Accessing a field in a closure makes rust want to copy ... something

[gwillen]

class example {
  let mut x: int;
  new() {
    self.x = 1;
  }

  drop {}
}

fn main(_args: [str]) {
  let x: example = example();
  let _z = fn@() {
    let _a: int = x.x;
  };
  x.x = 5;
}

noncopyable-field.rs:13:18: 13:19 error: copying a noncopyable value
noncopyable-field.rs:13     let _a: int = x.x;
                                          ^

Notably, it seems to be trying to copy the example object, not the int (as I first assumed), at least if you trust the error location marker.

So the question is, why does accessing a field of the object try to copy it?

This particular repro goes away if you remove the write to x.x after the closure is created, but I have another, larger one where the write isn't there and it still does it.

[gwillen]

Oh, in the example where I don't assign to x.x, I instead let z = @x after the closure is created, to trigger the same error.

[nikomatsakis]

This is not a bug. An fn@ closure copies all local variables which are referenced into the closure. If you only want the value of x.x, then make a temporary variable.

[gwillen]

Aha, mystery solved. I will think about what it would take to perhaps make this more intuitive.

In the case where I don't reference x after I close over it, and there's no error: is it detecting that x is dead, and moving it into the closure? Or what's happening there?

[nikomatsakis]

we discussed this on IRC, but for posterity's sake: if the variable is not used after the closure is constructed, it is considered a "last use" and is moved into the closure