Restore rustc shim error message

src/bootstrap/src/bin/rustc.rs

@@ -32,6 +32,9 @@ fn main() {
     let args = env::args_os().skip(1).collect::<Vec<_>>();
     let arg = |name| args.windows(2).find(|args| args[0] == name).and_then(|args| args[1].to_str());
 
+    // We don't use the stage in this shim, but let's parse it to make sure that we're invoked
+    // by bootstrap, or that we provide a helpful error message if not.
+    bin_helpers::parse_rustc_stage();
     let verbose = bin_helpers::parse_rustc_verbose();
 
     // Detect whether or not we're a build script depending on whether --target

src/bootstrap/src/utils/bin_helpers.rs

@@ -18,7 +18,6 @@ pub(crate) fn parse_rustc_verbose() -> usize {
 /// Parses the value of the "RUSTC_STAGE" environment variable and returns it as a `String`.
 ///
 /// If "RUSTC_STAGE" was not set, the program will be terminated with 101.
-#[allow(unused)]
 pub(crate) fn parse_rustc_stage() -> String {
     std::env::var("RUSTC_STAGE").unwrap_or_else(|_| {
         // Don't panic here; it's reasonable to try and run these shims directly. Give a helpful error instead.