diff --git a/compiler/rustc_data_structures/src/sync.rs b/compiler/rustc_data_structures/src/sync.rs
index bea87a6685d8d..703b30e237bc3 100644
--- a/compiler/rustc_data_structures/src/sync.rs
+++ b/compiler/rustc_data_structures/src/sync.rs
@@ -111,11 +111,7 @@ pub use std::sync::atomic::{AtomicBool, AtomicU32, AtomicUsize};
 pub use std::sync::{OnceLock, Weak};
 
 pub use mode::{is_dyn_thread_safe, set_dyn_thread_safe_mode};
-pub use parking_lot::{
-    MappedMutexGuard as MappedLockGuard, MappedRwLockReadGuard as MappedReadGuard,
-    MappedRwLockWriteGuard as MappedWriteGuard, RwLockReadGuard as ReadGuard,
-    RwLockWriteGuard as WriteGuard,
-};
+pub use parking_lot::MappedMutexGuard as MappedLockGuard;
 #[cfg(not(target_has_atomic = "64"))]
 pub use portable_atomic::AtomicU64;
 
@@ -151,12 +147,6 @@ impl<T> MTLock<T> {
     }
 }
 
-use parking_lot::RwLock as InnerRwLock;
-
-/// This makes locks panic if they are already held.
-/// It is only useful when you are running in a single thread
-const ERROR_CHECKING: bool = false;
-
 pub type MTLockRef<'a, T> = LRef<'a, MTLock<T>>;
 
 #[derive(Default)]
@@ -175,85 +165,7 @@ impl<K: Eq + Hash, V: Eq, S: BuildHasher> HashMapExt<K, V> for HashMap<K, V, S>
     }
 }
 
-#[derive(Debug, Default)]
-pub struct RwLock<T>(InnerRwLock<T>);
-
-impl<T> RwLock<T> {
-    #[inline(always)]
-    pub fn new(inner: T) -> Self {
-        RwLock(InnerRwLock::new(inner))
-    }
-
-    #[inline(always)]
-    pub fn into_inner(self) -> T {
-        self.0.into_inner()
-    }
-
-    #[inline(always)]
-    pub fn get_mut(&mut self) -> &mut T {
-        self.0.get_mut()
-    }
-
-    #[inline(always)]
-    pub fn read(&self) -> ReadGuard<'_, T> {
-        if ERROR_CHECKING {
-            self.0.try_read().expect("lock was already held")
-        } else {
-            self.0.read()
-        }
-    }
-
-    #[inline(always)]
-    #[track_caller]
-    pub fn with_read_lock<F: FnOnce(&T) -> R, R>(&self, f: F) -> R {
-        f(&*self.read())
-    }
-
-    #[inline(always)]
-    pub fn try_write(&self) -> Result<WriteGuard<'_, T>, ()> {
-        self.0.try_write().ok_or(())
-    }
-
-    #[inline(always)]
-    pub fn write(&self) -> WriteGuard<'_, T> {
-        if ERROR_CHECKING {
-            self.0.try_write().expect("lock was already held")
-        } else {
-            self.0.write()
-        }
-    }
-
-    #[inline(always)]
-    #[track_caller]
-    pub fn with_write_lock<F: FnOnce(&mut T) -> R, R>(&self, f: F) -> R {
-        f(&mut *self.write())
-    }
-
-    #[inline(always)]
-    #[track_caller]
-    pub fn borrow(&self) -> ReadGuard<'_, T> {
-        self.read()
-    }
-
-    #[inline(always)]
-    #[track_caller]
-    pub fn borrow_mut(&self) -> WriteGuard<'_, T> {
-        self.write()
-    }
-
-    #[inline(always)]
-    pub fn leak(&self) -> &T {
-        let guard = self.read();
-        let ret = unsafe { &*(&raw const *guard) };
-        std::mem::forget(guard);
-        ret
-    }
-}
-
-// FIXME: Probably a bad idea
-impl<T: Clone> Clone for RwLock<T> {
-    #[inline]
-    fn clone(&self) -> Self {
-        RwLock::new(self.borrow().clone())
-    }
-}
+mod rwlock;
+pub use rwlock::{
+    MappedReadGuard, MappedWriteGuard, ReadError, ReadGuard, RwLock, WriteError, WriteGuard,
+};
diff --git a/compiler/rustc_data_structures/src/sync/rwlock.rs b/compiler/rustc_data_structures/src/sync/rwlock.rs
new file mode 100644
index 0000000000000..beddc481a91a3
--- /dev/null
+++ b/compiler/rustc_data_structures/src/sync/rwlock.rs
@@ -0,0 +1,312 @@
+use std::cell::{Ref, RefCell, RefMut};
+use std::intrinsics::cold_path;
+use std::ops::{Deref, DerefMut};
+
+use crate::sync::mode::might_be_dyn_thread_safe;
+
+#[derive(Debug)]
+pub enum RwLock<T> {
+    Sync(parking_lot::RwLock<T>),
+    NoSync(RefCell<T>),
+}
+
+#[clippy::has_significant_drop]
+#[must_use = "if unused the RwLock will immediately unlock"]
+#[derive(Debug)]
+pub enum ReadGuard<'a, T> {
+    Sync(parking_lot::RwLockReadGuard<'a, T>),
+    NoSync(Ref<'a, T>),
+}
+
+#[clippy::has_significant_drop]
+#[must_use = "if unused the RwLock will immediately unlock"]
+#[derive(Debug)]
+pub enum WriteGuard<'a, T> {
+    Sync(parking_lot::RwLockWriteGuard<'a, T>),
+    NoSync(RefMut<'a, T>),
+}
+
+#[clippy::has_significant_drop]
+#[must_use = "if unused the RwLock will immediately unlock"]
+#[derive(Debug)]
+pub enum MappedReadGuard<'a, T> {
+    Sync(parking_lot::MappedRwLockReadGuard<'a, T>),
+    NoSync(Ref<'a, T>),
+}
+
+#[clippy::has_significant_drop]
+#[must_use = "if unused the RwLock will immediately unlock"]
+#[derive(Debug)]
+pub enum MappedWriteGuard<'a, T> {
+    Sync(parking_lot::MappedRwLockWriteGuard<'a, T>),
+    NoSync(RefMut<'a, T>),
+}
+
+#[derive(Debug)]
+pub struct ReadError;
+
+#[derive(Debug)]
+pub struct WriteError;
+
+impl<T> RwLock<T> {
+    #[inline(always)]
+    pub fn new(inner: T) -> Self {
+        if might_be_dyn_thread_safe() {
+            cold_path();
+            RwLock::Sync(parking_lot::RwLock::new(inner))
+        } else {
+            RwLock::NoSync(RefCell::new(inner))
+        }
+    }
+
+    #[inline(always)]
+    pub fn into_inner(self) -> T {
+        match self {
+            RwLock::Sync(inner) => {
+                cold_path();
+                parking_lot::RwLock::into_inner(inner)
+            }
+            RwLock::NoSync(inner) => RefCell::into_inner(inner),
+        }
+    }
+
+    #[inline(always)]
+    pub fn get_mut(&mut self) -> &mut T {
+        match self {
+            RwLock::Sync(inner) => {
+                cold_path();
+                parking_lot::RwLock::get_mut(inner)
+            }
+            RwLock::NoSync(inner) => RefCell::get_mut(inner),
+        }
+    }
+
+    #[inline(always)]
+    #[track_caller]
+    pub fn read(&self) -> ReadGuard<'_, T> {
+        match self {
+            RwLock::Sync(inner) => {
+                cold_path();
+                ReadGuard::Sync(inner.read())
+            }
+            RwLock::NoSync(inner) => ReadGuard::NoSync(inner.borrow()),
+        }
+    }
+
+    #[inline(always)]
+    pub fn try_read(&self) -> Result<ReadGuard<'_, T>, ReadError> {
+        match self {
+            RwLock::Sync(inner) => {
+                cold_path();
+                Ok(ReadGuard::Sync(inner.try_read().ok_or(ReadError)?))
+            }
+            RwLock::NoSync(inner) => {
+                Ok(ReadGuard::NoSync(inner.try_borrow().map_err(|_| ReadError)?))
+            }
+        }
+    }
+
+    #[inline(always)]
+    #[track_caller]
+    pub fn write(&self) -> WriteGuard<'_, T> {
+        match self {
+            RwLock::Sync(inner) => {
+                cold_path();
+                WriteGuard::Sync(inner.write())
+            }
+            RwLock::NoSync(inner) => WriteGuard::NoSync(inner.borrow_mut()),
+        }
+    }
+
+    #[inline(always)]
+    pub fn try_write(&self) -> Result<WriteGuard<'_, T>, WriteError> {
+        match self {
+            RwLock::Sync(inner) => {
+                cold_path();
+                Ok(WriteGuard::Sync(inner.try_write().ok_or(WriteError)?))
+            }
+            RwLock::NoSync(inner) => {
+                Ok(WriteGuard::NoSync(inner.try_borrow_mut().map_err(|_| WriteError)?))
+            }
+        }
+    }
+
+    #[inline(always)]
+    #[track_caller]
+    pub fn borrow(&self) -> ReadGuard<'_, T> {
+        self.read()
+    }
+
+    #[inline(always)]
+    pub fn try_borrow(&self) -> Result<ReadGuard<'_, T>, ReadError> {
+        self.try_read()
+    }
+
+    #[inline(always)]
+    #[track_caller]
+    pub fn borrow_mut(&self) -> WriteGuard<'_, T> {
+        self.write()
+    }
+
+    #[inline(always)]
+    pub fn try_borrow_mut(&self) -> Result<WriteGuard<'_, T>, WriteError> {
+        self.try_write()
+    }
+}
+
+impl<T: Default> Default for RwLock<T> {
+    #[inline(always)]
+    fn default() -> Self {
+        RwLock::<T>::new(Default::default())
+    }
+}
+
+impl<'a, T> ReadGuard<'a, T> {
+    #[inline(always)]
+    pub fn map<U, F>(s: Self, f: F) -> MappedReadGuard<'a, U>
+    where
+        F: FnOnce(&T) -> &U,
+    {
+        match s {
+            ReadGuard::Sync(guard) => {
+                cold_path();
+                MappedReadGuard::Sync(parking_lot::RwLockReadGuard::map(guard, f))
+            }
+            ReadGuard::NoSync(guard) => MappedReadGuard::NoSync(Ref::map(guard, f)),
+        }
+    }
+}
+
+impl<'a, T> WriteGuard<'a, T> {
+    #[inline(always)]
+    pub fn map<U, F>(s: Self, f: F) -> MappedWriteGuard<'a, U>
+    where
+        F: FnOnce(&mut T) -> &mut U,
+    {
+        match s {
+            WriteGuard::Sync(guard) => {
+                cold_path();
+                MappedWriteGuard::Sync(parking_lot::RwLockWriteGuard::map(guard, f))
+            }
+            WriteGuard::NoSync(guard) => MappedWriteGuard::NoSync(RefMut::map(guard, f)),
+        }
+    }
+}
+
+impl<'a, T> Deref for ReadGuard<'a, T> {
+    type Target = T;
+
+    #[inline(always)]
+    fn deref(&self) -> &T {
+        match self {
+            ReadGuard::Sync(guard) => {
+                cold_path();
+                Deref::deref(guard)
+            }
+            ReadGuard::NoSync(guard) => Deref::deref(guard),
+        }
+    }
+}
+
+impl<'a, T> Deref for WriteGuard<'a, T> {
+    type Target = T;
+
+    #[inline(always)]
+    fn deref(&self) -> &T {
+        match self {
+            WriteGuard::Sync(guard) => {
+                cold_path();
+                Deref::deref(guard)
+            }
+            WriteGuard::NoSync(guard) => Deref::deref(guard),
+        }
+    }
+}
+
+impl<'a, T> DerefMut for WriteGuard<'a, T> {
+    #[inline(always)]
+    fn deref_mut(&mut self) -> &mut T {
+        match self {
+            WriteGuard::Sync(guard) => {
+                cold_path();
+                DerefMut::deref_mut(guard)
+            }
+            WriteGuard::NoSync(guard) => DerefMut::deref_mut(guard),
+        }
+    }
+}
+
+impl<'a, T> MappedReadGuard<'a, T> {
+    #[inline(always)]
+    pub fn map<U, F>(s: Self, f: F) -> MappedReadGuard<'a, U>
+    where
+        F: FnOnce(&T) -> &U,
+    {
+        match s {
+            MappedReadGuard::Sync(guard) => {
+                cold_path();
+                MappedReadGuard::Sync(parking_lot::MappedRwLockReadGuard::map(guard, f))
+            }
+            MappedReadGuard::NoSync(guard) => MappedReadGuard::NoSync(Ref::map(guard, f)),
+        }
+    }
+}
+
+impl<'a, T> MappedWriteGuard<'a, T> {
+    #[inline(always)]
+    pub fn map<U, F>(s: Self, f: F) -> MappedWriteGuard<'a, U>
+    where
+        F: FnOnce(&mut T) -> &mut U,
+    {
+        match s {
+            MappedWriteGuard::Sync(guard) => {
+                cold_path();
+                MappedWriteGuard::Sync(parking_lot::MappedRwLockWriteGuard::map(guard, f))
+            }
+            MappedWriteGuard::NoSync(guard) => MappedWriteGuard::NoSync(RefMut::map(guard, f)),
+        }
+    }
+}
+
+impl<'a, T> Deref for MappedReadGuard<'a, T> {
+    type Target = T;
+
+    #[inline(always)]
+    fn deref(&self) -> &T {
+        match self {
+            MappedReadGuard::Sync(guard) => {
+                cold_path();
+                Deref::deref(guard)
+            }
+            MappedReadGuard::NoSync(guard) => Deref::deref(guard),
+        }
+    }
+}
+
+impl<'a, T> Deref for MappedWriteGuard<'a, T> {
+    type Target = T;
+
+    #[inline(always)]
+    fn deref(&self) -> &T {
+        match self {
+            MappedWriteGuard::Sync(guard) => {
+                cold_path();
+                Deref::deref(guard)
+            }
+            MappedWriteGuard::NoSync(guard) => Deref::deref(guard),
+        }
+    }
+}
+
+impl<'a, T> DerefMut for MappedWriteGuard<'a, T> {
+    #[inline(always)]
+    fn deref_mut(&mut self) -> &mut T {
+        match self {
+            MappedWriteGuard::Sync(guard) => {
+                cold_path();
+                DerefMut::deref_mut(guard)
+            }
+            MappedWriteGuard::NoSync(guard) => DerefMut::deref_mut(guard),
+        }
+    }
+}
diff --git a/compiler/rustc_data_structures/src/sync/vec.rs b/compiler/rustc_data_structures/src/sync/vec.rs
index 21ec5cf6c13bf..3da5008428221 100644
--- a/compiler/rustc_data_structures/src/sync/vec.rs
+++ b/compiler/rustc_data_structures/src/sync/vec.rs
@@ -2,6 +2,8 @@ use std::marker::PhantomData;
 
 use rustc_index::Idx;
 
+use crate::sync::RwLock;
+
 #[derive(Default)]
 pub struct AppendOnlyIndexVec<I: Idx, T: Copy> {
     vec: elsa::sync::LockFreeFrozenVec<T>,
@@ -26,7 +28,7 @@ impl<I: Idx, T: Copy> AppendOnlyIndexVec<I, T> {
 
 #[derive(Default)]
 pub struct AppendOnlyVec<T: Copy> {
-    vec: parking_lot::RwLock<Vec<T>>,
+    vec: RwLock<Vec<T>>,
 }
 
 impl<T: Copy> AppendOnlyVec<T> {