Enforce default discriminant type for all representations and across editions

CHANGELOG.md

@@ -10,8 +10,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Changed
 - Use safe casting in `PartialOrd` and `Ord` implementations in more cases when
   possible.
-- Avoid unnecessarily validating the discriminant type of enums with C
-  representations in some cases.
+- Avoid unnecessarily validating the default discriminant type in some cases.
+- Apply default enum discriminant type validation to all representations and
+  make it cross-edition safe.
 
 ## [1.2.4] - 2023-09-01
 

README.md

@@ -275,11 +275,9 @@ supported.
 Unions only support [`Clone`] and [`Copy`].
 
 [`PartialOrd`] and [`Ord`] need to determine the discriminant type to
-function correctly. Unfortunately, according to the specification, the C
-representation without an integer representation doesn't have a
-platform-independent discriminant type. Therefor a check is inserted to
-ascertain that discriminants of enums with a C representation have the
-[`isize`] type, which is currently the case for all known platforms.
+function correctly. To protect against a potential future change to the
+default discriminant type, some compile-time validation is inserted to
+ascertain that the type remains `isize`.
 
 ### `no_std` support
 

src/item.rs

@@ -99,18 +99,13 @@ impl Item<'_> {
 pub enum Discriminant {
 	/// The enum has only a single variant.
 	Single,
-	/// The enum uses the default or C representation and has only unit
-	/// variants.
-	Unit {
-		/// `true` if this is using the C representation.
-		c: bool,
-	},
-	/// The enum uses the default or C representation but has a non-unit
-	/// variant.
+	/// The enum has only unit variants.
+	Unit,
+	/// The enum has a non-unit variant.
 	Data,
-	/// The enum uses a non-default representation and has only unit variants.
+	/// The enum has only unit variants.
 	UnitRepr(Representation),
-	/// The enum uses a non-default representation and has a non-unit variant.
+	/// The enum has a non-unit variant.
 	DataRepr(Representation),
 }
 
@@ -122,7 +117,6 @@ impl Discriminant {
 		}
 
 		let mut has_repr = None;
-		let mut is_c = false;
 
 		for attr in attrs {
 			if attr.path().is_ident("repr") {
@@ -131,12 +125,10 @@ impl Discriminant {
 						list.parse_args_with(Punctuated::<Ident, Token![,]>::parse_terminated)?;
 
 					for ident in list {
-						if ident == "C" {
-							is_c = true;
-						} else if let Some(repr) = Representation::parse(&ident) {
+						if let Some(repr) = Representation::parse(&ident) {
 							has_repr = Some(repr);
 							break;
-						} else if ident != "Rust" && ident != "align" {
+						} else if ident != "C" && ident != "Rust" && ident != "align" {
 							return Err(Error::repr_unknown(ident.span()));
 						}
 					}
@@ -155,7 +147,7 @@ impl Discriminant {
 				Self::DataRepr(repr)
 			}
 		} else if is_unit {
-			Self::Unit { c: is_c }
+			Self::Unit
 		} else {
 			let discriminant = variants
 				.iter()

src/lib.rs

@@ -314,11 +314,9 @@
 //! Unions only support [`Clone`] and [`Copy`].
 //!
 //! [`PartialOrd`] and [`Ord`] need to determine the discriminant type to
-//! function correctly. Unfortunately, according to the specification, the C
-//! representation without an integer representation doesn't have a
-//! platform-independent discriminant type. Therefor a check is inserted to
-//! ascertain that discriminants of enums with a C representation have the
-//! [`isize`] type, which is currently the case for all known platforms.
+//! function correctly. To protect against a potential future change to the
+//! default discriminant type, some compile-time validation is inserted to
+//! ascertain that the type remains `isize`.
 //!
 //! ## `no_std` support
 //!

src/test/discriminant.rs

@@ -15,10 +15,14 @@ fn default() -> Result<()> {
 	#[cfg(not(feature = "nightly"))]
 	let partial_ord = quote! {
 		const fn __discriminant(__this: &Test) -> isize {
+			const __VALIDATE_ISIZE_A: isize = 0;
+			const __VALIDATE_ISIZE_B: isize = (0) + 1;
+			const __VALIDATE_ISIZE_C: isize = (0) + 2;
+
 			match __this {
-				Test::A => 0,
-				Test::B => (0) + 1,
-				Test::C => (0) + 2
+				Test::A => __VALIDATE_ISIZE_A,
+				Test::B => __VALIDATE_ISIZE_B,
+				Test::C => __VALIDATE_ISIZE_C
 			}
 		}
 
@@ -61,6 +65,10 @@ fn default_clone() -> Result<()> {
 	};
 	#[cfg(not(feature = "nightly"))]
 	let partial_ord = quote! {
+		const __VALIDATE_ISIZE_A: isize = 0;
+		const __VALIDATE_ISIZE_B: isize = (0) + 1;
+		const __VALIDATE_ISIZE_C: isize = (0) + 2;
+
 		::core::cmp::PartialOrd::partial_cmp(&(::core::clone::Clone::clone(self) as isize), &(::core::clone::Clone::clone(__other) as isize))
 	};
 
@@ -111,6 +119,10 @@ fn default_copy() -> Result<()> {
 	};
 	#[cfg(not(feature = "nightly"))]
 	let partial_ord = quote! {
+		const __VALIDATE_ISIZE_A: isize = 0;
+		const __VALIDATE_ISIZE_B: isize = (0) + 1;
+		const __VALIDATE_ISIZE_C: isize = (0) + 2;
+
 		::core::cmp::PartialOrd::partial_cmp(&(*self as isize), &(*__other as isize))
 	};
 
@@ -153,10 +165,14 @@ fn default_reverse() -> Result<()> {
 	#[cfg(not(feature = "nightly"))]
 	let partial_ord = quote! {
 		const fn __discriminant(__this: &Test) -> isize {
+			const __VALIDATE_ISIZE_A: isize = 2;
+			const __VALIDATE_ISIZE_B: isize = 1;
+			const __VALIDATE_ISIZE_C: isize = 0;
+
 			match __this {
-				Test::A => 2,
-				Test::B => 1,
-				Test::C => 0
+				Test::A => __VALIDATE_ISIZE_A,
+				Test::B => __VALIDATE_ISIZE_B,
+				Test::C => __VALIDATE_ISIZE_C
 			}
 		}
 
@@ -200,11 +216,16 @@ fn default_mix() -> Result<()> {
 	#[cfg(not(feature = "nightly"))]
 	let partial_ord = quote! {
 		const fn __discriminant(__this: &Test) -> isize {
+			const __VALIDATE_ISIZE_A: isize = 1;
+			const __VALIDATE_ISIZE_B: isize = 0;
+			const __VALIDATE_ISIZE_C: isize = 2;
+			const __VALIDATE_ISIZE_D: isize = (2) + 1;
+
 			match __this {
-				Test::A => 1,
-				Test::B => 0,
-				Test::C => 2,
-				Test::D => (2) + 1
+				Test::A => __VALIDATE_ISIZE_A,
+				Test::B => __VALIDATE_ISIZE_B,
+				Test::C => __VALIDATE_ISIZE_C,
+				Test::D => __VALIDATE_ISIZE_D
 			}
 		}
 
@@ -249,12 +270,18 @@ fn default_skip() -> Result<()> {
 	#[cfg(not(feature = "nightly"))]
 	let partial_ord = quote! {
 		const fn __discriminant(__this: &Test) -> isize {
+			const __VALIDATE_ISIZE_A: isize = 0;
+			const __VALIDATE_ISIZE_B: isize = 3;
+			const __VALIDATE_ISIZE_C: isize = (3) + 1;
+			const __VALIDATE_ISIZE_D: isize = (3) + 2;
+			const __VALIDATE_ISIZE_E: isize = (3) + 3;
+
 			match __this {
-				Test::A => 0,
-				Test::B => 3,
-				Test::C => (3) + 1,
-				Test::D => (3) + 2,
-				Test::E => (3) + 3
+				Test::A => __VALIDATE_ISIZE_A,
+				Test::B => __VALIDATE_ISIZE_B,
+				Test::C => __VALIDATE_ISIZE_C,
+				Test::D => __VALIDATE_ISIZE_D,
+				Test::E => __VALIDATE_ISIZE_E
 			}
 		}
 
@@ -300,10 +327,14 @@ fn default_expr() -> Result<()> {
 	#[cfg(not(feature = "nightly"))]
 	let partial_ord = quote! {
 		const fn __discriminant(__this: &Test) -> isize {
+			const __VALIDATE_ISIZE_A: isize = isize::MAX - 2;
+			const __VALIDATE_ISIZE_B: isize = (isize::MAX - 2) + 1;
+			const __VALIDATE_ISIZE_C: isize = (isize::MAX - 2) + 2;
+
 			match __this {
-				Test::A => isize::MAX - 2,
-				Test::B => (isize::MAX - 2) + 1,
-				Test::C => (isize::MAX - 2) + 2
+				Test::A => __VALIDATE_ISIZE_A,
+				Test::B => __VALIDATE_ISIZE_B,
+				Test::C => __VALIDATE_ISIZE_C
 			}
 		}
 
@@ -346,14 +377,15 @@ fn repr_c() -> Result<()> {
 	};
 	#[cfg(not(feature = "nightly"))]
 	let partial_ord = quote! {
-		#[repr(C)]
-		enum EnsureReprCIsIsize { Test = 0_isize }
-
 		const fn __discriminant(__this: &Test) -> isize {
+			const __VALIDATE_ISIZE_A: isize = 0;
+			const __VALIDATE_ISIZE_B: isize = (0) + 1;
+			const __VALIDATE_ISIZE_C: isize = (0) + 2;
+
 			match __this {
-				Test::A => 0,
-				Test::B => (0) + 1,
-				Test::C => (0) + 2
+				Test::A => __VALIDATE_ISIZE_A,
+				Test::B => __VALIDATE_ISIZE_B,
+				Test::C => __VALIDATE_ISIZE_C
 			}
 		}
 
@@ -445,8 +477,9 @@ fn repr_c_clone() -> Result<()> {
 	};
 	#[cfg(not(feature = "nightly"))]
 	let partial_ord = quote! {
-		#[repr(C)]
-		enum EnsureReprCIsIsize { Test = 0_isize }
+		const __VALIDATE_ISIZE_A: isize = 0;
+		const __VALIDATE_ISIZE_B: isize = (0) + 1;
+		const __VALIDATE_ISIZE_C: isize = (0) + 2;
 
 		::core::cmp::PartialOrd::partial_cmp(&(::core::clone::Clone::clone(self) as isize), &(::core::clone::Clone::clone(__other) as isize))
 	};
@@ -550,8 +583,9 @@ fn repr_c_copy() -> Result<()> {
 	};
 	#[cfg(not(feature = "nightly"))]
 	let partial_ord = quote! {
-		#[repr(C)]
-		enum EnsureReprCIsIsize { Test = 0_isize }
+		const __VALIDATE_ISIZE_A: isize = 0;
+		const __VALIDATE_ISIZE_B: isize = (0) + 1;
+		const __VALIDATE_ISIZE_C: isize = (0) + 2;
 
 		::core::cmp::PartialOrd::partial_cmp(&(*self as isize), &(*__other as isize))
 	};
@@ -637,14 +671,15 @@ fn repr_c_reverse() -> Result<()> {
 	};
 	#[cfg(not(feature = "nightly"))]
 	let partial_ord = quote! {
-		#[repr(C)]
-		enum EnsureReprCIsIsize { Test = 0_isize }
-
 		const fn __discriminant(__this: &Test) -> isize {
+			const __VALIDATE_ISIZE_A: isize = 2;
+			const __VALIDATE_ISIZE_B: isize = 1;
+			const __VALIDATE_ISIZE_C: isize = 0;
+
 			match __this {
-				Test::A => 2,
-				Test::B => 1,
-				Test::C => 0
+				Test::A => __VALIDATE_ISIZE_A,
+				Test::B => __VALIDATE_ISIZE_B,
+				Test::C => __VALIDATE_ISIZE_C
 			}
 		}
 
@@ -688,15 +723,17 @@ fn repr_c_mix() -> Result<()> {
 	};
 	#[cfg(not(feature = "nightly"))]
 	let partial_ord = quote! {
-		#[repr(C)]
-		enum EnsureReprCIsIsize { Test = 0_isize }
-
 		const fn __discriminant(__this: &Test) -> isize {
+			const __VALIDATE_ISIZE_A: isize = 1;
+			const __VALIDATE_ISIZE_B: isize = 0;
+			const __VALIDATE_ISIZE_C: isize = 2;
+			const __VALIDATE_ISIZE_D: isize = (2) + 1;
+
 			match __this {
-				Test::A => 1,
-				Test::B => 0,
-				Test::C => 2,
-				Test::D => (2) + 1
+				Test::A => __VALIDATE_ISIZE_A,
+				Test::B => __VALIDATE_ISIZE_B,
+				Test::C => __VALIDATE_ISIZE_C,
+				Test::D => __VALIDATE_ISIZE_D
 			}
 		}
 
@@ -741,16 +778,19 @@ fn repr_c_skip() -> Result<()> {
 	};
 	#[cfg(not(feature = "nightly"))]
 	let partial_ord = quote! {
-		#[repr(C)]
-		enum EnsureReprCIsIsize { Test = 0_isize }
-
 		const fn __discriminant(__this: &Test) -> isize {
+			const __VALIDATE_ISIZE_A: isize = 0;
+			const __VALIDATE_ISIZE_B: isize = 3;
+			const __VALIDATE_ISIZE_C: isize = (3) + 1;
+			const __VALIDATE_ISIZE_D: isize = (3) + 2;
+			const __VALIDATE_ISIZE_E: isize = (3) + 3;
+
 			match __this {
-				Test::A => 0,
-				Test::B => 3,
-				Test::C => (3) + 1,
-				Test::D => (3) + 2,
-				Test::E => (3) + 3
+				Test::A => __VALIDATE_ISIZE_A,
+				Test::B => __VALIDATE_ISIZE_B,
+				Test::C => __VALIDATE_ISIZE_C,
+				Test::D => __VALIDATE_ISIZE_D,
+				Test::E => __VALIDATE_ISIZE_E
 			}
 		}
 
@@ -796,14 +836,15 @@ fn repr_c_expr() -> Result<()> {
 	};
 	#[cfg(not(feature = "nightly"))]
 	let partial_ord = quote! {
-		#[repr(C)]
-		enum EnsureReprCIsIsize { Test = 0_isize }
-
 		const fn __discriminant(__this: &Test) -> isize {
+			const __VALIDATE_ISIZE_A: isize = isize::MAX - 2;
+			const __VALIDATE_ISIZE_B: isize = (isize::MAX - 2) + 1;
+			const __VALIDATE_ISIZE_C: isize = (isize::MAX - 2) + 2;
+
 			match __this {
-				Test::A => u64::MAX - 2,
-				Test::B => (u64::MAX - 2) + 1,
-				Test::C => (u64::MAX - 2) + 2
+				Test::A => __VALIDATE_ISIZE_A,
+				Test::B => __VALIDATE_ISIZE_B,
+				Test::C => __VALIDATE_ISIZE_C
 			}
 		}
 
@@ -815,7 +856,7 @@ fn repr_c_expr() -> Result<()> {
 			#[derive_where(PartialOrd)]
 			#[repr(C)]
 			enum Test {
-				A = u64::MAX - 2,
+				A = isize::MAX - 2,
 				B,
 				#[derive_where(incomparable)]
 				C,