error[E0277]: `std::rc::Rc<()>` cannot be sent between threads safely
  --> $DIR/issue-21763.rs:9:5
   |
LL |     foo::<HashMap<Rc<()>, Rc<()>>>();
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `std::rc::Rc<()>` cannot be sent between threads safely
   |
   = help: within `(std::rc::Rc<()>, std::rc::Rc<()>)`, the trait `std::marker::Send` is not implemented for `std::rc::Rc<()>`
   = note: required because it appears within the type `(std::rc::Rc<()>, std::rc::Rc<()>)`
   = note: required because of the requirements on the impl of `std::marker::Send` for `hashbrown::raw::RawTable<(std::rc::Rc<()>, std::rc::Rc<()>)>`
   = note: required because it appears within the type `hashbrown::map::HashMap<std::rc::Rc<()>, std::rc::Rc<()>, std::collections::hash_map::RandomState>`
   = note: required because it appears within the type `std::collections::HashMap<std::rc::Rc<()>, std::rc::Rc<()>>`
note: required by `foo`
  --> $DIR/issue-21763.rs:6:1
   |
LL | fn foo<T: Send>() {}
   | ^^^^^^^^^^^^^^^^^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.