Add test to verify noalias is not being added

src/test/codegen/noalias-rwlockreadguard.rs

@@ -0,0 +1,14 @@
+// compile-flags: -O -C no-prepopulate-passes -Z mutable-noalias=yes
+
+#![crate_type = "lib"]
+
+use std::cell::UnsafeCell;
+use std::sync::RwLockReadGuard;
+
+// Make sure that `RwLockReadGuard` does not get a `noalias` attribute, because
+// the `UnsafeCell` might alias writes after it is dropped.
+
+// CHECK-LABEL: @maybe_aliased(
+// CHECK-NOT: noalias
+#[no_mangle]
+pub unsafe fn maybe_aliased(_: RwLockReadGuard<'_, i32>, _data: &UnsafeCell<i32>) {}