A #[simd] attribute was applied to an empty or multi-field struct.

Erroneous code examples:

#![feature(repr_simd)]

#[repr(simd)]
struct Bad; // error!

#![feature(repr_simd)]

#[repr(simd)]
struct Bad([u32; 1], [u32; 1]); // error!

The #[simd] attribute can only be applied to a single-field struct, because the one field must be the array of values in the vector.

Fixed example:

#![feature(repr_simd)]

#[repr(simd)]
struct Good([u32; 2]); // ok!