Include git commit hash as string into Rust program

Question

I host a Rust project in git repository and I want to make it print the version on some command. How can I include the version into the program? I thought that the build script could set environment variables which can be used while compiling the project itself, but it does not work:

build.rs:

use std::env;

fn get_git_hash() -> Option<String> {
    use std::process::Command;

    let branch = Command::new("git")
                         .arg("rev-parse")
                         .arg("--abbrev-ref")
                         .arg("HEAD")
                         .output();
    if let Ok(branch_output) = branch {
        let branch_string = String::from_utf8_lossy(&branch_output.stdout);
        let commit = Command::new("git")
                             .arg("rev-parse")
                             .arg("--verify")
                             .arg("HEAD")
                             .output();
        if let Ok(commit_output) = commit {
            let commit_string = String::from_utf8_lossy(&commit_output.stdout);

            return Some(format!("{}, {}",
                        branch_string.lines().next().unwrap_or(""),
                        commit_string.lines().next().unwrap_or("")))
        } else {
            panic!("Can not get git commit: {}", commit_output.unwrap_err());
        }
    } else {
        panic!("Can not get git branch: {}", branch.unwrap_err());
    }
    None
}

fn main() {
    if let Some(git) = get_git_hash() {
        env::set_var("GIT_HASH", git);
    }
}

src/main.rs:

pub const GIT_HASH: &'static str = env!("GIT_HASH");

fm main() {
    println!("Git hash: {}", GIT_HASH);
}

The error message:

error: environment variable `GIT_HASH` not defined
  --> src/main.rs:10:25
   |
10 | pub const GIT_HASH: &'static str = env!("GIT_HASH");
   |   
                                        ^^^^^^^^^^^^^^^^

Is there a way to pass such data at compile time? How can I communicate between the build script and the source code if not with environment variables? I can only think about writing data to some file, but I think this is overkill for this case.

Answer 1

There is already an existing crate vergen that can calculate the git commit in the build script. As @DK's answer described, the build script cannot modify environment variable before Rust 1.19, so vergen still works by writing the result into OUT_DIR (i.e. vergen still won't solve OP's question, but it should be easier to use).

---

Usage:

# Cargo.toml
...
[build-dependencies]
vergen = "0.1"

// build.rs
extern crate vergen;
use vergen::*;
fn main() {
    vergen(SHORT_SHA | COMMIT_DATE).unwrap();
}

mod version {
    include!(concat!(env!("OUT_DIR"), "/version.rs"));
}
fn main() {
    println!("commit: {} {}", version::commit_date(), version::short_sha());
    // output something like:
    //        commit: 2017-05-03 a29c7e5
}