use Alloc not Box

to avoid uniqueness guarantees

Author: nikomatsakis

components/salsa-2022/src/alloc.rs

@@ -0,0 +1,42 @@
+use std::ptr::NonNull;
+
+/// A box but without the uniqueness guarantees.
+pub struct Alloc<T> {
+    data: NonNull<T>,
+}
+
+impl<T> Alloc<T> {
+    pub fn new(data: T) -> Self {
+        let data = Box::new(data);
+        let data = Box::into_raw(data);
+        Alloc {
+            data: unsafe { NonNull::new_unchecked(data) },
+        }
+    }
+}
+
+impl<T> Drop for Alloc<T> {
+    fn drop(&mut self) {
+        let data: *mut T = self.data.as_ptr();
+        let data: Box<T> = unsafe { Box::from_raw(data) };
+        drop(data);
+    }
+}
+
+impl<T> std::ops::Deref for Alloc<T> {
+    type Target = T;
+
+    fn deref(&self) -> &Self::Target {
+        unsafe { self.data.as_ref() }
+    }
+}
+
+impl<T> std::ops::DerefMut for Alloc<T> {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        unsafe { self.data.as_mut() }
+    }
+}
+
+unsafe impl<T> Send for Alloc<T> where T: Send {}
+
+unsafe impl<T> Sync for Alloc<T> where T: Sync {}

components/salsa-2022/src/interned.rs

@@ -3,6 +3,7 @@ use std::fmt;
 use std::hash::Hash;
 use std::marker::PhantomData;
 
+use crate::alloc::Alloc;
 use crate::durability::Durability;
 use crate::id::{AsId, LookupId};
 use crate::ingredient::{fmt_index, IngredientRequiresReset};
@@ -39,7 +40,7 @@ pub struct InternedIngredient<C: Configuration> {
     /// Maps from an interned id to its data.
     ///
     /// Deadlock requirement: We access `value_map` while holding lock on `key_map`, but not vice versa.
-    value_map: FxDashMap<Id, Box<ValueStruct<C>>>,
+    value_map: FxDashMap<Id, Alloc<ValueStruct<C>>>,
 
     /// counter for the next id.
     counter: AtomicCell<u32>,
@@ -121,7 +122,7 @@ where
                 let value = self
                     .value_map
                     .entry(next_id)
-                    .or_insert(Box::new(ValueStruct {
+                    .or_insert(Alloc::new(ValueStruct {
                         id: next_id,
                         fields: internal_data,
                     }));

components/salsa-2022/src/lib.rs

@@ -1,4 +1,5 @@
 pub mod accumulator;
+mod alloc;
 pub mod cancelled;
 pub mod cycle;
 pub mod database;

components/salsa-2022/src/tracked_struct/struct_map.rs

@@ -7,6 +7,7 @@ use crossbeam::queue::SegQueue;
 use dashmap::mapref::one::RefMut;
 
 use crate::{
+    alloc::Alloc,
     hash::{FxDashMap, FxHasher},
     plumbing::transmute_lifetime,
     Id, Runtime,
@@ -18,21 +19,21 @@ pub(crate) struct StructMap<C>
 where
     C: Configuration,
 {
-    map: Arc<FxDashMap<Id, Box<ValueStruct<C>>>>,
+    map: Arc<FxDashMap<Id, Alloc<ValueStruct<C>>>>,
 
     /// When specific entities are deleted, their data is added
     /// to this vector rather than being immediately freed. This is because we may` have
     /// references to that data floating about that are tied to the lifetime of some
     /// `&db` reference. This queue itself is not freed until we have an `&mut db` reference,
     /// guaranteeing that there are no more references to it.
-    deleted_entries: SegQueue<Box<ValueStruct<C>>>,
+    deleted_entries: SegQueue<Alloc<ValueStruct<C>>>,
 }
 
 pub(crate) struct StructMapView<C>
 where
     C: Configuration,
 {
-    map: Arc<FxDashMap<Id, Box<ValueStruct<C>>>>,
+    map: Arc<FxDashMap<Id, Alloc<ValueStruct<C>>>>,
 }
 
 /// Return value for [`StructMap`][]'s `update` method.
@@ -79,7 +80,7 @@ where
     pub fn insert<'db>(&'db self, runtime: &'db Runtime, value: ValueStruct<C>) -> &ValueStruct<C> {
         assert_eq!(value.created_at, runtime.current_revision());
 
-        let boxed_value = Box::new(value);
+        let boxed_value = Alloc::new(value);
         let pointer = std::ptr::addr_of!(*boxed_value);
 
         let old_value = self.map.insert(boxed_value.id, boxed_value);
@@ -165,7 +166,7 @@ where
     /// * If the value is not present in the map.
     /// * If the value has not been updated in this revision.
     fn get_from_map<'db>(
-        map: &'db FxDashMap<Id, Box<ValueStruct<C>>>,
+        map: &'db FxDashMap<Id, Alloc<ValueStruct<C>>>,
         runtime: &'db Runtime,
         id: Id,
     ) -> &'db ValueStruct<C> {
@@ -230,7 +231,7 @@ pub(crate) struct UpdateRef<'db, C>
 where
     C: Configuration,
 {
-    guard: RefMut<'db, Id, Box<ValueStruct<C>>, FxHasher>,
+    guard: RefMut<'db, Id, Alloc<ValueStruct<C>>, FxHasher>,
 }
 
 impl<'db, C> UpdateRef<'db, C>