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

[Gary Guo <gary@garyguo.net>]

On Wed, 28 Jun 2023 11:41:59 +0000
Benno Lossin <benno.lossin@proton.me> wrote:

> On 25.06.23 22:54, Gary Guo wrote:
> > On Sat, 24 Jun 2023 09:25:10 +0000
> > Benno Lossin <benno.lossin@proton.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@garyguo.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.

Yes, by default the hygiene of pasted macro is that of the group,
unless explicitly overriden.

Best,
Gary