allow custom derives on enums

Custom derives are just as useful on enums as they are on structs; not
supporting this was an oversight.

Adds a test that will fail to compile if the custom derive doesn't work
on enums. This test fails without the codegen fix.

Author: ericseppanen

bindgen-integration/build.rs

@@ -126,6 +126,10 @@ impl ParseCallbacks for MacroCallback {
             vec![
                 "PartialEq".into(),
             ]
+        } else if name == "MyOrderedEnum" {
+            vec![
+                "PartialOrd".into(),
+            ]
         } else {
             vec![]
         }

bindgen-integration/cpp/Test.h

@@ -234,3 +234,10 @@ typedef union {
 } Coord;
 
 Coord coord(double x, double y, double z, double t);
+
+// Used to test custom derives on enum. See `test_custom_derive`.
+enum MyOrderedEnum {
+  MICRON,
+  METER,
+  LIGHTYEAR,
+};

bindgen-integration/src/lib.rs

@@ -275,4 +275,14 @@ fn test_custom_derive() {
     let test1 = unsafe { bindings::Test::new(5) };
     let test2 = unsafe { bindings::Test::new(6) };
     assert_ne!(test1, test2);
+
+    // The `add_derives` callback should have added `#[derive(PartialOrd)]`
+    // to the `MyOrderedEnum` enum. If it didn't, this will fail to compile.
+
+    let micron = unsafe { bindings::MyOrderedEnum::MICRON };
+    let meter = unsafe { bindings::MyOrderedEnum::METER };
+    let lightyear = unsafe { bindings::MyOrderedEnum::LIGHTYEAR };
+
+    assert!(meter < lightyear);
+    assert!(meter > micron);
 }

src/codegen/mod.rs

@@ -3014,6 +3014,16 @@ impl CodeGenerator for Enum {
                     derives.push(derive);
                 }
             }
+
+            // The custom derives callback may return a list of derive attributes;
+            // add them to the end of the list.
+            let custom_derives;
+            if let Some(cb) = &ctx.options().parse_callbacks {
+                custom_derives = cb.add_derives(&name);
+                // In most cases this will be a no-op, since custom_derives will be empty.
+                derives.extend(custom_derives.iter().map(|s| s.as_str()));
+            };
+
             attrs.push(attributes::derives(&derives));
         }