Missing virtual destructors in C++ vtables on Linux x86_64

[DevilishSpirits]

Bug

I am using a large C++ library which makes heavy use of virtual functions and virtual destructor. I am using bindgen's vtable generation which miss virtual destructors on Linux x86_64.

I made a simple repository that exhibit this bug : https://github.com/DevilishSpirits/rust-bindgen-vtable-april25, clone and cargo run it. fn3() should be printed if bindgen correctly generated the vtable.

The src/main.cpp file define a class with pure virtual functions fn1(), fn2() and fn3(), then this class is derived with implementations that echo fnx() on screen. The level of indirection in the C++ is because I reproduced the style of the library I am using.

Both src/main.cpp and src/main.rs contain an example that create the object then call fn3() and destroy the object. While the C++ code echo fn3(), the Rust code echo fn1(). I inspected the vtables using gdb which show that there are 2 virtual destructor prepended in the vtable :

(gdb) info vtbl this
vtable for 'BugIllustrationImpl' @ 0x5555555a8eb0 (subobject @ 0x5555555bfa20):
[0]: 0x55555555b890 <BugIllustrationImpl::~BugIllustrationImpl()>
[1]: 0x55555555b8be <BugIllustrationImpl::~BugIllustrationImpl()>
[2]: 0x55555555b8ea <BugIllustrationImpl::fn1()>
[3]: 0x55555555b908 <BugIllustrationImpl::fn2()>

Rust does not include these virtual destructors and is off by 2 pointers and hence call fn1() instead of fn3() :

(gdb) p *bug_vtable
$1 = aa::BugIllustration__bindgen_vtable {
	BugIllustration_fn1: 0x55555555b890 <BugIllustrationImpl::~BugIllustrationImpl()>,
	BugIllustration_fn2: 0x55555555b8be <BugIllustrationImpl::~BugIllustrationImpl()>,
	BugIllustration_fn3: 0x55555555b8ea <BugIllustrationImpl::fn1()>
}

Expected behavior

I expect bindgen to include pointers to the virtual destructor in the vtable or an opaque padding to allow read-only usage of the vtable.

Expected behavior 2

If that is too difficult to implement, at least bindgen should detect that virtual destructor are in use and generate a fake vtable such as the one below :

/// Virtual destructors detected: vtable not generated
#[repr(C, align(8))] // ← shouldn't bindgen explicitely align the struct???
pub struct BugIllustration__bindgen_vtable {
	/// Virtual destructors detected: vtable not generated
	///
	/// Bindgen currently does not support vtable generation of C++ class with 
	/// virtual destructors. See https://example.com/some/bug/or/link
	pub __virtual_destructors_detected: std::convert::Infallible,
}

This clearly tell the user that bindgen attempted to generate the vtable but that it ran into a limitation.
The user can also use the vtable reference and pass it around for it own needs.
Then if bindgen get support for these vtable, existing code is unlikely to break.