Pass "-Wunused_extern_options" to stage1 compiler

Author: Aaron1011

src/bootstrap/bootstrap.py

@@ -691,6 +691,7 @@ def build_bootstrap(self):
         target_linker = self.get_toml("linker", build_section)
         if target_linker is not None:
             env["RUSTFLAGS"] += " -C linker=" + target_linker
+        # After the next cfg(bootstrap) bump, add "-Wunused_extern_options" to RUSTFLAGS
         env["RUSTFLAGS"] += " -Wrust_2018_idioms -Wunused_lifetimes"
         if self.get_toml("deny-warnings", "rust") != "false":
             env["RUSTFLAGS"] += " -Dwarnings"

src/bootstrap/builder.rs

@@ -1067,6 +1067,10 @@ impl<'a> Builder<'a> {
             // some code doesn't go through this `rustc` wrapper.
             rustflags.arg("-Wrust_2018_idioms");
             rustflags.arg("-Wunused_lifetimes");
+            // Remove this after the next cfg(bootstrap) bump
+            if stage != 0 {
+                rustflags.arg("-Wunused_extern_options");
+            }
 
             if self.config.deny_warnings {
                 rustflags.arg("-Dwarnings");