Change the logic for reserved field name verification to force a fully

qualified type name (ie starts with `.`).
This way we guarantee that the spelling the `.proto` file will be the same as
the spelling we see after crosslinking types.
Otherwise, the spellings might differ and it makes it harder to determine if
the name in the allowlist matches the name in the `.proto` file.

PiperOrigin-RevId: 497997383

src/google/protobuf/descriptor.cc

@@ -5860,9 +5860,13 @@ void DescriptorBuilder::BuildFieldOrExtension(const FieldDescriptorProto& proto,
     AddError(result->full_name(), proto, DescriptorPool::ErrorCollector::NUMBER,
              absl::Substitute(
                  "Field numbers $0 through $1 are reserved for the protocol "
-                 "buffer library implementation.",
+                 "buffer library implementation$2.",
                  FieldDescriptor::kFirstReservedNumber,
-                 FieldDescriptor::kLastReservedNumber));
+                 FieldDescriptor::kLastReservedNumber,
+                 absl::StartsWith(proto.type_name(), ".")
+                     ? ""
+                     : ", and the type name must be fully qualified with a `.` "
+                       "prefix"));
   }
 
   if (is_extension) {

src/google/protobuf/descriptor_unittest.cc

@@ -4585,21 +4585,42 @@ TEST_F(ValidationErrorTest, HugeFieldNumber) {
 
 TEST_F(ValidationErrorTest, ReservedFieldNumber) {
   BuildFileWithErrors(
-      "name: \"foo.proto\" "
-      "message_type {"
-      "  name: \"Foo\""
-      "  field {name:\"foo\" number: 18999 label:LABEL_OPTIONAL "
-      "type:TYPE_INT32 }"
-      "  field {name:\"bar\" number: 19000 label:LABEL_OPTIONAL "
-      "type:TYPE_INT32 }"
-      "  field {name:\"baz\" number: 19999 label:LABEL_OPTIONAL "
-      "type:TYPE_INT32 }"
-      "  field {name:\"moo\" number: 20000 label:LABEL_OPTIONAL "
-      "type:TYPE_INT32 }"
-      "}",
+      R"pb(
+        name: "foo.proto"
+        message_type {
+          name: "Foo"
+          field {
+            name: "foo"
+            number: 18999
+            label: LABEL_OPTIONAL
+            type: TYPE_INT32
+          }
+          field {
+            name: "bar"
+            number: 19000
+            label: LABEL_OPTIONAL
+            type: TYPE_MESSAGE
+            type_name: "Foo"
+          }
+          field {
+            name: "baz"
+            number: 19999
+            label: LABEL_OPTIONAL
+            type: TYPE_MESSAGE
+            type_name: ".Foo"
+          }
+          field {
+            name: "moo"
+            number: 20000
+            label: LABEL_OPTIONAL
+            type: TYPE_INT32
+          }
+        }
+      )pb",
 
       "foo.proto: Foo.bar: NUMBER: Field numbers 19000 through 19999 are "
-      "reserved for the protocol buffer library implementation.\n"
+      "reserved for the protocol buffer library implementation, and the type "
+      "name must be fully qualified with a `.` prefix.\n"
       "foo.proto: Foo.baz: NUMBER: Field numbers 19000 through 19999 are "
       "reserved for the protocol buffer library implementation.\n"
       "foo.proto: Foo: NUMBER: Suggested field numbers for Foo: 1, 2\n");