diff --git a/src/librustc/metadata/creader.rs b/src/librustc/metadata/creader.rs
index 64d4d5939d0..2b0d6479d0c 100644
--- a/src/librustc/metadata/creader.rs
+++ b/src/librustc/metadata/creader.rs
@@ -15,6 +15,7 @@ use metadata::cstore;
 use metadata::decoder;
 use metadata::loader;
 
+use std::cell::RefCell;
 use std::hashmap::HashMap;
 use syntax::ast;
 use syntax::abi;
@@ -347,5 +348,5 @@ fn resolve_crate_deps(e: &mut Env, cdata: &[u8]) -> cstore::cnum_map {
           }
         }
     }
-    return @mut cnum_map;
+    return @RefCell::new(cnum_map);
 }
diff --git a/src/librustc/metadata/cstore.rs b/src/librustc/metadata/cstore.rs
index 1804e8b1247..f567665256b 100644
--- a/src/librustc/metadata/cstore.rs
+++ b/src/librustc/metadata/cstore.rs
@@ -24,7 +24,7 @@ use syntax::parse::token::ident_interner;
 // local crate numbers (as generated during this session). Each external
 // crate may refer to types in other external crates, and each has their
 // own crate numbers.
-pub type cnum_map = @mut HashMap<ast::CrateNum, ast::CrateNum>;
+pub type cnum_map = @RefCell<HashMap<ast::CrateNum, ast::CrateNum>>;
 
 pub enum MetadataBlob {
     MetadataVec(~[u8]),
diff --git a/src/librustc/metadata/decoder.rs b/src/librustc/metadata/decoder.rs
index b9a3bc47eb3..a0b339cc91d 100644
--- a/src/librustc/metadata/decoder.rs
+++ b/src/librustc/metadata/decoder.rs
@@ -1197,9 +1197,15 @@ pub fn translate_def_id(cdata: Cmd, did: ast::DefId) -> ast::DefId {
         return ast::DefId { crate: cdata.cnum, node: did.node };
     }
 
-    match cdata.cnum_map.find(&did.crate) {
-      option::Some(&n) => ast::DefId { crate: n, node: did.node },
-      option::None => fail!("didn't find a crate in the cnum_map")
+    let cnum_map = cdata.cnum_map.borrow();
+    match cnum_map.get().find(&did.crate) {
+        Some(&n) => {
+            ast::DefId {
+                crate: n,
+                node: did.node,
+            }
+        }
+        None => fail!("didn't find a crate in the cnum_map")
     }
 }
 
diff --git a/src/librustc/middle/borrowck/mod.rs b/src/librustc/middle/borrowck/mod.rs
index ae867fe6727..dd886b3054e 100644
--- a/src/librustc/middle/borrowck/mod.rs
+++ b/src/librustc/middle/borrowck/mod.rs
@@ -83,7 +83,7 @@ pub fn check_crate(
         capture_map: capture_map,
         root_map: root_map(),
         write_guard_map: @RefCell::new(HashSet::new()),
-        stats: @mut BorrowStats {
+        stats: @BorrowStats {
             loaned_paths_same: Cell::new(0),
             loaned_paths_imm: Cell::new(0),
             stable_paths: Cell::new(0),
@@ -172,7 +172,7 @@ pub struct BorrowckCtxt {
     write_guard_map: write_guard_map,
 
     // Statistics:
-    stats: @mut BorrowStats
+    stats: @BorrowStats
 }
 
 pub struct BorrowStats {