@brson: some further follow-ups over at https://aur.archlinux.org/packages/rustup/.
The more I think about this, the trickier it seems. The biggest problem seems to be that, when building Rust packages from source (which Arch users do more than many other distros, but which all distros must do in order to provide binary artifacts), it is hard to have any guarantees about what version of Rust you'll get if you simply run
rustc. If you know that rustup is being used, then it's easy to override the version, but if you don't (and a build tool generally shouldn't need to -- it should just require "rust"), then you're sort of lost.
It'd be great if there was a way to tell rustup "ignore defaults and use at least this version", but without relying on actually running the rustup command. Perhaps an environment variable that rustup will use. That way, a build script could do something like:
cargo build --release
which works regardless of whether a "regular" rust install is used, or a rustup-managed version.
This is still not ideal, because it requires the build script (or the build tool, but that'd be worse) to specify the version dependency outside of the packaging system (in Arch, you'd normally just say
depends=(rust>=1.10) in the package's metadata), but it's a decent first step.