New upstream version 1.47.0+dfsg1

[rustc.git] / src / librustc_error_codes / error_codes / E0382.md

A variable was used after its contents have been moved elsewhere.

Erroneous code example:

```compile_fail,E0382
struct MyStruct { s: u32 }

fn main() {
    let mut x = MyStruct{ s: 5u32 };
    let y = x;
    x.s = 6;
    println!("{}", x.s);
}
```

Since `MyStruct` is a type that is not marked `Copy`, the data gets moved out
of `x` when we set `y`. This is fundamental to Rust's ownership system: outside
of workarounds like `Rc`, a value cannot be owned by more than one variable.

Sometimes we don't need to move the value. Using a reference, we can let another
function borrow the value without changing its ownership. In the example below,
we don't actually have to move our string to `calculate_length`, we can give it
a reference to it with `&` instead.

```
fn main() {
    let s1 = String::from("hello");

    let len = calculate_length(&s1);

    println!("The length of '{}' is {}.", s1, len);
}

fn calculate_length(s: &String) -> usize {
    s.len()
}
```

A mutable reference can be created with `&mut`.

Sometimes we don't want a reference, but a duplicate. All types marked `Clone`
can be duplicated by calling `.clone()`. Subsequent changes to a clone do not
affect the original variable.

Most types in the standard library are marked `Clone`. The example below
demonstrates using `clone()` on a string. `s1` is first set to "many", and then
copied to `s2`. Then the first character of `s1` is removed, without affecting
`s2`. "any many" is printed to the console.

```
fn main() {
    let mut s1 = String::from("many");
    let s2 = s1.clone();
    s1.remove(0);
    println!("{} {}", s1, s2);
}
```

If we control the definition of a type, we can implement `Clone` on it ourselves
with `#[derive(Clone)]`.

Some types have no ownership semantics at all and are trivial to duplicate. An
example is `i32` and the other number types. We don't have to call `.clone()` to
clone them, because they are marked `Copy` in addition to `Clone`.  Implicit
cloning is more convenient in this case. We can mark our own types `Copy` if
all their members also are marked `Copy`.

In the example below, we implement a `Point` type. Because it only stores two
integers, we opt-out of ownership semantics with `Copy`. Then we can
`let p2 = p1` without `p1` being moved.

```
#[derive(Copy, Clone)]
struct Point { x: i32, y: i32 }

fn main() {
    let mut p1 = Point{ x: -1, y: 2 };
    let p2 = p1;
    p1.x = 1;
    println!("p1: {}, {}", p1.x, p1.y);
    println!("p2: {}, {}", p2.x, p2.y);
}
```

Alternatively, if we don't control the struct's definition, or mutable shared
ownership is truly required, we can use `Rc` and `RefCell`:

```
use std::cell::RefCell;
use std::rc::Rc;

struct MyStruct { s: u32 }

fn main() {
    let mut x = Rc::new(RefCell::new(MyStruct{ s: 5u32 }));
    let y = x.clone();
    x.borrow_mut().s = 6;
    println!("{}", x.borrow().s);
}
```

With this approach, x and y share ownership of the data via the `Rc` (reference
count type). `RefCell` essentially performs runtime borrow checking: ensuring
that at most one writer or multiple readers can access the data at any one time.

If you wish to learn more about ownership in Rust, start with the
[Understanding Ownership][understanding-ownership] chapter in the Book.

[understanding-ownership]: https://doc.rust-lang.org/book/ch04-00-understanding-ownership.html