Unable to use hashbrown sctuctures between threads

[Shchvova]

It seems like hashbrown structures doesn't implement Send making it very hard to use them in multithreaded applications

For example, attempt to return them from thread via join handle:

extern crate hashbrown;
fn main () {
    std::thread::spawn(move || { hashbrown::HashSet::new() } );
    std::thread::spawn(move || { hashbrown::HashMap::new() } );
}

Wouldn't compile

➜  cargo build
error[E0277]: `std::ptr::NonNull<u8>` cannot be sent between threads safely
 --> src/main.rs:4:5
  |
4 |     std::thread::spawn(move || { hashbrown::HashSet::new() } );
  |     ^^^^^^^^^^^^^^^^^^ `std::ptr::NonNull<u8>` cannot be sent between threads safely
  |
  = help: within `hashbrown::HashSet<_>`, the trait `std::marker::Send` is not implemented for `std::ptr::NonNull<u8>`
  = note: required because it appears within the type `hashbrown::raw::RawTable<(_, ())>`
  = note: required because it appears within the type `hashbrown::HashMap<_, ()>`
  = note: required because it appears within the type `hashbrown::HashSet<_>`
  = note: required by `std::thread::spawn`

error[E0277]: `std::ptr::NonNull<(_, ())>` cannot be sent between threads safely
 --> src/main.rs:4:5
  |
4 |     std::thread::spawn(move || { hashbrown::HashSet::new() } );
  |     ^^^^^^^^^^^^^^^^^^ `std::ptr::NonNull<(_, ())>` cannot be sent between threads safely
  |
  = help: within `hashbrown::HashSet<_>`, the trait `std::marker::Send` is not implemented for `std::ptr::NonNull<(_, ())>`
  = note: required because it appears within the type `hashbrown::raw::RawTable<(_, ())>`

.....