Rust TinyPtrVec

tinyptrvec.rs

#![feature(untagged_unions)]

#[repr(C)]
struct MyVec<T> {
    data: *mut *const T,
    length: usize,
    capacity: usize,
}

union TinyPtrVec<T> {
    outline: MyVec<T>,
    inline: [*const T; 3],
    bytes: (u64, u64, u64),
}

impl<T> TinyPtrVec<T> {
    fn get(&self, index: usize) -> *const T {
        unsafe {
            let pointer_byte = self.bytes.0;
            let inline_len = pointer_byte & 0x4;
            if inline_len <= 3 { // We can use the inline vector
                if index == 0 { // Clear the low bits first
                    ((pointer_byte & !0x4) as *const T)
                } else {
                    self.inline[index]
                }
            } else { // Use the out-of-line vector
                std::slice::from_raw_parts(self.outline.data, self.outline.length)[index]
            }
        }
    }
}

fn main() {
    println!("{}", std::mem::size_of::<Vec<u8>>());
    println!("{}", std::mem::size_of::<TinyPtrVec<u8>>());
}