Using mem::replace(&mut _, mem::uninitialized()) or mem::replace(&mut _, mem::zeroed()) leads to undefined
behavior even if the value is overwritten later. This is because the uninitialized value might be observed in the case of a panic, which can lead to
unpredictable and dangerous consequences in your program.
Code examples
Noncompliant code example
use std::mem;
fn may_panic(v: Vec<i32>) -> Vec<i32> { v }
#[allow(deprecated, invalid_value)]
fn myfunc(v: &mut Vec<i32>) {
let taken_v = unsafe { mem::replace(v, mem::uninitialized()) }; // Noncompliant
let new_v = may_panic(taken_v); // undefined behavior on panic
mem::forget(mem::replace(v, new_v));
}
Compliant solution
use std::mem;
use take_mut::take;
fn may_panic(v: Vec<i32>) -> Vec<i32> { v }
fn myfunc(v: &mut Vec<i32>) {
let new_v = take(v, |old_v| may_panic(old_v)); // Compliant
mem::forget(mem::replace(v, new_v));
}
