[Pkg-rust-maintainers] Rust versions and the coming freeze for Stretch

[Josh Triplett]

On Wed, Jan 11, 2017 at 11:09:05AM -0500, Steve Klabnik wrote:
> Anyway, that's the last I'll say of it; I just _really_ don't want to
> end up having to tell people "no, please uninstall rustc through apt
> because it's too old", which I fear is gonna happen a lot. That might
> be unfounded. We'll see...

Stable Linux distributions will always have an older version of Rust;
Debian stable releases come out approximately every two years, while
Rust releases come out every six weeks.

1.15 brings in procedural macros, but during the lifetime of stretch,
we'll likely see many other features stabilize that the ecosystem starts
using, including "-> impl Trait", unions, the end of #[macro_use], the
trait for generalized '?', and perhaps even macros 2.0.

I think we need to develop a robust scheme for packages to declare Rust
and Cargo version requirements, and enhance the CI infrastructure to
make sure the declared minimum versions actually work, so people can
either fix packages or update requirements when the minimum version
changes.  That will avoid ever needing to tell people to install a new
version of Rust or Cargo; cargo will tell them that, instead.

See https://github.com/rust-lang/rfcs/pull/1707 for one possible
solution to that.  If the revised version of that gets accepted, I'd be
happy to do the corresponding work in Cargo to implement it.

- Josh Triplett