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

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Date:   Sun, 4 Dec 2022 10:31:53 +0000
From:   Gary Guo <gary@...yguo.net>
To:     Josh Triplett <josh@...htriplett.org>
Cc:     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>,
        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 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.

Best,
Gary