Add support for transparent enum

docs.md

@@ -133,10 +133,12 @@ You can learn about all of the different repr attributes [by reading Rust's refe
 
 * `#[repr(C)]`: give this struct/union/enum the same layout and ABI C would
 * `#[repr(u8, u16, ... etc)]`: give this enum the same layout and ABI as the given integer type
-* `#[repr(transparent)]`: give this single-field struct the same ABI as its field (useful for newtyping integers but keeping the integer ABI)
+* `#[repr(transparent)]`: give this single-field struct or enum the same ABI as its field (useful for newtyping integers but keeping the integer ABI)
 
 cbindgen supports the `#[repr(align(N))]` and `#[repr(packed)]` attributes, but currently does not support `#[repr(packed(N))]`.
 
+cbindgen supports using `repr(transparent)` on single-field structs and single-variant enums with fields. Transparent structs and enums are exported as typedefs that alias the underlying single field's type.
+
 cbindgen also supports using `repr(C)`/`repr(u8)` on non-C-like enums (enums with fields). This gives a C-compatible tagged union layout, as [defined by this RFC 2195][really-tagged-unions]. `repr(C)` will give a simpler layout that is perhaps more intuitive, while `repr(u8)` will produce a more compact layout.
 
 If you ensure everything has a guaranteed repr, then cbindgen will generate definitions for:
@@ -407,9 +409,17 @@ The rest are just local overrides for the same options found in the cbindgen.tom
 
 ### Enum Annotations
 
-* enum-trailing-values=\[variant1, variant2, ...\] -- add the following fieldless enum variants to the end of the enum's definition. These variant names *will* have the enum's renaming rules applied.
+* enum-trailing-values=\[variant1, variant2, ...\] -- add the following fieldless enum variants to
+  the end of the enum's definition. These variant names *will* have the enum's renaming rules
+  applied.
+
+  WARNING: if any of these values are ever passed into Rust, behaviour will be Undefined. Rust does
+  not know about them, and will assume they cannot happen.
 
-WARNING: if any of these values are ever passed into Rust, behaviour will be Undefined. Rust does not know about them, and will assume they cannot happen.
+* transparent-typedef -- when emitting the typedef for a transparent enum, mark it as
+  transparent. All references to the enum will be replaced with the type of its underlying NZST
+  variant field, effectively making the enum invisible on the FFI side. For exmaples of how this
+  works, see [Struct Annotations](#struct-annotations).
 
 The rest are just local overrides for the same options found in the cbindgen.toml:
 

src/bindgen/ir/enumeration.rs

@@ -12,7 +12,7 @@ use crate::bindgen::dependencies::Dependencies;
 use crate::bindgen::ir::{
     AnnotationSet, AnnotationValue, Cfg, ConditionWrite, DeprecatedNoteKind, Documentation, Field,
     GenericArgument, GenericParams, GenericPath, Item, ItemContainer, Literal, Path, Repr,
-    ReprStyle, Struct, ToCondition, TransparentTypeEraser, Type,
+    ReprStyle, Struct, ToCondition, TransparentTypeEraser, Type, Typedef,
 };
 use crate::bindgen::language_backend::LanguageBackend;
 use crate::bindgen::library::Library;
@@ -311,6 +311,10 @@ impl Enum {
         repr.style != ReprStyle::C
     }
 
+    pub fn is_transparent(&self) -> bool {
+        self.repr.style == ReprStyle::Transparent
+    }
+
     pub fn add_monomorphs(&self, library: &Library, out: &mut Monomorphs) {
         if self.is_generic() {
             return;
@@ -438,6 +442,31 @@ impl Enum {
             documentation,
         }
     }
+
+    /// Attempts to convert this enum to a typedef (only works for transparent enums).
+    pub fn as_typedef(&self) -> Option<Typedef> {
+        if self.is_transparent() {
+            // NOTE: A `#[repr(transparent)]` enum fails to compile unless it has
+            // exactly one NZT variant. So we can safely assume the variant exists.
+            if let Some(EnumVariant {
+                body: VariantBody::Body { ref body, .. },
+                ..
+            }) = self.variants.first()
+            {
+                // NOTE: Inline tagged enum has the tag field first, ignore it.
+                if let Some(field) = body.fields.last() {
+                    return Some(Typedef::new_from_item_field(self, field));
+                }
+            }
+        }
+        None
+    }
+
+    // Transparent enums become typedefs, so try converting to typedef and recurse on that.
+    pub fn as_transparent_alias(&self, generics: &[GenericArgument]) -> Option<Type> {
+        self.as_typedef()
+            .and_then(|t| t.as_transparent_alias(generics))
+    }
 }
 
 impl Item for Enum {
@@ -470,7 +499,9 @@ impl Item for Enum {
     }
 
     fn collect_declaration_types(&self, resolver: &mut DeclarationTypeResolver) {
-        if self.tag.is_some() {
+        if self.repr.style == ReprStyle::Transparent {
+            resolver.add_none(&self.path);
+        } else if self.tag.is_some() {
             if self.repr.style == ReprStyle::C {
                 resolver.add_struct(&self.path);
             } else {

src/bindgen/ir/structure.rs

@@ -150,7 +150,7 @@ impl Struct {
             // NOTE: A `#[repr(transparent)]` struct with 2+ NZT fields fails to compile, but 0
             // fields is allowed for some strange reason. Don't emit the typedef in that case.
             if let Some(field) = self.fields.first() {
-                return Some(Typedef::new_from_struct_field(self, field));
+                return Some(Typedef::new_from_item_field(self, field));
             } else {
                 error!(
                     "Cannot convert empty transparent struct {} to typedef",

src/bindgen/ir/ty.rs

@@ -569,6 +569,7 @@ impl Type {
                             ItemContainer::OpaqueItem(o) => o.as_transparent_alias(&generics),
                             ItemContainer::Typedef(t) => t.as_transparent_alias(&generics),
                             ItemContainer::Struct(s) => s.as_transparent_alias(&generics),
+                            ItemContainer::Enum(e) => e.as_transparent_alias(&generics),
                             _ => None,
                         };
                         if let Some(mut ty) = aliased_ty {

src/bindgen/ir/typedef.rs

@@ -11,7 +11,7 @@ use crate::bindgen::declarationtyperesolver::DeclarationTypeResolver;
 use crate::bindgen::dependencies::Dependencies;
 use crate::bindgen::ir::{
     AnnotationSet, Cfg, Documentation, Field, GenericArgument, GenericParams, Item, ItemContainer,
-    Path, Struct, TransparentTypeEraser, Type,
+    Path, TransparentTypeEraser, Type,
 };
 use crate::bindgen::library::Library;
 use crate::bindgen::mangle;
@@ -69,12 +69,12 @@ impl Typedef {
         }
     }
 
-    // Used to convert a transparent Struct to a Typedef.
-    pub fn new_from_struct_field(item: &Struct, field: &Field) -> Self {
+    // Used to convert a transparent Struct or Enum to a Typedef.
+    pub fn new_from_item_field(item: &impl Item, field: &Field) -> Self {
         Self {
             path: item.path().clone(),
             export_name: item.export_name().to_string(),
-            generic_params: item.generic_params.clone(),
+            generic_params: item.generic_params().clone(),
             aliased: field.ty.clone(),
             cfg: item.cfg().cloned(),
             annotations: item.annotations().clone(),

src/bindgen/language_backend/mod.rs

@@ -157,6 +157,27 @@ pub trait LanguageBackend: Sized {
         }
     }
 
+    /// If the enum is transparent, emit a typedef of its NZST field type instead.
+    fn write_enum_or_typedef<W: Write>(
+        &mut self,
+        out: &mut SourceWriter<W>,
+        e: &Enum,
+        _b: &Bindings,
+    ) {
+        if let Some(typedef) = e.as_typedef() {
+            self.write_type_def(out, &typedef);
+            // TODO: Associated constants are not supported for enums. Should they be? Rust
+            // enum exports as a union, and C/C++ at least supports static union members?
+            //
+            //for constant in &e.associated_constants {
+            //    out.new_line();
+            //    constant.write(b.config, self, out, Some(e));
+            //}
+        } else {
+            self.write_enum(out, e);
+        }
+    }
+
     fn write_items<W: Write>(&mut self, out: &mut SourceWriter<W>, b: &Bindings) {
         for item in &b.items {
             if item
@@ -172,7 +193,7 @@ pub trait LanguageBackend: Sized {
             match *item {
                 ItemContainer::Constant(..) => unreachable!(),
                 ItemContainer::Static(..) => unreachable!(),
-                ItemContainer::Enum(ref x) => self.write_enum(out, x),
+                ItemContainer::Enum(ref x) => self.write_enum_or_typedef(out, x, b),
                 ItemContainer::Struct(ref x) => self.write_struct_or_typedef(out, x, b),
                 ItemContainer::Union(ref x) => self.write_union(out, x),
                 ItemContainer::OpaqueItem(ref x) => self.write_opaque_item(out, x),

tests/expectations/const_transparent.compat.c

@@ -3,17 +3,29 @@
 #include <stdint.h>
 #include <stdlib.h>
 
+#define TransparentEnum_ASSOC_ENUM_FOO 8
+
 typedef uint8_t TransparentStruct;
 #define TransparentStruct_ASSOC_STRUCT_FOO 1
 #define TransparentStruct_ASSOC_STRUCT_BAR 2
 
 
 typedef uint8_t TransparentTupleStruct;
 
+typedef uint8_t TransparentEnum;
+
 #define STRUCT_FOO 4
 
 #define STRUCT_BAR 5
 
 
 
 
+
+
+
+
+
+
+
+

tests/expectations/const_transparent.cpp

@@ -4,6 +4,8 @@
 #include <ostream>
 #include <new>
 
+constexpr static const int64_t TransparentEnum_ASSOC_ENUM_FOO = 8;
+
 template<typename T>
 using Wrapper = T;
 
@@ -17,10 +19,23 @@ using TransparentTupleStruct = uint8_t;
 template<typename T>
 using TransparentStructWithErasedField = T;
 
+using TransparentEnum = uint8_t;
+
+template<typename T>
+using TransparentWrapperEnum = T;
+
 constexpr static const TransparentStruct STRUCT_FOO = 4;
 
 constexpr static const TransparentTupleStruct STRUCT_BAR = 5;
 
 constexpr static const TransparentStruct STRUCT_BAZ = 6;
 
 constexpr static const TransparentStructWithErasedField<TransparentStruct> COMPLEX = 7;
+
+
+
+
+
+
+
+

tests/expectations/const_transparent.pyx

@@ -6,17 +6,29 @@ cdef extern from *:
 
 cdef extern from *:
 
+  const int64_t TransparentEnum_ASSOC_ENUM_FOO # = 8
+
   ctypedef uint8_t TransparentStruct;
   const int64_t TransparentStruct_ASSOC_STRUCT_FOO # = 1
   const TransparentStruct TransparentStruct_ASSOC_STRUCT_BAR # = 2
 
 
   ctypedef uint8_t TransparentTupleStruct;
 
+  ctypedef uint8_t TransparentEnum;
+
   const TransparentStruct STRUCT_FOO # = 4
 
   const TransparentTupleStruct STRUCT_BAR # = 5
 
 
 
 
+
+
+
+
+
+
+
+

tests/expectations/transparent.c

@@ -13,48 +13,77 @@ typedef DummyStruct TransparentComplexWrappingStructTuple;
 
 typedef uint32_t TransparentPrimitiveWrappingStructTuple;
 
-typedef DummyStruct TransparentComplexWrappingStructure;
+typedef DummyStruct TransparentComplexWrappingStruct;
 
-typedef uint32_t TransparentPrimitiveWrappingStructure;
+typedef uint32_t TransparentPrimitiveWrappingStruct;
 
-typedef DummyStruct TransparentComplexWrapper_i32;
+typedef DummyStruct TransparentComplexWrapperStruct_i32;
 
-typedef uint32_t TransparentPrimitiveWrapper_i32;
+typedef uint32_t TransparentPrimitiveWrapperStruct_i32;
 
-typedef uint32_t TransparentPrimitiveWithAssociatedConstants;
-#define TransparentPrimitiveWithAssociatedConstants_ZERO 0
-#define TransparentPrimitiveWithAssociatedConstants_ONE 1
+typedef uint32_t TransparentPrimitiveStructWithAssociatedConstants;
+#define TransparentPrimitiveStructWithAssociatedConstants_STRUCT_ZERO 0
+#define TransparentPrimitiveStructWithAssociatedConstants_STRUCT_ONE 1
 
-typedef const uint32_t *TransparentPointerWrappingStructure;
+typedef const uint32_t *TransparentPointerWrappingStruct;
 
 typedef int32_t TransparentIntStruct;
 
 typedef DummyStruct TransparentComplexStruct;
 
-typedef TransparentPrimitiveWrappingStructure TransparentTransparentStruct;
+typedef TransparentPrimitiveWrappingStruct TransparentTransparentStruct;
 
 typedef uint32_t *TransparentNonNullStruct;
 
 typedef uint32_t *TransparentOptionNonNullStruct;
 
+typedef DummyStruct TransparentComplexWrappingEnumTuple;
+
+typedef uint32_t TransparentPrimitiveWrappingEnumTuple;
+
+typedef DummyStruct TransparentComplexWrappingEnum;
+
+typedef uint32_t TransparentPrimitiveWrappingEnum;
+
+typedef DummyStruct TransparentComplexWrapperEnum_i32;
+
+typedef uint32_t TransparentPrimitiveWrapperEnum_i32;
+
+typedef uint32_t TransparentPrimitiveEnumWithAssociatedConstants;
+
+typedef const uint32_t *TransparentPointerWrappingEnum;
+
+typedef int32_t TransparentIntEnum;
+
+typedef DummyStruct TransparentComplexEnum;
+
+typedef TransparentPrimitiveWrappingEnum TransparentTransparentEnum;
+
+typedef uint32_t *TransparentNonNullEnum;
+
+typedef uint32_t *TransparentOptionNonNullEnum;
+
 #define StructWithAssociatedConstantInImpl_STRUCT_TEN 10
 
+
+
+
+
 #define EnumWithAssociatedConstantInImpl_ENUM_TEN 10
 
-void root(TransparentComplexWrappingStructTuple a,
-          TransparentPrimitiveWrappingStructTuple b,
-          TransparentComplexWrappingStructure c,
-          TransparentPrimitiveWrappingStructure d,
-          TransparentComplexWrapper_i32 e,
-          TransparentPrimitiveWrapper_i32 f,
-          TransparentPrimitiveWithAssociatedConstants g,
-          TransparentPointerWrappingStructure h,
-          StructWithAssociatedConstantInImpl i,
-          EnumWithAssociatedConstantInImpl j);
+void struct_root(TransparentComplexWrappingStructTuple a,
+                 TransparentPrimitiveWrappingStructTuple b,
+                 TransparentComplexWrappingStruct c,
+                 TransparentPrimitiveWrappingStruct d,
+                 TransparentComplexWrapperStruct_i32 e,
+                 TransparentPrimitiveWrapperStruct_i32 f,
+                 TransparentPrimitiveStructWithAssociatedConstants g,
+                 TransparentPointerWrappingStruct h,
+                 StructWithAssociatedConstantInImpl i);
 
 void erased_root(uint32_t *a,
                  uint32_t *b,
-                 TransparentPrimitiveWrappingStructure c,
+                 TransparentPrimitiveWrappingStruct c,
                  uint32_t *d,
                  TransparentIntStruct e,
                  int32_t f,
@@ -66,3 +95,28 @@ void erased_root(uint32_t *a,
                  TransparentTransparentStruct l,
                  TransparentNonNullStruct m,
                  TransparentOptionNonNullStruct n);
+
+void enum_root(TransparentComplexWrappingEnumTuple a,
+               TransparentPrimitiveWrappingEnumTuple b,
+               TransparentComplexWrappingEnum c,
+               TransparentPrimitiveWrappingEnum d,
+               TransparentComplexWrapperEnum_i32 e,
+               TransparentPrimitiveWrapperEnum_i32 f,
+               TransparentPrimitiveEnumWithAssociatedConstants g,
+               TransparentPointerWrappingEnum h,
+               EnumWithAssociatedConstantInImpl i);
+
+void erased_enum_root(uint32_t *a,
+                      uint32_t *b,
+                      TransparentPrimitiveWrappingEnum c,
+                      uint32_t *d,
+                      TransparentIntEnum e,
+                      int32_t f,
+                      DummyStruct g,
+                      uint32_t *h,
+                      int32_t i,
+                      TransparentIntEnum j,
+                      TransparentComplexEnum k,
+                      TransparentTransparentEnum l,
+                      TransparentNonNullEnum m,
+                      TransparentOptionNonNullEnum n);