Emit more specific diagnostics when enums fail to cast with as

[gurry]

Fixes #120756

Changes this diagnostic reported in the issue:

error[E0605]: non-primitive cast: `Bad` as `u32`
  --> src/main.rs:18:10
   |
18 |     dbg!(bad as u32);
   |          ^^^^^^^^^^ an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object

to this:

error[E0605]: non-primitive cast: `Bad` as `u32`
  --> src/main.rs:18:10
   |
18 |     dbg!(bad as u32);
   |          ^^^^^^^^^^ an `as` expression can be used to convert enum types to numeric types only if the enum type is unit-only or field-less
   |
   = note: see https://doc.rust-lang.org/reference/items/enumerations.html#casting for more information

This change is only for enums. The diagnostic remains unchanged for all other cases.

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[petrochenkov]

@bors r+

[bors]

📌 Commit 6e37f95 has been approved by petrochenkov

It is now in the queue for this repository.