add validation for ignore_changes references

Ensure that ignore_changes only refers to arguments set in the
configuration.

terraform/context_validate_test.go

@@ -1712,3 +1712,47 @@ output "out" {
 		}
 	}
 }
+
+func TestContext2Validate_invalidIgnoreChanges(t *testing.T) {
+	// validate module and output depends_on
+	m := testModuleInline(t, map[string]string{
+		"main.tf": `
+resource "test_instance" "a" {
+  lifecycle {
+    ignore_changes = [foo]
+  }
+}
+
+`,
+	})
+
+	p := testProvider("test")
+	p.GetSchemaReturn = &ProviderSchema{
+		ResourceTypes: map[string]*configschema.Block{
+			"test_instance": {
+				Attributes: map[string]*configschema.Attribute{
+					"id":  {Type: cty.String, Computed: true},
+					"foo": {Type: cty.String, Computed: true, Optional: true},
+				},
+			},
+		},
+	}
+
+	ctx := testContext2(t, &ContextOpts{
+		Config: m,
+		Providers: map[addrs.Provider]providers.Factory{
+			addrs.NewDefaultProvider("test"): testProviderFuncFixed(p),
+		},
+	})
+	diags := ctx.Validate()
+	if !diags.HasErrors() {
+		t.Fatal("succeeded; want errors")
+	}
+
+	for _, d := range diags {
+		des := d.Description().Summary
+		if !strings.Contains(des, "Cannot ignore") {
+			t.Fatalf(`expected "Invalid depends_on reference", got %q`, des)
+		}
+	}
+}

terraform/eval_validate.go

@@ -410,8 +410,39 @@ func (n *EvalValidateResource) Validate(ctx EvalContext) error {
 
 		if cfg.Managed != nil { // can be nil only in tests with poorly-configured mocks
 			for _, traversal := range cfg.Managed.IgnoreChanges {
+				// This will error out if the traversal contains an invalid
+				// index step. That is OK if we want users to be able to ignore
+				// a key that is no longer specified in the config.
 				moreDiags := schema.StaticValidateTraversal(traversal)
 				diags = diags.Append(moreDiags)
+				if diags.HasErrors() {
+					continue
+				}
+
+				// first check to see if this assigned in the config
+				v, _ := traversal.TraverseRel(configVal)
+				if !v.IsNull() {
+					// it's assigned, so we can also assume it's not computed-only
+					continue
+				}
+
+				// We can't ignore changes that don't exist in the configuration.
+				// We're not checking specifically if the traversal resolves to
+				// a computed-only value, but we can hint to the user that it
+				// might also be the case.
+				sourceRange := traversal.SourceRange()
+				diags = diags.Append(&hcl.Diagnostic{
+					Severity: hcl.DiagError,
+					Summary:  "Cannot ignore argument not set in the configuration",
+					Detail: fmt.Sprintf("The ignore_changes argument is not set in the configuration.\n" +
+						"The ignore_changes mechanism only applies to changes " +
+						"within the configuration, and must be used with " +
+						"arguments set in the configuration and not computed by " +
+						"the provider.",
+					),
+					Subject: &sourceRange,
+				})
+				return diags.Err()
 			}
 		}