Support picking from multiple Rust version

[SimonSapin]

If Rust gains support for installing multiple versions in parallel (see rust-lang/rust#19263 for details), Cargo should have a mechanism to pick which one is used. Maybe an environment variable? RUSTC=rustc-beta cargo test

[SimonSapin]

RUSTC=rustc-beta cargo test doesn’t help Cargo find the corresponding rustdoc though, so it’s probably not the right mechanism. If the naming convention for rust-lang/rust#19263 is regular enough, it could be something like cargo --rust=beta test.

Whatever the mechanism for command-line invocations, .cargo/config could also have a key for setting the default.

[pwoolcoc]

how about defining a RUSTROOT in order to tell cargo the location of rustc and rustdoc, similar to GOROOT? So, defining it as RUSTROOT=/usr/local cargo test would cause cargo to look for rustc and rustdoc in /usr/local/bin?

[SimonSapin]

@pwoolcoc, this is assuming rust-lang/rust#19263 provides a way to have multiple Rust version in the same prefix/root. So there is not much to discuss in this bug until there’s a plan on how to resolve that one, but in the meantime this is a note that something will be needed.

[codyps]

Having a RUSTC env var would be useful to me as well.

[DeadDork]

+1

[alexcrichton]

I think that given the current state of affairs multirust is filling this niche quite nicely, although I would still like a RUSTC env var

[SimonSapin]

Multirust is based on having multiple "toolchains", each containing different versions of an arbitrary number of tools, including rustc, rustdoc, and Cargo.

Long term, I think it makes more sense to stabilize the rustc command-line interface and have a single version of Cargo that can drives multiples versions of rustc+rustdoc.