Re: [gentoo-dev] Add rust eclass to support multi-target compilation

[Dirkjan Ochtman]

On Mon, Jul 30, 2018 at 5:02 PM gibix <gibix@××××××.net> wrote:

> This will allows projects like rustfmt, clippy, bindgen that need runtime
> linking with the proper rust version to work correctly. Beyond this while
> rust is getting older as project we will see more projects that will
> require a specific rust version for compilation.
>

As I mentioned in the PR already, I'm not a fan of this change. (Context: I
have been writing Rust code for 2 years, I'm on the Rust team, and I've
done most of the maintenance on the dev-lang/rust ebuilds for the past year
or so.)

As far as I know, the Rust ecosystem is largely bimodal: stuff is either
compatible with stable and later, or it works only on nightly. It seems
very rare that code is actually tied to a particular Rust release and does
not compile against latest Rust stable. (Upstream actually promises they
won't break code except in case they need to fix a soundness issue in the
compiler.) So instead of building this whole target infrastructure (which
is definitely needed for something like Python or Ruby), we should be able
to just work with stable and nightly slots, and ebuilds can depend on those.

Upstream is also very clearly packaging their core tooling as a single
package made up of a number of components, and these are distributed and
compiled together in common usage on other platforms (with the rustup
tooling). That also means they are tightly coupled -- for example, rustfmt
can change formats over time, and CI that checks formatting will need to
align with whatever the current stable Rust version of rustfmt is.
Installing the latest (nightly or beta) version of rustfmt while you have a
stable Rust toolchain installed is thus not a good idea. As another
example, cargo is now tagged as 0.28, but when passing --version it will
report as 1.27.0 -- actually the Rust toolchain version.

For these reasons, I think it would be better to align our Rust ebuilds
with upstream and work with single ebuilds (dev-lang/rust and
dev-lang/rust-bin) that install all the tools belonging to a particular
version of the Rust toolchain. What tools are installed can be customized
with USE flags, and the default install can be minimal (just rustc and
cargo, which you need to build packages anyway).

Regards,

Dirkjan