Stack-allocated vector with capacity in bytes?

help
1

Is there a library containing the necessary unsafe code for something like this hypothetical ByteArrayVec, which owns an inline, fixed-length byte buffer and provides a container for a variable number of Ts?

struct ByteArrayVec<T, const CAP_BYTES: usize> {
    len: usize,
    storage: [MaybeUninit<u8>; CAP_BYTES],
    _phantom: PhantomData<T>,
}
impl ByteArrayVec {
    fn try_push(&mut self, value: T) -> Result<(), T> {...}
    fn as_slice(&self) -> &[T] {...}
    // ...
}

There are plenty of ArrayVec implementations, but all the ones I recall seeing take a capacity in elements, not bytes. For the application I have in mind, which is a buffer for batching items, I want to specify the capacity in bytes, and let the vector fit as many as possible. This way, the capacity parameter can be specified in terms of “reasonable amount of stack memory for this function to use” instead of in terms of the size of T.

(This would of course be trivial with const generic expressions — just compute CAP_BYTES / size_of::<T>() as the array length — but I want something that works in stable Rust.)

2

Something like this? crates.io: Rust Package Registry

3

fixed-slice-vec might work.

4

No, smallbytes only stores u8, not an arbitrary type T.

Thank you, that will do. I note that it requires that the buffer be borrowed rather than owned, which is not ideal but will do for the specific application I have in mind. (There’s a second place where a similar buffer needs to be owned, but it’s less bad in that case to require a capacity in elements.)