error message when trying to move from an Rc or Arc is ungreat

[nikomatsakis]

This example

use std::rc::Rc;

struct Bar { field: Vec<i32> }

fn main() {
    let x = Rc::new(Bar { field: vec![] });
    drop(x.field);
}

gives this error:

error[E0507]: cannot move out of borrowed content
 --> src/main.rs:7:10
  |
7 |     drop(x.field);
  |          ^ cannot move out of borrowed content

That is confusing -- after all, x is not borrowed!

The problem is that Rc only implements Deref, and hence we wind up lowering to Deref::deref(&x).field, and deref returns a &Bar.

We should probably say:

error[E0507]: cannot move out of `Rc`
 --> src/main.rs:7:10
  |
7 |     drop(x.field);
  |          ^ cannot move out of an `Rc`

[nikomatsakis]

This could be fixed in the (NLL) borrow check. It could maybe be fixed by altering typeck as well -- but borrowck seems perhaps easier. So I'm going to tag as an NLL bug, even though it's not specific to NLL.