start handling drop for PreloadMut

Author: ZuseZ4

compiler/rustc_codegen_ssa/src/mir/block.rs

@@ -591,6 +591,14 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         bx.ret(llval);
     }
 
+    fn is_preload_mut_type<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> bool {
+        let ty::Adt(adt_def, _) = ty.kind() else {
+            return false;
+        };
+
+        Some(adt_def.did()) == tcx.lang_items().preload_mut_type()
+    }
+
     #[tracing::instrument(level = "trace", skip(self, helper, bx))]
     fn codegen_drop_terminator(
         &mut self,
@@ -604,6 +612,12 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
     ) -> MergingSucc {
         let ty = location.ty(self.mir, bx.tcx()).ty;
         let ty = self.monomorphize(ty);
+
+        if is_preload_mut_type(bx.tcx(), ty) {
+            let place = self.codegen_place(bx, location.as_ref());
+
+            bx.codegen_offload_preload_mut_drop(ty, place);
+        }
         let drop_fn = Instance::resolve_drop_glue(bx.tcx(), ty);
 
         if let ty::InstanceKind::DropGlue(_, None) = drop_fn.def {

compiler/rustc_codegen_ssa/src/traits/intrinsic.rs

@@ -38,6 +38,12 @@ pub trait IntrinsicCallBuilderMethods<'tcx>: BackendTypes {
         is_mut: bool,
     );
 
+    fn codegen_offload_preload_mut_drop(
+        &mut self,
+        preload_ty: Ty<'tcx>,
+        place: PlaceRef<'tcx, Self::Value>,
+    );
+
     fn codegen_llvm_intrinsic_call(
         &mut self,
         instance: ty::Instance<'tcx>,

library/core/src/offload/mod.rs

@@ -31,3 +31,13 @@ pub fn preload<'a, T: ?Sized>(x: &'a T) -> Preload<'a, T> {
 pub fn preload_mut<'a, T: ?Sized>(x: &'a mut T) -> PreloadMut<'a, T> {
     PreloadMut { cpu_ptr: x as *mut T, _marker: PhantomData }
 }
+
+impl<T: ?Sized> Drop for PreloadMut<'_, T> {
+    fn drop(&mut self) {
+        // Intentionally empty.
+        //
+        // This exists so MIR creates Drop terminators for PreloadMut.
+        // rustc codegen intercepts those terminators and emits the
+        // offload return mapper.
+    }
+}