linux-kernel - Re: [PATCH 7/7] rust: sync: add support for dispatching on Arc and ArcBorrow.

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Date:   Sat, 31 Dec 2022 19:51:54 +0000
From:   Gary Guo <gary@...yguo.net>
To:     Laine Taffin Altman <alexanderaltman@...com>
Cc:     Wedson Almeida Filho <wedsonaf@...il.com>,
        rust-for-linux@...r.kernel.org, Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 7/7] rust: sync: add support for dispatching on Arc and
 ArcBorrow.

On Tue, 27 Dec 2022 23:24:37 -0800
Laine Taffin Altman <alexanderaltman@...com> wrote:

> > +// This is to allow `ArcBorrow<U>` to be dispatched on when `ArcBorrow<T>` can be coerced into
> > +// `ArcBorrow<U>`.
> > +impl<T: ?Sized + Unsize<U>, U: ?Sized> core::ops::DispatchFromDyn<ArcBorrow<'_, U>>
> > +    for ArcBorrow<'_, T>  
> 
> These lifetimes need to be tied together, I think.
> `impl<'a, T: ?Sized + Unsize<U> + 'a, U: ?Sized + 'a> core::ops::DispatchFromDyn<ArcBorrow<'a, U>> for ArcBorrow<'a, T>`

I don't think this is necessary, libcore has the following code
(https://doc.rust-lang.org/stable/src/core/ops/unsize.rs.html#123):

impl<'a, T: ?Sized + Unsize<U>, U: ?Sized> core::ops::DispatchFromDyn<&'a U> for &'a T {}

which should be equivalent to Wedson's lifetime elided version.

Best,
Gary