Support for struct-scoped type aliases

Hello! I hope you are doing well.

Love cbindgen! Phenomenal work that you all have accomplished.

I'm reaching out because I'm looking for very particular code generation behavior. I wanted to know if this is something that cbindgen can already do, is planning to do, or is explicitly not going to be supported.

Suppose that I'm trying to replace C++ code class-by-class in an existing codebase. Suppose also that the class definition is very simple and looks like this:

class FooBar {
public:
  using my_foo_t = uint32_t;

  FooBar(my_foo_t my_foo) : m_favorite_foo(my_foo) {}

  bool is_favorite_foo(const FooBar& foo_under_test) {
    return foo_under_test == m_favorite_foo;
  }

private:
  my_foo_t m_favorite_foo;
}

To port this class, I'd write something like following Rust code:

pub type my_foo_t = u32;

#[repr(C)]
pub struct FooBar {
  favorite_foo: my_foo_t,
}

#[unsafe(no_mangle)]
pub extern "C" fn RUST_is_favorite_foo(foo_bar: &FooBar, foo: my_foo_t) -> bool {
  foo.favorite_foo = other.foo
}

And cbindgen generates the following C++ header:

using my_foo_t = uint32_t;

struct FooBar {
  my_foo_t favorite_foo;
};

// ...

However, I would like it instead to generate the following C++ header where the type alias is scoped to the struct:

struct FooBar {
  using my_foo_t = uint32_t;
  my_foot_t favorite_foo;
};

// ...

If this is not something that can currently be done, I can't imagine it would be that hard (famous last works) to tell cbindgen to put the using statement inside of the struct instead of in the same scope as the struct definition using some kind of annotation in the lib.rs file.

Thank you again for taking the time to read my question!

Start by reading lib.rs and the surrounding cbindgen configuration and annotation handling mentioned in the issue. Trace how a Rust type alias and struct are emitted into the C++ header. Done means an opt-in annotation or equivalent produces the alias inside the generated struct without changing existing default output.