`#[serde(with = "serde_bytes")]` does not work in no_std.

[sosthene-nitrokey]

Hi!

I stumbled around some confusing behaviour with regards to no_std builds:

use serde::{Deserialize, Serialize};

#[derive(Deserialize, Serialize)]
struct Testing<'a> {
    #[serde(with = "serde_bytes")]
    testing: &'a [u8],
}

Compiles if the std or alloc feature is enabled, but doesn't compile if it isn't. I do not understand why the deserialize function is behind such a feature flag.

I understand that serde_bytes only changes the serialization of &[u8] since serde's implementation is already optimized, so you can get around the issue with

#[derive(Deserialize, Serialize)]
struct Testing<'a> {
    #[serde(serialize_with = "serde_bytes::serialize")]
    testing: &'a [u8],
}

But:

1. That is counter intuitive, since the doc's main example uses #[serde(with = "serde_bytes")]
2. It is more verbose
3. I do not understand why this should be gated behind the requirement of an allocator.
4. The error confusing since rustc can't understand the problem (though recent version do correctly point out the feature flag).

Can this feature-gate be removed?

Thanks

[sosthene-nitrokey]

Fixed by https://github.com/serde-rs/bytes/pull/47