It won't necessarily crash immediately. Inside unsafe blocks you can invoke undefined behavior by overwriting arbitrary memory locations. The effects of reading that modified memory (including crashes) can become visible much later, long after the unsafe block.

Rust undefined behavior can only happen within "unsafe" blocks.

You can read more about what things the programmer has to manually avoid while writing code in "unsafe" code blocks: https://doc.rust-lang.org/reference/behavior-considered-unde...

EDIT: this means that if you stick to safe rust, the type system ensures you cannot have data races (among other things)

I do mean the unsafe blocks.