Permit uninhabited enums to cast into ints

This essentially reverts part of #6204.

compiler/rustc_middle/src/ty/mod.rs

@@ -2481,8 +2481,10 @@ impl<'tcx> AdtDef {
         self.variants.iter().flat_map(|v| v.fields.iter())
     }
 
+    /// Whether the ADT lacks fields. Note that this includes uninhabited enums,
+    /// e.g., `enum Void {}` is considered payload free as well.
     pub fn is_payloadfree(&self) -> bool {
-        !self.variants.is_empty() && self.variants.iter().all(|v| v.fields.is_empty())
+        self.variants.iter().all(|v| v.fields.is_empty())
     }
 
     /// Return a `VariantDef` given a variant id.

src/test/ui/uninhabited/uninhabited-enum-cast.rs

@@ -1,7 +1,9 @@
+// check-pass
+
 enum E {}
 
 fn f(e: E) {
-    println!("{}", (e as isize).to_string());   //~ ERROR non-primitive cast
+    println!("{}", (e as isize).to_string());
 }
 
 fn main() {}

src/test/ui/uninhabited/uninhabited-enum-cast.stderr

@@ -1,9 +0,0 @@
-error[E0605]: non-primitive cast: `E` as `isize`
-  --> $DIR/uninhabited-enum-cast.rs:4:20
-   |
-LL |     println!("{}", (e as isize).to_string());
-   |                    ^^^^^^^^^^^^ an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object
-
-error: aborting due to previous error
-
-For more information about this error, try `rustc --explain E0605`.