fix: unsound cache upcast

Removes the unsafe code around cache upcasting by introducing traited
functions `as_ref` and `as_mut` on the AutoImplCacheRef trait. Also no
longer requires the `unsafe` attribute.

common/src/dataloader/cache.rs

@@ -1,4 +1,7 @@
-use std::collections::hash_map::RandomState;
+use std::{
+    collections::hash_map::RandomState,
+    ops::{Deref, DerefMut},
+};
 
 use crate::dataloader::LoaderOutput;
 
@@ -16,62 +19,70 @@ pub trait Cache<L: Loader<S>, S = RandomState> {
     }
 }
 
-/// # Safety
-///
-/// This trait is marked as unsafe because the implementor must ensure that the Cache is safe for concurrent access.
-/// This will almost always be with some kind of interior mutability. Such as a `RwLock` or `Mutex`. Or if the cache performs no-ops on mutation.
-/// Look at `SharedCache` for an example.
-pub unsafe trait AutoImplCacheRef<L: Loader<S>, S = RandomState>:
-    AutoImplCacheMutRef<L, S>
-{
+#[repr(transparent)]
+pub struct EmptyDerefMut<T>(T);
+
+impl<T> Deref for EmptyDerefMut<T> {
+    type Target = T;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl<T> DerefMut for EmptyDerefMut<T> {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.0
+    }
 }
 
-#[inline(always)]
-#[allow(clippy::mut_from_ref)]
-#[allow(invalid_reference_casting)]
-fn upcast<T: AutoImplCacheRef<L, S>, L: Loader<S>, S>(cache: &T) -> impl Cache<L, S> {
-    // Safety:
-    // This is safe because the trait `AutoImplCacheRef` is marked as unsafe and therefore the implementor must ensure that the
-    // Cache is safe for concurrent access. This is used to implement cache for &T where T is a Cache.
-    // The issue is we need to upcast the reference to a mutable reference, even though the implementor only ever requires a reference.
-    // This is not safe unless T has some kind of interior mutability.
-    unsafe { &mut *(cache as *const T as *mut T) }
+pub trait AutoImplCacheRef<L: Loader<S>, S = RandomState> {
+    type Cache: Cache<L, S>;
+    type Ref<'a>: Deref<Target = Self::Cache> + 'a
+    where
+        Self: 'a;
+    type MutRef<'a>: DerefMut<Target = Self::Cache> + 'a
+    where
+        Self: 'a;
+
+    fn as_ref(&self) -> Self::Ref<'_>;
+    fn as_mut(&self) -> Self::MutRef<'_>;
 }
 
 impl<L: Loader<S>, S, T: AutoImplCacheRef<L, S>> Cache<L, S> for &T {
     #[inline(always)]
     fn contains_key(&self, key: &L::Key) -> bool {
-        (**self).contains_key(key)
+        self.as_ref().contains_key(key)
     }
 
     #[inline(always)]
     fn get(&self, key: &L::Key) -> Option<L::Value> {
-        (**self).get(key)
+        self.as_ref().get(key)
     }
 
     #[inline(always)]
     fn insert(&mut self, key: &L::Key, value: &L::Value) {
-        upcast(*self).insert(key, value)
+        self.as_mut().insert(key, value)
     }
 
     #[inline(always)]
     fn clear(&mut self) {
-        upcast(*self).clear()
+        self.as_mut().clear()
     }
 
     #[inline(always)]
     fn len(&self) -> usize {
-        (**self).len()
+        self.as_ref().len()
     }
 
     #[inline(always)]
     fn delete(&mut self, key: &L::Key) -> Option<L::Value> {
-        upcast(*self).delete(key)
+        self.as_mut().delete(key)
     }
 
     #[inline(always)]
     fn is_empty(&self) -> bool {
-        (**self).is_empty()
+        self.as_ref().is_empty()
     }
 }
 
@@ -118,7 +129,19 @@ impl<L: Loader<S>, S, T: AutoImplCacheMutRef<L, S>> Cache<L, S> for &mut T {
 pub struct NoCache;
 
 /// Safety: The no cache is always for safe for concurrent access because it is a no-op cache and therefore has no internal state.
-unsafe impl<L: Loader<S>, S> AutoImplCacheRef<L, S> for NoCache {}
+impl<L: Loader<S>, S> AutoImplCacheRef<L, S> for NoCache {
+    type Cache = Self;
+    type Ref<'a> = EmptyDerefMut<Self>;
+    type MutRef<'a> = EmptyDerefMut<Self>;
+
+    fn as_mut(&self) -> Self::MutRef<'_> {
+        EmptyDerefMut(*self)
+    }
+
+    fn as_ref(&self) -> Self::Ref<'_> {
+        EmptyDerefMut(*self)
+    }
+}
 
 impl<L: Loader<S>, S> AutoImplCacheMutRef<L, S> for NoCache {}
 
@@ -234,8 +257,21 @@ impl<C: Default> Default for SharedCache<C> {
 }
 
 impl<C: Cache<L, S>, L: Loader<S>, S> AutoImplCacheMutRef<L, S> for SharedCache<C> {}
-/// Safety: The shared cache is always for concurrent access because it contains a RwLock, which is a safe concurrent access primitive.
-unsafe impl<'a, C: Cache<L, S> + 'a, L: Loader<S>, S> AutoImplCacheRef<L, S> for SharedCache<C> {}
+
+impl<C: Cache<L, S> + 'static, L: Loader<S>, S> AutoImplCacheRef<L, S> for SharedCache<C> {
+    type Cache = C;
+
+    type Ref<'a> = std::sync::RwLockReadGuard<'a, C>;
+    type MutRef<'a> = std::sync::RwLockWriteGuard<'a, C>;
+
+    fn as_ref(&self) -> Self::Ref<'_> {
+        self.0.read().unwrap()
+    }
+
+    fn as_mut(&self) -> Self::MutRef<'_> {
+        self.0.write().unwrap()
+    }
+}
 
 impl<C: Cache<L, S>, L: Loader<S>, S> Cache<L, S> for SharedCache<C> {
     #[inline(always)]
@@ -289,6 +325,16 @@ const _: () = {
         }
     }
 
+    const fn assert_size_of<T: Sized>() {
+        if std::mem::size_of::<T>() != std::mem::size_of::<EmptyDerefMut<T>>() {
+            panic!("T and EmptyDerefMut<T> have different sizes")
+        }
+    }
+
+    assert_size_of::<NoCache>();
+    assert_size_of::<HashMapCache<DummyLoader>>();
+    assert_size_of::<SharedCache<NoCache>>();
+
     const fn assert_auto_impl_cache_ref<C: Cache<L, S>, L: Loader<S>, S>() {}
 
     assert_auto_impl_cache_ref::<NoCache, DummyLoader, RandomState>();