#![allow(deprecated, invalid_value)] #![warn(clippy::all)] use std::mem; fn might_panic<X>(x: X) -> X { // in practice this would be a possibly-panicky operation x } fn main() { let mut v = vec![0i32; 4]; // the following is UB if `might_panic` panics unsafe { let taken_v = mem::replace(&mut v, mem::uninitialized()); let new_v = might_panic(taken_v); std::mem::forget(mem::replace(&mut v, new_v)); } unsafe { let taken_v = mem::replace(&mut v, mem::zeroed()); let new_v = might_panic(taken_v); std::mem::forget(mem::replace(&mut v, new_v)); } // this is silly but OK, because usize is a primitive type let mut u: usize = 42; let uref = &mut u; let taken_u = unsafe { mem::replace(uref, mem::zeroed()) }; *uref = taken_u + 1; // this is still not OK, because uninit let taken_u = unsafe { mem::replace(uref, mem::uninitialized()) }; *uref = taken_u + 1; }