#[repr(simd)] struct(isize, isize) not allowed

[gnzlbg]

Repro (playground: https://play.rust-lang.org/?gist=efa6e5caf752ccca651014ae5009feb8&version=nightly&mode=debug&edition=2015):

#![feature(repr_simd)] 

#[repr(simd)] struct A(isize, isize); // ERROR

#[cfg(target_pointer_width = "64")]
type isize_ = i64;

#[cfg(target_pointer_width = "32")]
type isize_ = i32;

#[repr(simd)] struct B(isize_, isize_); // Q: Is this ok?

produces

error[E0077]: SIMD vector element type should be machine type
 --> src/main.rs:3:15
  |
3 | #[repr(simd)] struct A(isize, isize);
  |               ^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

I am wondering why aren't isize and usize allowed ? I currently work around this to implement simd vectors of pointers, but maybe that's broken for reason I don't understand?

[eaglgenes101]

I think this can be made a thing (size type variables are just differences in memory addresses under the hood, after all), but the better question is if it should. In particular, what are the use cases of performing bulk arithmetic operations on structs of size types?

[gnzlbg]

what are the use cases of performing bulk arithmetic operations on size type structs?

The same as for normal pointers:

// vector of pointers
#[repr(simd)] struct x2<T>(T, T);
type const_ptr_x2 = x2<*const T>;
let a = const_ptr_x2(...);
let b = const_ptr_x2(...);

let c: ??? = a - b;  /// ??? should be a vector of isize

let d: ??? = ???(...); // wrapping_add takes a vector of usize
let e = a.wrapping_add(d);

[Centril]

Closing in favor of rust-lang/rust#55078.