Add explanatory comment to is_rvalue_or_immutable()

Author: nikomatsakis

src/librustc/middle/borrowck/gather_loans/lifetime.rs

@@ -170,6 +170,11 @@ impl GuaranteeLifetimeContext {
 
     fn is_rvalue_or_immutable(&self,
                               cmt: mc::cmt) -> bool {
+        //! We can omit the root on an `@T` value if the location
+        //! that holds the box is either (1) an rvalue, in which case
+        //! is is in a non-user-accessible temporary, or (2) an immutable
+        //! lvalue.
+
         cmt.mutbl.is_immutable() || match cmt.guarantor().cat {
             mc::cat_rvalue => true,
             _ => false