add test for add_derives

This test derives PartialEq for the Test struct, and then attempts to
use that by calling assert_ne! on two Test instances. If the derive
callback doesn't work, no PartialEq will be present and the test will
fail to compile.

bindgen-integration/build.rs

@@ -119,6 +119,17 @@ impl ParseCallbacks for MacroCallback {
             }
         }
     }
+
+    // Test the "custom derives" capability by adding `PartialEq` to the `Test` struct.
+    fn add_derives(&self, name: &str) -> Vec<String> {
+        if name == "Test" {
+            vec![
+                "PartialEq".into(),
+            ]
+        } else {
+            vec![]
+        }
+    }
 }
 
 impl Drop for MacroCallback {

bindgen-integration/src/lib.rs

@@ -267,3 +267,12 @@ fn test_homogeneous_aggregate_float_union() {
         assert_eq!([1., 2., 3., 4.], coord.v)
     }
 }
+
+#[test]
+fn test_custom_derive() {
+    // The `add_derives` callback should have added `#[derive(PartialEq)]`
+    // to the `Test` struct. If it didn't, this will fail to compile.
+    let test1 = unsafe { bindings::Test::new(5) };
+    let test2 = unsafe { bindings::Test::new(6) };
+    assert_ne!(test1, test2);
+}