How do I convert a binary value to a Redis command?

Question

I want to call a Redis function with a binary value of [u8] using the redis-rs crate using the following code:

let _ : () = redis::cmd("...").arg("...").arg(data).query(&mut con)?;

However, it results in this error when data is Vec<u8>:

the trait `redis::ToRedisArgs` is not implemented for `&std::vec::Vec<u8>`

And this error when data is [u8]:

the trait `redis::ToRedisArgs` is not implemented for `[u8]`

When looking at the documentation for ToRedisArgs, there should be one implementation for Vec<T> though.

There's an implementation for either &[u8] or Vec<u8>, but you apparently tried with [u8] and &Vec<u8>, so you need to dereference or reference as appropriate. — Sven Marnach
– Sven Marnach, Commented Oct 14, 2020 at 11:30
You can also trigger a deref coercion by explicitly specifying the argument type as &[u8] when passing in a &Vec<u8>. — Sven Marnach
– Sven Marnach, Commented Oct 14, 2020 at 11:32

Shepmaster · Accepted Answer · 2020-11-04 21:02:55Z

Here is a reproducible example of your problem:

fn main() {
    let data: &std::vec::Vec<u8> = &Vec::new();
    let _ = redis::cmd("...").arg(data);
}

Compiling it results in this error:

error[E0277]: the trait bound `&std::vec::Vec<u8>: redis::ToRedisArgs` is not satisfied
 --> src/main.rs:3:33
  |
3 |   let _ = redis::cmd("...").arg(data);
  |                                 ^^^^ the trait `redis::ToRedisArgs` is not implemented for `&std::vec::Vec<u8>`
  |
  = help: the following implementations were found:
            <std::vec::Vec<T> as redis::ToRedisArgs>

As the error message states, ToRedisArgs is implemented for Vec<T>, but not &Vec<T>. To get around this, you can clone data to get a Vec<u8>:

fn main() {
    let data: &std::vec::Vec<u8> = &Vec::new();
    let _ = redis::cmd("...").arg(data.clone());
}

Or, you can extract a slice from the vector because ToRedisArgs is implemented for slices:

fn main() {
    let data: &std::vec::Vec<u8> = &Vec::new();
    let _ = redis::cmd("...").arg(data.as_slice());
}

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