GODRIVER-3425 Reference ObjectIdAsHexString decoder option in excepti…

…on message. (mongodb#1896)

bson/decoder_test.go

@@ -647,7 +647,8 @@ func TestDecoderConfiguration(t *testing.T) {
 
 		var got objectIDTest
 		err := dec.Decode(&got)
-		assert.EqualError(t, err, "error decoding key id: decoding an object ID to a non-hexadecimal string representation is not supported")
+		const want = "error decoding key id: decoding an object ID into a string is not supported by default (set Decoder.ObjectIDAsHexString to enable decoding as a hexadecimal string)"
+		assert.EqualError(t, err, want)
 	})
 	t.Run("DefaultDocumentM top-level", func(t *testing.T) {
 		t.Parallel()

bson/string_codec.go

@@ -7,6 +7,7 @@
 package bson
 
 import (
+	"errors"
 	"fmt"
 	"reflect"
 )
@@ -50,14 +51,16 @@ func (sc *stringCodec) decodeType(dc DecodeContext, vr ValueReader, t reflect.Ty
 			return emptyValue, err
 		}
 	case TypeObjectID:
-		oid, err := vr.ReadObjectID()
-		if err != nil {
-			return emptyValue, err
-		}
 		if dc.objectIDAsHexString {
+			oid, err := vr.ReadObjectID()
+			if err != nil {
+				return emptyValue, err
+			}
 			str = oid.Hex()
 		} else {
-			return emptyValue, fmt.Errorf("decoding an object ID to a non-hexadecimal string representation is not supported")
+			const msg = "decoding an object ID into a string is not supported by default " +
+				"(set Decoder.ObjectIDAsHexString to enable decoding as a hexadecimal string)"
+			return emptyValue, errors.New(msg)
 		}
 	case TypeSymbol:
 		str, err = vr.ReadSymbol()