diff --git a/src/bootstrap/build.rs b/src/bootstrap/build.rs
index e0e32d3135354..d9810e899a043 100644
--- a/src/bootstrap/build.rs
+++ b/src/bootstrap/build.rs
@@ -1,6 +1,7 @@
 use std::env;
 
 fn main() {
+    // this is needed because `HOST` is only available to build scripts.
     let host = env::var("HOST").unwrap();
     println!("cargo:rerun-if-changed=build.rs");
     println!("cargo:rustc-env=BUILD_TRIPLE={host}");