Use $CARGO_PKG_VERSION from buildscript exec-time instead of build-time

Author: dtolnay

build.rs

@@ -1,9 +1,21 @@
+use std::env;
+use std::ffi::OsString;
+use std::process;
+
 fn main() {
     println!("cargo:rerun-if-changed=build.rs");
 
     println!("cargo:rustc-check-cfg=cfg(exhaustive)");
     println!("cargo:rustc-check-cfg=cfg(prettyplease_debug)");
     println!("cargo:rustc-check-cfg=cfg(prettyplease_debug_indent)");
 
-    println!(concat!("cargo:VERSION=", env!("CARGO_PKG_VERSION")));
+    let pkg_version = cargo_env_var("CARGO_PKG_VERSION");
+    println!("cargo:VERSION={}", pkg_version.to_str().unwrap());
+}
+
+fn cargo_env_var(key: &str) -> OsString {
+    env::var_os(key).unwrap_or_else(|| {
+        eprintln!("Environment variable ${key} is not set during execution of build script");
+        process::exit(1);
+    })
 }