I did see the note in the README about where to add them, however, I don't know enough about macros to add in these implementations where the imports look different.

If you have time or interest, could you consider adding these please:

https://docs.rs/tokio/latest/tokio/sync/oneshot/index.html

use tokio::sync::oneshot;

#[tokio::main]
async fn main() {
    let (tx, rx) = oneshot::channel();

https://docs.rs/tokio/latest/tokio/sync/mpsc/fn.channel.html

use tokio::sync::mpsc;

#[tokio::main]
async fn main() {
    let (tx, mut rx) = mpsc::channel(100);

https://docs.rs/futures-channel/latest/futures_channel/oneshot/fn.channel.html

use futures::channel::oneshot;
use std::{thread, time::Duration};

let (sender, receiver) = oneshot::channel::<i32>();

Ones without examples:
https://docs.rs/tokio/latest/tokio/sync/mpsc/fn.unbounded_channel.html
https://docs.rs/futures-channel/latest/futures_channel/mpsc/fn.channel.html
https://docs.rs/futures-channel/latest/futures_channel/mpsc/fn.unbounded.html

Also a few questions:
Is there such a thing as spmc?
What optimizations are possible for this use case... things that spmc in particular can take advantage of.

fn slice<S>(&self, offset: usize) -> &[S] {
        unsafe {
            let ptr = self.ptr.add(offset).cast();
            slice::from_raw_parts(ptr, self.capacity) 
        }
    }

should slice::from_raw_parts(ptr, self.capacity) be updated to slice::from_raw_parts(ptr, self.capacity-offset)?
and also offset need check range, otherwise it might have buffer overflow?

This should be implemented for an Errortype