Add comments

library/alloc/src/collections/mod.rs

@@ -1,5 +1,8 @@
 //! Collection types.
 
+// Note: This module is also included in the alloctests crate using #[path] to
+// run the tests. See the comment there for an explanation why this is the case.
+
 #![stable(feature = "rust1", since = "1.0.0")]
 
 #[cfg(not(no_global_oom_handling))]

library/alloc/src/raw_vec/mod.rs

@@ -1,6 +1,9 @@
 #![unstable(feature = "raw_vec_internals", reason = "unstable const warnings", issue = "none")]
 #![cfg_attr(test, allow(dead_code))]
 
+// Note: This module is also included in the alloctests crate using #[path] to
+// run the tests. See the comment there for an explanation why this is the case.
+
 use core::marker::PhantomData;
 use core::mem::{ManuallyDrop, MaybeUninit, SizedTypeProperties};
 use core::ptr::{self, NonNull, Unique};

library/alloctests/lib.rs

@@ -67,6 +67,9 @@ extern crate test;
 mod testing;
 use realalloc::*;
 
+// We are directly including collections and raw_vec here as both use non-public
+// methods and fields in tests and as such need to have the types to test in the
+// same crate as the tests themself.
 #[path = "../alloc/src/collections/mod.rs"]
 mod collections;