diff --git a/src/Cargo.lock b/src/Cargo.lock
index 3a27107f825d6..d5a1d18a67608 100644
--- a/src/Cargo.lock
+++ b/src/Cargo.lock
@@ -2044,6 +2044,7 @@ dependencies = [
  "parking_lot_core 0.2.14 (registry+https://github.com/rust-lang/crates.io-index)",
  "rustc-hash 1.0.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "rustc-rayon 0.1.0 (registry+https://github.com/rust-lang/crates.io-index)",
+ "rustc-rayon-core 0.1.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "rustc_cratesio_shim 0.0.0",
  "serialize 0.0.0",
  "stable_deref_trait 1.0.0 (registry+https://github.com/rust-lang/crates.io-index)",
diff --git a/src/libproc_macro/lib.rs b/src/libproc_macro/lib.rs
index 9d037cefceed6..befc1ba064aa2 100644
--- a/src/libproc_macro/lib.rs
+++ b/src/libproc_macro/lib.rs
@@ -36,6 +36,8 @@
 #![feature(lang_items)]
 #![feature(optin_builtin_traits)]
 
+#![recursion_limit="256"]
+
 extern crate syntax;
 extern crate syntax_pos;
 extern crate rustc_errors;
diff --git a/src/librustc/session/mod.rs b/src/librustc/session/mod.rs
index 8df66d8d68855..076d56fb80842 100644
--- a/src/librustc/session/mod.rs
+++ b/src/librustc/session/mod.rs
@@ -845,10 +845,10 @@ impl Session {
     /// We want to know if we're allowed to do an optimization for crate foo from -z fuel=foo=n.
     /// This expends fuel if applicable, and records fuel if applicable.
     pub fn consider_optimizing<T: Fn() -> String>(&self, crate_name: &str, msg: T) -> bool {
-        assert!(self.query_threads() == 1);
         let mut ret = true;
         match self.optimization_fuel_crate {
             Some(ref c) if c == crate_name => {
+                assert!(self.query_threads() == 1);
                 let fuel = self.optimization_fuel_limit.get();
                 ret = fuel != 0;
                 if fuel == 0 && !self.out_of_fuel.get() {
@@ -862,6 +862,7 @@ impl Session {
         }
         match self.print_fuel_crate {
             Some(ref c) if c == crate_name => {
+                assert!(self.query_threads() == 1);
                 self.print_fuel.set(self.print_fuel.get() + 1);
             }
             _ => {}
diff --git a/src/librustc/ty/context.rs b/src/librustc/ty/context.rs
index 68f55b4993301..35b2ce50da79d 100644
--- a/src/librustc/ty/context.rs
+++ b/src/librustc/ty/context.rs
@@ -58,7 +58,7 @@ use rustc_data_structures::stable_hasher::{HashStable, hash_stable_hashmap,
                                            StableVec};
 use arena::{TypedArena, SyncDroplessArena};
 use rustc_data_structures::indexed_vec::IndexVec;
-use rustc_data_structures::sync::{Lrc, Lock};
+use rustc_data_structures::sync::{self, Lrc, Lock, WorkerLocal};
 use std::any::Any;
 use std::borrow::Borrow;
 use std::cmp::Ordering;
@@ -80,14 +80,14 @@ use syntax_pos::Span;
 use hir;
 
 pub struct AllArenas<'tcx> {
-    pub global: GlobalArenas<'tcx>,
+    pub global: WorkerLocal<GlobalArenas<'tcx>>,
     pub interner: SyncDroplessArena,
 }
 
 impl<'tcx> AllArenas<'tcx> {
     pub fn new() -> Self {
         AllArenas {
-            global: GlobalArenas::new(),
+            global: WorkerLocal::new(|_| GlobalArenas::new()),
             interner: SyncDroplessArena::new(),
         }
     }
@@ -854,7 +854,7 @@ impl<'a, 'gcx, 'tcx> Deref for TyCtxt<'a, 'gcx, 'tcx> {
 }
 
 pub struct GlobalCtxt<'tcx> {
-    global_arenas: &'tcx GlobalArenas<'tcx>,
+    global_arenas: &'tcx WorkerLocal<GlobalArenas<'tcx>>,
     global_interners: CtxtInterners<'tcx>,
 
     cstore: &'tcx CrateStoreDyn,
@@ -1179,6 +1179,8 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
             output_filenames: Arc::new(output_filenames.clone()),
         };
 
+        sync::assert_send_val(&gcx);
+
         tls::enter_global(gcx, f)
     }
 
@@ -1704,7 +1706,7 @@ pub mod tls {
     use ty::maps;
     use errors::{Diagnostic, TRACK_DIAGNOSTICS};
     use rustc_data_structures::OnDrop;
-    use rustc_data_structures::sync::Lrc;
+    use rustc_data_structures::sync::{self, Lrc};
     use dep_graph::OpenTask;
 
     /// This is the implicit state of rustc. It contains the current
@@ -1832,6 +1834,10 @@ pub mod tls {
         if context == 0 {
             f(None)
         } else {
+            // We could get a ImplicitCtxt pointer from another thread.
+            // Ensure that ImplicitCtxt is Sync
+            sync::assert_sync::<ImplicitCtxt>();
+
             unsafe { f(Some(&*(context as *const ImplicitCtxt))) }
         }
     }
diff --git a/src/librustc/ty/mod.rs b/src/librustc/ty/mod.rs
index 646c60c139c85..f0f4adde7ee91 100644
--- a/src/librustc/ty/mod.rs
+++ b/src/librustc/ty/mod.rs
@@ -617,6 +617,8 @@ pub struct Slice<T> {
     opaque: OpaqueSliceContents,
 }
 
+unsafe impl<T: Sync> Sync for Slice<T> {}
+
 impl<T: Copy> Slice<T> {
     #[inline]
     fn from_arena<'tcx>(arena: &'tcx SyncDroplessArena, slice: &[T]) -> &'tcx Slice<T> {
diff --git a/src/librustc_codegen_utils/lib.rs b/src/librustc_codegen_utils/lib.rs
index 0c18571f4ffe8..d09e8f4845e5e 100644
--- a/src/librustc_codegen_utils/lib.rs
+++ b/src/librustc_codegen_utils/lib.rs
@@ -23,6 +23,8 @@
 #![feature(quote)]
 #![feature(rustc_diagnostic_macros)]
 
+#![recursion_limit="256"]
+
 extern crate ar;
 extern crate flate2;
 #[macro_use]
diff --git a/src/librustc_data_structures/Cargo.toml b/src/librustc_data_structures/Cargo.toml
index 15956829ff9a5..17ee771e52940 100644
--- a/src/librustc_data_structures/Cargo.toml
+++ b/src/librustc_data_structures/Cargo.toml
@@ -17,6 +17,7 @@ cfg-if = "0.1.2"
 stable_deref_trait = "1.0.0"
 parking_lot_core = "0.2.8"
 rustc-rayon = "0.1.0"
+rustc-rayon-core = "0.1.0"
 rustc-hash = "1.0.1"
 
 [dependencies.parking_lot]
diff --git a/src/librustc_data_structures/lib.rs b/src/librustc_data_structures/lib.rs
index 23a920739b964..7046a2a2a493d 100644
--- a/src/librustc_data_structures/lib.rs
+++ b/src/librustc_data_structures/lib.rs
@@ -44,6 +44,7 @@ extern crate parking_lot;
 extern crate cfg_if;
 extern crate stable_deref_trait;
 extern crate rustc_rayon as rayon;
+extern crate rustc_rayon_core as rayon_core;
 extern crate rustc_hash;
 
 // See librustc_cratesio_shim/Cargo.toml for a comment explaining this.
diff --git a/src/librustc_data_structures/sync.rs b/src/librustc_data_structures/sync.rs
index 3661763133014..6f7d9e1b54b1e 100644
--- a/src/librustc_data_structures/sync.rs
+++ b/src/librustc_data_structures/sync.rs
@@ -36,7 +36,6 @@ use std::marker::PhantomData;
 use std::fmt::Debug;
 use std::fmt::Formatter;
 use std::fmt;
-use std;
 use std::ops::{Deref, DerefMut};
 use owning_ref::{Erased, OwningRef};
 
@@ -100,6 +99,33 @@ cfg_if! {
 
         use std::cell::Cell;
 
+        #[derive(Debug)]
+        pub struct WorkerLocal<T>(OneThread<T>);
+
+        impl<T> WorkerLocal<T> {
+            /// Creates a new worker local where the `initial` closure computes the
+            /// value this worker local should take for each thread in the thread pool.
+            #[inline]
+            pub fn new<F: FnMut(usize) -> T>(mut f: F) -> WorkerLocal<T> {
+                WorkerLocal(OneThread::new(f(0)))
+            }
+
+            /// Returns the worker-local value for each thread
+            #[inline]
+            pub fn into_inner(self) -> Vec<T> {
+                vec![OneThread::into_inner(self.0)]
+            }
+        }
+
+        impl<T> Deref for WorkerLocal<T> {
+            type Target = T;
+
+            #[inline(always)]
+            fn deref(&self) -> &T {
+                &*self.0
+            }
+        }
+
         #[derive(Debug)]
         pub struct MTLock<T>(T);
 
@@ -200,9 +226,12 @@ cfg_if! {
         use parking_lot::Mutex as InnerLock;
         use parking_lot::RwLock as InnerRwLock;
 
+        use std;
         use std::thread;
         pub use rayon::{join, scope};
 
+        pub use rayon_core::WorkerLocal;
+
         pub use rayon::iter::ParallelIterator;
         use rayon::iter::IntoParallelIterator;
 
@@ -638,7 +667,9 @@ pub struct OneThread<T> {
     inner: T,
 }
 
+#[cfg(parallel_queries)]
 unsafe impl<T> std::marker::Sync for OneThread<T> {}
+#[cfg(parallel_queries)]
 unsafe impl<T> std::marker::Send for OneThread<T> {}
 
 impl<T> OneThread<T> {
diff --git a/src/librustc_metadata/lib.rs b/src/librustc_metadata/lib.rs
index 7ecf2eba43ddf..d76ca5bdf2710 100644
--- a/src/librustc_metadata/lib.rs
+++ b/src/librustc_metadata/lib.rs
@@ -23,6 +23,8 @@
 #![feature(specialization)]
 #![feature(rustc_private)]
 
+#![recursion_limit="256"]
+
 extern crate libc;
 #[macro_use]
 extern crate log;
diff --git a/src/librustc_privacy/lib.rs b/src/librustc_privacy/lib.rs
index bfb8c282d377b..f32f6eda8ff59 100644
--- a/src/librustc_privacy/lib.rs
+++ b/src/librustc_privacy/lib.rs
@@ -14,6 +14,8 @@
 
 #![feature(rustc_diagnostic_macros)]
 
+#![recursion_limit="256"]
+
 #[macro_use] extern crate rustc;
 #[macro_use] extern crate syntax;
 extern crate rustc_typeck;
diff --git a/src/librustc_save_analysis/lib.rs b/src/librustc_save_analysis/lib.rs
index e57a793ff426f..64dcd3e51751c 100644
--- a/src/librustc_save_analysis/lib.rs
+++ b/src/librustc_save_analysis/lib.rs
@@ -15,6 +15,8 @@
 #![cfg_attr(stage0, feature(macro_lifetime_matcher))]
 #![allow(unused_attributes)]
 
+#![recursion_limit="256"]
+
 #[macro_use]
 extern crate rustc;
 
diff --git a/src/librustc_traits/lib.rs b/src/librustc_traits/lib.rs
index 733d8e1708ec0..7fa69cb98338d 100644
--- a/src/librustc_traits/lib.rs
+++ b/src/librustc_traits/lib.rs
@@ -17,6 +17,8 @@
 #![feature(iterator_find_map)]
 #![feature(in_band_lifetimes)]
 
+#![recursion_limit="256"]
+
 extern crate chalk_engine;
 #[macro_use]
 extern crate log;