diff --git a/src/tools/compiletest/src/runtest.rs b/src/tools/compiletest/src/runtest.rs
index db08d9b54ba2c..41972b6994c60 100644
--- a/src/tools/compiletest/src/runtest.rs
+++ b/src/tools/compiletest/src/runtest.rs
@@ -3566,7 +3566,7 @@ impl<'test> TestCx<'test> {
             .env(dylib_env_var(), &dylib_env_paths)
             .env("TARGET", &self.config.target)
             .env("PYTHON", &self.config.python)
-            .env("S", &src_root)
+            .env("SOURCE_ROOT", &src_root)
             .env("RUST_BUILD_STAGE", &self.config.stage_id)
             .env("RUSTC", cwd.join(&self.config.rustc_path))
             .env("TMPDIR", &rmake_out_dir)
diff --git a/src/tools/run-make-support/src/lib.rs b/src/tools/run-make-support/src/lib.rs
index d74a0272a627e..0f9d5a3b6c365 100644
--- a/src/tools/run-make-support/src/lib.rs
+++ b/src/tools/run-make-support/src/lib.rs
@@ -89,7 +89,7 @@ pub fn htmldocck() -> Command {
 
 /// Path to the root rust-lang/rust source checkout.
 pub fn source_root() -> PathBuf {
-    env_var("S").into()
+    env_var("SOURCE_ROOT").into()
 }
 
 /// Construct the static library name based on the platform.