f7ef1c9f41
... if they use arbitrary enum discriminant. Code like
```rust
enum Enum {
Foo = 1,
Bar(),
Baz{}
}
```
seems to be unintentionally allowed so we couldn't disallow them now,
but we could disallow them if arbitrary enum discriminant is used before
1.56 hits stable.
For more information about how rustc works, see the rustc dev guide.