Unsafe Rust

Undefined behavior

What the compiler assumes never happens, and how to catch it when it does

What it means

Undefined behavior means the compiler is allowed to assume it never happens. A program that triggers it can print the expected answer, garbage, or crash, and change between builds

Common causes

Reading or writing memory that has been freed (use-after-free)
Reading or writing outside an allocation (buffer overflow)
Two threads touching the same memory at once with at least one write (data race)
Breaking the aliasing rules, e.g. writing through a raw pointer while a &mut to the same data is still live
Producing an invalid value, such as a bool that isn't 0 or 1

Resources

The Rust Reference: Behavior considered undefinedThe full list of what counts as undefined behavior
MiriAn interpreter that detects undefined behavior