offset_of_slice

The tracking issue for this feature is: #126151

When the offset_of_slice feature is enabled, the offset_of! macro may be used to determine the offset of fields whose type is [T], that is, a slice of dynamic size.

In general, fields whose type is dynamically sized do not have statically known offsets because they do not have statically known alignments. However, [T] has the same alignment as T, so it specifically may be allowed.

#![feature(offset_of_slice)]

#[repr(C)]
pub struct Struct {
    head: u32,
    tail: [u8],
}

fn main() {
    assert_eq!(std::mem::offset_of!(Struct, tail), 4);
}