An unknown intrinsic function was declared.

Erroneous code example:

#![feature(intrinsics)]
#![allow(internal_features)]

extern "rust-intrinsic" {
    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 compiler/rustc_codegen_llvm/src/intrinsic.rs and in library/core/src/intrinsics.rs in the Rust source code. Example:

#![feature(intrinsics)]
#![allow(internal_features)]

extern "rust-intrinsic" {
    fn atomic_fence_seqcst(); // ok!
}

fn main() {
    unsafe {
        atomic_fence_seqcst();
    }
}