remove retag from drop shim

Author: WaffleLapkin

compiler/rustc_mir_transform/src/shim.rs

@@ -298,35 +298,17 @@ fn local_decls_for_sig<'tcx>(
 fn dropee_emit_retag<'tcx>(
     tcx: TyCtxt<'tcx>,
     body: &mut Body<'tcx>,
-    mut dropee_ptr: Place<'tcx>,
+    dropee_ptr: Place<'tcx>,
     span: Span,
-) -> Place<'tcx> {
+) {
     if tcx.sess.opts.unstable_opts.mir_emit_retag {
         let source_info = SourceInfo::outermost(span);
-        // 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 { kind: MutBorrowKind::Default },
-            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::new(source_info, s));
-        }
+        let new_statement = StatementKind::Retag(RetagKind::FnEntry, Box::new(dropee_ptr));
+
+        body.basic_blocks_mut()[START_BLOCK]
+            .statements
+            .push(Statement::new(source_info, new_statement));
     }
-    dropee_ptr
 }
 
 fn build_drop_shim<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId, ty: Option<Ty<'tcx>>) -> Body<'tcx> {
@@ -359,7 +341,7 @@ fn build_drop_shim<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId, ty: Option<Ty<'tcx>>)
 
     // The first argument (index 0), but add 1 for the return value.
     let dropee_ptr = Place::from(Local::new(1 + 0));
-    let dropee_ptr = dropee_emit_retag(tcx, &mut body, dropee_ptr, span);
+    dropee_emit_retag(tcx, &mut body, dropee_ptr, span);
 
     if ty.is_some() {
         let patch = {

compiler/rustc_mir_transform/src/shim/async_destructor_ctor.rs

@@ -124,13 +124,13 @@ pub(super) fn build_async_drop_shim<'tcx>(
         return body;
     }
 
-    let mut dropee_ptr = Place::from(body.local_decls.push(LocalDecl::new(drop_ptr_ty, span)));
+    let dropee_ptr = Place::from(body.local_decls.push(LocalDecl::new(drop_ptr_ty, span)));
     let st_kind = StatementKind::Assign(Box::new((
         dropee_ptr,
         Rvalue::Use(Operand::Move(coroutine_layout_dropee)),
     )));
     body.basic_blocks_mut()[START_BLOCK].statements.push(Statement::new(source_info, st_kind));
-    dropee_ptr = dropee_emit_retag(tcx, &mut body, dropee_ptr, span);
+    dropee_emit_retag(tcx, &mut body, dropee_ptr, span);
 
     let dropline = body.basic_blocks.last_index();