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

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Date:   Mon, 5 Dec 2022 15:00:05 -0800
From:   Josh Triplett <josh@...htriplett.org>
To:     Wedson Almeida Filho <wedsonaf@...il.com>
Cc:     Gary Guo <gary@...yguo.net>, 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, Dec 04, 2022 at 05:36:08PM +0000, Wedson Almeida Filho wrote:
> 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.

I'd expect it to be in alloc rather than core (though I suppose it
doesn't *fundamentally* depend on an allocator itself), but I'd be happy
to see a fallible version proposed, yeah.