diff --git a/compiler/rustc_hir_typeck/src/cast.rs b/compiler/rustc_hir_typeck/src/cast.rs
index 58823ea30ce..f21de1609cb 100644
--- a/compiler/rustc_hir_typeck/src/cast.rs
+++ b/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");
}
diff --git a/tests/ui/cast/enum-to-numeric-cast.rs b/tests/ui/cast/enum-to-numeric-cast.rs
new file mode 100644
index 00000000000..d5ab2a6a1fe
--- /dev/null
+++ b/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`
+}
diff --git a/tests/ui/cast/enum-to-numeric-cast.stderr b/tests/ui/cast/enum-to-numeric-cast.stderr
new file mode 100644
index 00000000000..1a49cb97451
--- /dev/null
+++ b/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`.
diff --git a/tests/ui/cast/issue-88621.stderr b/tests/ui/cast/issue-88621.stderr
index 0459ce5eabd..201651a207a 100644
--- a/tests/ui/cast/issue-88621.stderr
+++ b/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
diff --git a/tests/ui/tag-variant-cast-non-nullary.stderr b/tests/ui/tag-variant-cast-non-nullary.stderr
index 560dd7e8164..2e1dde27d0f 100644
--- a/tests/ui/tag-variant-cast-non-nullary.stderr
+++ b/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