build: add git hash

This commit makes the git hash ripgrep was built with available for use
in the version string.

We also do a few minor touchups in build.rs and src/app.rs.

build.rs

@@ -5,6 +5,7 @@ extern crate lazy_static;
 
 use std::env;
 use std::fs;
+use std::process;
 
 use clap::Shell;
 
@@ -13,14 +14,34 @@ use clap::Shell;
 mod app;
 
 fn main() {
+    // OUT_DIR is set by Cargo and it's where any additional build artifacts
+    // are written.
     let outdir = match env::var_os("OUT_DIR") {
-        None => return,
         Some(outdir) => outdir,
+        None => {
+            eprintln!(
+                "OUT_DIR environment variable not defined. \
+                 Please file a bug: \
+                 https://github.com/BurntSushi/ripgrep/issues/new");
+            process::exit(1);
+        }
     };
     fs::create_dir_all(&outdir).unwrap();
 
+    // Use clap to build completion files.
     let mut app = app::app();
     app.gen_completions("rg", Shell::Bash, &outdir);
     app.gen_completions("rg", Shell::Fish, &outdir);
     app.gen_completions("rg", Shell::PowerShell, &outdir);
+    // Note that we do not use clap's support for zsh. Instead, zsh completions
+    // are manually maintained in `complete/_rg`.
+
+    // Make the current git hash available to the build.
+    let result = process::Command::new("git")
+        .args(&["rev-parse", "--short=10", "HEAD"])
+        .output();
+    if let Ok(output) = result {
+        let hash = String::from_utf8_lossy(&output.stdout);
+        println!("cargo:rustc-env=RIPGREP_BUILD_GIT_HASH={}", hash);
+    }
 }

src/app.rs

@@ -44,6 +44,14 @@ OPTIONS:
 /// This is an intentionally stand-alone module so that it can be used easily
 /// in a `build.rs` script to build shell completion files.
 pub fn app() -> App<'static, 'static> {
+    // We need to specify our version in a static because we've painted clap
+    // into a corner. We've told it that every string we give it will be
+    // 'static, but we need to build the version string dynamically. We can
+    // fake the 'static lifetime with lazy_static.
+    lazy_static! {
+        static ref LONG_VERSION: String = long_version();
+    }
+
     let arg = |name| {
         Arg::with_name(name)
             .help(USAGES[name].short)
@@ -194,6 +202,30 @@ pub fn app() -> App<'static, 'static> {
         .arg(flag("search-zip").short("z"))
 }
 
+/// Return the "long" format of ripgrep's version string.
+fn long_version() -> String {
+    // Let's say whether faster CPU instructions are enabled or not.
+    let mut features = vec![];
+    if cfg!(feature = "simd-accel") {
+        features.push("+SIMD");
+    } else {
+        features.push("-SIMD");
+    }
+    if cfg!(feature = "avx-accel") {
+        features.push("+AVX");
+    } else {
+        features.push("-AVX");
+    }
+    // Do we have a git hash?
+    // (Yes, if ripgrep was built on a machine with `git` installed.)
+    let hash = match option_env!("RIPGREP_BUILD_GIT_HASH") {
+        None => String::new(),
+        Some(githash) => format!(" (rev {})", githash),
+    };
+    // Put everything together.
+    format!("{}{}\n{}", crate_version!(), hash, features.join(" "))
+}
+
 struct Usage {
     short: &'static str,
     long: &'static str,
@@ -212,24 +244,6 @@ macro_rules! doc {
 }
 
 lazy_static! {
-    static ref LONG_VERSION: String = {
-        let mut features: Vec<&str> = vec![];
-
-        if cfg!(feature = "avx-accel") {
-            features.push("+AVX");
-        } else {
-            features.push("-AVX");
-        }
-
-        if cfg!(feature = "simd-accel") {
-            features.push("+SIMD");
-        } else {
-            features.push("-SIMD");
-        }
-
-        format!("{}\n{}", crate_version!(), features.join(" "))
-    };
-
     static ref USAGES: HashMap<&'static str, Usage> = {
         let mut h = HashMap::new();
         doc!(h, "help-short",