Function well_defined_copy_from 

pub unsafe fn well_defined_copy_from<const SYNC_OPT: u8, const WORST_CASE_ALIGNMENT: usize>(
    dst: *mut u8,
    src: *const u8,
    size_bytes: usize,
)

Copy size_bytes bytes from src to dst using atomic load operations to load the element from src so that the behavior of the system is always well defined, even if there is a well_defined_copy_to operation writing to the memory pointed to by src concurrent with this well_defined_copy_from operation.

well_defined_copy_from has memcpy semantics, not memmove semantics. In other words, it is illegal for src or dst to overlap in any way.

While it is not required, by default, that src and dst have any specific alignment, both src and dst must have the same alignment.

IOW: (src & 0x7) must equal (dst & 0x7)

Const Generic Args

SYNC_OPT Controls the options for memory order synchronization. See the comments on crate::common::SyncOpt for details.

WORST_CASE_ALIGNMENT An explicit guarantee of the worst case alignment that src/dst will obey. When this alignment guarantee is greater than or equal to the maximum internal transfer granularity of 64 bits, the initial explicit alignment step of the operation can be optimized away for a minor performance gain.

Safety

• src and dst must be valid for reads and writes (respectively) of size_bytes bytes, must not overlap, and must have the same alignment modulo 8.
• Both pointers must be aligned to at least WORST_CASE_ALIGNMENT.
• src must only be accessed concurrently through well-defined copy operations, and dst must not be concurrently accessed.