Expose rustc version to build.rs

[mitsuhiko]

Currently the only way to find the rustc version is to invoke rustc. Which is for instance what the rustc_version crate does. It would be great if RUSTC_VERSION or something similar was set automatically so that build scripts could customize what happens based on the version of Rust it compiles with.

See also #4408

[alexcrichton]

I'm a little wary of piling too much information onto build script environment variables. Right now we added exposing RUSTC unconditionally as the compiler being used, would that be enough?

[nagisa]

Link to the crate for the lazy: https://crates.io/crates/rustc_version

[mitsuhiko]

@alexcrichton that's what I'm using currently to shell out to rustc. Just seems like not particularly the best idea to parse compiler output manually for something that I would assume is a common problem. Alternatively one could make the argument that rust itself should provide a macro maybe that exposes the version?

[alexcrichton]

Yeah I guess a macro would perhaps be better, one day Cargo may not always run rustc ahead of compiling crates, so it may not even have this information on hand at all time.

[dwijnand]

#4408 tracks the more general idea of setting an env var for the rust version.

[epage]

I wonder if rust-lang/rfcs#2523 would be more appropriate. I feel like we should at least have a cohesive plan between cargo/lang on how we should be handled version detection.

[epage]

Considering rust-lang/rfcs#2523 is an approved RFC that is meant to target these kinds of use cases, I'm going to close in favor of that. If there is a reason we should reconsider this, let us know!

Note that we still have #4408 for gathering the rustc version for reporting purposes (like a --bugreport flag)./