Use pointer arithmetic to remove dynamic dispatch

src/lib.rs

@@ -1,4 +1,5 @@
 #![allow(clippy::all)]
+#![feature(pointer_byte_offsets)]
 
 use bevy::{
     ecs::{
@@ -13,9 +14,8 @@ pub trait DynQuery: 'static {}
 
 pub trait DynQueryMarker<Trait: ?Sized + DynQuery> {
     type Covered: Component<Storage = TableStorage>;
-    /// Casts an untyped pointer to a trait object pointer,
-    /// with a vtable corresponding to `Self::Covered`.
-    fn cast(_: *mut u8) -> *mut Trait;
+    /// Returns a pointer to a trait object with concrete type `Self::Covered`
+    fn null_ptr() -> *mut Trait;
 }
 
 pub trait RegisterExt {
@@ -39,7 +39,9 @@ impl RegisterExt for World {
         registry.components.push(component_id);
         registry.meta.push(TraitImplMeta {
             size_bytes: std::mem::size_of::<C>(),
-            dyn_ctor: DynCtor { cast: <(C,)>::cast },
+            dyn_ctor: DynCtor {
+                null_ptr: <(C,)>::null_ptr(),
+            },
         });
         self
     }
@@ -92,8 +94,8 @@ macro_rules! impl_dyn_query {
             $crate::DynQueryMarker<dyn $trait> for (T,)
         {
             type Covered = T;
-            fn cast(ptr: *mut u8) -> *mut dyn $trait {
-                ptr as *mut T as *mut _
+            fn null_ptr() -> *mut dyn $trait {
+                ::std::ptr::null_mut::<T>() as *mut _
             }
         }
 
@@ -172,7 +174,7 @@ pub struct ReadTraitComponentsFetch<'w, Trait: ?Sized + DynQuery> {
 /// Turns an untyped pointer into a trait object pointer,
 /// for a specific erased concrete type.
 struct DynCtor<Trait: ?Sized> {
-    cast: unsafe fn(*mut u8) -> *mut Trait,
+    null_ptr: *mut Trait,
 }
 
 impl<T: ?Sized> Copy for DynCtor<T> {}
@@ -182,12 +184,15 @@ impl<T: ?Sized> Clone for DynCtor<T> {
     }
 }
 
+unsafe impl<T: ?Sized> Send for DynCtor<T> {}
+unsafe impl<T: ?Sized> Sync for DynCtor<T> {}
+
 impl<Trait: ?Sized> DynCtor<Trait> {
     unsafe fn cast(self, ptr: Ptr) -> &Trait {
-        &*(self.cast)(ptr.as_ptr())
+        &*self.null_ptr.byte_add(ptr.as_ptr() as usize)
     }
     unsafe fn cast_mut(self, ptr: PtrMut) -> &mut Trait {
-        &mut *(self.cast)(ptr.as_ptr())
+        &mut *self.null_ptr.byte_add(ptr.as_ptr() as usize)
     }
 }