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Currently in Rust master (0.10-pre), when you move one element of a unique vector and try to move a different element, the compiler complains:

let x = ~[~1, ~2, ~3];
let z0 = x[0];
let z1 = x[1]; // error: use of partially moved value: `x`

This error message is somewhat different from if you were to move the entire vector:

let y = ~[~1, ~2, ~3];
let y1 = y;
let y2 = y; // error: use of moved value `y`

Why the different message? If x is only "partially moved" in the first example, is there any way to "partially move" different parts of x? If not, why not just say that x is moved?

tshepang
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nham
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  • Even if there is no way to partially move elements, it would still make sense to have a different error message because moving an element isn't the same as moving the entire vector. It never hurts to have accurate error messages. – A.B. Jan 26 '14 at 21:49

1 Answers1

2

If x is only "partially moved" in the first example, is there any way to "partially move" different parts of x?

Yes, there is, but only when you move these parts all at once:

let x = ~[~1, ~2, ~3];
match x {
    [x1, x2, x3] => println!("{} {} {}", *x1, *x2, *x3),
    _ => unreachable!()
}

It can be easily observed that xNs are really moved out because if you add additional line after the match:

println!("{:?}", x);

The compiler will throw an error:

main3.rs:16:22: 16:23 error: use of partially moved value: `x`
main3.rs:16     println!("{:?}", x);
                                 ^
note: in expansion of format_args!
<std-macros>:195:27: 195:81 note: expansion site
<std-macros>:194:5: 196:6 note: in expansion of println!
main3.rs:16:5: 16:25 note: expansion site
main3.rs:13:10: 13:12 note: `(*x)[]` moved here because it has type `~int`, which is moved by default (use `ref` to override)
main3.rs:13         [x1, x2, x3] => println!("{} {} {}", *x1, *x2, *x3),
                     ^~
error: aborting due to previous error

This is also true for structures and enums - they also can be partially moved.

Vladimir Matveev
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