linux-kernel - Re: [PATCH v2 27/28] rust: types: add `Either` type

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Message-ID: <CANeycqpNW0E5fQqD1Qzqr0p-3cgs5k5KgLusragaSWbrmcx2Nw@mail.gmail.com>
Date:   Sun, 4 Dec 2022 17:36:08 +0000
From:   Wedson Almeida Filho <wedsonaf@...il.com>
To:     Gary Guo <gary@...yguo.net>
Cc:     Josh Triplett <josh@...htriplett.org>, ojeda@...nel.org,
        Alex Gaynor <alex.gaynor@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
        patches@...ts.linux.dev, Wei Liu <wei.liu@...nel.org>
Subject: Re: [PATCH v2 27/28] rust: types: add `Either` type

On Sun, 4 Dec 2022 at 10:31, Gary Guo <gary@...yguo.net> wrote:
>
> On Fri, 2 Dec 2022 15:41:59 -0800
> Josh Triplett <josh@...htriplett.org> wrote:
>
> > On Fri, Dec 02, 2022 at 05:14:58PM +0100, ojeda@...nel.org wrote:
> > > From: Wedson Almeida Filho <wedsonaf@...il.com>
> > >
> > > Introduce the new `types` module of the `kernel` crate with
> > > `Either` as its first type.
> > >
> > > `Either<L, R>` is a sum type that always holds either a value
> > > of type `L` (`Left` variant) or `R` (`Right` variant).
> > >
> > > For instance:
> > >
> > >     struct Executor {
> > >         queue: Either<BoxedQueue, &'static Queue>,
> > >     }
> >
> > This specific example seems like it would be better served by the
> > existing `Cow` type.
>
> We use `no_global_oom_handling`, which gates most `ToOwned`
> implementations (e.g. `str` cannot implement `to_owned()` because it
> cannot guarantee allocation success).
>
> So the Rust `Cow` is pretty much useless in the kernel.

It's also implemented in `std`, which the kernel doesn't include.
(Which is actually good for us, since we can't really use it.)

Josh, how do you feel about adding a `TryToOwned` trait to
`core::borrow`? This would be similar to the precedent of `TryFrom` in
addition to `From` for the fallible case, and would be usable by the
kernel.