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Update the guidance on uninitialized data with ptr::addr_of_mut #325

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24 changes: 19 additions & 5 deletions src/unchecked-uninit.md
Original file line number Diff line number Diff line change
Expand Up @@ -135,11 +135,24 @@ to compute the address of array index `idx`. This relies on
how arrays are laid out in memory.
* For a struct, however, in general we do not know how it is laid out, and we
also cannot use `&mut base_ptr.field` as that would be creating a
reference. Thus, it is currently not possible to create a raw pointer to a field
of a partially initialized struct, and also not possible to initialize a single
field of a partially initialized struct. (a
[solution to this problem](https://github.com/rust-lang/rust/issues/64490) is being
worked on).
reference. So, you must carefully use the [`addr_of_mut`] macro. This creates
a raw pointer to the field without creating an intermediate reference:

```rust
use std::{ptr, mem::MaybeUninit};

struct Demo {
field: bool,
}

let mut uninit = MaybeUninit::<Demo>::uninit();
// `&uninit.as_mut().field` would create a reference to an uninitialized `bool`,
// and thus be Undefined Behavior!
let f1_ptr = unsafe { ptr::addr_of_mut!((*uninit.as_mut_ptr()).field) };
unsafe { f1_ptr.write(true); }

let init = unsafe { uninit.assume_init() };
```

One last remark: when reading old Rust code, you might stumble upon the
deprecated `mem::uninitialized` function. That function used to be the only way
Expand All @@ -155,6 +168,7 @@ it around at all, be sure to be *really* careful.
[`MaybeUninit`]: ../core/mem/union.MaybeUninit.html
[assume_init]: ../core/mem/union.MaybeUninit.html#method.assume_init
[`ptr`]: ../core/ptr/index.html
[`addr_of_mut`]: ../core/ptr/macro.addr_of_mut.html
[`write`]: ../core/ptr/fn.write.html
[`copy`]: ../std/ptr/fn.copy.html
[`copy_nonoverlapping`]: ../std/ptr/fn.copy_nonoverlapping.html