rustbuild: Deny and fix warnings

Turned out this lint uncovered an actual bug!

Closes #38484

src/bootstrap/bin/rustc.rs

@@ -25,6 +25,8 @@
 //! switching compilers for the bootstrap and for build scripts will probably
 //! never get replaced.
 
+#![deny(warnings)]
+
 extern crate bootstrap;
 
 use std::env;

src/bootstrap/bin/rustdoc.rs

@@ -12,6 +12,8 @@
 //!
 //! See comments in `src/bootstrap/rustc.rs` for more information.
 
+#![deny(warnings)]
+
 extern crate bootstrap;
 
 use std::env;

src/bootstrap/flags.rs

@@ -280,7 +280,7 @@ To learn more about a subcommand, run `./x.py <command> -h`
 
         Flags {
             verbose: m.opt_count("v"),
-            stage: m.opt_str("stage").map(|j| j.parse().unwrap()),
+            stage: stage,
             keep_stage: m.opt_str("keep-stage").map(|j| j.parse().unwrap()),
             build: m.opt_str("build").unwrap_or_else(|| {
                 env::var("BUILD").unwrap()

src/bootstrap/lib.rs

@@ -64,6 +64,8 @@
 //! More documentation can be found in each respective module below, and you can
 //! also check out the `src/bootstrap/README.md` file for more information.
 
+#![deny(warnings)]
+
 extern crate build_helper;
 extern crate cmake;
 extern crate filetime;