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Override Box::<[T]>::clone_from #72499

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merged 3 commits into from May 30, 2020
Merged

Override Box::<[T]>::clone_from #72499

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a5734ca
Override Box::<[T]>::clone_from
mendess May 23, 2020
32eedad
Add tests
mendess May 23, 2020
dbf32e2
Remove flaky test and document the other's flakiness
mendess May 29, 2020
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8 changes: 8 additions & 0 deletions src/liballoc/boxed.rs
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Expand Up @@ -1090,6 +1090,14 @@ impl<T: Clone> Clone for Box<[T]> {
fn clone(&self) -> Self {
self.to_vec().into_boxed_slice()
}

fn clone_from(&mut self, other: &Self) {
if self.len() == other.len() {
self.clone_from_slice(&other);
} else {
*self = other.clone();
}
}
}

#[stable(feature = "box_borrow", since = "1.1.0")]
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36 changes: 36 additions & 0 deletions src/liballoc/tests/boxed.rs
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Expand Up @@ -16,3 +16,39 @@ fn unitialized_zero_size_box() {
NonNull::<MaybeUninit<String>>::dangling().as_ptr(),
);
}

#[derive(Clone, PartialEq, Eq, Debug)]
struct Dummy {
_data: u8,
}

#[test]
fn box_clone_and_clone_from_equivalence() {
for size in (0..8).map(|i| 2usize.pow(i)) {
let control = vec![Dummy { _data: 42 }; size].into_boxed_slice();
let clone = control.clone();
let mut copy = vec![Dummy { _data: 84 }; size].into_boxed_slice();
copy.clone_from(&control);
assert_eq!(control, clone);
assert_eq!(control, copy);
}
}

#[test]
fn box_clone_from_ptr_stability() {
for size in (0..8).map(|i| 2usize.pow(i)) {
let control = vec![Dummy { _data: 42 }; size].into_boxed_slice();
let mut copy = vec![Dummy { _data: 84 }; size].into_boxed_slice();
let copy_raw = copy.as_ptr() as usize;
copy.clone_from(&control);
assert_eq!(copy.as_ptr() as usize, copy_raw);
}

for size in (0..8).map(|i| 2usize.pow(i)) {
let control = vec![Dummy { _data: 42 }; size].into_boxed_slice();
let mut copy = vec![Dummy { _data: 84 }; size + 1].into_boxed_slice();
let copy_raw = copy.as_ptr() as usize;
copy.clone_from(&control);
assert_ne!(copy.as_ptr() as usize, copy_raw);
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Seems like this would be flaky. The allocator can put these at the same address if the compiler decides to drop the old one before allocating the new one.

What sort of regression is this test hoping to catch?

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Fair enough, I made this test while I was trying to write them without UB or unsafe. It doesn't make much sense now that you point that out

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The other test on the other hand might not catch a regression for the same reason 馃 but I can't think of a another way to test this. Basically it might give a false positive but never a false negative

}
}