In Rust, pointers are not simply an "integer" or "address". For instance, a "use after free" is undefined behavior even if you "get lucky" and the freed memory gets reallocated before your read/write. As another example, writing through a pointer derived from an &i32 reference is undefined behavior, even if writing to the same address via a different pointer is legal. The underlying pattern here is that the way a pointer is computed matters, not just the address that results from this computation. For this reason, we say that pointers have provenance: to fully characterize pointer-related undefined behavior in Rust, we have to know not only the address the pointer points to, but also track which other pointer(s) it is "derived from". Most of the time, programmers do not need to worry much about provenance, and it is very clear how a pointer got derived. However, when casting pointers to integers and back, the provenance of the resulting pointer is underspecified. With this release, Rust is adding a set of APIs that can in many cases replace the use of integer-pointer-casts, and therefore avoid the ambiguities inherent to such casts. In particular, the pattern of using the lowest bits of an aligned pointer to store extra information can now be implemented without ever casting a pointer to an integer or back. This makes the code easier to reason about, easier to analyze for the compiler, and also benefits tools like Miri and architectures like CHERI that aim to detect and diagnose pointer misuse.