diff --git a/src/librustc/dep_graph/dep_node.rs b/src/librustc/dep_graph/dep_node.rs
index 810355dc2c0f8..caa912c7c8baf 100644
--- a/src/librustc/dep_graph/dep_node.rs
+++ b/src/librustc/dep_graph/dep_node.rs
@@ -574,6 +574,7 @@ define_dep_nodes!( <'tcx>
     [] MaybeUnusedTraitImport(HirId),
     [] MaybeUnusedExternCrates,
     [] StabilityIndex,
+    [] AllCrateNums,
 );
 
 trait DepNodeParams<'a, 'gcx: 'tcx + 'a, 'tcx: 'a> : fmt::Debug {
diff --git a/src/librustc/middle/cstore.rs b/src/librustc/middle/cstore.rs
index 68d871c4b30ec..a7d874386d1c9 100644
--- a/src/librustc/middle/cstore.rs
+++ b/src/librustc/middle/cstore.rs
@@ -255,7 +255,7 @@ pub trait CrateStore {
 
     // This is basically a 1-based range of ints, which is a little
     // silly - I may fix that.
-    fn crates(&self) -> Vec<CrateNum>;
+    fn crates_untracked(&self) -> Vec<CrateNum>;
 
     // utility functions
     fn encode_metadata<'a, 'tcx>(&self,
@@ -334,9 +334,7 @@ impl CrateStore for DummyCrateStore {
     }
     fn load_macro_untracked(&self, did: DefId, sess: &Session) -> LoadedMacro { bug!("load_macro") }
 
-    // This is basically a 1-based range of ints, which is a little
-    // silly - I may fix that.
-    fn crates(&self) -> Vec<CrateNum> { vec![] }
+    fn crates_untracked(&self) -> Vec<CrateNum> { vec![] }
 
     // utility functions
     fn extern_mod_stmt_cnum_untracked(&self, emod_id: ast::NodeId) -> Option<CrateNum> { None }
@@ -370,8 +368,9 @@ pub trait CrateLoader {
 // positions.
 pub fn used_crates<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
                              prefer: LinkagePreference) -> Vec<(CrateNum, LibSource)> {
-    let mut libs = tcx.sess.cstore.crates()
-        .into_iter()
+    let mut libs = tcx.crates()
+        .iter()
+        .cloned()
         .filter_map(|cnum| {
             if tcx.dep_kind(cnum).macros_only() {
                 return None
diff --git a/src/librustc/middle/dependency_format.rs b/src/librustc/middle/dependency_format.rs
index 81c406f3cb183..1c7d0b76a643e 100644
--- a/src/librustc/middle/dependency_format.rs
+++ b/src/librustc/middle/dependency_format.rs
@@ -132,7 +132,7 @@ fn calculate_type<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
             if let Some(v) = attempt_static(tcx) {
                 return v;
             }
-            for cnum in sess.cstore.crates() {
+            for &cnum in tcx.crates().iter() {
                 if tcx.dep_kind(cnum).macros_only() { continue }
                 let src = tcx.used_crate_source(cnum);
                 if src.rlib.is_some() { continue }
@@ -165,7 +165,7 @@ fn calculate_type<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
     // Sweep all crates for found dylibs. Add all dylibs, as well as their
     // dependencies, ensuring there are no conflicts. The only valid case for a
     // dependency to be relied upon twice is for both cases to rely on a dylib.
-    for cnum in sess.cstore.crates() {
+    for &cnum in tcx.crates().iter() {
         if tcx.dep_kind(cnum).macros_only() { continue }
         let name = tcx.crate_name(cnum);
         let src = tcx.used_crate_source(cnum);
@@ -181,7 +181,7 @@ fn calculate_type<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
     }
 
     // Collect what we've got so far in the return vector.
-    let last_crate = sess.cstore.crates().len();
+    let last_crate = tcx.crates().len();
     let mut ret = (1..last_crate+1).map(|cnum| {
         match formats.get(&CrateNum::new(cnum)) {
             Some(&RequireDynamic) => Linkage::Dynamic,
@@ -195,7 +195,7 @@ fn calculate_type<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
     //
     // If the crate hasn't been included yet and it's not actually required
     // (e.g. it's an allocator) then we skip it here as well.
-    for cnum in sess.cstore.crates() {
+    for &cnum in tcx.crates().iter() {
         let src = tcx.used_crate_source(cnum);
         if src.dylib.is_none() &&
            !formats.contains_key(&cnum) &&
@@ -281,7 +281,7 @@ fn attempt_static<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) -> Option<DependencyLis
 
     // All crates are available in an rlib format, so we're just going to link
     // everything in explicitly so long as it's actually required.
-    let last_crate = sess.cstore.crates().len();
+    let last_crate = tcx.crates().len();
     let mut ret = (1..last_crate+1).map(|cnum| {
         if tcx.dep_kind(CrateNum::new(cnum)) == DepKind::Explicit {
             Linkage::Static
diff --git a/src/librustc/middle/lang_items.rs b/src/librustc/middle/lang_items.rs
index d7cc47f9ee78d..cb59d9870faac 100644
--- a/src/librustc/middle/lang_items.rs
+++ b/src/librustc/middle/lang_items.rs
@@ -208,7 +208,7 @@ pub fn extract(attrs: &[ast::Attribute]) -> Option<Symbol> {
 
 pub fn collect<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) -> LanguageItems {
     let mut collector = LanguageItemCollector::new(tcx);
-    for cnum in tcx.sess.cstore.crates() {
+    for &cnum in tcx.crates().iter() {
         for &(index, item_index) in tcx.defined_lang_items(cnum).iter() {
             let def_id = DefId { krate: cnum, index: index };
             collector.collect_item(item_index, def_id);
diff --git a/src/librustc/middle/weak_lang_items.rs b/src/librustc/middle/weak_lang_items.rs
index ad6702ed21f8d..50fb584070262 100644
--- a/src/librustc/middle/weak_lang_items.rs
+++ b/src/librustc/middle/weak_lang_items.rs
@@ -83,7 +83,7 @@ fn verify<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
     }
 
     let mut missing = HashSet::new();
-    for cnum in tcx.sess.cstore.crates() {
+    for &cnum in tcx.crates().iter() {
         for &item in tcx.missing_lang_items(cnum).iter() {
             missing.insert(item);
         }
diff --git a/src/librustc/ty/context.rs b/src/librustc/ty/context.rs
index 15e649e7cec0c..18f286ebf5576 100644
--- a/src/librustc/ty/context.rs
+++ b/src/librustc/ty/context.rs
@@ -994,14 +994,14 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         let interners = CtxtInterners::new(arena);
         let common_types = CommonTypes::new(&interners);
         let dep_graph = hir.dep_graph.clone();
-        let max_cnum = s.cstore.crates().iter().map(|c| c.as_usize()).max().unwrap_or(0);
+        let max_cnum = s.cstore.crates_untracked().iter().map(|c| c.as_usize()).max().unwrap_or(0);
         let mut providers = IndexVec::from_elem_n(extern_providers, max_cnum + 1);
         providers[LOCAL_CRATE] = local_providers;
 
         let def_path_hash_to_def_id = if s.opts.build_dep_graph() {
             let upstream_def_path_tables: Vec<(CrateNum, Rc<_>)> = s
                 .cstore
-                .crates()
+                .crates_untracked()
                 .iter()
                 .map(|&cnum| (cnum, s.cstore.def_path_table(cnum)))
                 .collect();
@@ -1121,6 +1121,10 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
             self.stability_index(LOCAL_CRATE)
         })
     }
+
+    pub fn crates(self) -> Rc<Vec<CrateNum>> {
+        self.all_crate_nums(LOCAL_CRATE)
+    }
 }
 
 impl<'gcx: 'tcx, 'tcx> GlobalCtxt<'gcx> {
diff --git a/src/librustc/ty/maps.rs b/src/librustc/ty/maps.rs
index 29e5e4e343134..48b92d101edb3 100644
--- a/src/librustc/ty/maps.rs
+++ b/src/librustc/ty/maps.rs
@@ -754,6 +754,12 @@ impl<'tcx> QueryDescription for queries::stability_index<'tcx> {
     }
 }
 
+impl<'tcx> QueryDescription for queries::all_crate_nums<'tcx> {
+    fn describe(_tcx: TyCtxt, _: CrateNum) -> String {
+        format!("fetching all foreign CrateNum instances")
+    }
+}
+
 // If enabled, send a message to the profile-queries thread
 macro_rules! profq_msg {
     ($tcx:expr, $msg:expr) => {
@@ -1376,6 +1382,7 @@ define_maps! { <'tcx>
         -> Rc<Vec<(HirId, Span)>>,
 
     [] fn stability_index: stability_index_node(CrateNum) -> Rc<stability::Index<'tcx>>,
+    [] fn all_crate_nums: all_crate_nums_node(CrateNum) -> Rc<Vec<CrateNum>>,
 }
 
 fn type_param_predicates<'tcx>((item_id, param_id): (DefId, DefId)) -> DepConstructor<'tcx> {
@@ -1485,3 +1492,7 @@ fn maybe_unused_extern_crates_node<'tcx>(_: CrateNum) -> DepConstructor<'tcx> {
 fn stability_index_node<'tcx>(_: CrateNum) -> DepConstructor<'tcx> {
     DepConstructor::StabilityIndex
 }
+
+fn all_crate_nums_node<'tcx>(_: CrateNum) -> DepConstructor<'tcx> {
+    DepConstructor::AllCrateNums
+}
diff --git a/src/librustc/ty/trait_def.rs b/src/librustc/ty/trait_def.rs
index 4687fc6540924..3aadacfe826fd 100644
--- a/src/librustc/ty/trait_def.rs
+++ b/src/librustc/ty/trait_def.rs
@@ -146,7 +146,7 @@ pub(super) fn trait_impls_of_provider<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
     // Traits defined in the current crate can't have impls in upstream
     // crates, so we don't bother querying the cstore.
     if !trait_id.is_local() {
-        for cnum in tcx.sess.cstore.crates() {
+        for &cnum in tcx.crates().iter() {
             let impls = tcx.implementations_of_trait((cnum, trait_id));
             remote_impls.extend(impls.iter().cloned());
         }
diff --git a/src/librustc_metadata/cstore_impl.rs b/src/librustc_metadata/cstore_impl.rs
index 61881bc033bea..776fd35829000 100644
--- a/src/librustc_metadata/cstore_impl.rs
+++ b/src/librustc_metadata/cstore_impl.rs
@@ -281,6 +281,11 @@ pub fn provide_local<'tcx>(providers: &mut Providers<'tcx>) {
             tcx.sess.cstore.extern_mod_stmt_cnum_untracked(id)
         },
 
+        all_crate_nums: |tcx, cnum| {
+            assert_eq!(cnum, LOCAL_CRATE);
+            Rc::new(tcx.sess.cstore.crates_untracked())
+        },
+
         // Returns a map from a sufficiently visible external item (i.e. an
         // external item that is visible from at least one local module) to a
         // sufficiently visible parent (considering modules that re-export the
@@ -292,7 +297,7 @@ pub fn provide_local<'tcx>(providers: &mut Providers<'tcx>) {
             assert_eq!(cnum, LOCAL_CRATE);
             let mut visible_parent_map: DefIdMap<DefId> = DefIdMap();
 
-            for cnum in tcx.sess.cstore.crates() {
+            for &cnum in tcx.crates().iter() {
                 // Ignore crates without a corresponding local `extern crate` item.
                 if tcx.missing_extern_crate_item(cnum) {
                     continue
@@ -481,7 +486,7 @@ impl CrateStore for cstore::CStore {
         })
     }
 
-    fn crates(&self) -> Vec<CrateNum>
+    fn crates_untracked(&self) -> Vec<CrateNum>
     {
         let mut result = vec![];
         self.iter_crate_data(|cnum, _| result.push(cnum));
diff --git a/src/librustc_metadata/encoder.rs b/src/librustc_metadata/encoder.rs
index d3b3396413c13..a27a85a63e8a7 100644
--- a/src/librustc_metadata/encoder.rs
+++ b/src/librustc_metadata/encoder.rs
@@ -1288,7 +1288,7 @@ impl<'a, 'b: 'a, 'tcx: 'b> IsolatedEncoder<'a, 'b, 'tcx> {
     }
 
     fn encode_crate_deps(&mut self, _: ()) -> LazySeq<CrateDep> {
-        let crates = self.tcx.sess.cstore.crates();
+        let crates = self.tcx.crates();
 
         let mut deps = crates
             .iter()
diff --git a/src/librustc_save_analysis/lib.rs b/src/librustc_save_analysis/lib.rs
index 3dc3bad602a90..3de5fda0d45c1 100644
--- a/src/librustc_save_analysis/lib.rs
+++ b/src/librustc_save_analysis/lib.rs
@@ -109,7 +109,7 @@ impl<'l, 'tcx: 'l> SaveContext<'l, 'tcx> {
     pub fn get_external_crates(&self) -> Vec<ExternalCrateData> {
         let mut result = Vec::new();
 
-        for n in self.tcx.sess.cstore.crates() {
+        for &n in self.tcx.crates().iter() {
             let span = match *self.tcx.extern_crate(n.as_def_id()) {
                 Some(ref c) => c.span,
                 None => {
diff --git a/src/librustc_trans/back/symbol_export.rs b/src/librustc_trans/back/symbol_export.rs
index 68f30b589c2c5..b546059b4c51d 100644
--- a/src/librustc_trans/back/symbol_export.rs
+++ b/src/librustc_trans/back/symbol_export.rs
@@ -110,7 +110,7 @@ impl ExportedSymbols {
         let mut exports = FxHashMap();
         exports.insert(LOCAL_CRATE, local_crate);
 
-        for cnum in tcx.sess.cstore.crates() {
+        for &cnum in tcx.crates().iter() {
             debug_assert!(cnum != LOCAL_CRATE);
 
             // If this crate is a plugin and/or a custom derive crate, then
diff --git a/src/librustc_trans/base.rs b/src/librustc_trans/base.rs
index b3db5a8730a0a..6cfde9c7bbc73 100644
--- a/src/librustc_trans/base.rs
+++ b/src/librustc_trans/base.rs
@@ -1524,7 +1524,7 @@ impl CrateInfo {
             used_crate_source: FxHashMap(),
         };
 
-        for cnum in tcx.sess.cstore.crates() {
+        for &cnum in tcx.crates().iter() {
             info.native_libraries.insert(cnum, tcx.native_libraries(cnum));
             info.crate_name.insert(cnum, tcx.crate_name(cnum).to_string());
             info.used_crate_source.insert(cnum, tcx.used_crate_source(cnum));
diff --git a/src/librustc_typeck/check/method/suggest.rs b/src/librustc_typeck/check/method/suggest.rs
index 5b327566a13ed..7fa3dd7472db8 100644
--- a/src/librustc_typeck/check/method/suggest.rs
+++ b/src/librustc_typeck/check/method/suggest.rs
@@ -559,7 +559,7 @@ pub fn all_traits<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>) -> AllTraits<'a>
                 _ => {}
             }
         }
-        for cnum in tcx.sess.cstore.crates() {
+        for &cnum in tcx.crates().iter() {
             let def_id = DefId {
                 krate: cnum,
                 index: CRATE_DEF_INDEX,
diff --git a/src/librustdoc/clean/inline.rs b/src/librustdoc/clean/inline.rs
index 11fe8a64d4102..70563b3d26713 100644
--- a/src/librustdoc/clean/inline.rs
+++ b/src/librustdoc/clean/inline.rs
@@ -236,7 +236,7 @@ pub fn build_impls(cx: &DocContext, did: DefId) -> Vec<clean::Item> {
 
     cx.populated_all_crate_impls.set(true);
 
-    for cnum in tcx.sess.cstore.crates() {
+    for &cnum in tcx.crates().iter() {
         for did in tcx.all_trait_implementations(cnum).iter() {
             build_impl(cx, *did, &mut impls);
         }
diff --git a/src/librustdoc/clean/mod.rs b/src/librustdoc/clean/mod.rs
index e8927e28d753b..3a5786f668919 100644
--- a/src/librustdoc/clean/mod.rs
+++ b/src/librustdoc/clean/mod.rs
@@ -131,7 +131,7 @@ impl<'a, 'tcx> Clean<Crate> for visit_ast::RustdocVisitor<'a, 'tcx> {
         }
 
         let mut externs = Vec::new();
-        for cnum in cx.sess().cstore.crates() {
+        for &cnum in cx.tcx.crates().iter() {
             externs.push((cnum, cnum.clean(cx)));
             // Analyze doc-reachability for extern items
             LibEmbargoVisitor::new(cx).visit_lib(cnum);