Add --version switch

[JanLikar]

Add --version switch, which prints a short hash of the current commit. Fix #612

[sanxiyn]

Doesn't this print the commit when rustfmt is run, not when rustfmt is compiled? That seems wrong.

[nrc]

Yeah, I think the right approach is to get the commit hash at compile time. Probably the best way to do this is to make a small syntax extension, however, this would mean we can't use stable Rust :-( Alternatively, we could use a Cargo build script - I remember we used to have one, but it was removed because it was annoying for various reasons. I wonder how rustc gets its version hash - I assume some combination of unstable Rust and its horrific system of make files - but it might be worth checking out to see if there is some way of passing the data in.

[sanxiyn]

rustc sets CFG_VER_HASH environment variable in makefile and uses env! macro in source. env! is stable.

[nrc]

Ah cool, I wonder if Cargo lets you set environment variables without having to use a build script?

[JanLikar]

You're right, my commit is wrong. Nonetheless, I have learned that this approach might be problematic.
huon: JanLikar: the build script approach will be the easiest with stable Rust, however note that it won't always work (e.g. if the crate is published to crates.io there's no longer any git info) So if someone would install rustfmt from Cargo, he wouldn't even be able to get the hash. We could hardcode the hash into some file or have a git hook do it when we commit, but that's not really great. Could we just use crate versioning?

[nrc]

Using a git hook to save the hash into a file and then #includeing that file to get the hash sounds like a non-awful option if we can get the git hook working well. I don't know much about git hooks and afaik GH does not support them. Maybe worth looking into? Otherwise we could just return 0.0.1 for now and think about proper versioning sooner rather than later.

[JanLikar]

Github only has webhooks, so we would need a server to process each of the commits. I think that would be an overkill. "--version" or "-V" now prints version as defined in Cargo.toml. Now we need to agree on a versioning scheme.

[mbrt]

Why to use commit hashes? Isn't better to use the Cargo version? With std::option_env (as already done by @JanLikar) you get that version at compile time. It's simpler and other projects already followed that path (e.g. clap).

[nrc]

@JanLikar sorry for the lack of attention, I have been on vacation the last week, but this has been sitting even longer than that. I think we should land this, the PR looks good and we can bump versions when we feel like it. Thanks for the PR, and sorry again it took so long to land.

@mrbt that is the current state of the PR

[JanLikar]

@nrc It's ok, no harm done. I hope you had a great vacation!