Improve UnknownFieldError message

Signed-off-by: Tim Barry <tim.barry.business+github@gmail.com>

Author: tim-barry

decode.go

@@ -207,11 +207,13 @@ func (e *DupMapKeyError) Error() string {
 
 // UnknownFieldError describes detected unknown field in CBOR map when decoding to Go struct.
 type UnknownFieldError struct {
-	Index int
+	Struct string // name of the struct being decoded
+	Field  string // field of the CBOR map that was unexpected
+	Index  int    // index of the field in the CBOR map
 }
 
 func (e *UnknownFieldError) Error() string {
-	return fmt.Sprintf("cbor: found unknown field at map element index %d", e.Index)
+	return fmt.Sprintf("cbor: found unknown field: struct '%s' has no field '%s', at map element index %d", e.Struct, e.Field, e.Index)
 }
 
 // UnacceptableDataItemError is returned when unmarshaling a CBOR input that contains a data item
@@ -2617,8 +2619,8 @@ MapEntryLoop:
 		var k any
 
 		t := d.nextCBORType()
+		var keyBytes []byte
 		if t == cborTypeTextString || (t == cborTypeByteString && d.dm.fieldNameByteString == FieldNameByteStringAllowed) {
-			var keyBytes []byte
 			if t == cborTypeTextString {
 				keyBytes, lastErr = d.parseTextString()
 				if lastErr != nil {
@@ -2767,7 +2769,7 @@ MapEntryLoop:
 
 		if f == nil {
 			if errOnUnknownField {
-				err = &UnknownFieldError{j}
+				err = &UnknownFieldError{Struct: tInfo.typ.String(), Field: string(keyBytes), Index: j}
 				d.skip() // Skip value
 				j++
 				// skip the rest of the map

decode_test.go

@@ -6825,7 +6825,7 @@ func TestExtraErrorCondUnknownField(t *testing.T) {
 			name:         "CBOR map unknown field with ExtraDecErrorUnknownField",
 			data:         hexDecode("a461416161614261626143616361446164"), // map[string]string{"A": "a", "B": "b", "C": "c", "D": "d"}
 			dm:           dmUnknownFieldError,
-			wantErrorMsg: "cbor: found unknown field at map element index 3",
+			wantErrorMsg: "cbor: found unknown field: struct 'cbor.s' has no field 'D', at map element index 3",
 		},
 	}
 	for _, tc := range testCases {
@@ -6885,7 +6885,7 @@ func TestStreamExtraErrorCondUnknownField(t *testing.T) {
 	}
 
 	data := hexDecode("a461416161614461646142616261436163a3614161616142616261436163") // map[string]string{"A": "a", "D": "d", "B": "b", "C": "c"}, map[string]string{"A": "a", "B": "b", "C": "c"}
-	wantErrorMsg := "cbor: found unknown field at map element index 1"
+	wantErrorMsg := "cbor: found unknown field: struct 'cbor.s' has no field 'D', at map element index 1"
 	wantObj := s{A: "a", B: "b", C: "c"}
 
 	dmUnknownFieldError, _ := DecOptions{ExtraReturnErrors: ExtraDecErrorUnknownField}.DecMode()