Re: [Libguestfs] [libnbd PATCH v5 01/12] rust: create basic Rust bindings

On Thu, Aug 03, 2023 at 03:36:05PM +0000, Tage Johansson wrote: > This commit creates basic Rust bindings in the rust directory. > The bindings are generated by generator/Rust.ml and > generator/RustSys.ml. > --- > --- /dev/null > +++ b/generator/RustSys.ml > +(** Print the struct for a closure. *) > +let print_closure_struct { cbname; cbargs } = > + pr "#[repr(C)]\n"; > + pr "#[derive(Debug, Clone, Copy)]\n"; > + pr "pub struct nbd_%s_callback {\n" cbname; > + pr " pub callback: \n"; > + pr " Option<unsafe extern \"C\" fn(*mut c_void, %s) -> c_int>,\n" > + (cbargs |> List.map cbarg_types |> List.flatten |> String.concat ", "); > + pr " pub user_data: *mut c_void,\n"; > + pr " pub free: Option<unsafe extern \"C\" fn(*mut c_void)>,\n"; > + pr "}\n" Why is 'callback' an Option<> rather than a mandatory argument? I get that 'free' must be an Option<> (because it corresponds to an OClosure, which is an optional callback), but 'callback' is a mandatory function pointer in the C API; why would it ever be acceptable to pass None instead of Some<function>?