Re: [RFC PATCH v3 1/5] build-sys: Add rust feature option

[Paolo Bonzini <pbonzini@redhat.com>]

On 6/25/24 23:47, Manos Pitsidianakis wrote:
> On Mon, 24 Jun 2024 20:14, Paolo Bonzini <pbonzini@redhat.com> wrote:
>> Yes, I agree. However, considering we haven't even checked the situation
>> with what language features are required by any idiomatic bindings vs the
>> 1.63 version that we need to support for Debian, I think it's a bit
>> premature to make it mandatory.
>>
> 
> FWIW, I just ran
> 
> `cargo msrv -- cargo check --target x86_64-unknown-linux-gnu`
> 
> And the result was `The MSRV is: 1.77.2`
> 
> and the most important issue was that the mem::offset_of! macro was 
> unstable till then.

I looked for a way to avoid it and the most promising is 
https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=10a22a9b8393abd7b541d8fc844bc0df

Basically, you replace

     pub struct Foo {
         foo: i32,
         bar: i32
     }

with

     with_offsets! {
         #[repr(C)]  // mandatory
         pub struct Foo {
             foo: i32,
             bar: i32,
         }
     }

The macro walks the struct twice, once to actually declare it and once 
to determine the offsets using mem::size_of and mem::align_of.  The 
result is something like

         #[repr(C)]  // mandatory
         pub struct Foo {
             foo: i32,
             bar: i32,
         }

         pub struct FooOffsets {
             foo: usize,
             bar: usize,
         }

         impl Foo {
             pub const offset_to: FooOffsets = FooOffsets {
                 foo: 0,
                 bar: 4,
             }
         }

(where 0 and 4 are actually the aforementioned computation based on 
size_of and align_of).

There are some limitations but the trick is really well done; the need 
for #[repr(C)] is not a problem for us (C<->Rust interoperability needs 
it anyway), and the implementation is fully "const".  And though it only 
works for structs that use "with_offsets!", and with just one level of 
fields, the implementation of offset_of is trivial:

     macro_rules! offset_of {
         ($Container:ty, $field:ident) => {
             <$Container>::offset_to.$field
         };
     }

Anyhow, this should _not_ be in the first version that is 
committed---which, as you remarked in the v2, should focus on working 
build system integration.  As long as we know it is doable, it can be 
left for later.

Paolo