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Jun 27, 2023
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Remove obsolete note about soundness hole in type-layout.md #1367

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Remove note about soundness hole in type-layout.md
yjhn Jun 14, 2023
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yjhn Jun 15, 2023
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yjhn Jun 15, 2023
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30 changes: 22 additions & 8 deletions src/type-layout.md
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Expand Up @@ -549,13 +549,28 @@ The `align` modifier can also be applied on an `enum`.
When it is, the effect on the `enum`'s alignment is the same as if the `enum`
was wrapped in a newtype `struct` with the same `align` modifier.

<div class="warning">

***Warning:*** Dereferencing an unaligned pointer is [undefined behavior] and
it is possible to [safely create unaligned pointers to `packed` fields][27060].
Like all ways to create undefined behavior in safe Rust, this is a bug.

</div>
> Note: References to unaligned fields are not allowed because it is [undefined behavior].
> When fields are unaligned due to an alignment modifier, consider the following options for using references and dereferences:
>
> ```rust
> #[repr(packed)]
> struct Packed {
> f1: u8,
> f2: u16,
> }
> let mut e = Packed { f1: 1, f2: 2 };
> // Instead of creating a reference to a field, copy the value to a local variable.
> let x = e.f2;
> // Or in situations like `println!` which creates a reference, use braces
> // to change it to a copy of the value.
> println!("{}", {e.f2});
> // Or if you need a pointer, use the unaligned methods for reading and writing
> // instead of dereferencing the pointer directly.
> let ptr: *const u16 = std::ptr::addr_of!(e.f2);
> let value = unsafe { ptr.read_unaligned() };
> let mut_ptr: *mut u16 = std::ptr::addr_of_mut!(e.f2);
> unsafe { mut_ptr.write_unaligned(3) }
> ```

### The `transparent` Representation

Expand Down Expand Up @@ -587,7 +602,6 @@ used with any other representation.
[enumerations]: items/enumerations.md
[zero-variant enums]: items/enumerations.md#zero-variant-enums
[undefined behavior]: behavior-considered-undefined.md
[27060]: https://github.com/rust-lang/rust/issues/27060
[55149]: https://github.com/rust-lang/rust/issues/55149
[`PhantomData<T>`]: special-types-and-traits.md#phantomdatat
[Default]: #the-default-representation
Expand Down