Retag argument to drop_in_place unconditionally

Author: JakobDegen

compiler/rustc_mir_transform/src/shim.rs

@@ -174,35 +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);
 
-    if ty.is_some() {
-        // 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 });
-            }
+    // 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() {
         let patch = {
             let param_env = tcx.param_env_reveal_all_normalized(def_id);
             let mut elaborator =

src/tools/miri/tests/fail/stacked_borrows/drop_in_place_protector.rs

@@ -10,7 +10,7 @@ impl Drop for HasDrop {
     fn drop(&mut self) {
         unsafe {
             let _val = *P;
-            //~^ ERROR: /not granting access .* because that would remove .* which is protected/
+            //~^ ERROR: /not granting access .* because that would remove .* which is strongly protected/
         }
     }
 }

src/tools/miri/tests/fail/stacked_borrows/drop_in_place_protector.stderr

@@ -1,8 +1,8 @@
-error: Undefined Behavior: not granting access to tag <TAG> because that would remove [Unique for <TAG>] which is protected because it is an argument of call ID
+error: Undefined Behavior: not granting access to tag <TAG> because that would remove [Unique for <TAG>] which is strongly protected because it is an argument of call ID
   --> $DIR/drop_in_place_protector.rs:LL:CC
    |
 LL |             let _val = *P;
-   |                        ^^ not granting access to tag <TAG> because that would remove [Unique for <TAG>] which is protected because it is an argument of call ID
+   |                        ^^ not granting access to tag <TAG> because that would remove [Unique for <TAG>] which is strongly protected because it is an argument of call ID
    |
    = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
    = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information

src/tools/miri/tests/fail/stacked_borrows/drop_in_place_retag.rs

@@ -3,16 +3,9 @@
 
 //@error-pattern: /retag .* for Unique permission .* only grants SharedReadOnly permission/
 
-#[repr(transparent)]
-struct HasDrop;
-
-impl Drop for HasDrop {
-    fn drop(&mut self) {}
-}
-
 fn main() {
     unsafe {
-        let x = (0u8, HasDrop);
+        let x = 0u8;
         let x = core::ptr::addr_of!(x);
         core::ptr::drop_in_place(x.cast_mut());
     }

src/tools/miri/tests/fail/stacked_borrows/drop_in_place_retag.stderr

@@ -15,7 +15,7 @@ help: <TAG> was created by a SharedReadOnly retag at offsets [0x0..0x1]
 LL |         let x = core::ptr::addr_of!(x);
    |                 ^^^^^^^^^^^^^^^^^^^^^^
    = note: BACKTRACE:
-   = note: inside `std::ptr::drop_in_place::<(u8, HasDrop)> - shim(Some((u8, HasDrop)))` at RUSTLIB/core/src/ptr/mod.rs:LL:CC
+   = note: inside `std::ptr::drop_in_place::<u8> - shim(None)` at RUSTLIB/core/src/ptr/mod.rs:LL:CC
 note: inside `main` at $DIR/drop_in_place_retag.rs:LL:CC
   --> $DIR/drop_in_place_retag.rs:LL:CC
    |