What Is Undefined Behavior in Rust?

Undefined behavior in Rust occurs when code violates memory safety rules, such as accessing invalid memory or data races, causing the program to crash or behave unpredictably. This typically happens in unsafe blocks where the compiler skips checks, like when multiple threads access a static mut variable without synchronization.

static mut COUNTER: u32 = 0;

/// SAFETY: Calling this from more than a single thread at a time is undefined
/// behavior, so you *must* guarantee you only call it from a single thread at
/// a time.
unsafe fn add_to_count(inc: u32) {
    unsafe {
        COUNTER += inc;
    }
}

In this example, calling add_to_count from multiple threads simultaneously creates a data race, which is undefined behavior.

Undefined behavior is when your code breaks the rules of how memory should be used, leading to crashes or random results. It usually happens when you manually manage memory or share data between threads without proper safety checks. Think of it like driving a car without a seatbelt: if everything goes right, you're fine, but one small mistake can cause a catastrophic accident.