Simple nested message fails

[Erhannis]

Consider the following proto file.

syntax = "proto3";

message Address {
  uint64 lsb_six_bytes = 1;
}

message MTest1 {
  Address address = 1;
}

message MTest2 {
  oneof val {
    Address ADDRESS = 1;
  }
}

MTest2 works, and MTest1 does not. Specifically, encoding MTest1 doesn't include address. See example project:

https://github.com/Erhannis/mpb_bug_example

It's based off the micropb basic example. MTest2 passes the assertion (and appears to actually de/serialize correctly), while MTest1 fails the assertion (and in fact serializes to 0 bytes). Assertion fail:

thread 'main' panicked at src/main.rs:35:3:
assertion `left == right` failed
  left: MTest1 { address: Address { lsb_six_bytes: 6618611909121 }, _has: _Hazzer([0]) }
 right: MTest1 { address: Address { lsb_six_bytes: 0 }, _has: _Hazzer([0]) }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[Erhannis]

Ok, I think I figured it out. In the readme you mention "Note that a field will be considered empty (and ignored by the encoder) if its bit in the hazzer is not set, even if the field itself has been written." This appears to be the case for nested messages; it works if I do either

  obj.set_address(Address {lsb_six_bytes: 0x060504030201});

or

  obj.address = Address {lsb_six_bytes: 0x060504030201};
  obj._has = obj._has.init_address();

It's not clear to me though why I need init or w/e for an Address, but not for a plain uint64.

[YuhanLiin]

The reason is because a plain uint64 is not considered an optional field, but a nested message such as Address is always optional, so the hazzer bit is required to track presence. Easiest way to set it is via obj._has.set_address().

I am considering changing this API in the future, since it's becoming a common footgun.