If you omit the `#[no_mangle]` definition then the compiler will end up emitting a mangled symbol (e.g. one with a long hash at the end that's hard to guess).
Many C APIs take a callback function pointer, which in rust has the type `extern fn()`, and this is the primary use case for an `extern` function defined in Rust which doesn't have `#[no_mangle]` on it.
Many C APIs take a callback function pointer, which in rust has the type `extern fn()`, and this is the primary use case for an `extern` function defined in Rust which doesn't have `#[no_mangle]` on it.