diff --git a/compiler/rustc_data_structures/Cargo.toml b/compiler/rustc_data_structures/Cargo.toml
index 24cf9812a25a1..6056dab5a5d48 100644
--- a/compiler/rustc_data_structures/Cargo.toml
+++ b/compiler/rustc_data_structures/Cargo.toml
@@ -14,7 +14,7 @@ indexmap = { version = "1.9.1" }
 jobserver_crate = { version = "0.1.13", package = "jobserver" }
 libc = "0.2"
 measureme = "10.0.0"
-rayon-core = { version = "0.4.0", package = "rustc-rayon-core", optional = true }
+rayon-core = { version = "0.4.0", package = "rustc-rayon-core" }
 rayon = { version = "0.4.0", package = "rustc-rayon", optional = true }
 rustc_graphviz = { path = "../rustc_graphviz" }
 rustc-hash = "1.1.0"
@@ -50,4 +50,4 @@ features = [
 memmap2 = "0.2.1"
 
 [features]
-rustc_use_parallel_compiler = ["indexmap/rustc-rayon", "rayon", "rayon-core"]
+rustc_use_parallel_compiler = ["indexmap/rustc-rayon", "rayon"]
diff --git a/compiler/rustc_data_structures/src/sync.rs b/compiler/rustc_data_structures/src/sync.rs
index 31323c21df009..1382216449199 100644
--- a/compiler/rustc_data_structures/src/sync.rs
+++ b/compiler/rustc_data_structures/src/sync.rs
@@ -30,6 +30,8 @@ pub use vec::AppendOnlyVec;
 
 mod vec;
 
+static PARALLEL: std::sync::atomic::AtomicBool = std::sync::atomic::AtomicBool::new(false);
+
 cfg_if! {
     if #[cfg(not(parallel_compiler))] {
         pub auto trait Send {}
@@ -182,33 +184,6 @@ 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
-            }
-        }
-
         pub type MTRef<'a, T> = &'a mut T;
 
         #[derive(Debug, Default)]
@@ -328,8 +303,6 @@ cfg_if! {
             };
         }
 
-        pub use rayon_core::WorkerLocal;
-
         pub use rayon::iter::ParallelIterator;
         use rayon::iter::IntoParallelIterator;
 
@@ -364,6 +337,49 @@ cfg_if! {
     }
 }
 
+#[derive(Debug)]
+pub enum WorkerLocal<T> {
+    SingleThread(T),
+    Rayon(rayon_core::WorkerLocal<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> {
+        if !PARALLEL.load(Ordering::Relaxed) {
+            WorkerLocal::SingleThread(f(0))
+        } else {
+            WorkerLocal::Rayon(rayon_core::WorkerLocal::new(f))
+        }
+    }
+
+    /// Returns the worker-local value for each thread
+    #[inline]
+    pub fn into_inner(self) -> Vec<T> {
+        match self {
+            WorkerLocal::SingleThread(inner) => vec![inner],
+            WorkerLocal::Rayon(mt_inner) => mt_inner.into_inner(),
+        }
+    }
+}
+
+impl<T> Deref for WorkerLocal<T> {
+    type Target = T;
+
+    #[inline(always)]
+    fn deref(&self) -> &T {
+        match self {
+            WorkerLocal::SingleThread(inner) => inner,
+            WorkerLocal::Rayon(mt_inner) => mt_inner.deref(),
+        }
+    }
+}
+
+// Just for speed test
+unsafe impl<T: Send> std::marker::Sync for WorkerLocal<T> {}
+
 pub fn assert_sync<T: ?Sized + Sync>() {}
 pub fn assert_send<T: ?Sized + Send>() {}
 pub fn assert_send_val<T: ?Sized + Send>(_t: &T) {}