C++ class method pointer treated as C function pointer

Input C/C++ Header

class Thing {
    void (Thing::*method_ptr)(void);
};

Bindgen Invocation

$ bindgen header.hpp -o src/bindings.rs

Actual Results

#[repr(C)]
#[derive(Debug, Copy, Clone)]
pub struct Thing {
    pub method_ptr: ::std::option::Option<unsafe extern "C" fn()>,
}

Expected Results

From C++, sizeof(Thing::method_ptr) == 16, however from Rust it's 8 (which appropriately causes the generated test for the size of Thing to fail). I'd expect either to get an error on binding generation if this isn't supported, or a generated method_ptr field with the same size as the C++ equivalent. This is with bindgen 0.55.1 on x86-64.

Start with the header.hpp reproduction and the bindgen invocation, then inspect the generated src/bindings.rs and the generated Thing size test. Trace how C++ member-function-pointer fields are represented and determine whether unsupported input should fail generation or preserve the field's C++ layout. Done means the incorrect 8-byte representation is no longer emitted without an explicit failure.