Add source annotation to express explicit derives for a type.

This allows for explicit selection of which traits are derived for a
type, e.g. deriving custom traits on particular types that need it.

Example usage:

```C++
/// <div rustbindgen derive="Clone"></div>
/// <div rustbindgen derive="MyDerivableTrait"></div>
struct foo { ...  };
```

generates into

```Rust
 #[derive(Clone,MyDerivableTrait)]
 struct foo { ... }
```

4 files changed, 138 insertions, 0 deletions

diff --git a/src/codegen/mod.rs b/src/codegen/mod.rs
index 073745b1..9405f11b 100644
--- a/src/codegen/mod.rs
+++ b/src/codegen/mod.rs
@@ -1764,6 +1764,8 @@ impl CodeGenerator for CompInfo {
             derives.push("Eq");
         }
 
+        derives.extend(item.annotations().derives().iter().map(String::as_str));
+
         if !derives.is_empty() {
             attributes.push(attributes::derives(&derives))
         }
diff --git a/src/ir/annotations.rs b/src/ir/annotations.rs
index bc57b555..654e1686 100644
--- a/src/ir/annotations.rs
+++ b/src/ir/annotations.rs
@@ -58,6 +58,8 @@ pub struct Annotations {
     /// In that case, bindgen will generate a constant for `Bar` instead of
     /// `Baz`.
     constify_enum_variant: bool,
+    /// List of explicit derives for this type.
+    derives: Vec<String>,
 }
 
 fn parse_accessor(s: &str) -> FieldAccessorKind {
@@ -79,6 +81,7 @@ impl Default for Annotations {
             private_fields: None,
             accessor_kind: None,
             constify_enum_variant: false,
+            derives: vec![],
         }
     }
 }
@@ -130,6 +133,11 @@ impl Annotations {
         self.use_instead_of.as_ref().map(|s| &**s)
     }
 
+    /// The list of derives that have been specified in this annotation.
+    pub fn derives(&self) -> &[String] {
+        &self.derives
+    }
+
     /// Should we avoid implementing the `Copy` trait?
     pub fn disallow_copy(&self) -> bool {
         self.disallow_copy
@@ -165,6 +173,9 @@ impl Annotations {
                                 attr.value.split("::").map(Into::into).collect(),
                             )
                     }
+                    "derive" => {
+                        self.derives.push(attr.value)
+                    }
                     "private" => {
                         self.private_fields = Some(attr.value != "false")
                     }
diff --git a/tests/expectations/tests/derive-custom.rs b/tests/expectations/tests/derive-custom.rs
new file mode 100644
index 00000000..1184e435
--- /dev/null
+++ b/tests/expectations/tests/derive-custom.rs
@@ -0,0 +1,97 @@
+/* automatically generated by rust-bindgen */
+
+#![allow(
+    dead_code,
+    non_snake_case,
+    non_camel_case_types,
+    non_upper_case_globals
+)]
+
+/// <div rustbindgen derive="Debug"></div>
+#[repr(C)]
+#[derive(Default, Debug)]
+pub struct my_type {
+    pub a: ::std::os::raw::c_int,
+}
+#[test]
+fn bindgen_test_layout_my_type() {
+    assert_eq!(
+        ::std::mem::size_of::<my_type>(),
+        4usize,
+        concat!("Size of: ", stringify!(my_type))
+    );
+    assert_eq!(
+        ::std::mem::align_of::<my_type>(),
+        4usize,
+        concat!("Alignment of ", stringify!(my_type))
+    );
+    assert_eq!(
+        unsafe { &(*(::std::ptr::null::<my_type>())).a as *const _ as usize },
+        0usize,
+        concat!(
+            "Offset of field: ",
+            stringify!(my_type),
+            "::",
+            stringify!(a)
+        )
+    );
+}
+/// <div rustbindgen derive="Debug"></div>
+/// <div rustbindgen derive="Clone"></div>
+#[repr(C)]
+#[derive(Default, Debug, Clone)]
+pub struct my_type2 {
+    pub a: ::std::os::raw::c_uint,
+}
+#[test]
+fn bindgen_test_layout_my_type2() {
+    assert_eq!(
+        ::std::mem::size_of::<my_type2>(),
+        4usize,
+        concat!("Size of: ", stringify!(my_type2))
+    );
+    assert_eq!(
+        ::std::mem::align_of::<my_type2>(),
+        4usize,
+        concat!("Alignment of ", stringify!(my_type2))
+    );
+    assert_eq!(
+        unsafe { &(*(::std::ptr::null::<my_type2>())).a as *const _ as usize },
+        0usize,
+        concat!(
+            "Offset of field: ",
+            stringify!(my_type2),
+            "::",
+            stringify!(a)
+        )
+    );
+}
+/// <div rustbindgen derive="Debug" derive="Clone"></div>
+#[repr(C)]
+#[derive(Default, Debug, Clone)]
+pub struct my_type3 {
+    pub a: ::std::os::raw::c_ulong,
+}
+#[test]
+fn bindgen_test_layout_my_type3() {
+    assert_eq!(
+        ::std::mem::size_of::<my_type3>(),
+        8usize,
+        concat!("Size of: ", stringify!(my_type3))
+    );
+    assert_eq!(
+        ::std::mem::align_of::<my_type3>(),
+        8usize,
+        concat!("Alignment of ", stringify!(my_type3))
+    );
+    assert_eq!(
+        unsafe { &(*(::std::ptr::null::<my_type3>())).a as *const _ as usize },
+        0usize,
+        concat!(
+            "Offset of field: ",
+            stringify!(my_type3),
+            "::",
+            stringify!(a)
+        )
+    );
+}
diff --git a/tests/headers/derive-custom.h b/tests/headers/derive-custom.h
new file mode 100644
index 00000000..83f2ce08
--- /dev/null
+++ b/tests/headers/derive-custom.h
@@ -0,0 +1,28 @@
+// bindgen-flags: --no-derive-debug --no-derive-copy --default-enum-style rust
+
+/** <div rustbindgen derive="Debug"></div> */
+struct my_type;
+
+/** <div rustbindgen derive="Clone"></div> */
+struct my_type;
+
+struct my_type {
+    int a;
+};
+
+/**
+ * <div rustbindgen derive="Debug"></div>
+ * <div rustbindgen derive="Clone"></div>
+ */
+struct my_type2;
+
+struct my_type2 {
+    unsigned a;
+};
+
+/**
+ * <div rustbindgen derive="Debug" derive="Clone"></div>
+ */
+struct my_type3 {
+    unsigned long a;
+};