What is safe and unsafe really?

“You are allowed to do this, but you had better know what you are doing.”

- A Rustacean

When we talk about safety in programming languages, it is a property that spans different levels. A language can be memory-safe, type-safe, or it can be concurrent-safe. Memory safety means that a program doesn't write to a forbidden memory address and it doesn't access invalid memory. Type safety means that a program doesn't allow you to assign a number to a string variable and that this check happens at compile time, while concurrent-safe means that the program does not lead to race conditions when multiple threads are executing and modifying a shared state. If a language provides all of these levels of safety by itself, then it is said to be safe. To put it more generally, a program is deemed safe if, in all possible executions...