Auto merge of #103957 - JakobDegen:drop-retag, r=RalfJung

Retag as FnEntry on `drop_in_place`

This commit changes the mir drop shim to always retag its argument as if it were a `&mut`.

cc rust-lang/unsafe-code-guidelines#373

compiler/rustc_const_eval/src/interpret/terminator.rs

@@ -119,11 +119,20 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             }
 
             Drop { place, target, unwind } => {
-                let place = self.eval_place(place)?;
-                let ty = place.layout.ty;
-                trace!("TerminatorKind::drop: {:?}, type {}", place, ty);
-
+                let frame = self.frame();
+                let ty = place.ty(&frame.body.local_decls, *self.tcx).ty;
+                let ty = self.subst_from_frame_and_normalize_erasing_regions(frame, ty)?;
                 let instance = Instance::resolve_drop_in_place(*self.tcx, ty);
+                if let ty::InstanceDef::DropGlue(_, None) = instance.def {
+                    // This is the branch we enter if and only if the dropped type has no drop glue
+                    // whatsoever. This can happen as a result of monomorphizing a drop of a
+                    // generic. In order to make sure that generic and non-generic code behaves
+                    // roughly the same (and in keeping with Mir semantics) we do nothing here.
+                    self.go_to_block(target);
+                    return Ok(());
+                }
+                let place = self.eval_place(place)?;
+                trace!("TerminatorKind::drop: {:?}, type {}", place, ty);
                 self.drop_in_place(&place, instance, target, unwind)?;
             }
 

compiler/rustc_middle/src/mir/syntax.rs

@@ -564,14 +564,13 @@ pub enum TerminatorKind<'tcx> {
     Unreachable,
 
     /// The behavior of this statement differs significantly before and after drop elaboration.
-    /// After drop elaboration, `Drop` executes the drop glue for the specified place, after which
-    /// it continues execution/unwinds at the given basic blocks. It is possible that executing drop
-    /// glue is special - this would be part of Rust's memory model. (**FIXME**: due we have an
-    /// issue tracking if drop glue has any interesting semantics in addition to those of a function
-    /// call?)
     ///
-    /// `Drop` before drop elaboration is a *conditional* execution of the drop glue. Specifically, the
-    /// `Drop` will be executed if...
+    /// After drop elaboration: `Drop` terminators are a complete nop for types that have no drop
+    /// glue. For other types, `Drop` terminators behave exactly like a call to
+    /// `core::mem::drop_in_place` with a pointer to the given place.
+    ///
+    /// `Drop` before drop elaboration is a *conditional* execution of the drop glue. Specifically,
+    /// the `Drop` will be executed if...
     ///
     /// **Needs clarification**: End of that sentence. This in effect should document the exact
     /// behavior of drop elaboration. The following sounds vaguely right, but I'm not quite sure:

compiler/rustc_mir_transform/src/shim.rs

@@ -174,9 +174,36 @@ fn build_drop_shim<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId, ty: Option<Ty<'tcx>>)
     let mut body =
         new_body(source, blocks, local_decls_for_sig(&sig, span), sig.inputs().len(), span);
 
+    // The first argument (index 0), but add 1 for the return value.
+    let mut dropee_ptr = Place::from(Local::new(1 + 0));
+    if tcx.sess.opts.unstable_opts.mir_emit_retag {
+        // We want to treat the function argument as if it was passed by `&mut`. As such, we
+        // generate
+        // ```
+        // temp = &mut *arg;
+        // Retag(temp, FnEntry)
+        // ```
+        // It's important that we do this first, before anything that depends on `dropee_ptr`
+        // has been put into the body.
+        let reborrow = Rvalue::Ref(
+            tcx.lifetimes.re_erased,
+            BorrowKind::Mut { allow_two_phase_borrow: false },
+            tcx.mk_place_deref(dropee_ptr),
+        );
+        let ref_ty = reborrow.ty(body.local_decls(), tcx);
+        dropee_ptr = body.local_decls.push(LocalDecl::new(ref_ty, span)).into();
+        let new_statements = [
+            StatementKind::Assign(Box::new((dropee_ptr, reborrow))),
+            StatementKind::Retag(RetagKind::FnEntry, Box::new(dropee_ptr)),
+        ];
+        for s in new_statements {
+            body.basic_blocks_mut()[START_BLOCK]
+                .statements
+                .push(Statement { source_info, kind: s });
+        }
+    }
+
     if ty.is_some() {
-        // The first argument (index 0), but add 1 for the return value.
-        let dropee_ptr = Place::from(Local::new(1 + 0));
         let patch = {
             let param_env = tcx.param_env_reveal_all_normalized(def_id);
             let mut elaborator =