Deny warnings in std stage 0

src/bootstrap/builder.rs

@@ -1018,8 +1018,7 @@ pub fn cargo(
 
         cargo.env("RUSTC_VERBOSE", self.verbosity.to_string());
 
-        // in std, we want to avoid denying warnings for stage 0 as that makes cfg's painful.
-        if self.config.deny_warnings && !(mode == Mode::Std && stage == 0) {
+        if self.config.deny_warnings {
             cargo.env("RUSTC_DENY_WARNINGS", "1");
         }