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use core::mem::{self, MaybeUninit};
/// An array of at most `N` elements.
struct ArrayBuilder<T, const N: usize> {
/// The (possibly uninitialized) elements of the `ArrayBuilder`.
///
/// # Safety
///
/// The elements of `arr[..len]` are valid `T`s.
arr: [MaybeUninit<T>; N],
/// The number of leading elements of `arr` that are valid `T`s, len <= N.
len: usize,
}
impl<T, const N: usize> ArrayBuilder<T, N> {
/// Initializes a new, empty `ArrayBuilder`.
pub fn new() -> Self {
// SAFETY: The safety invariant of `arr` trivially holds for `len = 0`.
Self {
arr: [(); N].map(|_| MaybeUninit::uninit()),
len: 0,
}
}
/// Pushes `value` onto the end of the array.
///
/// # Panics
///
/// This panics if `self.len >= N`.
#[inline(always)]
pub fn push(&mut self, value: T) {
// PANICS: This will panic if `self.len >= N`.
let place = &mut self.arr[self.len];
// SAFETY: The safety invariant of `self.arr` applies to elements at
// indices `0..self.len` — not to the element at `self.len`. Writing to
// the element at index `self.len` therefore does not violate the safety
// invariant of `self.arr`. Even if this line panics, we have not
// created any intermediate invalid state.
*place = MaybeUninit::new(value);
// Lemma: `self.len < N`. By invariant, `self.len <= N`. Above, we index
// into `self.arr`, which has size `N`, at index `self.len`. If `self.len == N`
// at that point, that index would be out-of-bounds, and the index
// operation would panic. Thus, `self.len != N`, and since `self.len <= N`,
// that means that `self.len < N`.
//
// PANICS: Since `self.len < N`, and since `N <= usize::MAX`,
// `self.len + 1 <= usize::MAX`, and so `self.len += 1` will not
// overflow. Overflow is the only panic condition of `+=`.
//
// SAFETY:
// - We are required to uphold the invariant that `self.len <= N`.
// Since, by the preceding lemma, `self.len < N` at this point in the
// code, `self.len += 1` results in `self.len <= N`.
// - We are required to uphold the invariant that `self.arr[..self.len]`
// are valid instances of `T`. Since this invariant already held when
// this method was called, and since we only increment `self.len`
// by 1 here, we only need to prove that the element at
// `self.arr[self.len]` (using the value of `self.len` before incrementing)
// is valid. Above, we construct `place` to point to `self.arr[self.len]`,
// and then initialize `*place` to `MaybeUninit::new(value)`, which is
// a valid `T` by construction.
self.len += 1;
}
/// Consumes the elements in the `ArrayBuilder` and returns them as an array
/// `[T; N]`.
///
/// If `self.len() < N`, this returns `None`.
pub fn take(&mut self) -> Option<[T; N]> {
if self.len == N {
// SAFETY: Decreasing the value of `self.len` cannot violate the
// safety invariant on `self.arr`.
self.len = 0;
// SAFETY: Since `self.len` is 0, `self.arr` may safely contain
// uninitialized elements.
let arr = mem::replace(&mut self.arr, [(); N].map(|_| MaybeUninit::uninit()));
Some(arr.map(|v| {
// SAFETY: We know that all elements of `arr` are valid because
// we checked that `len == N`.
unsafe { v.assume_init() }
}))
} else {
None
}
}
}
impl<T, const N: usize> AsMut<[T]> for ArrayBuilder<T, N> {
fn as_mut(&mut self) -> &mut [T] {
let valid = &mut self.arr[..self.len];
// SAFETY: By invariant on `self.arr`, the elements of `self.arr` at
// indices `0..self.len` are in a valid state. Since `valid` references
// only these elements, the safety precondition of
// `slice_assume_init_mut` is satisfied.
unsafe { slice_assume_init_mut(valid) }
}
}
impl<T, const N: usize> Drop for ArrayBuilder<T, N> {
// We provide a non-trivial `Drop` impl, because the trivial impl would be a
// no-op; `MaybeUninit<T>` has no innate awareness of its own validity, and
// so it can only forget its contents. By leveraging the safety invariant of
// `self.arr`, we do know which elements of `self.arr` are valid, and can
// selectively run their destructors.
fn drop(&mut self) {
// SAFETY:
// - by invariant on `&mut [T]`, `self.as_mut()` is:
// - valid for reads and writes
// - properly aligned
// - non-null
// - the dropped `T` are valid for dropping; they do not have any
// additional library invariants that we've violated
// - no other pointers to `valid` exist (since we're in the context of
// `drop`)
unsafe { core::ptr::drop_in_place(self.as_mut()) }
}
}
/// Assuming all the elements are initialized, get a mutable slice to them.
///
/// # Safety
///
/// The caller guarantees that the elements `T` referenced by `slice` are in a
/// valid state.
unsafe fn slice_assume_init_mut<T>(slice: &mut [MaybeUninit<T>]) -> &mut [T] {
// SAFETY: Casting `&mut [MaybeUninit<T>]` to `&mut [T]` is sound, because
// `MaybeUninit<T>` is guaranteed to have the same size, alignment and ABI
// as `T`, and because the caller has guaranteed that `slice` is in the
// valid state.
unsafe { &mut *(slice as *mut [MaybeUninit<T>] as *mut [T]) }
}
/// Equivalent to `it.next_array()`.
pub(crate) fn next_array<I, const N: usize>(it: &mut I) -> Option<[I::Item; N]>
where
I: Iterator,
{
let mut builder = ArrayBuilder::new();
for _ in 0..N {
builder.push(it.next()?);
}
builder.take()
}
#[cfg(test)]
mod test {
use super::ArrayBuilder;
#[test]
fn zero_len_take() {
let mut builder = ArrayBuilder::<(), 0>::new();
let taken = builder.take();
assert_eq!(taken, Some([(); 0]));
}
#[test]
#[should_panic]
fn zero_len_push() {
let mut builder = ArrayBuilder::<(), 0>::new();
builder.push(());
}
#[test]
fn push_4() {
let mut builder = ArrayBuilder::<(), 4>::new();
assert_eq!(builder.take(), None);
builder.push(());
assert_eq!(builder.take(), None);
builder.push(());
assert_eq!(builder.take(), None);
builder.push(());
assert_eq!(builder.take(), None);
builder.push(());
assert_eq!(builder.take(), Some([(); 4]));
}
#[test]
fn tracked_drop() {
use std::panic::{catch_unwind, AssertUnwindSafe};
use std::sync::atomic::{AtomicU16, Ordering};
static DROPPED: AtomicU16 = AtomicU16::new(0);
#[derive(Debug, PartialEq)]
struct TrackedDrop;
impl Drop for TrackedDrop {
fn drop(&mut self) {
DROPPED.fetch_add(1, Ordering::Relaxed);
}
}
{
let builder = ArrayBuilder::<TrackedDrop, 0>::new();
assert_eq!(DROPPED.load(Ordering::Relaxed), 0);
drop(builder);
assert_eq!(DROPPED.load(Ordering::Relaxed), 0);
}
{
let mut builder = ArrayBuilder::<TrackedDrop, 2>::new();
builder.push(TrackedDrop);
assert_eq!(builder.take(), None);
assert_eq!(DROPPED.load(Ordering::Relaxed), 0);
drop(builder);
assert_eq!(DROPPED.swap(0, Ordering::Relaxed), 1);
}
{
let mut builder = ArrayBuilder::<TrackedDrop, 2>::new();
builder.push(TrackedDrop);
builder.push(TrackedDrop);
assert!(matches!(builder.take(), Some(_)));
assert_eq!(DROPPED.swap(0, Ordering::Relaxed), 2);
drop(builder);
assert_eq!(DROPPED.load(Ordering::Relaxed), 0);
}
{
let mut builder = ArrayBuilder::<TrackedDrop, 2>::new();
builder.push(TrackedDrop);
builder.push(TrackedDrop);
assert!(catch_unwind(AssertUnwindSafe(|| {
builder.push(TrackedDrop);
}))
.is_err());
assert_eq!(DROPPED.load(Ordering::Relaxed), 1);
drop(builder);
assert_eq!(DROPPED.swap(0, Ordering::Relaxed), 3);
}
{
let mut builder = ArrayBuilder::<TrackedDrop, 2>::new();
builder.push(TrackedDrop);
builder.push(TrackedDrop);
assert!(catch_unwind(AssertUnwindSafe(|| {
builder.push(TrackedDrop);
}))
.is_err());
assert_eq!(DROPPED.load(Ordering::Relaxed), 1);
assert!(matches!(builder.take(), Some(_)));
assert_eq!(DROPPED.load(Ordering::Relaxed), 3);
builder.push(TrackedDrop);
builder.push(TrackedDrop);
assert!(matches!(builder.take(), Some(_)));
assert_eq!(DROPPED.swap(0, Ordering::Relaxed), 5);
}
}
}