Wrong size of class with virtual methods

[kdanecki]

input C++ header

class Base 
{
public:
      virtual void fun();
      int a;
};

class Derived : public Base
{
public:
      void fun() override;
      int x;
};

bindgen command

bindgen -o foo.rs foo.h -- -x c++

actual result

#[repr(C)]
pub struct Base__bindgen_vtable(::std::os::raw::c_void);
#[repr(C)]
#[derive(Debug, Copy, Clone)]
pub struct Base {
    pub vtable_: *const Base__bindgen_vtable,
    pub a: ::std::os::raw::c_int,
}
#[allow(clippy::unnecessary_operation, clippy::identity_op)]
const _: () = {
    ["Size of Base"][::std::mem::size_of::<Base>() - 16usize];
    ["Alignment of Base"][::std::mem::align_of::<Base>() - 8usize];
    ["Offset of field: Base::a"][::std::mem::offset_of!(Base, a) - 8usize];
};
unsafe extern "C" {
    #[link_name = "\u{1}_ZN4Base3funEv"]
    pub fn Base_fun(this: *mut ::std::os::raw::c_void);
}
#[repr(C)]
#[derive(Debug, Copy, Clone)]
pub struct Derived {
    pub _base: Base,
    pub x: ::std::os::raw::c_int,
}
#[allow(clippy::unnecessary_operation, clippy::identity_op)]
const _: () = {
    ["Size of Derived"][::std::mem::size_of::<Derived>() - 16usize];
    ["Alignment of Derived"][::std::mem::align_of::<Derived>() - 8usize];
    ["Offset of field: Derived::x"][::std::mem::offset_of!(Derived, x) - 12usize];
};
unsafe extern "C" {
    #[link_name = "\u{1}_ZN7Derived3funEv"]
    pub fn Derived_fun(this: *mut ::std::os::raw::c_void);
}

compiler error

error[E0080]: evaluation of constant value failed
  --> foo.rs:29:5
   |
29 |     ["Size of Derived"][::std::mem::size_of::<Derived>() - 16usize];
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ index out of bounds: the length is 1 but the index is 8

description

When a class with virtual methods is derived, the fields of derived class can occupy padding of the Base class. But rust representation makes sizeof Derived == sizeof Base + sizeof all fields of derived.