# Error code E0795

Invalid argument for the `offset_of!` macro.

Erroneous code example:

    #![allow(unused)]
    #![feature(offset_of, offset_of_enum)]

    fn main() {
    let x = std::mem::offset_of!(Option<u8>, Some);
    }

The `offset_of!` macro gives the offset of a field within a type. It can navigate through enum variants, but the final component of its second argument must be a field and not a variant.

The offset of the contained `u8` in the `Option<u8>` can be found by specifying the field name `0`:

    #![allow(unused)]
    #![feature(offset_of, offset_of_enum)]

    fn main() {
    let x: usize = std::mem::offset_of!(Option<u8>, Some.0);
    }

The discriminant of an enumeration may be read with `core::mem::discriminant`, but this is not always a value physically present within the enum.

Further information about enum layout may be found at https://rust-lang.github.io/unsafe-code-guidelines/layout/enums.html.