On 6/25/24 23:47, Manos Pitsidianakis wrote:
On Mon, 24 Jun 2024 20:14, Paolo Bonzini <pbonz...@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
