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

[Josh Triplett]

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@xxxxxxxxxxx> wrote:
> >
> > On Fri, 2 Dec 2022 15:41:59 -0800
> > Josh Triplett <josh@xxxxxxxxxxxxxxxx> wrote:
> >
> > > On Fri, Dec 02, 2022 at 05:14:58PM +0100, ojeda@xxxxxxxxxx wrote:
> > > > From: Wedson Almeida Filho <wedsonaf@xxxxxxxxx>
> > > >
> > > > 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.