Guarantee that Vec::default() does not allocate?

help
19

I guess I could do this:

    pub fn finalize(self) -> BufReadGuard<T> {
        let mut this = std::mem::ManuallyDrop::new(self);
        BufReadGuard::new(take(&mut this.buffer), this.recycler.clone())
    }

(Playground)

But wouldn't this be even nicer?

    pub fn finalize(self) -> BufReadGuard<T> {
        let this = std::mem::ManuallyDrop::new(self);
        BufReadGuard::new(this.buffer, this.recycler)
    }

(Playground)

Unfortunately:

   Compiling playground v0.0.1 (/playground)
error[E0507]: cannot move out of dereference of `ManuallyDrop<BufWriteGuard<T>>`
  --> src/main.rs:54:27
   |
54 |         BufReadGuard::new(this.buffer, this.recycler)
   |                           ^^^^^^^^^^^ move occurs because value has type `Vec<T>`, which does not implement the `Copy` trait

error[E0507]: cannot move out of dereference of `ManuallyDrop<BufWriteGuard<T>>`
  --> src/main.rs:54:40
   |
54 |         BufReadGuard::new(this.buffer, this.recycler)
   |                                        ^^^^^^^^^^^^^ move occurs because value has type `UnboundedSender<Vec<T>>`, which does not implement the `Copy` trait

For more information about this error, try `rustc --explain E0507`.
error: could not compile `playground` due to 2 previous errors

I guess that's because ManuallyDrop is an ordinary wrapper and not some sort of language-built-in feature.