helper/schema: Disallow validation+diff suppression on computed-only fields

helper/schema/schema.go

@@ -645,6 +645,19 @@ func (m schemaMap) InternalValidate(topSchemaMap schemaMap) error {
 			}
 		}
 
+		// Computed-only field
+		if v.Computed && !v.Optional {
+			if v.ValidateFunc != nil {
+				return fmt.Errorf("%s: ValidateFunc is for validating user input, "+
+					"there's nothing to validate on computed-only field", k)
+			}
+			if v.DiffSuppressFunc != nil {
+				return fmt.Errorf("%s: DiffSuppressFunc is for suppressing differences"+
+					" between config and state representation. "+
+					"There is no config for computed-only field, nothing to compare.", k)
+			}
+		}
+
 		if v.ValidateFunc != nil {
 			switch v.Type {
 			case TypeList, TypeSet:

helper/schema/schema_test.go

@@ -3325,16 +3325,46 @@ func TestSchemaMap_InternalValidate(t *testing.T) {
 			},
 			true,
 		},
+
+		"computed-only field with validateFunc": {
+			map[string]*Schema{
+				"string": &Schema{
+					Type:     TypeString,
+					Computed: true,
+					ValidateFunc: func(v interface{}, k string) (ws []string, es []error) {
+						es = append(es, fmt.Errorf("this is not fine"))
+						return
+					},
+				},
+			},
+			true,
+		},
+
+		"computed-only field with diffSuppressFunc": {
+			map[string]*Schema{
+				"string": &Schema{
+					Type:     TypeString,
+					Computed: true,
+					DiffSuppressFunc: func(k, old, new string, d *ResourceData) bool {
+						// Always suppress any diff
+						return false
+					},
+				},
+			},
+			true,
+		},
 	}
 
 	for tn, tc := range cases {
+		t.Run(tn, func(t *testing.T) {
 			err := schemaMap(tc.In).InternalValidate(nil)
 			if err != nil != tc.Err {
 				if tc.Err {
 					t.Fatalf("%q: Expected error did not occur:\n\n%#v", tn, tc.In)
 				}
-			t.Fatalf("%q: Unexpected error occurred:\n\n%#v", tn, tc.In)
+				t.Fatalf("%q: Unexpected error occurred: %s\n\n%#v", tn, err, tc.In)
 			}
+		})
 	}
 
 }