error[E0759]: `items` has an anonymous lifetime `'_` but it needs to satisfy a `'static` lifetime requirement
  --> $DIR/dyn-trait-underscore.rs:8:20
   |
LL | fn a<T>(items: &[T]) -> Box<dyn Iterator<Item=&T>> {
   |                ---- this data with an anonymous lifetime `'_`...
LL |     //                      ^^^^^^^^^^^^^^^^^^^^^ bound *here* defaults to `'static`
LL |     Box::new(items.iter())
   |     ---------------^^^^--- ...is captured and required to live as long as `'static` here
   |
help: to declare that the trait object captures data from argument `items`, you can add an explicit `'_` lifetime bound
   |
LL | fn a<T>(items: &[T]) -> Box<dyn Iterator<Item=&T> + '_> {
   |                                                   ^^^^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0759`.