8 files changed, 28 insertions, 10 deletions
diff --git a/src/librustc/dep_graph/dep_node.rs b/src/librustc/dep_graph/dep_node.rs
index 7bf8c616817..56b21888985 100644
--- a/src/librustc/dep_graph/dep_node.rs
+++ b/src/librustc/dep_graph/dep_node.rs
@@ -500,6 +500,7 @@ define_dep_nodes!( <'tcx>
     [] InherentImpls(DefId),
     [] TypeckBodiesKrate,
     [] TypeckTables(DefId),
+    [] UsedTraitImports(DefId),
     [] HasTypeckTables(DefId),
     [] ConstEval { param_env: ParamEnvAnd<'tcx, (DefId, &'tcx Substs<'tcx>)> },
     [] SymbolName(DefId),
diff --git a/src/librustc/ty/context.rs b/src/librustc/ty/context.rs
index 5e9396068c8..47ca7b11731 100644
--- a/src/librustc/ty/context.rs
+++ b/src/librustc/ty/context.rs
@@ -386,8 +386,10 @@ pub struct TypeckTables<'tcx> {
     cast_kinds: ItemLocalMap<ty::cast::CastKind>,
 
     /// Set of trait imports actually used in the method resolution.
-    /// This is used for warning unused imports.
-    pub used_trait_imports: DefIdSet,
+    /// This is used for warning unused imports. During type
+    /// checking, this `Rc` should not be cloned: it must have a ref-count
+    /// of 1 so that we can insert things into the set mutably.
+    pub used_trait_imports: Rc<DefIdSet>,
 
     /// If any errors occurred while type-checking this body,
     /// this field will be set to `true`.
@@ -417,7 +419,7 @@ impl<'tcx> TypeckTables<'tcx> {
             liberated_fn_sigs: ItemLocalMap(),
             fru_field_types: ItemLocalMap(),
             cast_kinds: ItemLocalMap(),
-            used_trait_imports: DefIdSet(),
+            used_trait_imports: Rc::new(DefIdSet()),
             tainted_by_errors: false,
             free_region_map: FreeRegionMap::new(),
         }
diff --git a/src/librustc/ty/maps/mod.rs b/src/librustc/ty/maps/mod.rs
index e588cdc52d8..851d126056c 100644
--- a/src/librustc/ty/maps/mod.rs
+++ b/src/librustc/ty/maps/mod.rs
@@ -183,6 +183,8 @@ define_maps! { <'tcx>
 
     [] fn typeck_tables_of: TypeckTables(DefId) -> &'tcx ty::TypeckTables<'tcx>,
 
+    [] fn used_trait_imports: UsedTraitImports(DefId) -> Rc<DefIdSet>,
+
     [] fn has_typeck_tables: HasTypeckTables(DefId) -> bool,
 
     [] fn coherent_trait: coherent_trait_dep_node((CrateNum, DefId)) -> (),
diff --git a/src/librustc/ty/maps/plumbing.rs b/src/librustc/ty/maps/plumbing.rs
index cce96817717..7edad824cdc 100644
--- a/src/librustc/ty/maps/plumbing.rs
+++ b/src/librustc/ty/maps/plumbing.rs
@@ -754,6 +754,7 @@ pub fn force_from_dep_node<'a, 'gcx, 'lcx>(tcx: TyCtxt<'a, 'gcx, 'lcx>,
         DepKind::InherentImpls => { force!(inherent_impls, def_id!()); }
         DepKind::TypeckBodiesKrate => { force!(typeck_item_bodies, LOCAL_CRATE); }
         DepKind::TypeckTables => { force!(typeck_tables_of, def_id!()); }
+        DepKind::UsedTraitImports => { force!(used_trait_imports, def_id!()); }
         DepKind::HasTypeckTables => { force!(has_typeck_tables, def_id!()); }
         DepKind::SymbolName => { force!(def_symbol_name, def_id!()); }
         DepKind::SpecializationGraph => { force!(specialization_graph_of, def_id!()); }
diff --git a/src/librustc_typeck/check/method/mod.rs b/src/librustc_typeck/check/method/mod.rs
index d4eda13c6cd..58d72e37d51 100644
--- a/src/librustc_typeck/check/method/mod.rs
+++ b/src/librustc_typeck/check/method/mod.rs
@@ -25,6 +25,8 @@ use syntax_pos::Span;
 
 use rustc::hir;
 
+use std::rc::Rc;
+
 pub use self::MethodError::*;
 pub use self::CandidateSource::*;
 
@@ -163,7 +165,8 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
         if let Some(import_id) = pick.import_id {
             let import_def_id = self.tcx.hir.local_def_id(import_id);
             debug!("used_trait_import: {:?}", import_def_id);
-            self.tables.borrow_mut().used_trait_imports.insert(import_def_id);
+            Rc::get_mut(&mut self.tables.borrow_mut().used_trait_imports)
+                                        .unwrap().insert(import_def_id);
         }
 
         self.tcx.check_stability(pick.item.def_id, call_expr.id, span);
@@ -361,7 +364,8 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
         if let Some(import_id) = pick.import_id {
             let import_def_id = self.tcx.hir.local_def_id(import_id);
             debug!("used_trait_import: {:?}", import_def_id);
-            self.tables.borrow_mut().used_trait_imports.insert(import_def_id);
+            Rc::get_mut(&mut self.tables.borrow_mut().used_trait_imports)
+                                        .unwrap().insert(import_def_id);
         }
 
         let def = pick.item.def();
diff --git a/src/librustc_typeck/check/mod.rs b/src/librustc_typeck/check/mod.rs
index d4616a7c304..c82cafd3a62 100644
--- a/src/librustc_typeck/check/mod.rs
+++ b/src/librustc_typeck/check/mod.rs
@@ -106,9 +106,10 @@ use session::{CompileIncomplete, Session};
 use TypeAndSubsts;
 use lint;
 use util::common::{ErrorReported, indenter};
-use util::nodemap::{DefIdMap, FxHashMap, NodeMap};
+use util::nodemap::{DefIdMap, DefIdSet, FxHashMap, NodeMap};
 
 use std::cell::{Cell, RefCell, Ref, RefMut};
+use std::rc::Rc;
 use std::collections::hash_map::Entry;
 use std::cmp;
 use std::fmt::Display;
@@ -742,6 +743,7 @@ pub fn provide(providers: &mut Providers) {
         closure_kind,
         generator_sig,
         adt_destructor,
+        used_trait_imports,
         ..*providers
     };
 }
@@ -845,6 +847,12 @@ fn has_typeck_tables<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
     primary_body_of(tcx, id).is_some()
 }
 
+fn used_trait_imports<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
+                              def_id: DefId)
+                              -> Rc<DefIdSet> {
+    tcx.typeck_tables_of(def_id).used_trait_imports.clone()
+}
+
 fn typeck_tables_of<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
                               def_id: DefId)
                               -> &'tcx ty::TypeckTables<'tcx> {
diff --git a/src/librustc_typeck/check/writeback.rs b/src/librustc_typeck/check/writeback.rs
index b3648d357e5..ce2ac73a27e 100644
--- a/src/librustc_typeck/check/writeback.rs
+++ b/src/librustc_typeck/check/writeback.rs
@@ -23,6 +23,7 @@ use rustc::util::nodemap::DefIdSet;
 use syntax::ast;
 use syntax_pos::Span;
 use std::mem;
+use std::rc::Rc;
 
 ///////////////////////////////////////////////////////////////////////////
 // Entry point
@@ -49,7 +50,7 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
         wbcx.visit_generator_interiors();
 
         let used_trait_imports = mem::replace(&mut self.tables.borrow_mut().used_trait_imports,
-                                              DefIdSet());
+                                              Rc::new(DefIdSet()));
         debug!("used_trait_imports({:?}) = {:?}", item_def_id, used_trait_imports);
         wbcx.tables.used_trait_imports = used_trait_imports;
 
diff --git a/src/librustc_typeck/check_unused.rs b/src/librustc_typeck/check_unused.rs
index 0c35b5e6834..b867a655b4a 100644
--- a/src/librustc_typeck/check_unused.rs
+++ b/src/librustc_typeck/check_unused.rs
@@ -66,10 +66,9 @@ pub fn check_crate<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) {
     let mut used_trait_imports = DefIdSet();
     for &body_id in tcx.hir.krate().bodies.keys() {
         let item_def_id = tcx.hir.body_owner_def_id(body_id);
-        let tables = tcx.typeck_tables_of(item_def_id);
-        let imports = &tables.used_trait_imports;
+        let imports = tcx.used_trait_imports(item_def_id);
         debug!("GatherVisitor: item_def_id={:?} with imports {:#?}", item_def_id, imports);
-        used_trait_imports.extend(imports);
+        used_trait_imports.extend(imports.iter());
     }
 
     let mut visitor = CheckVisitor { tcx, used_trait_imports };