rust/src/doc/trpl/intrinsics.md

% Intrinsics

> **Note**: intrinsics will forever have an unstable interface, it is
> recommended to use the stable interfaces of libcore rather than intrinsics
> directly.

These are imported as if they were FFI functions, with the special
`rust-intrinsic` ABI. For example, if one was in a freestanding
context, but wished to be able to `transmute` between types, and
perform efficient pointer arithmetic, one would import those functions
via a declaration like

```rust
#![feature(intrinsics)]
# fn main() {}

extern "rust-intrinsic" {
    fn transmute<T, U>(x: T) -> U;

    fn offset<T>(dst: *const T, offset: isize) -> *const T;
}
```

As with any other FFI functions, these are always `unsafe` to call.