They’re not functionally equivalent for one reason: `panic` unwinding. Rust can catch panics, and if you catch a panic while calling the latter, you will effect

They’re not functionally equivalent for one reason: `panic` unwinding. Rust can catch panics, and if you catch a panic while calling the latter, you will effectively drop `T`. Otherwise, if you have a temporary value (e.g. `Default`), you can convert the former to the latter via `std::mem::replace` or `std::mem::take` (which replaces with default) fn f1(x: &mut T) { let y = std::mem::take(x); *x = f2(y); } and you can always convert the latter to the former fn f2(mut x: T) -> T { f1(&mut x); return x; } See also the `replace_with` crate: https://docs.rs/replace_with https://docs.rs/replace_with