Drop vergen_git2 dependency (#36439)

Manually call `git rev-parse --short HEAD` to retrieve the commit hash,
instead of using Emitter from vergen_git2. This helps remove
dependencies and shorten compile time.

Testing: It doesn't require tests because it is a refactoring for
removing dependencies
Fixes: #36435

Signed-off-by: Kingsley Yung <kingsley@kkoyung.dev>

ports/servoshell/Cargo.toml

@@ -22,7 +22,6 @@ bench = false
 # since build-scripts can't detect the cargo target os at build-time, we
 # must unconditionally add these dependencies. See https://github.com/rust-lang/cargo/issues/4932
 [build-dependencies]
-vergen-git2 = { version = "1.0.5", features = ["build"] }
 # MacOS only
 cc = "1.2"
 

ports/servoshell/build.rs

@@ -6,18 +6,20 @@ use std::error::Error;
 use std::fs::File;
 use std::io::Write;
 use std::path::Path;
+use std::process::Command;
 
-use vergen_git2::{Emitter, Git2Builder};
-
-fn emit_git_sha() -> Result<(), String> {
-    let git_options = Git2Builder::default()
-        .sha(true /* short */)
-        .build()
+fn git_sha() -> Result<String, String> {
+    let output = Command::new("git")
+        .args(["rev-parse", "--short", "HEAD"])
+        .output()
         .map_err(|e| e.to_string())?;
-    Emitter::default()
-        .add_instructions(&git_options)
-        .and_then(|emitter| emitter.fail_on_error().emit())
-        .map_err(|e| e.to_string())
+    if output.status.success() {
+        let hash = String::from_utf8(output.stdout).map_err(|e| e.to_string())?;
+        Ok(hash.trim().to_owned())
+    } else {
+        let stderr = String::from_utf8(output.stderr).map_err(|e| e.to_string())?;
+        Err(stderr)
+    }
 }
 
 fn main() -> Result<(), Box<dyn Error>> {
@@ -72,12 +74,15 @@ fn main() -> Result<(), Box<dyn Error>> {
         println!("cargo:rustc-link-search=native={}", out.display());
     }
 
-    if let Err(error) = emit_git_sha() {
-        println!(
-            "cargo:warning=Could not generate git version information: {:?}",
-            error
-        );
-        println!("cargo:rustc-env=VERGEN_GIT_SHA=nogit");
+    match git_sha() {
+        Ok(hash) => println!("cargo:rustc-env=GIT_SHA={}", hash),
+        Err(error) => {
+            println!(
+                "cargo:warning=Could not generate git version information: {:?}",
+                error
+            );
+            println!("cargo:rustc-env=GIT_SHA=nogit");
+        },
     }
 
     // On MacOS, all dylib dependencies are shipped along with the binary

ports/servoshell/lib.rs

@@ -97,11 +97,7 @@ pub fn init_tracing(filter_directives: Option<&str>) {
 }
 
 pub fn servo_version() -> String {
-    format!(
-        "Servo {}-{}",
-        env!("CARGO_PKG_VERSION"),
-        env!("VERGEN_GIT_SHA")
-    )
+    format!("Servo {}-{}", env!("CARGO_PKG_VERSION"), env!("GIT_SHA"))
 }
 
 /// Plumbs tracing spans into HiTrace, with the following caveats: