command/diff: Small additional context about deposed objects

In the very unusual situation where we end up planning to destroy a
deposed object alone, it's likely that we're exposing users to this idea
of "deposed" for the very first time.

This additional sentence will hopefully give some extra context for what
that means. We don't really have room here for a lengthy explanation about
how deposed objects come to exist but this will hopefully be enough of
a hook to help users connect this with an error they saw on a previous
run, or at least to have some additional keywords to search for if they
want to research further.

backend/local/backend_plan_test.go

@@ -427,6 +427,7 @@ Terraform will perform the following actions:
     }
 
   # test_instance.foo (deposed object 00000000) will be destroyed
+  # (left over from a partially-failed replacement of this instance)
   - resource "test_instance" "foo" {
       - ami = "bar" -> null
 

command/format/diff.go

@@ -67,6 +67,10 @@ func ResourceChange(
 		}
 	case plans.Delete:
 		buf.WriteString(color.Color(fmt.Sprintf("[bold]  # %s[reset] will be [bold][red]destroyed", dispAddr)))
+		if change.DeposedKey != states.NotDeposed {
+			// Some extra context about this unusual situation.
+			buf.WriteString(color.Color(fmt.Sprint("\n  # (left over from a partially-failed replacement of this instance)")))
+		}
 	default:
 		// should never happen, since the above is exhaustive
 		buf.WriteString(fmt.Sprintf("%s has an action the plan renderer doesn't support (this is a bug)", dispAddr))

command/format/diff_test.go

@@ -8,6 +8,7 @@ import (
 	"github.com/hashicorp/terraform/addrs"
 	"github.com/hashicorp/terraform/configs/configschema"
 	"github.com/hashicorp/terraform/plans"
+	"github.com/hashicorp/terraform/states"
 	"github.com/mitchellh/colorstring"
 	"github.com/zclconf/go-cty/cty"
 )
@@ -88,6 +89,27 @@ func TestResourceChange_primitiveTypes(t *testing.T) {
   - resource "test_instance" "example" {
       - id = "i-02ae66f368e8518a9" -> null
     }
+`,
+		},
+		"deletion of deposed object": {
+			Action:     plans.Delete,
+			Mode:       addrs.ManagedResourceMode,
+			DeposedKey: states.DeposedKey("byebye"),
+			Before: cty.ObjectVal(map[string]cty.Value{
+				"id": cty.StringVal("i-02ae66f368e8518a9"),
+			}),
+			After: cty.NullVal(cty.EmptyObject),
+			Schema: &configschema.Block{
+				Attributes: map[string]*configschema.Attribute{
+					"id": {Type: cty.String, Computed: true},
+				},
+			},
+			RequiredReplace: cty.NewPathSet(),
+			ExpectedOutput: `  # test_instance.example (deposed object byebye) will be destroyed
+  # (left over from a partially-failed replacement of this instance)
+  - resource "test_instance" "example" {
+      - id = "i-02ae66f368e8518a9" -> null
+    }
 `,
 		},
 		"deletion (empty string)": {
@@ -4081,6 +4103,7 @@ type testCase struct {
 	Action          plans.Action
 	ActionReason    plans.ResourceInstanceChangeActionReason
 	Mode            addrs.ResourceMode
+	DeposedKey      states.DeposedKey
 	Before          cty.Value
 	BeforeValMarks  []cty.PathValueMarks
 	AfterValMarks   []cty.PathValueMarks
@@ -4127,6 +4150,7 @@ func runTestCases(t *testing.T, testCases map[string]testCase) {
 					Type: "test_instance",
 					Name: "example",
 				}.Instance(addrs.NoKey).Absolute(addrs.RootModuleInstance),
+				DeposedKey: tc.DeposedKey,
 				ProviderAddr: addrs.AbsProviderConfig{
 					Provider: addrs.NewDefaultProvider("test"),
 					Module:   addrs.RootModule,
@@ -4143,9 +4167,8 @@ func runTestCases(t *testing.T, testCases map[string]testCase) {
 			}
 
 			output := ResourceChange(change, tc.Schema, color)
-			if output != tc.ExpectedOutput {
-				t.Errorf("Unexpected diff.\ngot:\n%s\nwant:\n%s\n", output, tc.ExpectedOutput)
-				t.Errorf("%s", cmp.Diff(output, tc.ExpectedOutput))
+			if diff := cmp.Diff(output, tc.ExpectedOutput); diff != "" {
+				t.Errorf("wrong output\n%s", diff)
 			}
 		})
 	}