error[E0746]: return type cannot have an unboxed trait object
  --> $DIR/issue-91803.rs:3:43
   |
LL | fn or<'a>(first: &'static dyn Foo<'a>) -> dyn Foo<'a> {
   |                                           ^^^^^^^^^^^ doesn't have a size known at compile-time
   |
help: return an `impl Trait` instead of a `dyn Trait`, if all returned values are the same type
   |
LL | fn or<'a>(first: &'static dyn Foo<'a>) -> impl Foo<'a> {
   |                                           ~~~~
help: box the return type, and wrap all of the returned values in `Box::new`
   |
LL | fn or<'a>(first: &'static dyn Foo<'a>) -> Box<dyn Foo<'a>> {
   |                                           ++++           +

error: aborting due to previous error

For more information about this error, try `rustc --explain E0746`.