Error code E0793

An unaligned reference 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.