[Libguestfs] The way of implementing structs of Rust bindings

[Richard W.M. Jones]

On Wed, Jun 05, 2019 at 03:33:51PM +0900, Hiroyuki Katsura wrote:
> Hi, I'm now implementing generators of Rust, and I faced a problem.
> 
> In order for the wrapper to 'understand' the struct passed from C API, it
> is required to incorporate the definition of the struct in C into Rust
> code.
> 
> I have two approaches.

I don't know a lot about Rust bindings, but ...

> 1. Create raw struct(#[repr(C)]), which has the equivalent memory mapping
> to C struct and access through this struct in Rust

... this is what I did in nbdkit (although it is not generated, but
written by hand in this case):

https://github.com/libguestfs/nbdkit/blob/a5d94ab65110136680598677610868196ed71145/plugins/rust/src/lib.rs#L38

> 2. Use bindgen to create ffi struct from guestfs.h

We have tended to avoid this in other languages because the results
can be very poor quality (eg. swig bindings are usually terrible).
However if the resulting code from bindgen is high quality and
accurately covers all the features that libguestfs bindings need then
we can consider it.

> Each of them has advantages and disadvantages.
> 
> # 1st approach
> This is highly dependent on the implementation of API because it is subject
> to the memory mapping of API struct. When the way struct is structured
> changes in API, you have to modify Rust bindings. In order to avoid this
> situation, you can define the same struct in C and Rust and then access the
> API struct in C and then Rust accesses the data through this struct. This
> means that

Sure, I understand all this.  We use the default C memory
representation for the C compiler in use.  Rust should be able to deal
with this naturally.

> # 2nd approach
> 
> The above is easily done by 'bindgen', which automatically generates rust
> ffi bindings to C library. By using this, API struct is automatically
> generated. However, it requires a new dependency: libclang.
> 
> # Question
> 
> Which of these approaches is more preferable?

Depends on the quality of the bindgen output IMO.  If it's good, and
covers all the features needed, use it.  If not, generate the repr(C)
structs.

Rich.

-- 
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