E0793 - Error codes index
Description: Book listing all Rust error codes

# Error code E0793

An unaligned references to a field of a packed struct got created.

Erroneous code example:

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

    unsafe {
        let foo = Foo { field1: 0, field2: 0 };
        // Accessing the field directly is fine.
        let val = foo.field1;
        // A reference to a packed field causes a error.
        let val = &foo.field1; // ERROR
        // An implicit `&` is added in format strings, causing the same error.
        println!("{}", foo.field1); // ERROR
    }
    }

Creating a reference to an insufficiently aligned packed field is undefined behavior and therefore disallowed. Using an `unsafe` block does not change anything about this. Instead, the code should do a copy of the data in the packed field or use raw pointers and unaligned accesses.

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

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

        // Instead of a reference, we can create a raw pointer...
        let ptr = std::ptr::addr_of!(foo.field1);
        // ... and then (crucially!) access it in an explicitly unaligned way.
        let val = unsafe { ptr.read_unaligned() };
        // This would *NOT* be correct:
        // let val = unsafe { *ptr }; // Undefined Behavior due to unaligned load!

        // For formatting, we can create a copy to avoid the direct reference.
        let copy = foo.field1;
        println!("{}", copy);
        // Creating a copy can be written in a single line with curly braces.
        // (This is equivalent to the two lines above.)
        println!("{}", { foo.field1 });
    }
    }

### Additional information

Note that this error is specifically about references to packed fields. Direct by-value access of those fields is fine, since then the compiler has enough information to generate the correct kind of access.

See issue #82523 for more information.