THRIFT-4953: Missing Field Identifiers

When identifiers are not specified, negative id will be converted to a valid
rust identifier.
diff --git a/test/Identifiers.thrift b/test/Identifiers.thrift
new file mode 100644
index 0000000..aa3e4ea
--- /dev/null
+++ b/test/Identifiers.thrift
@@ -0,0 +1,6 @@
+// THRIFT-4953
+struct NoFieldIdentifiersTest {
+  string field_without_id1,
+  string field_without_id2,
+  1: string field_with_id
+}
\ No newline at end of file