Prevent default derives for Forward declared types.

We do not know the layout for forward declared types.
Deriving Copy, Clone, Hash, PartialEq etc. is simply wrong.
Deriving Debug is debatable. The default derive might give a wrong
impression, since it prints out the inner unused field.
The opt-in manual implementation would print opaque, which seems
friendlier.

Changed tests:
- func-return-must-use: Don't use forward declared structs in the
  test, since that would result in wrong bindings!
- issue-1238-fwd-no-copy test no longer requires an annotation,
  since Forward declared types should always be no copy

Author: jschwe

bindgen-tests/tests/expectations/tests/anon_union.rs

@@ -4,7 +4,9 @@ typedef int MustUseInt;
 
 MustUseInt return_int();
 
-struct MustUseStruct;
+struct MustUseStruct {
+    int a;
+};
 
 struct MustUseStruct return_struct();
 
@@ -20,11 +22,15 @@ int return_plain_int();
 /**
  * <div rustbindgen mustusetype></div>
  */
-struct AnnotatedStruct {};
+struct AnnotatedStruct {
+    int a;
+};
 
 struct AnnotatedStruct return_annotated_struct();
 
-struct PlainStruct {};
+struct PlainStruct {
+    int a;
+};
 
 /**
  * <div rustbindgen mustusetype></div>

bindgen-tests/tests/expectations/tests/forward-declaration-autoptr.rs

@@ -1,3 +1,2 @@
-// bindgen-flags: --no-copy MyType
 
 typedef struct MyType MyTypeT;

bindgen-tests/tests/expectations/tests/forward_declared_complex_types.rs

@@ -2444,7 +2444,14 @@ impl CodeGenerator for CompInfo {
             }
         }
 
-        let derivable_traits = derives_of_item(item, ctx, packed);
+        let derivable_traits = if self.is_forward_declaration() {
+            // The only trait we could derive for forward declared types is `Debug`.
+            // However, since we don't actually know anything about the type, the
+            // Debug implementation would be useless.
+            DerivableTraits::empty()
+        } else {
+            derives_of_item(item, ctx, packed)
+        };
         if !derivable_traits.contains(DerivableTraits::DEBUG) {
             needs_debug_impl = ctx.options().derive_debug &&
                 ctx.options().impl_debug &&