Code shared from the Rust Playground

playground.rs

#![feature(unsize)]

use std::alloc::Layout;
use std::marker::{PhantomData, Unsize};
use std::mem;

/// Assumes pointer-sized metadata
#[repr(C)]
struct DstPointerRepr {
    data: *mut (),
    meta: *mut (),
}

pub struct VecOfDst<DST: ?Sized> {
    storage: heterogeneous_vec::HeterogeneousVec,
    _phantom: PhantomData<DST>,
}

impl<DST: ?Sized> VecOfDst<DST> {
    pub fn new() -> Self {
        Self {
            storage: heterogeneous_vec::HeterogeneousVec::new(),
            _phantom: PhantomData,
        }
    }

    pub fn push<T: Unsize<DST>>(&mut self, value: T) {
        assert_eq!(mem::size_of::<&DST>(),
                   mem::size_of::<DstPointerRepr>());
        let repr = unsafe {
            let dst_ref: &DST = &value;
            mem::transmute_copy::<&DST, DstPointerRepr>(&dst_ref)
        };
        self.storage.push(repr.meta);
        self.storage.push(value);
    }

    pub fn iter(&self) -> impl Iterator<Item=&DST> {
        self.iter_raw().map(|ptr| unsafe { &*ptr })
    }

    pub fn iter_mut(&mut self) -> impl Iterator<Item=&mut DST> {
        self.iter_raw().map(|ptr| unsafe { &mut *ptr })
    }

    fn iter_raw<'a>(&'a self) -> impl Iterator<Item=*mut DST> + 'a {
        struct Iter<'a, DST: ?Sized> {
            iter: heterogeneous_vec::Iter<'a>,
            _phantom: PhantomData<DST>,
        }

        impl<'a, DST: ?Sized> Iterator for Iter<'a, DST> {
            type Item = *mut DST;

            fn next(&mut self) -> Option<Self::Item> {
                const POINTER_BYTES: usize = mem::size_of::<*mut ()>();
                const POINTER_BITS: usize = POINTER_BYTES * 8;
                const HYPER_ALIGNED: *mut () = (1_usize << (POINTER_BITS - 1)) as _;
                unsafe {
                    self.iter.read_next::<*mut ()>().map(|meta| {
                        let fake_repr = DstPointerRepr { data: HYPER_ALIGNED, meta };
                        let fake_ref = mem::transmute_copy::<DstPointerRepr, &DST>(&fake_repr);
                        let data = self.iter.next(Layout::for_value(fake_ref)).unwrap();
                        let repr = DstPointerRepr { data, meta };
                        mem::transmute_copy::<DstPointerRepr, *mut DST>(&repr)
                    })
                }
            }
        }

        Iter {
            iter: self.storage.iter(),
            _phantom: PhantomData,
        }
    }
}

impl<DST: ?Sized> Drop for VecOfDst<DST> {
    fn drop(&mut self) {
        for item in self.iter_raw() {
            unsafe {
                item.drop_in_place()
            }
        }
    }
}

#[test]
fn trait_objets() {
    #[repr(align(1024))]
    #[derive(Debug)]
    struct SuperAligned(u32);

    let mut v = VecOfDst::<std::fmt::Debug>::new();
    v.push(4);
    v.push(SuperAligned(7));
    v.push("foo");
    assert_eq!(debugs(v.iter()), ["4", "SuperAligned(7)", "\"foo\""]);
}

#[test]
fn slices() {
    let mut v = VecOfDst::<[u32]>::new();
    v.push([4]);
    v.push([9000, 1, 3]);
    assert_eq!(debugs(v.iter()), ["[4]", "[9000, 1, 3]"]);
}

#[cfg(test)]
fn debugs<I>(iter: I) -> Vec<String>
    where I: Iterator, I::Item: std::fmt::Debug
{
    iter.map(|x| format!("{:?}", x)).collect()
}

mod heterogeneous_vec {
    use std::alloc::{self, Layout};
    use std::mem;
    use std::num::NonZeroUsize;
    use std::ptr;

    pub struct HeterogeneousVec {
        ptr: *mut u8,
        // In bytes:
        len: usize,
        capacity: usize,
        align: NonZeroUsize,
    }

    unsafe fn realloc_with_align(ptr: *mut u8, old_layout: Layout, new_layout: Layout)
                                 -> *mut u8 {
        if new_layout.align() == old_layout.align() {
            alloc::realloc(ptr, old_layout, new_layout.size())
        } else {
            let new_ptr = alloc::alloc(new_layout);
            if !new_ptr.is_null() {
                let size = old_layout.size().min(new_layout.size());
                ptr::copy_nonoverlapping(ptr, new_ptr, size);
                alloc::dealloc(ptr, old_layout);
            }
            new_ptr
        }
    }

    fn align_to(position: usize, align: usize) -> usize {
        let remainder = position % align;
        if remainder > 0 {
            position.checked_add(align - remainder).unwrap()
        } else {
            position
        }
    }

    impl HeterogeneousVec {
        pub fn new() -> Self {
            Self {
                ptr: ptr::null_mut(),
                len: 0,
                capacity: 0,
                align: NonZeroUsize::new(1).unwrap(),
            }
        }

        pub fn push<T>(&mut self, value: T) {
            let value_size = mem::size_of::<T>();
            let value_align = mem::align_of::<T>();
            let value_position = align_to(self.len, value_align);
            let available = self.capacity.saturating_sub(value_position);
            let align = self.align.get();
            if available < value_size || align < value_align {
                let required_capacity = self.len.checked_add(value_size).unwrap();
                let new_capacity = self.capacity.max(required_capacity).next_power_of_two();
                let new_align = align.max(value_align);
                let new_layout = Layout::from_size_align(new_capacity, new_align).unwrap();
                let ptr = unsafe {
                    if self.ptr.is_null() {
                        alloc::alloc(new_layout)
                    } else {
                        let old_layout = Layout::from_size_align(self.capacity, align).unwrap();
                        realloc_with_align(self.ptr, old_layout, new_layout)
                    }
                };
                if ptr.is_null() {
                    alloc::handle_alloc_error(new_layout)
                }
                self.ptr = ptr;
                self.capacity = new_capacity;
            }
            unsafe {
                let next = self.ptr.add(value_position) as *mut T;
                debug_assert!(next as usize % value_align == 0);
                next.write(value);
            }
            self.len = value_position + value_size;
        }

        pub fn iter(&self) -> Iter {
            Iter { vec: self, position: 0 }
        }
    }

    pub struct Iter<'a> {
        vec: &'a HeterogeneousVec,
        position: usize,
    }

    impl<'a> Iter<'a> {
        pub unsafe fn read_next<T>(&mut self) -> Option<T> {
            self.next(Layout::new::<T>()).map(|ptr| (ptr as *mut T).read())
        }

        pub unsafe fn next(&mut self, layout: Layout) -> Option<*mut ()> {
            if self.position < self.vec.len {
                let value_position = align_to(self.position, layout.align());
                let ptr = self.vec.ptr.add(value_position);
                self.position = value_position + layout.size();
                debug_assert!(self.position <= self.vec.len);
                Some(ptr as *mut ())
            } else {
                None
            }
        }
    }
}