It would be possible to work around that by changing `#![cfg_attr(not(feature = "std"), no_std)]` to
```
#![no_std]
#[cfg(feature = "std")]
extern crate std;
```
That avoids importing the prelude, and therefore the warning. But it also requires manual imports of several symbols (conditional on `#[cfg(feature = "std")]`), so it's not looking nice.