Allow moving instances to new resources

If a state mv target happens to be a resource that doesn't exist, allow
the creation of the new resource inferring the EachMode from the target
address.

command/state_mv.go

@@ -278,7 +278,9 @@ func (c *StateMvCommand) Run(args []string) int {
 			c.Ui.Output(fmt.Sprintf("%s %q to %q", prefix, addrFrom.String(), args[1]))
 			if !dryRun {
 				fromResourceAddr := addrFrom.ContainingResource()
-				fromProviderAddr := ssFrom.Resource(fromResourceAddr).ProviderConfig
+				fromResource := ssFrom.Resource(fromResourceAddr)
+				fromProviderAddr := fromResource.ProviderConfig
+				fromEachMode := fromResource.EachMode
 				ssFrom.ForgetResourceInstanceAll(addrFrom)
 				ssFrom.RemoveResourceIfEmpty(fromResourceAddr)
 
@@ -287,22 +289,17 @@ func (c *StateMvCommand) Run(args []string) int {
 					// If we're moving to an address without an index then that
 					// suggests the user's intent is to establish both the
 					// resource and the instance at the same time (since the
-					// address covers both), but if there's an index in the
-					// target then the resource must already exist.
+					// address covers both). If there's an index in the
+					// target then allow creating the new instance here,
+					// inferring the mode from how the new address was parsed.
 					if addrTo.Resource.Key != addrs.NoKey {
-						diags = diags.Append(tfdiags.Sourceless(
-							tfdiags.Error,
-							msgInvalidTarget,
-							fmt.Sprintf("Cannot move to %s: %s does not exist in the current state.", addrTo, addrTo.ContainingResource()),
-						))
-						c.showDiagnostics(diags)
-						return 1
+						fromEachMode = eachModeForInstanceKey(addrTo.Resource.Key)
 					}
 
 					resourceAddr := addrTo.ContainingResource()
 					stateTo.SyncWrapper().SetResourceMeta(
 						resourceAddr,
-						states.NoEach,
+						fromEachMode,
 						fromProviderAddr, // in this case, we bring the provider along as if we were moving the whole resource
 					)
 					rs = stateTo.Resource(resourceAddr)
@@ -358,6 +355,20 @@ func (c *StateMvCommand) Run(args []string) int {
 	return 0
 }
 
+func eachModeForInstanceKey(key addrs.InstanceKey) states.EachMode {
+	switch key.(type) {
+	case addrs.IntKey:
+		return states.EachList
+	case addrs.StringKey:
+		return states.EachMap
+	default:
+		if key == addrs.NoKey {
+			return states.NoEach
+		}
+		panic(fmt.Sprintf("don't know an each mode for instance key %#v", key))
+	}
+}
+
 // sourceObjectAddrs takes a single source object address and expands it to
 // potentially multiple objects that need to be handled within it.
 //

command/state_mv_test.go

@@ -237,6 +237,74 @@ test_instance.foo.0:
 `)
 }
 
+func TestStateMv_instanceToNewResource(t *testing.T) {
+	state := states.BuildState(func(s *states.SyncState) {
+		s.SetResourceInstanceCurrent(
+			addrs.Resource{
+				Mode: addrs.ManagedResourceMode,
+				Type: "test_instance",
+				Name: "foo",
+			}.Instance(addrs.IntKey(0)).Absolute(addrs.RootModuleInstance),
+			&states.ResourceInstanceObjectSrc{
+				AttrsJSON: []byte(`{"id":"bar","foo":"value","bar":"value"}`),
+				Status:    states.ObjectReady,
+			},
+			addrs.ProviderConfig{Type: "test"}.Absolute(addrs.RootModuleInstance),
+		)
+	})
+	statePath := testStateFile(t, state)
+
+	p := testProvider()
+	ui := new(cli.MockUi)
+	c := &StateMvCommand{
+		StateMeta{
+			Meta: Meta{
+				testingOverrides: metaOverridesForProvider(p),
+				Ui:               ui,
+			},
+		},
+	}
+
+	args := []string{
+		"-state", statePath,
+		"test_instance.foo[0]",
+		"test_instance.bar[\"new\"]",
+	}
+	if code := c.Run(args); code != 0 {
+		t.Fatalf("bad: %d\n\n%s", code, ui.ErrorWriter.String())
+	}
+
+	// Test it is correct
+	testStateOutput(t, statePath, `
+test_instance.bar["new"]:
+  ID = bar
+  provider = provider.test
+  bar = value
+  foo = value
+`)
+
+	// now move the instance to a new resource in a new module
+	args = []string{
+		"-state", statePath,
+		"test_instance.bar[\"new\"]",
+		"module.test.test_instance.baz[\"new\"]",
+	}
+	if code := c.Run(args); code != 0 {
+		t.Fatalf("bad: %d\n\n%s", code, ui.ErrorWriter.String())
+	}
+
+	// Test it is correct
+	testStateOutput(t, statePath, `
+<no state>
+module.test:
+  test_instance.baz["new"]:
+    ID = bar
+    provider = provider.test
+    bar = value
+    foo = value
+`)
+}
+
 func TestStateMv_differentResourceTypes(t *testing.T) {
 	state := states.BuildState(func(s *states.SyncState) {
 		s.SetResourceInstanceCurrent(