error[E0310]: the parameter type `E` may not live long enough
  --> $DIR/unconstrained-closure-lifetime-trait-object.rs:7:29
   |
LL |     Box::new(|e| Box::new(e.is::<E>()))
   |                             ^^
   |                             |
   |                             the parameter type `E` must be valid for the static lifetime...
   |                             ...so that the type `E` will meet its required lifetime bounds
   |
help: consider adding an explicit lifetime bound
   |
LL | fn foo<E: 'static>() -> B {
   |         +++++++++

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0310`.