Pointers Are Complicated, or: What's in a Byte?

[Tom-Phinney]

@kornel

Buut maybe the language definition can be stretched a bit. If you make usize 128 bit (I think Rust still keeps that possibility open by forbidding u64::from(usize) ) but keep offsets/indexes de-facto limited to 64-bit (even though index is still usize , it has to be <= u64::max_value() ). This way casts to and from 128-bit integers will be lossless.

Making usize 128-bit for purpose of UB checking will also catch tons of programs that do ptr as u64 as *const (which is another kind of error, right? )

Rust will need to support usize == u128 anyway, since one of the defined base instruction sets for RISC-V, RV128I, provides 128-bit addressing. LLVM does not yet support that option, which presumably is why Rust's recent 128-bit support in 1.26.0 did not include usize == u128. Here (Edit 2) is a list of files where u128 support in 1.26.0 was inadequate to cover usize == u128.