Build Cargo with Thin LTO and 1 CGU

Author: Kobzol

src/bootstrap/src/core/build_steps/tool.rs

@@ -229,6 +229,9 @@ pub fn prepare_tool_cargo(
         // `RUSTFLAGS` causes unnecessary tool rebuilds due to cache invalidation from building e.g.
         // cargo *without* `-Zon-broken-pipe=kill` but then rustdoc *with* `-Zon-broken-pipe=kill`.
         cargo.env("FORCE_ON_BROKEN_PIPE_KILL", "-Zon-broken-pipe=kill");
+    } else {
+        cargo.env("CARGO_PROFILE_RELEASE_LTO", "thin");
+        cargo.env("CARGO_PROFILE_RELEASE_CODEGEN_UNITS", "1");
     }
 
     cargo