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Add comment about UnsafeCell in MaybeUninit #66051

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Add comment about UnsafeCell in MaybeUninit #66051

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Add comment about UnsafeCell in MaybeUninit
pitdicker Nov 3, 2019
ae91b69
Add example to UnsafeCell
pitdicker Nov 4, 2019
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2 changes: 1 addition & 1 deletion src/libcore/mem/maybe_uninit.rs
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Expand Up @@ -324,7 +324,7 @@ impl<T> MaybeUninit<T> {
/// Gets a pointer to the contained value. Reading from this pointer or turning it
/// into a reference is undefined behavior unless the `MaybeUninit<T>` is initialized.
/// Writing to memory that this pointer (non-transitively) points to is undefined behavior
/// (except inside an `UnsafeCell<T>`).
/// (except inside an `UnsafeCell<T>` when already initialized).
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I don't really understand the goal of this change. It is also not correct; the interior mutability exception is entirely orthogonal to initialization.

Could you elaborate a bit more?

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Hm, maybe this change isn't helping to make things more clear...
This is roughly what I was doing:

let value: MaybeUninit<UnsafeCell<bool>> = MaybeUninit::uninit();
let slot = (&*value.as_ptr()).get() as *mut T;
slot.write(true);

But because UnsafeCell::get takes a reference, this would be invalid, right? There would temporary be a reference to an uninitialized UnsafeCell<bool>.

So I would think the exception that you can write through MaybeUninit::as_ptr when it wraps an UnsafeCell would only be correct if it was already initialized.

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So I would think the exception that you can write through MaybeUninit::as_ptr when it wraps an UnsafeCell would only be correct if it was already initialized.

No, the exception always applies. It's just you cannot make use of it through UnsafeCell::get. When writing unsafe code, you have to get all clauses of all constructs you use satisfied. Your example is fine as far as MaybeUninit::as_ptr is concerned -- it it what you are doing later that breaks it. Under some proposed memory model, if you use transmute to turn *const UnsafeCell<bool> into *mut bool, that is allowed and then you may use ptr:.write to initialize the bool.

///
/// # Examples
///
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