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

[Eric Blake]

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>?

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.
Virtualization:  qemu.org | libguestfs.org