read protoc version pin instead of reading from env vars

build.rs

@@ -2,7 +2,7 @@
 extern crate protoc_rust;
 
 #[cfg(feature = "serialization-protobuf")]
-fn assert_protobuf_version(version: &str) {
+fn assert_protobuf_version(version: String) {
     use std::process::{Command, Stdio};
     let protoc = Command::new("protoc")
         .stdin(Stdio::null())
@@ -15,8 +15,7 @@ fn assert_protobuf_version(version: &str) {
     assert!(version_output.status.success());
     assert_eq!(
         String::from_utf8(version_output.stdout).unwrap().trim(),
-        version.to_string().trim(),
-        "protoc version must be 3.5.1"
+        version.trim()
     );
 }
 
@@ -33,7 +32,13 @@ fn build_protobuf(out_dir: &str, input: &[&str], includes: &[&str]) {
 
 #[cfg(feature = "serialization-protobuf")]
 fn build_protobuf_schemata() {
-    assert_protobuf_version(env!("PROTOC_VERSION"));
+    use std::fs::File;
+    use std::io::Read;
+    let mut version_string = String::new();
+    let mut version_pin =
+        File::open("PROTOC_VERSION").expect("protoc version pin `PROTOC_VERSION` file is missing");
+    version_pin.read_to_string(&mut version_string).expect("cannot read protoc pin file");
+    assert_protobuf_version(version_string);
     build_protobuf("src/proto", &["protobuf/proof.proto"], &[]);
 }