Warning about Rust 2024 with the newest Nightly Compiler: this function depends on never type fallback being ()

[kamulos]

In my code I often use code that ignores the return value of operations like set:

use redis::{Commands, Connection, RedisError};

fn do_something(connection: &mut Connection) -> Result<(), RedisError> {
    connection.set("test", "test_data")?;
    Ok(())
}

Now with the newest nightly compiler I get the following warning:

warning: this function depends on never type fallback being `()`
 --> src/main.rs:3:1
  |
3 | pub fn do_something(connection: &mut Connection) -> Result<(), RedisError> {
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = 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/main.rs:4:16
  |
4 |     connection.set("test", "test_data")?;
  |                ^^^
  = note: `#[warn(dependency_on_unit_never_type_fallback)]` on by default

Now I don't entirely understand what's going on here, but this warning seems to be fixable for me like this:

- connection.set("test", "test_data")?;
+ let _: () = connection.set("test", "test_data")?;

This is just not very convenient nor pretty. Am I doing something wrong? Is there any way the redis crate can mute this warning?

[nihohit]

I'm looking into this, but ATM I'm not sure how to solve this on the crate side.

[kamulos]

Hmm, just tried it again. I think this could basically be the following situation:

#![feature(never_type)]

trait Constructible {
    fn construct() -> Self;
}

impl Constructible for () {
    fn construct() -> Self {
        ()
    }
}

// impl Constructible for ! {
//     fn construct() -> Self {
//         loop {}
//     }
// }

fn generic_return<T: Constructible>() -> Result<T, ()> {
    Ok(T::construct())
}

fn main() -> Result<(), ()> {
    generic_return()?;
    Ok(())
}

If I enable the implementation for !, the warning goes away. Unfortunately I still don't understand it...

[nihohit]

Unfortunately I still don't understand it...

https://discord.com/channels/273534239310479360/1253055949598363760

join the club 😅, I received explanations and I still don't fully grasp it.

[kamulos]

I don't think I have access via your link. Do I need to join a specific channel before I can read that?

What I also noticed in my example is as soon as I replace the ? by .unwrap() it stops working entirely without an explicit specification of the generic return type.

fn main() -> Result<(), ()> {
    generic_return().unwrap();
    Ok(())
}

To me it feels like these two should be interchangeable without such different type inference.

[nihohit]

I don't think I have access via your link

You need to join the Rust Programming Language server - https://discord.com/invite/rust-lang-community

To me it feels like these two should be interchangeable without such different type inference.

No, because ? desugars to a match, see rust-lang/rust#51125 (comment)