error[E0277]: the size for values of type `(dyn A + 'static)` cannot be known at compilation time
  --> $DIR/issue-5883.rs:8:5
   |
LL |     r: dyn A + 'static
   |     ^ doesn't have a size known at compile-time
   |
   = help: the trait `Sized` is not implemented for `(dyn A + 'static)`
   = help: unsized fn params are gated as an unstable feature
help: you can use `impl Trait` as the argument type
   |
LL |     r: impl A + 'static
   |        ~~~~
help: function arguments must have a statically known size, borrowed types always have a known size
   |
LL |     r: &dyn A + 'static
   |        +

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.