Require cargo promised environment variables to be present

Author: dtolnay

build.rs

@@ -1,8 +1,9 @@
 #![allow(clippy::option_if_let_else)]
 
 use std::env;
+use std::ffi::OsString;
 use std::path::Path;
-use std::process::{Command, ExitStatus, Stdio};
+use std::process::{self, Command, ExitStatus, Stdio};
 use std::str;
 
 #[cfg(all(feature = "backtrace", not(feature = "std")))]
@@ -66,8 +67,8 @@ fn compile_probe() -> Option<ExitStatus> {
         return None;
     }
 
-    let rustc = env::var_os("RUSTC")?;
-    let out_dir = env::var_os("OUT_DIR")?;
+    let rustc = cargo_env_var("RUSTC");
+    let out_dir = cargo_env_var("OUT_DIR");
     let probefile = Path::new("build").join("probe.rs");
 
     // Make sure to pick up Cargo rustc configuration.
@@ -115,3 +116,13 @@ fn rustc_minor_version() -> Option<u32> {
     }
     pieces.next()?.parse().ok()
 }
+
+fn cargo_env_var(key: &str) -> OsString {
+    env::var_os(key).unwrap_or_else(|| {
+        eprintln!(
+            "Environment variable ${} is not set during execution of build script",
+            key,
+        );
+        process::exit(1);
+    })
+}