Environment variable for the Rust version

[stanislav-tkach]

Cargo sets some environment variables, for example, CARGO_PKG_VERSION. I suppose it will be handy to have environment variable with the Rust version.

I would like to try to add it if it is appropriate.

See also #2903

[alexcrichton]

Thanks for the report! I'm not sure we're quite ready to do this in the sense that it would entail parsing the output of rustc -vV, which I'm not sure is a "stable" format right now.

[stanislav-tkach]

For my purpose it will be OK to have a version in any format: I'm trying to form some "user agent" string in our application that should include the Rust version. But I understand that such solution may not be appropriate for the Cargo. I can still get the Rust version inside build.rs anyway. Do you think that it makes no sense to have such functionality in the Cargo right now?

[alexcrichton]

Nah I think eventually we may wish to expose this, but I'd prefer to ensure there's a stable method for Cargo to learn the compiler's version info (e.g. a structured format) before we expose it.

[dwijnand]

RFC rust-lang/rfcs#2523 might be of interest, as it proposes #[cfg(version(1.27))].

[dekellum]

See also the version_check crate. See the nom crate's build.rs for good example of using it.

[epage]

Looking back at this, this request is for the string being available for process and is independent of then using that in build.rs for conditional compilation, which is what #2903 is about.

If we did this, I'd either want rust-lang/rfcs#2523 stabilized first and/or to disallow this env varaible in build scripts so it doesn't become an alternative mechanism to rust-lang/rfcs#2523