A tuple struct's element isn't a machine type when using the #[simd]
attribute.
Erroneous code example:
#![feature(repr_simd)]
#[repr(simd)]
struct Bad([String; 2]); // error!
When using the #[simd] attribute on a tuple struct, the elements in the tuple
must be machine types so SIMD operations can be applied to them.
Fixed example:
#![feature(repr_simd)]
#[repr(simd)]
struct Good([u32; 4]); // ok!