Issue #5967: Make rvalues mutable

Author: nikomatsakis

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

@@ -93,7 +93,7 @@ impl GuaranteeLifetimeContext {
                 let omit_root = (
                     ptr_mutbl == m_imm &&
                     self.bccx.is_subregion_of(self.loan_region, base_scope) &&
-                    base.mutbl.is_immutable() &&
+                    self.is_rvalue_or_immutable(base) &&
                     !self.is_moved(base)
                 );
 
@@ -168,6 +168,14 @@ impl GuaranteeLifetimeContext {
         }
     }
 
+    fn is_rvalue_or_immutable(&self,
+                              cmt: mc::cmt) -> bool {
+        cmt.mutbl.is_immutable() || match cmt.guarantor().cat {
+            mc::cat_rvalue => true,
+            _ => false
+        }
+    }
+
     fn check_root(&self,
                   cmt_deref: mc::cmt,
                   cmt_base: mc::cmt,

src/librustc/middle/mem_categorization.rs

@@ -551,7 +551,7 @@ pub impl mem_categorization_ctxt {
             id:elt.id(),
             span:elt.span(),
             cat:cat_rvalue,
-            mutbl:McImmutable,
+            mutbl:McDeclared,
             ty:expr_ty
         }
     }