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Date:   Sun, 12 Jul 2020 12:39:44 -0700
From:   Josh Triplett <josh@joshtriplett.org>
To:     Adrian Bunk <bunk@kernel.org>
Cc:     Nick Desaulniers <ndesaulniers@google.com>, alex.gaynor@gmail.com,
        geofft@ldpreload.com, jbaublitz@redhat.com,
        Masahiro Yamada <masahiroy@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Greg KH <gregkh@linuxfoundation.org>,
        Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        LKML <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>
Subject: Re: Linux kernel in-tree Rust support
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On Sun, Jul 12, 2020 at 03:31:51PM +0300, Adrian Bunk wrote:
> On Thu, Jul 09, 2020 at 11:41:47AM -0700, Nick Desaulniers wrote:
> >...
> > but also a larger question of "should we do
> > this?" or "how might we place limits on where this can be used?"
> >...
> 
> I won't attend, but I do have a topic that should be covered:
> 
> Firefox always depends on recent Rust, which forces distributions to 
> update Rust in stable releases.
> 
> As an example:
> Ubuntu LTS releases upgrade to a new Rust version every 1-2 months.
> Ubuntu 16.04 started with Rust 1.7.0 and is now at Rust 1.41.0.
> 
> It would not sound good to me if security updates of distribution
> kernels might additionally end up using a different version of the
> Rust compiler - the toolchain for the kernel should be stable.
> 
> Would Rust usage in the kernel require distributions to ship
> a "Rust for Firefox" and a "Rust for the kernel"?

Rust has hard stability guarantees when upgrading from one stable
version to the next. If code compiles with a given stable version of
Rust, it'll compile with a newer stable version of Rust. Given that, a
stable distribution will just need a single sufficiently up-to-date Rust
that meets the minimum version requirements of both Firefox and Linux.

(That would not apply if the kernel used nightly Rust, since
nightly-only features are allowed to change before becoming stable;
that's one reason why we should use stable Rust, and try to get Firefox
to stick to stable Rust.)