Emit more specific diagnostics when enums fail to cast with as

compiler/rustc_hir_typeck/src/cast.rs

@@ -448,13 +448,35 @@ impl<'a, 'tcx> CastCheck<'tcx> {
                             );
                         }
                     }
-                    let msg = "an `as` expression can only be used to convert between primitive \
-                               types or to coerce to a specific trait object";
+
+                    let (msg, note) = if let ty::Adt(adt, _) = self.expr_ty.kind()
+                        && adt.is_enum()
+                        && self.cast_ty.is_numeric()
+                    {
+                        (
+                            "an `as` expression can be used to convert enum types to numeric \
+                             types only if the enum type is unit-only or field-less",
+                            Some(
+                                "see https://doc.rust-lang.org/reference/items/enumerations.html#casting for more information",
+                            ),
+                        )
+                    } else {
+                        (
+                            "an `as` expression can only be used to convert between primitive \
+                             types or to coerce to a specific trait object",
+                            None,
+                        )
+                    };
+
                     if label {
                         err.span_label(self.span, msg);
                     } else {
                         err.note(msg);
                     }
+
+                    if let Some(note) = note {
+                        err.note(note);
+                    }
                 } else {
                     err.span_label(self.span, "invalid cast");
                 }

tests/ui/cast/enum-to-numeric-cast.rs

@@ -0,0 +1,46 @@
+// Tests that `as` casts from enums to numeric types succeed
+// only if the enum type is "unit-only" or "fieldless" as
+// described here: https://doc.rust-lang.org/reference/items/enumerations.html#casting
+
+pub enum UnitOnly {
+    Foo,
+    Bar,
+    Baz,
+}
+
+pub enum Fieldless {
+    Tuple(),
+    Struct{},
+    Unit,
+}
+
+pub enum NotUnitOnlyOrFieldless {
+    Foo,
+    Bar(u8),
+    Baz
+}
+
+fn main() {
+    let unit_only = UnitOnly::Foo;
+
+    let _ = unit_only as isize;
+    let _ = unit_only as i32;
+    let _ = unit_only as usize;
+    let _ = unit_only as u32;
+
+
+    let fieldless = Fieldless::Struct{};
+
+    let _ = fieldless as isize;
+    let _ = fieldless as i32;
+    let _ = fieldless as usize;
+    let _ = fieldless as u32;
+
+
+    let not_unit_only_or_fieldless = NotUnitOnlyOrFieldless::Foo;
+
+    let _ = not_unit_only_or_fieldless as isize; //~ ERROR non-primitive cast: `NotUnitOnlyOrFieldless` as `isize`
+    let _ = not_unit_only_or_fieldless as i32; //~ ERROR non-primitive cast: `NotUnitOnlyOrFieldless` as `i32`
+    let _ = not_unit_only_or_fieldless as usize; //~ ERROR non-primitive cast: `NotUnitOnlyOrFieldless` as `usize`
+    let _ = not_unit_only_or_fieldless as u32; //~ ERROR non-primitive cast: `NotUnitOnlyOrFieldless` as `u32`
+}

tests/ui/cast/enum-to-numeric-cast.stderr

@@ -0,0 +1,35 @@
+error[E0605]: non-primitive cast: `NotUnitOnlyOrFieldless` as `isize`
+  --> $DIR/enum-to-numeric-cast.rs:42:13
+   |
+LL |     let _ = not_unit_only_or_fieldless as isize;
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 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
+
+error[E0605]: non-primitive cast: `NotUnitOnlyOrFieldless` as `i32`
+  --> $DIR/enum-to-numeric-cast.rs:43:13
+   |
+LL |     let _ = not_unit_only_or_fieldless as i32;
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 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
+
+error[E0605]: non-primitive cast: `NotUnitOnlyOrFieldless` as `usize`
+  --> $DIR/enum-to-numeric-cast.rs:44:13
+   |
+LL |     let _ = not_unit_only_or_fieldless as usize;
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 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
+
+error[E0605]: non-primitive cast: `NotUnitOnlyOrFieldless` as `u32`
+  --> $DIR/enum-to-numeric-cast.rs:45:13
+   |
+LL |     let _ = not_unit_only_or_fieldless 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
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0605`.

tests/ui/cast/issue-88621.stderr

@@ -2,7 +2,9 @@ error[E0605]: non-primitive cast: `Kind2` as `u8`
   --> $DIR/issue-88621.rs:9:13
    |
 LL |     let _ = Kind2::Foo() as u8;
-   |             ^^^^^^^^^^^^^^^^^^ an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object
+   |             ^^^^^^^^^^^^^^^^^^ 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
 
 error: aborting due to 1 previous error
 

tests/ui/tag-variant-cast-non-nullary.stderr

@@ -4,7 +4,8 @@ error[E0605]: non-primitive cast: `NonNullary` as `isize`
 LL |     let val = v as isize;
    |               ^^^^^^^^^^ help: consider using the `From` trait instead: `isize::from(v)`
    |
-   = note: an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object
+   = note: 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
 
 error: aborting due to 1 previous error