This function depends on never type fallback being `()`

[EvilWatermelon]

What does this mean and how can I fix this warning?

warning: this function depends on never type fallback being `()`
  --> src/service/redis_service.rs:31:5
   |
31 |     pub async fn set_str<V: AsRef<[u8]>>(&self, key: &str, value: V) -> Result<(), RedisErr> {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
   = note: for more information, see issue #123748 <https://github.com/rust-lang/rust/issues/123748>
   = help: specify the types explicitly
note: in edition 2024, the requirement `!: FromRedisValue` will fail
  --> src/service/redis_service.rs:39:41
   |
39 |         let _ = &mut Self::init().await.set(key, &encoded)?;
   |                                         ^^^
   = note: `#[warn(dependency_on_unit_never_type_fallback)]` on by default

I figured out that I can fix this by adding let _: &mut () = &mut Self::init().await.set(key, &encoded)?; but this is way more confusing than before.

[SebastianJL]

Can you please post the offending code example. Its hard to help you solely based on the error message.

[EvilWatermelon]

Sure.

pub async fn set_str<V: AsRef<[u8]>>(&self, key: &str, value: V) -> Result<(), RedisErr> {

        info!("Hash new value...");

        let encoded: String = encode(&value);

        info!("Write into redis cache: {}, {}", &key, &encoded);

        let _: &mut () = &mut Self::init().await.set(key, &encoded)?;
        
        let expire = 16*16*27*25*7; // Registration URLs expire after 2 weeks
        
        if key.eq("uuids") {
            let _: &mut () = &mut Self::init().await.expire(String::from("uuids"), expire)?;
        }

        Ok(())
    }

[kpreid]

What is the signature (fn ... {) of the set() method you are calling?

[EvilWatermelon]

This redis::Commands::set()

[kpreid]

Ah so the return type is set by the type parameter RV: FromRedisValue and isn't otherwise constrained. I can reproduce the error in this self-contained program:

trait ReturnTrait {
    fn value() -> Self;
}
struct Error;
impl ReturnTrait for () {
    fn value() -> Self {}
}
// ensure we’re not using the only-one-trait-impl inference rule
impl ReturnTrait for bool {
    fn value() -> Self { false }
}

fn set<R: ReturnTrait>() -> Result<R, Error> {
    Ok(R::value())
}

async fn usage() -> Result<(), Error> {
    let _ = &mut set()?;
    Ok(())
}

The part of the warning that says that this will fail in 2024 seems to be correct. I’m not sure why ! is involved — ah, apparently something about the ? operator gets it involved: this function depends on never type fallback being `()` · Issue #126466 · rust-lang/rust · GitHub

As to your actual question, here is how to properly and concisely constrain the return type to ():

let () = Self::init().await.set(key, &encoded)?;

It can also be written as:

() = Self::init().await.set(key, &encoded)?;

but I personally don’t care for that (it’s technically an assignment to a tuple of zero variables). In either case, the &mut was doing you no good but adding verbosity.

[SkiFire13]

Another solution would be to just specify the generic type argument:

Self::init().await.set::<_, _, ()>(key, &encoded)?;