Polymorphic Containers

The API is subject to change until the next major release.

CompanionStack

CompanionStack is a polymorphic container that allows you to store and manipulate a collection of elements of different types in the stack. The sizes of elements are not necessarily known at compile time, and they can vary dynamically during runtime.

Examples

Instances of dynamically sized types can be allocated in the stack using CompanionStack.

• References to two differently sized/typed impl Future<Output = ()> instances can be coerced into a dyn Future<Output = ()> reference without allocating heap memory.
• Dynamically sized buffers can be stored and they can be uniformly referenced.
• Nightly-only feature: +nightly --features nightly allows RefMut<T> to be coerced into RefMut<U> if T can be coerced into U.

use pcc::CompanionStack;
use pcc::companion_stack::RefMut;
use std::time::SystemTime;

let start = SystemTime::now();

let mut dyn_stack = CompanionStack::default();

// Different `impl Future<Output = ()>` can be used in the code, and
// either of them can be referred to as `dyn Future<Output = ()>` without
// boxing them.
let mut dyn_future: RefMut<dyn Future<Output = ()>> = if start == SystemTime::now() {
    dyn_stack.push_one(|| {
        Ok::<_, ()>(async {
            println!("On time");
        })
    })
    .unwrap()
    .into()
} else {
    dyn_stack.push_one(|| {
        Ok::<_, ()>(async {
            println!("Late");
        })
    })
    .unwrap()
    .into()
};

// The `CompanionStack` instance can be retrieved, but the reference's
// lifetime is limited to the scope of the `dyn_future` variable.
let (dyn_future, dyn_stack) = dyn_future.retrieve_stack();

// The buffer is allocated on the stack.
let mut buffer_size = 1024;
let mut dyn_buffer: RefMut<[u8]> =
    dyn_stack.push_many(|_| Ok::<_, ()>(0_u8), buffer_size).unwrap();
assert_eq!(dyn_buffer.len(), 1024);

// The buffer is popped from the stack.
drop(dyn_buffer);

// Another buffer of a different size is allocated on the stack.
buffer_size *= 2;
let mut dyn_buffer: RefMut<[u8]> =
    dyn_stack.push_many(|_| Ok::<_, ()>(0_u8), buffer_size).unwrap();
assert_eq!(dyn_buffer.len(), 2048);

#[cfg(not(feature = "nightly"))]
fn nightly_example(_dyn_stack: &mut CompanionStack) {
    // `cargo +nightly build --features nightly` will enable the nightly feature.
}

#[cfg(feature = "nightly")]
fn nightly_example(dyn_stack: &mut CompanionStack) {
    let start = SystemTime::now();

    trait Len {
        fn len(&self) -> usize;
    }

    struct Data1(usize);
    impl Len for Data1 {
        fn len(&self) -> usize {
            1
        }
    }

    struct Data2(Vec<u8>);
    impl Len for Data2 {
        fn len(&self) -> usize {
            self.0.len()
        }
    }

    // `RefMut<Data1>` and `RefMut<Data2>` are coerced into `RefMut<dyn Len>`.
    let dyn_len: RefMut<dyn Len> = if start == SystemTime::now() {
        dyn_stack.push_one(|| Ok::<_, ()>(Data1(11))).unwrap()
    } else {
        dyn_stack
            .push_one(|| Ok::<_, ()>(Data2(vec![1, 2, 3, 4])))
            .unwrap()
    };
    assert!(dyn_len.len() == 1 || dyn_len.len() == 4);
}

nightly_example(dyn_buffer.retrieve_stack().1);