ipv4.rs: Potentially incomplete implementation of Result in `impl TryFrom<Ipv4Addr> for Mask {}`

[Guy Turcotte]

Hello,

I am implementing an instantiation of Wifi using specific IP address, subnet mask and gateway. The following code is close to be complete but I can't retrieve the error result from the try_from call, I can only use the unwrap() call (see line with // <======= comment):

pub struct Wireless {
    running: bool,
    wifi: Box<EspWifi>,
}

impl Wireless {
    pub fn new(cfg: Config) -> Result<Wireless, anyhow::Error> {
      let netif_stack = Arc::new(EspNetifStack::new()?);
      let sys_loop_stack = Arc::new(EspSysLoopStack::new()?);
      let default_nvs = Arc::new(EspDefaultNvs::new()?);

      let ssid = cfg.ssid.clone();
      let psw = cfg.wifi_password.clone();

      let mut wireless = Wireless {
        running: false,
        wifi: Box::new(EspWifi::new(netif_stack, sys_loop_stack, default_nvs)?),
      };

      let ap_infos = wireless.wifi.scan()?;

      let ours = ap_infos.into_iter().find(|a| a.ssid == ssid);

      let channel = if let Some(ours) = ours {
        info!(
          "Found configured access point {} on channel {}",
          ssid, ours.channel
        );
        Some(ours.channel)
      } else {
        info!(
          "Configured access point {} not found during scanning, will go with unknown channel",
          ssid
        );
        None
      };

      wireless
        .wifi
        .set_configuration(&Configuration::Client(ClientConfiguration {
          ssid: ssid.clone(),
          password: psw.clone(),
          ip_conf: Some(
            if cfg.ip.is_empty() { ipv4::ClientConfiguration::DHCP(Default::default()) }
            else { ipv4::ClientConfiguration::Fixed(ipv4::ClientSettings {
              ip: cfg.ip.parse::<Ipv4Addr>()?,
              subnet: ipv4::Subnet {
                gateway: cfg.gateway.parse::<Ipv4Addr>()?,
                mask: ipv4::Mask::try_from(cfg.subnet_mask.parse::<Ipv4Addr>()?).unwrap(),  // <==========
              },
              dns: Some(cfg.dns.parse::<Ipv4Addr>()?),
              ..Default::default()
            }) }
          ),
          ..Default::default()
        }))?;

I can't replace the .unwrap() call with ? as I receive the following message from the analyser:

[Result]()<[Mask](), ()>
Go to Result | Mask

error: the trait bound `(): StdError` is not satisfied
label: the trait `StdError` is not implemented for `()`

note: required because of the requirements on the impl of `From<()>` for `anyhow::Error`
label: the trait `StdError` is not implemented for `()`

note: required because of the requirements on the impl of `FromResidual<Result<Infallible, ()>>` for `Result<Wireless, anyhow::Error>`
label: the trait `StdError` is not implemented for `()`

the trait bound `(): StdError` is not satisfied
required because of the requirements on the impl of `From<()>` for `anyhow::Error`
required because of the requirements on the impl of `FromResidual<Result<Infallible, ()>>` for `Result<Wireless, anyhow::Error>`rustc[E0277](https://doc.rust-lang.org/error-index.html#E0277

Thanks for your help!!
Guy

[ivmarkov]

That's because the implementation of TryFrom returns an error of type () which is not compatible with the STD Error trait, and as such the conversion of it to anyhow::Error(via ?) fails.

Now - I can implement a custom Error type which also implements the STD Error trait (if STD is enabled) and then use it in the TryFrom trait, but that's a lot of work and might be an overkill for a conversion - which if it fails - cannot return anything meaningful besides "I failed". :)

Regardless, and in the meantime - a quick remedy would be something like:

mask: ipv4::Mask::try_from(cfg.subnet_mask.parse::<Ipv4Addr>()?).map_err(|_| anyhow::anyhow!("Invalid mask"))?

Let me know if that works for you.

UPDATE: Or even

mask: ipv4::Mask::try_from(cfg.subnet_mask.parse::<Ipv4Addr>()?).map_err(|_| anyhow::anyhow!("Invalid mask {}", cfg.subnet_mask))?

[Guy Turcotte]

Thanks a lot! I've learned something today!! It worked fine!

Cheers!
Guy

[ivmarkov]

Closing for now, if more folks complain, I may implement a better compat with STD.