An unknown intrinsic function was declared.

Erroneous code example:

```compile_fail,E0093
#![feature(intrinsics)]
#![allow(internal_features)]

#[rustc_intrinsic]
unsafe fn foo(); // error: unrecognized intrinsic function: `foo`

fn main() {
    unsafe {
        foo();
    }
}
```

Please check you didn't make a mistake in the function's name. All intrinsic
functions are defined in `library/core/src/intrinsics` in the Rust source code.