Do not derive Default on vtable types

#597 (comment)

Author: SimonSapin

src/codegen/mod.rs

@@ -706,11 +706,7 @@ impl<'a> CodeGenerator for Vtable<'a> {
         assert_eq!(item.id(), self.item_id);
         // For now, generate an empty struct, later we should generate function
         // pointers and whatnot.
-        let mut attributes = vec![attributes::repr("C")];
-
-        if ctx.options().derive_default {
-            attributes.push(attributes::derives(&["Default"]))
-        }
+        let attributes = vec![attributes::repr("C")];
 
         let vtable = aster::AstBuilder::new()
             .item()

tests/expectations/tests/enum_and_vtable_mangling.rs

@@ -10,7 +10,6 @@ pub const whatever_else: _bindgen_ty_1 = _bindgen_ty_1::whatever_else;
 #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
 pub enum _bindgen_ty_1 { match_ = 0, whatever_else = 1, }
 #[repr(C)]
-#[derive(Default)]
 pub struct C__bindgen_vtable {
 }
 #[repr(C)]

tests/expectations/tests/nested_vtable.rs

@@ -5,7 +5,6 @@
 
 
 #[repr(C)]
-#[derive(Default)]
 pub struct nsISupports__bindgen_vtable {
 }
 #[repr(C)]

tests/expectations/tests/ref_argument_array.rs

@@ -6,7 +6,6 @@
 
 pub const NSID_LENGTH: ::std::os::raw::c_uint = 10;
 #[repr(C)]
-#[derive(Default)]
 pub struct nsID__bindgen_vtable {
 }
 #[repr(C)]

tests/expectations/tests/virtual_dtor.rs

@@ -5,7 +5,6 @@
 
 
 #[repr(C)]
-#[derive(Default)]
 pub struct nsSlots__bindgen_vtable {
 }
 #[repr(C)]

tests/expectations/tests/virtual_inheritance.rs

@@ -25,7 +25,6 @@ impl Clone for A {
     fn clone(&self) -> Self { *self }
 }
 #[repr(C)]
-#[derive(Default)]
 pub struct B__bindgen_vtable {
 }
 #[repr(C)]
@@ -53,7 +52,6 @@ impl Default for B {
     fn default() -> Self { unsafe { ::std::mem::zeroed() } }
 }
 #[repr(C)]
-#[derive(Default)]
 pub struct C__bindgen_vtable {
 }
 #[repr(C)]

tests/expectations/tests/virtual_overloaded.rs

@@ -5,7 +5,6 @@
 
 
 #[repr(C)]
-#[derive(Default)]
 pub struct C__bindgen_vtable {
 }
 #[repr(C)]

tests/expectations/tests/vtable_recursive_sig.rs

@@ -23,7 +23,6 @@ impl Default for Derived {
     fn default() -> Self { unsafe { ::std::mem::zeroed() } }
 }
 #[repr(C)]
-#[derive(Default)]
 pub struct Base__bindgen_vtable {
 }
 #[repr(C)]