Function unimplemented_native 

pub fn unimplemented_native(name: &str, rust_path: Option<&str>) -> String

What a call to a ## To native declaration no engine implements is refused with, on every tier. A declaration naming a Rust path (is native "crate::module::function"), or a native only the Rust backends link (rust_only_native), runs only where Rust links it, as Escape to Rust: does; one naming no path and no system native is the Rust backends’ own compile-time refusal, in their words.