[marcianx]

Yes, it's completely safe and its rust equivalent is

    let x = vec![1, 2, 3];
    process(std::mem::replace(&mut x, Vec::new()));
    x.push(4);

https://doc.rust-lang.org/std/mem/fn.replace.html

[khuey]

There's no guarantee that std::move moves elements out of x. (and thus no guarantee that it's equivalent to a Rust mem::replace with a new Vec) The only requirement the C++ standard imposes is that the vector is left in a "valid but unspecified state".