Assert protoc version while building with cargo

Cargo.toml

@@ -24,13 +24,13 @@ serde = { version = "^1.0.55", optional = true }
 serde_derive = { version = "^1.0.55", optional = true }
 
 [build-dependencies]
-protoc-rust = "1.7.1"
+protoc-rust = { version = "1.7.1", optional = true }
 
 [dev-dependencies]
 serde_json = "1.0.17"
 
 [features]
-serialization-protobuf = [ "protobuf" ]
+serialization-protobuf = [ "protobuf", "protoc-rust" ]
 serialization-serde = [ "serde", "serde_derive" ]
 
 [package.metadata.release]

build.rs

@@ -1,6 +1,26 @@
+#[cfg(feature = "serialization-protobuf")]
 extern crate protoc_rust;
 
-fn build_protobuf<'a>(out_dir: &'a str, input: &'a [&'a str], includes: &'a [&'a str]) {
+#[cfg(feature = "serialization-protobuf")]
+fn assert_protobuf_version(version: &str) {
+    use std::process::{Command, Stdio};
+    let protoc = Command::new("protoc")
+        .stdin(Stdio::null())
+        .stdout(Stdio::piped())
+        .stderr(Stdio::piped())
+        .args(&["--version"])
+        .spawn()
+        .unwrap();
+    let version_output = protoc.wait_with_output().unwrap();
+    assert!(version_output.status.success());
+    assert_eq!(
+        String::from_utf8(version_output.stdout).unwrap().trim(),
+        version
+    );
+}
+
+#[cfg(feature = "serialization-protobuf")]
+fn build_protobuf(out_dir: &str, input: &[&str], includes: &[&str]) {
     use self::protoc_rust::{run, Args};
     run(Args {
         out_dir,
@@ -9,7 +29,13 @@ fn build_protobuf<'a>(out_dir: &'a str, input: &'a [&'a str], includes: &'a [&'a
     }).expect("protoc");
 }
 
-fn main() {
-    #[cfg(feature = "serialization-protobuf")]
+#[cfg(feature = "serialization-protobuf")]
+fn build_protobuf_schemata() {
+    assert_protobuf_version("libprotoc 3.5.1");
     build_protobuf("src/proto", &["protobuf/proof.proto"], &[]);
 }
+
+fn main() {
+    #[cfg(feature = "serialization-protobuf")]
+    build_protobuf_schemata();
+}