C++ std::function types are translated into [u128; 3usize]

header.hpp:

using example_type = std::function<void()>;

bindgen invocation:

let bindings = bindgen::Builder::default()
        .header("header.hpp")
        .opaque_type("std::.*")
        .whitelist_type("example_type")
        .rust_target(bindgen::RustTarget::Nightly)
        .parse_callbacks(Box::new(bindgen::CargoCallbacks))
        .generate()
        .expect("Unable to generate bindings");

Output binding:

pub type example_type = [u128; 3usize];

With just a plain typedef like this all is well, but in many situations bindgen also needs to use this type (possibly indirectly) in an extern "C" {...} block, resulting in warnings like this:

warning: `extern` block uses type `[u128; 3usize]`, which is not FFI-safe
    --> src/../bindings.rs:9772:18
     |
9772 |             root::example_type;
     |             ^^^^^^^^^^^^^^^^^^ not FFI-safe
     |
     = help: consider passing a pointer to the array
     = note: passing raw arrays by value is not FFI-safe

warning: `extern` block uses type `u128`, which is not FFI-safe
    --> src/../bindings.rs:1230:27
     |
1230 |                     this: *const root::example_type,
     |                           ^^^^^^^^^^^^^^^^^^^^^^^^^ not FFI-safe
     |
     = note: 128-bit integers don't currently have a known stable ABI

While not strictly an error, it makes me uneasy that function pointers may or may not be interpreted correctly between the C++ library I need to use and my own code.

Is there a way to get bindgen to generate, say, &[u64; 6usize] or something for std::functions instead? Is this intended behavior or unexpected?

I have a feeling this could be related to the use of .opaque_type("std::.*"), but whitelisting std::function doesn't seem to change anything and I'd greatly prefer not to generate bindings for all of std.

Reproduce the report with the shown header.hpp and bindgen::Builder invocation, then inspect the generated example_type alias and the resulting FFI warnings. Review how bindgen translates std::function and decide whether the output is intentional or needs a supported representation. Done means the expected behavior is established and documented or demonstrated by a regression test.