Include git commit hash as string into Rust program

Since Rust 1.19 (cargo 0.20.0), thanks to https://github.com/rust-lang/cargo/pull/3929, you can now define a compile-time environment variable (env!(…)) for rustc and rustdoc via:

println!("cargo:rustc-env=KEY=value");

So OP’s program can be written as:

// build.rs
use std::process::Command;
fn main() {
    // note: add error checking yourself.
    let output = Command::new("git").args(&["rev-parse", "HEAD"]).output().unwrap();
    let git_hash = String::from_utf8(output.stdout).unwrap();
    println!("cargo:rustc-env=GIT_HASH={}", git_hash);
}

// main.rs
fn main() {
    println!("{}", env!("GIT_HASH"));
    // output something like:
    // 7480b50f3c75eeed88323ec6a718d7baac76290d
}

Note that you still cannot use this if you still want to support 1.18 or below.