error[E0277]: `Rc>` cannot be sent between threads safely --> $DIR/issue-7013.rs:24:19 | LL | let a = A {v: Box::new(B{v: None}) as Box}; | ^^^^^^^^^^^^^^^^^^^^ `Rc>` cannot be sent between threads safely | = help: within `B`, the trait `Send` is not implemented for `Rc>` note: required because it appears within the type `Option>>` --> $SRC_DIR/core/src/option.rs:LL:COL note: required because it appears within the type `B` --> $DIR/issue-7013.rs:8:8 | LL | struct B { | ^ = note: required for the cast from `Box` to `Box` error: aborting due to 1 previous error For more information about this error, try `rustc --explain E0277`.