Implement RFC 2645 (transparent enums and unions)

Tracking issue: #60405

src/doc/unstable-book/src/language-features/transparent-enums.md

@@ -0,0 +1,93 @@
+# `transparent_enums`
+
+The tracking issue for this feature is [#60405]
+
+[60405]: https://github.com/rust-lang/rust/issues/60405
+
+----
+
+The `transparent_enums` feature allows you mark `enum`s as
+`#[repr(transparent)]`. An `enum` may be `#[repr(transparent)]` if it has
+exactly one variant, and that variant matches the same conditions which `struct`
+requires for transparency. Some concrete illustrations follow.
+
+```rust
+#![feature(transparent_enums)]
+
+// This enum has the same representation as `f32`.
+#[repr(transparent)]
+enum SingleFieldEnum {
+    Variant(f32)
+}
+
+// This enum has the same representation as `usize`.
+#[repr(transparent)]
+enum MultiFieldEnum {
+    Variant { field: usize, nothing: () },
+}
+```
+
+For consistency with transparent `struct`s, `enum`s must have exactly one
+non-zero-sized field. If all fields are zero-sized, the `enum` must not be
+`#[repr(transparent)]`:
+
+```rust
+#![feature(transparent_enums)]
+
+// This (non-transparent) enum is already valid in stable Rust:
+pub enum GoodEnum {
+    Nothing,
+}
+
+// Error: transparent enum needs exactly one non-zero-sized field, but has 0
+// #[repr(transparent)]
+// pub enum BadEnum {
+//     Nothing(()),
+// }
+
+// Error: transparent enum needs exactly one non-zero-sized field, but has 0
+// #[repr(transparent)]
+// pub enum BadEmptyEnum {
+//     Nothing,
+// }
+```
+
+The one exception is if the `enum` is generic over `T` and has a field of type
+`T`, it may be `#[repr(transparent)]` even if `T` is a zero-sized type:
+
+```rust
+#![feature(transparent_enums)]
+
+// This enum has the same representation as `T`.
+#[repr(transparent)]
+pub enum GenericEnum<T> {
+    Variant(T, ()),
+}
+
+// This is okay even though `()` is a zero-sized type.
+pub const THIS_IS_OKAY: GenericEnum<()> = GenericEnum::Variant((), ());
+```
+
+Transparent `enum`s require exactly one variant:
+
+```rust
+// Error: transparent enum needs exactly one variant, but has 0
+// #[repr(transparent)]
+// pub enum TooFewVariants {
+// }
+
+// Error: transparent enum needs exactly one variant, but has 2
+// #[repr(transparent)]
+// pub enum TooManyVariants {
+//     First(usize),
+//     Second,
+// }
+```
+
+Like transarent `struct`s, a transparent `enum` of type `E` has the same layout,
+size, and ABI as its single non-ZST field. If it is generic over a type `T`, and
+all its fields are ZSTs except for exactly one field of type `T`, then it has
+the same layout and ABI as `T` (even if `T` is a ZST when monomorphized).
+
+Like transparent `struct`s, transparent `enum`s are FFI-safe if and only if
+their underlying representation type is also FFI-safe.

src/doc/unstable-book/src/language-features/transparent-unions.md

@@ -0,0 +1,83 @@
+# `transparent_unions`
+
+The tracking issue for this feature is [#60405]
+
+[60405]: https://github.com/rust-lang/rust/issues/60405
+
+----
+
+The `transparent_unions` feature allows you mark `union`s as
+`#[repr(transparent)]`. A `union` may be `#[repr(transparent)]` in exactly the
+same conditions in which a `struct` may be `#[repr(transparent)]` (generally,
+this means the `union` must have exactly one non-zero-sized field). Some
+concrete illustrations follow.
+
+```rust
+#![feature(transparent_unions)]
+
+// This union has the same representation as `f32`.
+#[repr(transparent)]
+union SingleFieldUnion {
+    field: f32,
+}
+
+// This union has the same representation as `usize`.
+#[repr(transparent)]
+union MultiFieldUnion {
+    field: usize,
+    nothing: (),
+}
+```
+
+For consistency with transparent `struct`s, `union`s must have exactly one
+non-zero-sized field. If all fields are zero-sized, the `union` must not be
+`#[repr(transparent)]`:
+
+```rust
+#![feature(transparent_unions)]
+
+// This (non-transparent) union is already valid in stable Rust:
+pub union GoodUnion {
+    pub nothing: (),
+}
+
+// Error: transparent union needs exactly one non-zero-sized field, but has 0
+// #[repr(transparent)]
+// pub union BadUnion {
+//     pub nothing: (),
+// }
+```
+
+The one exception is if the `union` is generic over `T` and has a field of type
+`T`, it may be `#[repr(transparent)]` even if `T` is a zero-sized type:
+
+```rust
+#![feature(transparent_unions)]
+
+// This union has the same representation as `T`.
+#[repr(transparent)]
+pub union GenericUnion<T: Copy> { // Unions with non-`Copy` fields are unstable.
+    pub field: T,
+    pub nothing: (),
+}
+
+// This is okay even though `()` is a zero-sized type.
+pub const THIS_IS_OKAY: GenericUnion<()> = GenericUnion { field: () };
+```
+
+Like transarent `struct`s, a transparent `union` of type `U` has the same
+layout, size, and ABI as its single non-ZST field. If it is generic over a type
+`T`, and all its fields are ZSTs except for exactly one field of type `T`, then
+it has the same layout and ABI as `T` (even if `T` is a ZST when monomorphized).
+
+Like transparent `struct`s, transparent `union`s are FFI-safe if and only if
+their underlying representation type is also FFI-safe.
+
+A `union` may not be eligible for the same nonnull-style optimizations that a
+`struct` or `enum` (with the same fields) are eligible for. Adding
+`#[repr(transparent)]` to  `union` does not change this. To give a more concrete
+example, it is unspecified whether `size_of::<T>()` is equal to
+`size_of::<Option<T>>()`, where `T` is a `union` (regardless of whether or not
+it is transparent). The Rust compiler is free to perform this optimization if
+possible, but is not required to, and different compiler versions may differ in
+their application of these optimizations.

src/librustc/hir/check_attr.rs

@@ -181,12 +181,9 @@ impl<'a, 'tcx> CheckAttrVisitor<'a, 'tcx> {
             let (article, allowed_targets) = match hint.name_or_empty() {
                 name @ sym::C | name @ sym::align => {
                     is_c |= name == sym::C;
-                    if target != Target::Struct &&
-                            target != Target::Union &&
-                            target != Target::Enum {
-                                ("a", "struct, enum or union")
-                    } else {
-                        continue
+                    match target {
+                        Target::Struct | Target::Union | Target::Enum => continue,
+                        _ => ("a", "struct, enum, or union"),
                     }
                 }
                 sym::packed => {
@@ -207,10 +204,9 @@ impl<'a, 'tcx> CheckAttrVisitor<'a, 'tcx> {
                 }
                 sym::transparent => {
                     is_transparent = true;
-                    if target != Target::Struct {
-                        ("a", "struct")
-                    } else {
-                        continue
+                    match target {
+                        Target::Struct | Target::Union | Target::Enum => continue,
+                        _ => ("a", "struct, enum, or union"),
                     }
                 }
                 sym::i8  | sym::u8  | sym::i16 | sym::u16 |
@@ -241,7 +237,7 @@ impl<'a, 'tcx> CheckAttrVisitor<'a, 'tcx> {
         if is_transparent && hints.len() > 1 {
             let hint_spans: Vec<_> = hint_spans.clone().collect();
             span_err!(self.tcx.sess, hint_spans, E0692,
-                      "transparent struct cannot have other repr hints");
+                      "transparent {} cannot have other repr hints", target);
         }
         // Warn on repr(u8, u16), repr(C, simd), and c-like-enum-repr(C, u8)
         if (int_reprs > 1)
@@ -277,7 +273,7 @@ impl<'a, 'tcx> CheckAttrVisitor<'a, 'tcx> {
                         attr.span,
                         stmt.span,
                         "attribute should not be applied to a statement",
-                        "not a struct, enum or union",
+                        "not a struct, enum, or union",
                     );
                 }
             }
@@ -298,7 +294,7 @@ impl<'a, 'tcx> CheckAttrVisitor<'a, 'tcx> {
                     attr.span,
                     expr.span,
                     "attribute should not be applied to an expression",
-                    "not defining a struct, enum or union",
+                    "not defining a struct, enum, or union",
                 );
             }
         }

src/librustc/ty/mod.rs

@@ -2303,7 +2303,7 @@ impl<'a, 'gcx, 'tcx> AdtDef {
     /// Returns an iterator over all fields contained
     /// by this ADT.
     #[inline]
-    pub fn all_fields<'s>(&'s self) -> impl Iterator<Item = &'s FieldDef> {
+    pub fn all_fields<'s>(&'s self) -> impl Iterator<Item = &'s FieldDef> + Clone {
         self.variants.iter().flat_map(|v| v.fields.iter())
     }
 

src/librustc_lint/types.rs

@@ -531,8 +531,8 @@ fn ty_is_known_nonnull<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, ty: Ty<'tcx>) -> b
     match ty.sty {
         ty::FnPtr(_) => true,
         ty::Ref(..) => true,
-        ty::Adt(field_def, substs) if field_def.repr.transparent() && field_def.is_struct() => {
-            for field in &field_def.non_enum_variant().fields {
+        ty::Adt(field_def, substs) if field_def.repr.transparent() && !field_def.is_union() => {
+            for field in field_def.all_fields() {
                 let field_ty = tcx.normalize_erasing_regions(
                     ParamEnv::reveal_all(),
                     field.ty(tcx, substs),
@@ -627,8 +627,8 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
                             return FfiUnsafe {
                                 ty: ty,
                                 reason: "this struct has unspecified layout",
-                                help: Some("consider adding a #[repr(C)] or #[repr(transparent)] \
-                                            attribute to this struct"),
+                                help: Some("consider adding a `#[repr(C)]` or \
+                                            `#[repr(transparent)]` attribute to this struct"),
                             };
                         }
 
@@ -668,11 +668,12 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
                         if all_phantom { FfiPhantom(ty) } else { FfiSafe }
                     }
                     AdtKind::Union => {
-                        if !def.repr.c() {
+                        if !def.repr.c() && !def.repr.transparent() {
                             return FfiUnsafe {
                                 ty: ty,
                                 reason: "this union has unspecified layout",
-                                help: Some("consider adding a #[repr(C)] attribute to this union"),
+                                help: Some("consider adding a `#[repr(C)]` or \
+                                            `#[repr(transparent)]` attribute to this union"),
                             };
                         }
 
@@ -690,6 +691,11 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
                                 ParamEnv::reveal_all(),
                                 field.ty(cx, substs),
                             );
+                            // repr(transparent) types are allowed to have arbitrary ZSTs, not just
+                            // PhantomData -- skip checking all ZST fields.
+                            if def.repr.transparent() && is_zst(cx, field.did, field_ty) {
+                                continue;
+                            }
                             let r = self.check_type_for_ffi(cache, field_ty);
                             match r {
                                 FfiSafe => {
@@ -712,26 +718,32 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
 
                         // Check for a repr() attribute to specify the size of the
                         // discriminant.
-                        if !def.repr.c() && def.repr.int.is_none() {
+                        if !def.repr.c() && !def.repr.transparent() && def.repr.int.is_none() {
                             // Special-case types like `Option<extern fn()>`.
                             if !is_repr_nullable_ptr(cx, ty, def, substs) {
                                 return FfiUnsafe {
                                     ty: ty,
                                     reason: "enum has no representation hint",
-                                    help: Some("consider adding a #[repr(...)] attribute \
-                                                to this enum"),
+                                    help: Some("consider adding a `#[repr(C)]`, \
+                                                `#[repr(transparent)]`, or integer `#[repr(...)]` \
+                                                attribute to this enum"),
                                 };
                             }
                         }
 
                         // Check the contained variants.
                         for variant in &def.variants {
                             for field in &variant.fields {
-                                let arg = cx.normalize_erasing_regions(
+                                let field_ty = cx.normalize_erasing_regions(
                                     ParamEnv::reveal_all(),
                                     field.ty(cx, substs),
                                 );
-                                let r = self.check_type_for_ffi(cache, arg);
+                                // repr(transparent) types are allowed to have arbitrary ZSTs, not
+                                // just PhantomData -- skip checking all ZST fields.
+                                if def.repr.transparent() && is_zst(cx, field.did, field_ty) {
+                                    continue;
+                                }
+                                let r = self.check_type_for_ffi(cache, field_ty);
                                 match r {
                                     FfiSafe => {}
                                     FfiUnsafe { .. } => {