librustc: De-@mut the used_mut_nodes table in the type context

src/librustc/middle/borrowck/check_loans.rs

@@ -357,7 +357,10 @@ fn mark_variable_as_used_mut(this: &CheckLoanCtxt,
                     mc::cat_local(id) |
                     mc::cat_arg(id) |
                     mc::cat_self(id) => {
-                        this.tcx().used_mut_nodes.insert(id);
+                        let mut used_mut_nodes = this.tcx()
+                                                     .used_mut_nodes
+                                                     .borrow_mut();
+                        used_mut_nodes.get().insert(id);
                         return;
                     }
 

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

@@ -607,7 +607,10 @@ pub fn mark_loan_path_as_mutated(&self, loan_path: @LoanPath) {
 
         match *loan_path {
             LpVar(local_id) => {
-                self.tcx().used_mut_nodes.insert(local_id);
+                let mut used_mut_nodes = self.tcx()
+                                             .used_mut_nodes
+                                             .borrow_mut();
+                used_mut_nodes.get().insert(local_id);
             }
             LpExtend(base, mc::McInherited, _) => {
                 self.mark_loan_path_as_mutated(base);

src/librustc/middle/lint.rs

@@ -1038,7 +1038,8 @@ fn check_unused_mut_pat(cx: &Context, p: &ast::Pat) {
                 }
             };
 
-            if !initial_underscore && !cx.tcx.used_mut_nodes.contains(&p.id) {
+            let used_mut_nodes = cx.tcx.used_mut_nodes.borrow();
+            if !initial_underscore && !used_mut_nodes.get().contains(&p.id) {
                 cx.span_lint(unused_mut, p.span,
                              "variable does not need to be mutable");
             }

src/librustc/middle/ty.rs

@@ -353,7 +353,7 @@ struct ctxt_ {
     // Set of nodes which mark locals as mutable which end up getting used at
     // some point. Local variable definitions not in this set can be warned
     // about.
-    used_mut_nodes: @mut HashSet<ast::NodeId>,
+    used_mut_nodes: RefCell<HashSet<ast::NodeId>>,
 
     // vtable resolution information for impl declarations
     impl_vtables: typeck::impl_vtable_map,
@@ -1005,7 +1005,7 @@ pub fn mk_ctxt(s: session::Session,
         inherent_impls: RefCell::new(HashMap::new()),
         impls: RefCell::new(HashMap::new()),
         used_unsafe: RefCell::new(HashSet::new()),
-        used_mut_nodes: @mut HashSet::new(),
+        used_mut_nodes: RefCell::new(HashSet::new()),
         impl_vtables: RefCell::new(HashMap::new()),
         populated_external_types: @mut HashSet::new(),
         populated_external_traits: @mut HashSet::new(),