error[E0282]: type annotations needed
  --> $DIR/recursive-coroutine-boxed.rs:14:23
   |
LL |         let mut gen = Box::pin(foo());
   |                       ^^^^^^^^ cannot infer type of the type parameter `T` declared on the struct `Box`
LL |
LL |         let mut r = gen.as_mut().resume(());
   |                         ------ type must be known at this point
   |
help: consider specifying the generic argument
   |
LL |         let mut gen = Box::<T>::pin(foo());
   |                          +++++

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0282`.