linux-kernel - Re: [PATCH 1/4] rust: revocable: support fallible PinInit types

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Message-Id: <DAKOUVS4YP4L.3I6FD9JKN02EU@kernel.org>
Date: Thu, 12 Jun 2025 18:17:40 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Danilo Krummrich" <dakr@...nel.org>
Cc: <gregkh@...uxfoundation.org>, <rafael@...nel.org>, <ojeda@...nel.org>,
 <alex.gaynor@...il.com>, <boqun.feng@...il.com>, <gary@...yguo.net>,
 <bjorn3_gh@...tonmail.com>, <benno.lossin@...ton.me>,
 <a.hindborg@...nel.org>, <aliceryhl@...gle.com>, <tmgross@...ch.edu>,
 <rust-for-linux@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/4] rust: revocable: support fallible PinInit types

On Thu Jun 12, 2025 at 5:58 PM CEST, Danilo Krummrich wrote:
> On Thu, Jun 12, 2025 at 05:48:36PM +0200, Benno Lossin wrote:
>> On Thu Jun 12, 2025 at 4:51 PM CEST, Danilo Krummrich wrote:
>> > diff --git a/rust/kernel/revocable.rs b/rust/kernel/revocable.rs
>> > index fa1fd70efa27..41b8fe374af6 100644
>> > --- a/rust/kernel/revocable.rs
>> > +++ b/rust/kernel/revocable.rs
>> > @@ -82,8 +82,11 @@ unsafe impl<T: Sync + Send> Sync for Revocable<T> {}
>> >  
>> >  impl<T> Revocable<T> {
>> >      /// Creates a new revocable instance of the given data.
>> > -    pub fn new(data: impl PinInit<T>) -> impl PinInit<Self> {
>> > -        pin_init!(Self {
>> > +    pub fn new<E>(data: impl PinInit<T, E>) -> impl PinInit<Self, Error>
>> > +    where
>> > +        Error: From<E>,
>> 
>> I don't think we need this bound as you don't use it in the function
>> body.
>
> I think it's needed by try_pin_init!() below, no?
>
> Without it I get the compilation error in [1].
>
>> > +    {
>> > +        try_pin_init!(Self {
>> >              is_available: AtomicBool::new(true),
>> >              data <- Opaque::pin_init(data),
>> >          })

Does it work with this?

    try_pin_init!(Self {
        is_available: AtomicBool::new(true),
        data <- Opaque::pin_init(data),
    }? E)

---
Cheers,
Benno

>> 
>
> [1]
>
> error[E0277]: `?` couldn't convert the error to `error::Error`
>   --> rust/kernel/revocable.rs:87:9
>    |
> 87 | /         try_pin_init!(Self {
> 88 | |             is_available: AtomicBool::new(true),
> 89 | |             data <- Opaque::pin_init(data),
> 90 | |         })
>    | |          ^
>    | |          |
>    | |__________this can't be annotated with `?` because it has type `Result<_, E>`
>    |            the trait `core::convert::From<E>` is not implemented for `error::Error`
>    |
>    = note: the question mark operation (`?`) implicitly performs a conversion on the error value using the `From` trait
>    = note: required for `core::result::Result<revocable::Revocable<T>::new::__InitOk, error::Error>` to implement `core::ops::FromResidual<core::result::Result<core::convert::Infallible, E>>`
>    = note: this error originates in the macro `$crate::__init_internal` which comes from the expansion of the macro `try_pin_init` (in Nightly builds, run with -Z macro-backtrace for more info)
> help: consider introducing a `where` clause, but there might be an alternative better way to express this requirement
>    |
> 83 | impl<T> Revocable<T> where error::Error: core::convert::From<E> {
>    |                      ++++++++++++++++++++++++++++++++++++++++++
>
> error[E0277]: the trait bound `impl PinInit<Revocable<T>, Error>: PinInit<Revocable<T>, E>` is not satisfied
>   --> rust/kernel/revocable.rs:85:48
>    |
> 85 |     pub fn new<E>(data: impl PinInit<T, E>) -> impl PinInit<Self, E>
>    |                                                ^^^^^^^^^^^^^^^^^^^^^ the trait `pin_init::PinInit<revocable::Revocable<T>, E>` is not implemented for `impl pin_init::PinInit<revocable::Revocable<T>, error::Error>`
>    |
>    = help: the following other types implement trait `pin_init::PinInit<T, E>`:
>              `core::result::Result<T, E>` implements `pin_init::PinInit<T, E>`
>              `pin_init::ChainInit<I, F, T, E>` implements `pin_init::PinInit<T, E>`
>              `pin_init::ChainPinInit<I, F, T, E>` implements `pin_init::PinInit<T, E>`
>              `pin_init::__internal::AlwaysFail<T>` implements `pin_init::PinInit<T, ()>`
>    = note: the full name for the type has been written to 'kernel.long-type-441004638990533407.txt'
>    = note: consider using `--verbose` to print the full type name to the console