Function std::intrinsics::align_offset

pub unsafe extern "rust-intrinsic" fn align_offset(
    ptr: *const (), 
    align: usize
) -> usize

🔬 This is a nightly-only experimental API. (core_intrinsics)

intrinsics are unlikely to ever be stabilized, instead they should be used through stabilized interfaces in the rest of the standard library

Computes the byte offset that needs to be applied to ptr in order to make it aligned to align. If it is not possible to align ptr, the implementation returns usize::max_value().

There are no guarantees whatsover that offsetting the pointer will not overflow or go beyond the allocation that ptr points into. It is up to the caller to ensure that the returned offset is correct in all terms other than alignment.

Examples

Accessing adjacent u8 as u16

let x = [5u8, 6u8, 7u8, 8u8, 9u8];
let ptr = &x[n] as *const u8;
let offset = align_offset(ptr as *const (), align_of::<u16>());
if offset < x.len() - n - 1 {
    let u16_ptr = ptr.offset(offset as isize) as *const u16;
    assert_ne!(*u16_ptr, 500);
} else {
    // while the pointer can be aligned via `offset`, it would point
    // outside the allocation
}