Rust for C-Programmers

25.12 Example: A Bug Miri Might Catch

Consider a function that returns a pointer to a local variable:

fn return_dangling_pointer() -> *const i32 {
    let x = 10;
    &x as *const i32
}

fn main() {
    let ptr = return_dangling_pointer();
    unsafe {
        // Danger: 'x' is out of scope, so dereferencing 'ptr' is undefined behavior.
        println!("Value is {}", *ptr);
    }
}

Although this code might occasionally print 10 and appear to work, it exhibits undefined behavior because x is out of scope. Tools like Miri can detect this error before it leads to more severe problems.