Changing a moved struct

[jbornschein]

I was very surprised that this code compiles and outputs 1. When I try to print a.val after the assignment I get a compile error -- as I expected.

Assuming A has move semantics, this should IMHO never compile ... and/or, (according to my newbie mental model of move semantics) output a 2.

struct A {
    val: i32
}

fn main() {
    let mut a = A{val: 1};
    let a_ = a;

    a.val = 2;
    println!("a_: {}", a_.val);
}

[steveklabnik]

/cc @rust-lang/lang

This isn't unsafe or anything, but we may want to change that? Or have a diagnostic of some kind?

[huonw]

Local variables are semantically memory locations on the stack, which can own values. Move semantics means that something like let a_ = a; moves the value out of a (into a new independent stack slot a_), leaving the latter as an "empty" stack slot. It's possible to reinitialise the slot (i.e. put a new value into it) and continue to use it, e.g. a = A { val: 2 }; println!("{}", a.val); works perfectly OK. Rust somewhat tracks the initialisation state of structs on a per-field basis but it doesn't do this fully, as demonstrated by a.val not working, but it could in future. In other words, Rust allows writing to the individual fields of an empty stack slot, but doesn't then follow this through to allow reading the ones that have been initialised.

It would be unsafe if a.val = 2 caused a_.val to also be 2, as it would be a source of mutable sharing which is the core thing Rust tries to control/protect against.

Closing as not-a-bug, but thanks for filing!

[jbornschein]

Thanks for the explanation!