stacks: error when removing a component still in config

internal/stacks/stackruntime/apply_test.go

@@ -3147,7 +3147,6 @@ func TestApply_RemovedBlocks(t *testing.T) {
 	)
 
 	// TODO: Add tests for and implement the following cases:
-	//   - Removed and component blocks that target the same instance.
 	//   - Validate what happens when a removed block foreach evaluates to
 	//     unknown.
 	//   - Add a test for a removed block targeting state that has already been

internal/stacks/stackruntime/internal/stackeval/removed_instance.go

@@ -76,6 +76,24 @@ func (r *RemovedInstance) ModuleTreePlan(ctx context.Context) (*plans.Plan, tfdi
 	return doOnceWithDiags(ctx, &r.moduleTreePlan, r.main, func(ctx context.Context) (*plans.Plan, tfdiags.Diagnostics) {
 		var diags tfdiags.Diagnostics
 
+		component := r.main.Stack(ctx, r.Addr().Stack, PlanPhase).Component(ctx, r.Addr().Item.Component)
+		if component != nil {
+			insts, unknown := component.Instances(ctx, PlanPhase)
+			if !unknown {
+				if _, exists := insts[r.key]; exists {
+					// The instance we're planning to remove is also targeted
+					// by a component block. We won't remove it, and we'll
+					// report a diagnostic to that effect.
+					return nil, diags.Append(&hcl.Diagnostic{
+						Severity: hcl.DiagError,
+						Summary:  "Cannot remove component instance",
+						Detail:   fmt.Sprintf("The component instance %s is targeted by a component block and cannot be removed. The relevant component is defined at %s.", r.Addr(), component.Declaration(ctx).DeclRange.ToHCL()),
+						Subject:  r.DeclRange(ctx),
+					})
+				}
+			}
+		}
+
 		known, unknown, moreDiags := EvalProviderValues(ctx, r.main, r.call.Config(ctx).config.ProviderConfigs, PlanPhase, r)
 		if moreDiags.HasErrors() {
 			// We won't actually add the diagnostics here, they should be

internal/stacks/stackruntime/plan_test.go

@@ -4437,6 +4437,114 @@ func TestPlan_RemovedBlocks(t *testing.T) {
 				},
 			},
 		},
+		"duplicate component": {
+			source: filepath.Join("with-single-input", "removed-component-instance"),
+			initialState: stackstate.NewStateBuilder().
+				AddComponentInstance(stackstate.NewComponentInstanceBuilder(mustAbsComponentInstance("component.self[\"a\"]")).
+					AddInputVariable("id", cty.StringVal("a")).
+					AddInputVariable("input", cty.StringVal("a"))).
+				AddResourceInstance(stackstate.NewResourceInstanceBuilder().
+					SetAddr(mustAbsResourceInstanceObject("component.self[\"a\"].testing_resource.data")).
+					SetProviderAddr(mustDefaultRootProvider("testing")).
+					SetResourceInstanceObjectSrc(states.ResourceInstanceObjectSrc{
+						Status: states.ObjectReady,
+						AttrsJSON: mustMarshalJSONAttrs(map[string]any{
+							"id":    "a",
+							"value": "a",
+						}),
+					})).
+				Build(),
+			store: stacks_testing_provider.NewResourceStoreBuilder().
+				AddResource("a", cty.ObjectVal(map[string]cty.Value{
+					"id":    cty.StringVal("a"),
+					"value": cty.StringVal("a"),
+				})).
+				Build(),
+			inputs: map[string]cty.Value{
+				"input": cty.SetVal([]cty.Value{
+					cty.StringVal("a"),
+				}),
+				"removed": cty.SetVal([]cty.Value{
+					cty.StringVal("a"),
+				}),
+			},
+			wantPlanChanges: []stackplan.PlannedChange{
+				&stackplan.PlannedChangeApplyable{
+					Applyable: false, // No! The removed block is a duplicate of the component!
+				},
+				&stackplan.PlannedChangeComponentInstance{
+					Addr:          mustAbsComponentInstance("component.self[\"a\"]"),
+					PlanComplete:  true,
+					PlanApplyable: false, // no changes
+					Action:        plans.Update,
+					PlannedInputValues: map[string]plans.DynamicValue{
+						"id":    mustPlanDynamicValueDynamicType(cty.StringVal("a")),
+						"input": mustPlanDynamicValueDynamicType(cty.StringVal("a")),
+					},
+					PlannedInputValueMarks: map[string][]cty.PathValueMarks{
+						"input": nil,
+						"id":    nil,
+					},
+					PlannedOutputValues: make(map[string]cty.Value),
+					PlannedCheckResults: &states.CheckResults{},
+					PlanTimestamp:       fakePlanTimestamp,
+				},
+				&stackplan.PlannedChangeResourceInstancePlanned{
+					ResourceInstanceObjectAddr: mustAbsResourceInstanceObject("component.self[\"a\"].testing_resource.data"),
+					ChangeSrc: &plans.ResourceInstanceChangeSrc{
+						Addr:        mustAbsResourceInstance("testing_resource.data"),
+						PrevRunAddr: mustAbsResourceInstance("testing_resource.data"),
+						ChangeSrc: plans.ChangeSrc{
+							Action: plans.NoOp,
+							Before: mustPlanDynamicValue(cty.ObjectVal(map[string]cty.Value{
+								"id":    cty.StringVal("a"),
+								"value": cty.StringVal("a"),
+							})),
+							After: mustPlanDynamicValue(cty.ObjectVal(map[string]cty.Value{
+								"id":    cty.StringVal("a"),
+								"value": cty.StringVal("a"),
+							})),
+						},
+						ProviderAddr: mustDefaultRootProvider("testing"),
+					},
+					PriorStateSrc: &states.ResourceInstanceObjectSrc{
+						AttrsJSON: mustMarshalJSONAttrs(map[string]any{
+							"id":    "a",
+							"value": "a",
+						}),
+						Dependencies: make([]addrs.ConfigResource, 0),
+						Status:       states.ObjectReady,
+					},
+					ProviderConfigAddr: mustDefaultRootProvider("testing"),
+					Schema:             stacks_testing_provider.TestingResourceSchema,
+				},
+				&stackplan.PlannedChangeHeader{
+					TerraformVersion: version.SemVer,
+				},
+				&stackplan.PlannedChangePlannedTimestamp{
+					PlannedTimestamp: fakePlanTimestamp,
+				},
+				&stackplan.PlannedChangeRootInputValue{
+					Addr: stackaddrs.InputVariable{Name: "input"},
+					Value: cty.SetVal([]cty.Value{
+						cty.StringVal("a"),
+					}),
+				},
+				&stackplan.PlannedChangeRootInputValue{
+					Addr: stackaddrs.InputVariable{Name: "removed"},
+					Value: cty.SetVal([]cty.Value{
+						cty.StringVal("a"),
+					}),
+				},
+			},
+			wantPlanDiags: []expectedDiagnostic{
+				{
+					severity: tfdiags.Error,
+					summary:  "Cannot remove component instance",
+					detail:   "The component instance component.self[\"a\"] is targeted by a component block and cannot be removed. The relevant component is defined at git::https://example.com/test.git//with-single-input/removed-component-instance/removed-component-instance.tfstack.hcl:18,1-17.",
+				},
+			},
+		},
 	}
 
 	for name, tc := range tcs {