impl dyn Trait on an auto trait produces a confusing diagnostic

[PatchMixolydic]

Given the following code (playground):

impl dyn Unpin {}

The current output is:

error[E0118]: no nominal type found for inherent implementation
 --> src/lib.rs:1:6
  |
1 | impl dyn Unpin {}
  |      ^^^^^^^^^ impl requires a nominal type
  |
  = note: either implement a trait on it or create a newtype to wrap it instead

This is rather confusing. It's not immediately clear what the difference between impl dyn Unpin and this working code is (playground):

trait A {}
impl dyn A {}

Since impl dyn A is accepted, the user might be confused why dyn A would be considered a nominal type when dyn Unpin isn't. The real reason why impl dyn Unpin is an error is because it is an impl block for a type [edit: a dynamically dispatched auto trait] from another crate, which should be handled by E0116.

Ideally the output should look like:

error[E0116]: cannot define inherent `impl` for a type outside of the crate where the type is defined
 --> src/lib.rs:1:1
  |
1 | impl dyn Unpin {}
  | ^^^^^^^^^^^^^^^^^ impl for type defined outside of crate.
  |
  = note: define and implement a trait or new type instead

[PatchMixolydic]

It seems like this only happens with auto traits. Regular traits already emit E0116 in this case (playground):

use std::panic::UnwindSafe;
impl dyn Copy {}
impl dyn UnwindSafe {}

error[E0116]: cannot define inherent `impl` for a type outside of the crate where the type is defined
 --> src/lib.rs:3:1
  |
3 | impl dyn Copy {}
  | ^^^^^^^^^^^^^^^^ impl for type defined outside of crate.
  |
  = note: define and implement a trait or new type instead

error[E0118]: no nominal type found for inherent implementation
 --> src/lib.rs:4:6
  |
4 | impl dyn UnwindSafe {}
  |      ^^^^^^^^^^^^^^ impl requires a nominal type
  |
  = note: either implement a trait on it or create a newtype to wrap it instead

Note that this still happens with auto traits in the same crate, but in this case E0116 wouldn't be applicable (playground):

#![feature(auto_traits)]

auto trait X {}
impl dyn X {}

error[E0118]: no nominal type found for inherent implementation
 --> src/lib.rs:4:6
  |
4 | impl dyn X {}
  |      ^^^^^ impl requires a nominal type
  |
  = note: either implement a trait on it or create a newtype to wrap it instead

The diagnostic is still confusing in this case and could be rewritten to explicitly point out that impl dyn AutoTrait is not allowed.

@rustbot modify labels: +D-confusing +F-auto_traits