Move handling of -Dwarnings to builder.rs

src/bootstrap/bin/rustc.rs

@@ -111,9 +111,6 @@ fn main() {
             cmd.arg("-Zunstable-options");
             cmd.arg("-Wrustc::internal");
         }
-        if env::var_os("RUSTC_DENY_WARNINGS").is_some() {
-            cmd.arg("-Dwarnings");
-        }
     }
 
     if let Some(target) = target {

src/bootstrap/builder.rs

@@ -1055,8 +1055,8 @@ pub fn cargo(
 
         cargo.env("RUSTC_VERBOSE", self.verbosity.to_string());
 
-        if self.config.deny_warnings {
-            cargo.env("RUSTC_DENY_WARNINGS", "1");
+        if self.config.deny_warnings && !mode.is_tool() {
+            rustflags.arg("-Dwarnings");
         }
 
         // Throughout the build Cargo can execute a number of build scripts