Rust error E0511: invalid monomorphization of an intrinsic function was used

The compiler team's explanation

Invalid monomorphization of an intrinsic function was used.

Erroneous code example:

#![feature(intrinsics)]

#[rustc_intrinsic]
unsafe fn simd_add<T>(a: T, b: T) -> T;

fn main() {
    unsafe { simd_add(0, 1); }
    // error: invalid monomorphization of `simd_add` intrinsic
}

The generic type has to be a SIMD type. Example:

#![feature(repr_simd)]
#![feature(intrinsics)]

#[repr(simd)]
#[derive(Copy, Clone)]
struct i32x2([i32; 2]);

#[rustc_intrinsic]
unsafe fn simd_add<T>(a: T, b: T) -> T;

unsafe { simd_add(i32x2([0, 0]), i32x2([1, 2])); } // ok!

From rustc --explain E0511 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.