linux-kernel - Re: [PATCH 3/7] rust: init: make guards in the init macros hygienic

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Date:   Wed, 28 Jun 2023 11:41:59 +0000
From:   Benno Lossin <benno.lossin@...ton.me>
To:     Gary Guo <gary@...yguo.net>
Cc:     Miguel Ojeda <ojeda@...nel.org>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Alice Ryhl <aliceryhl@...gle.com>,
        Andreas Hindborg <nmi@...aspace.dk>,
        rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
        patches@...ts.linux.dev, Asahi Lina <lina@...hilina.net>
Subject: Re: [PATCH 3/7] rust: init: make guards in the init macros hygienic

On 25.06.23 22:54, Gary Guo wrote:
> On Sat, 24 Jun 2023 09:25:10 +0000
> Benno Lossin <benno.lossin@...ton.me> wrote:
> 
>> Use hygienic identifiers for the guards instead of the field names. This
>> makes the init macros feel more like normal struct initializers, since
>> assigning identifiers with the name of a field does not create
>> conflicts.
>> Also change the internals of the guards, no need to make the `forget`
>> function `unsafe`, since users cannot access the guards anyways. Now the
>> guards are carried directly on the stack and have no extra `Cell<bool>`
>> field that marks if they have been forgotten or not, instead they are
>> just forgotten via `mem::forget`.
> 
> The code LGTM, so:
> 
> Reviewed-by: Gary Guo <gary@...yguo.net>
> 
> Although this will cause the new expansion we have to be no longer
> compatible with a totally-proc-macro impl, if we want to do everything
> in proc macro in the future.
> 
> If we have the paste macro upstream (
> https://github.com/nbdd0121/linux/commit/fff00461b0be7fd3ec218dcc428f25886b5ec04a
> ) then we can replace the `guard` with `paste!([<$field>])` and keep
> the expansion identical.
> 

I tried it and it seems to work, but I am not sure why the hygiene is
set correctly. Could you maybe explain why this works?
```
        $crate::__internal::paste!{
            let [<$field>] = unsafe {
                $crate::__internal::DropGuard::new(::core::ptr::addr_of_mut!((*$slot).$field))
            };
            $crate::__init_internal!(init_slot($use_data):
                @data($data),
                @slot($slot),
                @guards([<$field>], $($guards,)*),
                @munch_fields($($rest)*),
            );
        }
```

i.e. why can't a user access the guard? I think it is because the hygiene of the `[<>]`
is used, but not sure why that works.

-- 
Cheers,
Benno