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, since it can't print anything
useful. Since we previously would derive Debug, lets just
keep it, to avoid needless breaking changes.

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
- forward-declaration-autoptr.rs: Manually implement clone.
  The inner type is unknown, so requiring clone/copy is not correct.

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 +2444,17 @@ 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 can derive for forward declared types is `Debug`,
+            // since we don't know anything about the layout or type.
+            let mut derivable_traits = DerivableTraits::empty();
+            if !item.annotations().disallow_debug() {
+                derivable_traits |= DerivableTraits::DEBUG;
+            };
+            derivable_traits
+        } else {
+            derives_of_item(item, ctx, packed)
+        };
         if !derivable_traits.contains(DerivableTraits::DEBUG) {
             needs_debug_impl = ctx.options().derive_debug &&
                 ctx.options().impl_debug &&