Discrepancy regarding how closures capture Copy variables

[ross-pure]

There seems to be a discrepancy between the description in the RBE section on capturing (9.2.1) and current stable implementation of rust regarding what happens to a copy variable when it needs to be captured by value in a closure. The RBE says that

mem::drop requires T so this must take by value. A copy type would copy into the closure leaving the original untouched.

However, it appears that the compiler does not do this precisely. Consider the following example:

#![allow(unused)]

#[derive(Clone, Copy)]
struct IsCopy {}

fn main() {
    let y = {
        let x = IsCopy {};

        // x is not copied into the closure here, only taken by reference
        || drop(x)
    };
}

fn foo() -> impl FnOnce() {
    let x = IsCopy {};

    // Also here x is only taken by reference and not copied into the closure
    || drop(x)
}

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
error[E0597]: `x` does not live long enough
  --> src/main.rs:11:17
   |
7  |     let y = {
   |         - borrow later stored here
...
11 |         || drop(x)
   |         --      ^ borrowed value does not live long enough
   |         |
   |         value captured here
12 |     };
   |     - `x` dropped here while still borrowed

error[E0597]: `x` does not live long enough
  --> src/main.rs:19:13
   |
15 | fn foo() -> impl FnOnce() {
   |             ------------- opaque type requires that `x` is borrowed for `'static`
...
19 |     || drop(x)
   |     --      ^ borrowed value does not live long enough
   |     |
   |     value captured here
20 | }
   | - `x` dropped here while still borrowed

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0597`.
error: could not compile `playground`.

To learn more, run the command again with --verbose.

From the example it seems (I'm not actually 100% sure what is going on behind the scenes) that the closure only takes a reference, and will perform the copy later when it needs to. In any case, this is different to what is implied by the RBE.