Introducing unsafe Rust

So far in this book, we've seen and used Rust language that enforces memory and type safety at compilation time and prevents various kinds of undefined behavior, such as memory overflows, null or invalid pointer constructions, and data races. This is safe Rust. In fact, the Rust Standard Library gives us good tools and utilities to write safe, idiomatic Rust, and helps to keep the program safe (and you sane!).

But in some situations, the compiler can get in the way. The Rust compiler performs static analysis of code that is conservative (meaning the Rust compiler does not mind generating a few false positives and rejecting valid code, as long as it does not let bad code get through). You, as a programmer, know that a piece of code is safe, but the compiler thinks it is risky, so it rejects this code. This includes operations such as system calls, type coercions, and direct manipulations of memory pointers, which are used in the development of several...