error[E0106]: missing lifetime specifier
  --> $DIR/issue-63388-2.rs:12:10
   |
LL |         foo: &dyn Foo, bar: &'a dyn Foo
   |              --------       -----------
LL |     ) -> &dyn Foo
   |          ^ expected named lifetime parameter
   |
   = help: this function's return type contains a borrowed value, but the signature does not say whether it is borrowed from `foo` or `bar`
help: consider using the `'a` lifetime
   |
LL |     ) -> &'a dyn Foo
   |          ~~~

error: aborting due to previous error

For more information about this error, try `rustc --explain E0106`.