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use std::ops::{Deref, DerefMut};
use std::ptr;
#[repr(u8)]
enum Flag<T> {
Alive(T),
Dropped(u8),
}
pub struct NoDrop<T>(Flag<T>);
impl<T> NoDrop<T> {
#[inline]
pub fn new(value: T) -> NoDrop<T> {
NoDrop(Flag::Alive(value))
}
}
impl<T> Drop for NoDrop<T> {
fn drop(&mut self) {
unsafe {
ptr::write(&mut self.0, Flag::Dropped(0));
}
}
}
impl<T> Deref for NoDrop<T> {
type Target = T;
#[inline]
fn deref(&self) -> &T {
match self.0 {
Flag::Alive(ref inner) => inner,
_ => unsafe { debug_assert_unreachable() },
}
}
}
impl<T> DerefMut for NoDrop<T> {
#[inline]
fn deref_mut(&mut self) -> &mut T {
match self.0 {
Flag::Alive(ref mut inner) => inner,
_ => unsafe { debug_assert_unreachable() },
}
}
}
#[cfg(test)]
#[test]
fn test_no_nonnullable_opt() {
use std::mem;
assert!(mem::size_of::<Flag<&i32>>() > mem::size_of::<&i32>());
assert!(mem::size_of::<Flag<Vec<i32>>>() > mem::size_of::<Vec<i32>>());
}
#[inline]
unsafe fn debug_assert_unreachable() -> ! {
debug_assert!(false, "Reached unreachable section: this is a bug!");
enum Void {}
match *(1 as *const Void) {}
}
#[cfg(test)]
mod tests {
use super::NoDrop;
#[test]
fn test_drop() {
use std::cell::Cell;
let flag = &Cell::new(0);
struct Bump<'a>(&'a Cell<i32>);
impl<'a> Drop for Bump<'a> {
fn drop(&mut self) {
let n = self.0.get();
self.0.set(n + 1);
}
}
{
let _ = NoDrop::new([Bump(flag), Bump(flag)]);
}
assert_eq!(flag.get(), 0);
flag.set(0);
{
let mut array = NoDrop::new(Vec::new());
array.push(vec![Bump(flag)]);
array.push(vec![Bump(flag), Bump(flag)]);
array.push(vec![]);
array.push(vec![Bump(flag)]);
drop(array.pop());
assert_eq!(flag.get(), 1);
drop(array.pop());
assert_eq!(flag.get(), 1);
drop(array.pop());
assert_eq!(flag.get(), 3);
}
assert_eq!(flag.get(), 3);
}
}