Packed unsized struct with drop: Confusing error message

[RalfJung]

The following type

struct UnsizedDrop(str);

impl Drop for UnsizedDrop {
    fn drop(&mut self) {
    }
}

#[repr(packed)]
struct Foo {
    x: u32,
    y: UnsizedDrop,
}

raises the following error

error[E0277]: the trait bound `str: std::marker::Sized` is not satisfied in `UnsizedDrop`
  --> src/main.rs:11:5
   |
11 |     y: UnsizedDrop,
   |     ^^^^^^^^^^^^^^ `str` does not have a constant size known at compile-time
   |
   = help: within `UnsizedDrop`, the trait `std::marker::Sized` is not implemented for `str`
   = note: required because it appears within the type `UnsizedDrop`
   = note: only the last field of a struct may have a dynamically sized type

The last line of the note here is pretty confusing, because this is the last field! The actual problem is that a packed struct implementing Drop copies things around for dropping to get the right alignment, and that requires the field to be sized.

[RalfJung]

Cc @arielb1 who wrote the code emitting the above error in #44884.

[estebank]

Current output:

error[[E0277]](https://doc.rust-lang.org/stable/error_codes/E0277.html): the size for values of type `str` cannot be known at compilation time
  --> src/lib.rs:11:8
   |
11 |     y: UnsizedDrop,
   |        ^^^^^^^^^^^ doesn't have a size known at compile-time
   |
   = help: within `UnsizedDrop`, the trait `Sized` is not implemented for `str`
note: required because it appears within the type `UnsizedDrop`
  --> src/lib.rs:1:8
   |
1  | struct UnsizedDrop(str);
   |        ^^^^^^^^^^^
   = note: the last field of a packed struct may only have a dynamically sized type if it does not need drop to be run
   = help: change the field's type to have a statically known size
help: borrowed types always have a statically known size
   |
11 |     y: &UnsizedDrop,
   |        +
help: the `Box` type always has a statically known size and allocates its contents in the heap
   |
11 |     y: Box<UnsizedDrop>,
   |        ++++           +