Derive from any other trait only when deriving from Copy

It's impossible to #[derive] from any other trair when not deriving from
Copy when using the newest Rust nightly. Any attempt to do that results
in the following error:

  error: `#[derive]` can't be used on a `#[repr(packed)]` struct that does not derive Copy (error E0133)

Fixes: rust-lang#2083
Signed-off-by: Michal Rostecki <[email protected]>

Author: vadorovsky

src/codegen/mod.rs

@@ -113,14 +113,6 @@ bitflags! {
 fn derives_of_item(item: &Item, ctx: &BindgenContext) -> DerivableTraits {
     let mut derivable_traits = DerivableTraits::empty();
 
-    if item.can_derive_debug(ctx) && !item.annotations().disallow_debug() {
-        derivable_traits |= DerivableTraits::DEBUG;
-    }
-
-    if item.can_derive_default(ctx) && !item.annotations().disallow_default() {
-        derivable_traits |= DerivableTraits::DEFAULT;
-    }
-
     let all_template_params = item.all_template_params(ctx);
 
     if item.can_derive_copy(ctx) && !item.annotations().disallow_copy() {
@@ -137,26 +129,34 @@ fn derives_of_item(item: &Item, ctx: &BindgenContext) -> DerivableTraits {
             // It's not hard to fix though.
             derivable_traits |= DerivableTraits::CLONE;
         }
-    }
 
-    if item.can_derive_hash(ctx) {
-        derivable_traits |= DerivableTraits::HASH;
-    }
+        if item.can_derive_debug(ctx) && !item.annotations().disallow_debug() {
+            derivable_traits |= DerivableTraits::DEBUG;
+        }
 
-    if item.can_derive_partialord(ctx) {
-        derivable_traits |= DerivableTraits::PARTIAL_ORD;
-    }
+        if item.can_derive_default(ctx) && !item.annotations().disallow_default() {
+            derivable_traits |= DerivableTraits::DEFAULT;
+        }
 
-    if item.can_derive_ord(ctx) {
-        derivable_traits |= DerivableTraits::ORD;
-    }
+        if item.can_derive_hash(ctx) {
+            derivable_traits |= DerivableTraits::HASH;
+        }
 
-    if item.can_derive_partialeq(ctx) {
-        derivable_traits |= DerivableTraits::PARTIAL_EQ;
-    }
+        if item.can_derive_partialord(ctx) {
+            derivable_traits |= DerivableTraits::PARTIAL_ORD;
+        }
 
-    if item.can_derive_eq(ctx) {
-        derivable_traits |= DerivableTraits::EQ;
+        if item.can_derive_ord(ctx) {
+            derivable_traits |= DerivableTraits::ORD;
+        }
+
+        if item.can_derive_partialeq(ctx) {
+            derivable_traits |= DerivableTraits::PARTIAL_EQ;
+        }
+
+        if item.can_derive_eq(ctx) {
+            derivable_traits |= DerivableTraits::EQ;
+        }
     }
 
     derivable_traits