the meaningless changes will be ignored
altogether during a subsequent apply anyway. The primary goal here is just
to ensure we can converge on a fixpoint when there are no explicit changes
in the configuration.

helper/plugin/grpc_provider.go

@@ -564,6 +564,20 @@ func (s *GRPCProviderServer) PlanResourceChange(_ context.Context, req *proto.Pl
 
 	plannedStateVal = copyTimeoutValues(plannedStateVal, proposedNewStateVal)
 
+	// The old SDK code has some inprecisions that cause it to sometimes
+	// generate differences that the SDK itself does not consider significant
+	// but Terraform Core would. To avoid producing weird do-nothing diffs
+	// in that case, we'll check if the provider as produced something we
+	// think is "equivalent" to the prior state and just return the prior state
+	// itself if so, thus ensuring that Terraform Core will treat this as
+	// a no-op. See the docs for ValuesSDKEquivalent for some caveats on its
+	// accuracy.
+	forceNoChanges := false
+	if hcl2shim.ValuesSDKEquivalent(priorStateVal, plannedStateVal) {
+		plannedStateVal = priorStateVal
+		forceNoChanges = true
+	}
+
 	plannedMP, err := msgpack.Marshal(plannedStateVal, block.ImpliedType())
 	if err != nil {
 		resp.Diagnostics = convert.AppendProtoDiag(resp.Diagnostics, err)
@@ -600,9 +614,11 @@ func (s *GRPCProviderServer) PlanResourceChange(_ context.Context, req *proto.Pl
 	// collect the attributes that require instance replacement, and convert
 	// them to cty.Paths.
 	var requiresNew []string
-	for attr, d := range diff.Attributes {
-		if d.RequiresNew {
-			requiresNew = append(requiresNew, attr)
+	if !forceNoChanges {
+		for attr, d := range diff.Attributes {
+			if d.RequiresNew {
+				requiresNew = append(requiresNew, attr)
+			}
 		}
 	}