error[E0277]: `Rc<()>` cannot be sent between threads safely --> $DIR/issue-21763.rs:11:11 | LL | foo::, Rc<()>>>(); | ^^^^^^^^^^^^^^^^^^^^^^^ `Rc<()>` cannot be sent between threads safely | = help: within `(Rc<()>, Rc<()>)`, the trait `Send` is not implemented for `Rc<()>` = note: required because it appears within the type `(Rc<()>, Rc<()>)` = note: required for `hashbrown::raw::RawTable<(Rc<()>, Rc<()>)>` to implement `Send` note: required because it appears within the type `HashMap, Rc<()>, RandomState>` --> $HASHBROWN_SRC_LOCATION note: required because it appears within the type `HashMap, Rc<()>>` --> $SRC_DIR/std/src/collections/hash/map.rs:LL:COL note: required by a bound in `foo` --> $DIR/issue-21763.rs:8:11 | LL | fn foo() {} | ^^^^ required by this bound in `foo` error: aborting due to previous error For more information about this error, try `rustc --explain E0277`.