Add test ensuring that rust keywords can be message names

PiperOrigin-RevId: 561057538

rust/test/reserved.proto

@@ -34,4 +34,7 @@ package naming;
 
 message Reserved {
   optional int32 for = 1; // for is a reserved word in rust, let's make sure we can compile it
+
+  message Inner {}
+  optional Inner pub = 2;  // pub is reserved too!
 }

rust/test/shared/reserved_test.rs

@@ -37,3 +37,9 @@ fn test_reserved_keyword_in_accessors() {
     let res = msg.r#for();
     assert_eq!(res, 0);
 }
+
+#[test]
+fn test_reserved_keyword_in_messages() {
+    let msg = Reserved::new();
+    let _ = msg.r#pub();
+}