Add and use InadmissibleTagContentTypeError

Currently *errors.errorString is returned when unmarshalling
built-in tags (tag 0-3 and 21-23) with inadmissible content type.

This commit adds InadmissibleTagContentTypeError and returns this
error with the same error message as before when unmarshalling
build-in tags with inadmissible content type.

common.go

@@ -139,34 +139,33 @@ func validBuiltinTag(tagNum uint64, contentHead byte) error {
 	case tagNumRFC3339Time:
 		// Tag content (date/time text string in RFC 3339 format) must be string type.
 		if t != cborTypeTextString {
-			return fmt.Errorf(
-				"cbor: tag number %d must be followed by text string, got %s",
+			return newInadmissibleTagContentTypeError(
 				tagNumRFC3339Time,
-				t.String(),
-			)
+				"text string",
+				t.String())
 		}
 		return nil
 
 	case tagNumEpochTime:
 		// Tag content (epoch date/time) must be uint, int, or float type.
 		if t != cborTypePositiveInt && t != cborTypeNegativeInt && (contentHead < 0xf9 || contentHead > 0xfb) {
-			return fmt.Errorf(
-				"cbor: tag number %d must be followed by integer or floating-point number, got %s",
+			return newInadmissibleTagContentTypeError(
 				tagNumEpochTime,
-				t.String(),
-			)
+				"integer or floating-point number",
+				t.String())
 		}
 		return nil
 
 	case tagNumUnsignedBignum, tagNumNegativeBignum:
 		// Tag content (bignum) must be byte type.
 		if t != cborTypeByteString {
-			return fmt.Errorf(
-				"cbor: tag number %d or %d must be followed by byte string, got %s",
-				tagNumUnsignedBignum,
-				tagNumNegativeBignum,
-				t.String(),
-			)
+			return newInadmissibleTagContentTypeErrorf(
+				fmt.Sprintf(
+					"tag number %d or %d must be followed by byte string, got %s",
+					tagNumUnsignedBignum,
+					tagNumNegativeBignum,
+					t.String(),
+				))
 		}
 		return nil
 

decode.go

@@ -254,6 +254,45 @@ func (e *ByteStringExpectedFormatError) Unwrap() error {
 	return e.err
 }
 
+// InadmissibleTagContentTypeError is returned when unmarshaling built-in CBOR tags
+// fails because of inadmissible type for tag content. Currently, the built-in
+// CBOR tags in this codec are tags 0-3 and 21-23.
+// See "Tag validity" in RFC 8949 Section 5.3.2.
+type InadmissibleTagContentTypeError struct {
+	s                      string
+	tagNum                 int
+	expectedTagContentType string
+	gotTagContentType      string
+}
+
+func newInadmissibleTagContentTypeError(
+	tagNum int,
+	expectedTagContentType string,
+	gotTagContentType string,
+) *InadmissibleTagContentTypeError {
+	return &InadmissibleTagContentTypeError{
+		tagNum:                 tagNum,
+		expectedTagContentType: expectedTagContentType,
+		gotTagContentType:      gotTagContentType,
+	}
+}
+
+func newInadmissibleTagContentTypeErrorf(s string) *InadmissibleTagContentTypeError {
+	return &InadmissibleTagContentTypeError{s: "cbor: " + s}
+}
+
+func (e *InadmissibleTagContentTypeError) Error() string {
+	if e.s == "" {
+		return fmt.Sprintf(
+			"cbor: tag number %d must be followed by %s, got %s",
+			e.tagNum,
+			e.expectedTagContentType,
+			e.gotTagContentType,
+		)
+	}
+	return e.s
+}
+
 // DupMapKeyMode specifies how to enforce duplicate map key. Two map keys are considered duplicates if:
 //  1. When decoding into a struct, both keys match the same struct field. The keys are also
 //     considered duplicates if neither matches any field and decoding to interface{} would produce

diagnose.go

@@ -401,11 +401,10 @@ func (di *diagnose) item() error { //nolint:gocyclo
 		switch tagNum {
 		case tagNumUnsignedBignum:
 			if nt := di.d.nextCBORType(); nt != cborTypeByteString {
-				return fmt.Errorf(
-					"cbor: tag number %d must be followed by byte string, got %s",
+				return newInadmissibleTagContentTypeError(
 					tagNumUnsignedBignum,
-					nt.String(),
-				)
+					"byte string",
+					nt.String())
 			}
 
 			b, _ := di.d.parseByteString()
@@ -415,9 +414,9 @@ func (di *diagnose) item() error { //nolint:gocyclo
 
 		case tagNumNegativeBignum:
 			if nt := di.d.nextCBORType(); nt != cborTypeByteString {
-				return fmt.Errorf(
-					"cbor: tag number %d must be followed by byte string, got %s",
+				return newInadmissibleTagContentTypeError(
 					tagNumNegativeBignum,
+					"byte string",
 					nt.String(),
 				)
 			}