rustc suggests Box<Option<T>>, but could specialize to suggest Option<Box<T>> instead

[Noratrieb]

Given the following code: playground link

struct X {
    x: Option<X>,
}

The current output is:

error[E0072]: recursive type `X` has infinite size
 --> src/lib.rs:1:1
  |
1 | struct X {
  | ^^^^^^^^ recursive type has infinite size
2 |     x: Option<X>,
  |        --------- recursive without indirection
  |
help: insert some indirection (e.g., a `Box`, `Rc`, or `&`) to make `X` representable
  |
2 |     x: Box<Option<X>>,
  |        ++++         +

For more information about this error, try `rustc --explain E0072`.
error: could not compile `playground` due to previous error

Ideally the output should look like:

error[E0072]: recursive type `X` has infinite size
 --> src/lib.rs:1:1
  |
1 | struct X {
  | ^^^^^^^^ recursive type has infinite size
2 |     x: Option<X>,
  |        --------- recursive without indirection
  |
help: insert some indirection (e.g., a `Box`, `Rc`, or `&`) to make `X` representable
  |
2 |     x: Option<Box<X>>,
  |               ++++ +

For more information about this error, try `rustc --explain E0072`.
error: could not compile `playground` due to previous error

This allows rustc to use the nullpointer niche optimization, and is also easier to handle, and what the user probably wants in general.

[compiler-errors]

Gave this a shot in the attached PR, so

@rustbot claim