--explain E0120 constrained much more than how it's emitted

[CAD97]

Location

https://doc.rust-lang.org/nightly/error-index.html#E0120

Summary

E0120 says

Drop was implemented on a trait, which is not allowed: only structs and enums can implement Drop.

The inline note is

the Drop trait may only be implemented for structs, enums, and unions

(Bonus: --explain should include unions as allowed.)

However, E0120 is emitted for impl target types which are fundamental types but not trait objects:

error[E0120]: the `Drop` trait may only be implemented for structs, enums, and unions
 --> src/lib.rs:2:15
  |
2 | impl Drop for &'_ mut Concrete {
  |               ^^^^^^^^^^^^^^^^ must be a struct, enum, or union

error[E0210]: type parameter `T` must be used as the type parameter for some local type (e.g., `MyStruct<T>`)
 --> src/lib.rs:2:6
  |
2 | impl<T: Trait> Drop for T {
  |      ^ type parameter `T` must be used as the type parameter for some local type
  |
  = note: implementing a foreign trait is only possible if at least one of the types for which it is implemented is local
  = note: only traits defined in the current crate can be implemented for a type parameter

error[E0120]: the `Drop` trait may only be implemented for structs, enums, and unions
 --> src/lib.rs:2:25
  |
2 | impl<T: Trait> Drop for T {
  |                         ^ must be a struct, enum, or union

(see also #36061; E0210 should probably be preferred here and suppress E0120)

error[E0117]: only traits defined in the current crate can be implemented for arbitrary types
 --> src/lib.rs:2:1
  |
2 | impl Drop for *mut Concrete {
  | ^^^^^^^^^^^^^^-------------
  | |             |
  | |             `*mut Concrete` is not defined in the current crate
  | impl doesn't use only types from inside the current crate
  |
  = note: define and implement a trait or new type instead

error[E0120]: the `Drop` trait may only be implemented for structs, enums, and unions
 --> src/lib.rs:2:15
  |
2 | impl Drop for *mut Concrete {
  |               ^^^^^^^^^^^^^ must be a struct, enum, or union

(E0117 should probably be preferred here and suppress E0120)

The --explain text should be adjusted to something along the lines of

Drop was implemented on a trait object or reference, which is not allowed; only structs, enums, and unions can implement Drop.

and have its examples updated to match the new byline.