Add enum_class option

This option allows specifying that a C++ enum should be emitted with
plain `enum` rather than `enum class`. It's true that `enum class` should
generally be preferred, but sometimes there's existing code which is
already using plain `enum`, and porting that code to Rust becomes easier when
`cbindgen` can emit plain `enum`.

This option can be overridden on a per-enum basis using a `cbindgen:` annotation.

It defaults to true for two reasons:

* Backward compatibility.
* `enum class` is probably actually what you want, in general.

docs.md

@@ -269,6 +269,7 @@ The rest are just local overrides for the same options found in the cbindgen.tom
 * derive-mut-casts
 * derive-tagged-enum-destructor
 * derive-tagged-enum-copy-constructor
+* enum-class
 * prefix-with-name
 * private-default-tagged-enum-constructor
 
@@ -672,6 +673,10 @@ add_sentinel = false
 # default: false
 prefix_with_name = false
 
+# Whether to emit enums using "enum class" when targeting C++.
+# default: true
+enum_class = true
+
 # Whether to generate static `::MyVariant(..)` constructors and `bool IsMyVariant()`
 # methods for enums with fields.
 #

src/bindgen/config.rs

@@ -391,7 +391,7 @@ impl StructConfig {
 }
 
 /// Settings to apply to generated enums.
-#[derive(Debug, Clone, Default, Deserialize)]
+#[derive(Debug, Clone, Deserialize)]
 #[serde(rename_all = "snake_case")]
 #[serde(deny_unknown_fields)]
 #[serde(default)]
@@ -425,11 +425,34 @@ pub struct EnumConfig {
     /// This is only generated if a copy constructor for the same tagged enum is
     /// generated as well.
     pub derive_tagged_enum_copy_assignment: bool,
+    /// Declare the enum as an enum class.
+    /// Only relevant when targeting C++.
+    pub enum_class: bool,
     /// Whether to generate empty, private default-constructors for tagged
     /// enums.
     pub private_default_tagged_enum_constructor: bool,
 }
 
+impl Default for EnumConfig {
+    fn default() -> EnumConfig {
+        EnumConfig {
+            rename_variants: None,
+            add_sentinel: false,
+            prefix_with_name: false,
+            derive_helper_methods: false,
+            derive_const_casts: false,
+            derive_mut_casts: false,
+            cast_assert_name: None,
+            must_use: None,
+            derive_tagged_enum_destructor: false,
+            derive_tagged_enum_copy_constructor: false,
+            derive_tagged_enum_copy_assignment: false,
+            enum_class: true,
+            private_default_tagged_enum_constructor: false,
+        }
+    }
+}
+
 impl EnumConfig {
     pub(crate) fn add_sentinel(&self, annotations: &AnnotationSet) -> bool {
         if let Some(x) = annotations.bool("add-sentinel") {
@@ -473,6 +496,12 @@ impl EnumConfig {
         }
         self.derive_tagged_enum_copy_assignment
     }
+    pub(crate) fn enum_class(&self, annotations: &AnnotationSet) -> bool {
+        if let Some(x) = annotations.bool("enum-class") {
+            return x;
+        }
+        self.enum_class
+    }
     pub(crate) fn private_default_tagged_enum_constructor(
         &self,
         annotations: &AnnotationSet,

src/bindgen/ir/enumeration.rs

@@ -559,7 +559,11 @@ impl Source for Enum {
                 }
             }
         } else {
-            out.write("enum class");
+            if config.enumeration.enum_class(&self.annotations) {
+                out.write("enum class");
+            } else {
+                out.write("enum");
+            }
 
             if self.annotations.must_use {
                 if let Some(ref anno) = config.enumeration.must_use {

template.toml

@@ -103,6 +103,7 @@ derive_mut_casts = false
 # cast_assert_name = "ASSERT"
 derive_tagged_enum_destructor = false
 derive_tagged_enum_copy_constructor = false
+enum_class = true
 private_default_tagged_enum_constructor = false
 
 

tests/expectations/both/enum.c

@@ -65,6 +65,21 @@ enum M {
 };
 typedef int8_t M;
 
+typedef enum N {
+  n1,
+  n2,
+  n3,
+  n4,
+} N;
+
+enum O {
+  o1,
+  o2,
+  o3,
+  o4,
+};
+typedef int8_t O;
+
 typedef struct J J;
 
 typedef struct K K;
@@ -142,4 +157,19 @@ typedef struct I {
   };
 } I;
 
-void root(Opaque *o, A a, B b, C c, D d, E e, F f, G g, H h, I i, J j, K k, L l, M m);
+void root(Opaque *opaque,
+          A a,
+          B b,
+          C c,
+          D d,
+          E e,
+          F f,
+          G g,
+          H h,
+          I i,
+          J j,
+          K k,
+          L l,
+          M m,
+          N n,
+          O o);

tests/expectations/both/enum.compat.c

@@ -107,6 +107,27 @@ enum M
 typedef int8_t M;
 #endif // __cplusplus
 
+typedef enum N {
+  n1,
+  n2,
+  n3,
+  n4,
+} N;
+
+enum O
+#ifdef __cplusplus
+  : int8_t
+#endif // __cplusplus
+ {
+  o1,
+  o2,
+  o3,
+  o4,
+};
+#ifndef __cplusplus
+typedef int8_t O;
+#endif // __cplusplus
+
 typedef struct J J;
 
 typedef struct K K;
@@ -200,7 +221,22 @@ typedef struct I {
 extern "C" {
 #endif // __cplusplus
 
-void root(Opaque *o, A a, B b, C c, D d, E e, F f, G g, H h, I i, J j, K k, L l, M m);
+void root(Opaque *opaque,
+          A a,
+          B b,
+          C c,
+          D d,
+          E e,
+          F f,
+          G g,
+          H h,
+          I i,
+          J j,
+          K k,
+          L l,
+          M m,
+          N n,
+          O o);
 
 #ifdef __cplusplus
 } // extern "C"

tests/expectations/enum.c

@@ -65,6 +65,21 @@ enum M {
 };
 typedef int8_t M;
 
+typedef enum {
+  n1,
+  n2,
+  n3,
+  n4,
+} N;
+
+enum O {
+  o1,
+  o2,
+  o3,
+  o4,
+};
+typedef int8_t O;
+
 typedef struct J J;
 
 typedef struct K K;
@@ -142,4 +157,19 @@ typedef struct {
   };
 } I;
 
-void root(Opaque *o, A a, B b, C c, D d, E e, F f, G g, H h, I i, J j, K k, L l, M m);
+void root(Opaque *opaque,
+          A a,
+          B b,
+          C c,
+          D d,
+          E e,
+          F f,
+          G g,
+          H h,
+          I i,
+          J j,
+          K k,
+          L l,
+          M m,
+          N n,
+          O o);

tests/expectations/enum.compat.c

@@ -107,6 +107,27 @@ enum M
 typedef int8_t M;
 #endif // __cplusplus
 
+typedef enum {
+  n1,
+  n2,
+  n3,
+  n4,
+} N;
+
+enum O
+#ifdef __cplusplus
+  : int8_t
+#endif // __cplusplus
+ {
+  o1,
+  o2,
+  o3,
+  o4,
+};
+#ifndef __cplusplus
+typedef int8_t O;
+#endif // __cplusplus
+
 typedef struct J J;
 
 typedef struct K K;
@@ -200,7 +221,22 @@ typedef struct {
 extern "C" {
 #endif // __cplusplus
 
-void root(Opaque *o, A a, B b, C c, D d, E e, F f, G g, H h, I i, J j, K k, L l, M m);
+void root(Opaque *opaque,
+          A a,
+          B b,
+          C c,
+          D d,
+          E e,
+          F f,
+          G g,
+          H h,
+          I i,
+          J j,
+          K k,
+          L l,
+          M m,
+          N n,
+          O o);
 
 #ifdef __cplusplus
 } // extern "C"

tests/expectations/enum.cpp

@@ -58,6 +58,20 @@ enum class M : int8_t {
   m3 = 1,
 };
 
+enum N {
+  n1,
+  n2,
+  n3,
+  n4,
+};
+
+enum O : int8_t {
+  o1,
+  o2,
+  o3,
+  o4,
+};
+
 struct J;
 
 struct K;
@@ -137,6 +151,21 @@ struct I {
 
 extern "C" {
 
-void root(Opaque *o, A a, B b, C c, D d, E e, F f, G g, H h, I i, J j, K k, L l, M m);
+void root(Opaque *opaque,
+          A a,
+          B b,
+          C c,
+          D d,
+          E e,
+          F f,
+          G g,
+          H h,
+          I i,
+          J j,
+          K k,
+          L l,
+          M m,
+          N n,
+          O o);
 
 } // extern "C"

tests/expectations/tag/enum.c

@@ -65,6 +65,21 @@ enum M {
 };
 typedef int8_t M;
 
+enum N {
+  n1,
+  n2,
+  n3,
+  n4,
+};
+
+enum O {
+  o1,
+  o2,
+  o3,
+  o4,
+};
+typedef int8_t O;
+
 struct J;
 
 struct K;
@@ -142,7 +157,7 @@ struct I {
   };
 };
 
-void root(struct Opaque *o,
+void root(struct Opaque *opaque,
           A a,
           B b,
           C c,
@@ -155,4 +170,6 @@ void root(struct Opaque *o,
           struct J j,
           struct K k,
           enum L l,
-          M m);
+          M m,
+          enum N n,
+          O o);

tests/expectations/tag/enum.compat.c

@@ -107,6 +107,27 @@ enum M
 typedef int8_t M;
 #endif // __cplusplus
 
+enum N {
+  n1,
+  n2,
+  n3,
+  n4,
+};
+
+enum O
+#ifdef __cplusplus
+  : int8_t
+#endif // __cplusplus
+ {
+  o1,
+  o2,
+  o3,
+  o4,
+};
+#ifndef __cplusplus
+typedef int8_t O;
+#endif // __cplusplus
+
 struct J;
 
 struct K;
@@ -200,7 +221,7 @@ struct I {
 extern "C" {
 #endif // __cplusplus
 
-void root(struct Opaque *o,
+void root(struct Opaque *opaque,
           A a,
           B b,
           C c,
@@ -213,7 +234,9 @@ void root(struct Opaque *o,
           struct J j,
           struct K k,
           enum L l,
-          M m);
+          M m,
+          enum N n,
+          O o);
 
 #ifdef __cplusplus
 } // extern "C"

tests/rust/enum.rs

@@ -103,9 +103,27 @@ enum M {
     m3 = 1,
 }
 
+/// cbindgen:enum-class=false
+#[repr(C)]
+enum N {
+    n1,
+    n2,
+    n3,
+    n4,
+}
+
+/// cbindgen:enum-class=false
+#[repr(i8)]
+enum O {
+    o1,
+    o2,
+    o3,
+    o4,
+}
+
 #[no_mangle]
 pub extern "C" fn root(
-    o: *mut Opaque,
+    opaque: *mut Opaque,
     a: A,
     b: B,
     c: C,
@@ -119,5 +137,7 @@ pub extern "C" fn root(
     k: K,
     l: L,
     m: M,
+    n: N,
+    o: O,
 ) {
 }