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Understanding Rust Error E0793

Overview of Error E0793

Rust Error E0793 occurs when an unaligned reference to a field of a packed struct is created. A packed struct is a struct that conserves space by removing any padding bytes between its fields. Accessing an aligned field of a packed struct directly is fine, but creating an unaligned reference is considered undefined behavior and disallowed in Rust.

Understanding Packed Structs

Packed structs in Rust are created using the `#[repr(packed)]` attribute. This attribute tells the compiler to optimize the memory layout by removing any padding between the fields. However, this can lead to problems when creating references to the fields, as it may violate the alignment requirements expected by the CPU.

Code Example

Here is an example of Rust code that generates Error E0793:

```rust
#![allow(unused)]
fn main() {
#[repr(packed)]
pub struct Foo {
    field1: u64,
    field2: u8,
}

unsafe {
    let foo = Foo { field1: 0, field2: 0 };
    let val = foo.field1;
    let val = &foo.field1; // ERROR
    println!("{}", foo.field1); // ERROR
}
}
```

In this code, the error occurs when creating a reference to `foo.field1`, a field of the packed struct. The `println!()` macro also causes the error because it implicitly adds a reference to its arguments.

Solutions to Error E0793

To resolve Rust Error E0793, avoid creating an unaligned reference to a packed field. Instead, either copy the data from the packed field or use raw pointers with unaligned accesses as demonstrated in this code example:

```rust
#![allow(unused)]
fn main() {
#[repr(packed)]
pub struct Foo {
    field1: u64,
    field2: u8,
}

unsafe {
    let foo = Foo { field1: 0, field2: 0 };

    let ptr = std::ptr::addr_of!(foo.field1);
    let val = unsafe { ptr.read_unaligned() };

    let copy = foo.field1;
    println!("{}", copy);
    println!("{}", { foo.field1 });
}
}
```

In this code, instead of creating an unaligned reference, a raw pointer is created to access the packed field. The pointer is then used with an explicitly unaligned access, which avoids undefined behavior and resolves the error.

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