offset_of.rs

#[macro_export]
macro_rules! offset_of_unsafe {
    ($container:path, $field:ident) => {{
        // Make sure the field actually exists. This line ensures that a
        // compile-time error is generated if $field is accessed through a
        // Deref impl.
        let $container { $field : _, .. };

        // Create an instance of the container and calculate the offset to its
        // field. Although we are creating references to uninitialized data this
        // is fine since we are not dereferencing them.
        let val: $container = $crate::__core::mem::uninitialized();
        let result = &val.$field as *const _ as isize - &val as *const _ as isize;
        $crate::__core::mem::forget(val);
        result
    }};
}