Add comment to `AllocRef` implementation for `System`

diff --git a/src/libstd/alloc.rs b/src/libstd/alloc.rs
index f295565bec3481279d032febbd489cad5139e560..6e8ac7c90363f1ce1f822753c1a93e8e3d7ef848 100644 (file)
--- a/src/libstd/alloc.rs
+++ b/src/libstd/alloc.rs
@@ -134,6 +134,8 @@
 #[derive(Debug, Default, Copy, Clone)]
 pub struct System;
 
+// The AllocRef impl checks the layout size to be non-zero and forwards to the GlobalAlloc impl,
+// which is in `std::sys::*::alloc`.
 #[unstable(feature = "allocator_api", issue = "32838")]
 unsafe impl AllocRef for System {
     #[inline]