Auto merge of #142170 - RalfJung:sb-raw-retag, r=<try>

Stacked Borrows: raw pointers inherit the tag from their parent pointer

The biggest design mistake of SB in my opinion was the decision to have raw pointers be distinct from the reference they are derived from. This has multiple undesirable consequences:
- Within a function, a `let mut x` and `&raw mut x` are not allowed to be used in an interleaving way, which is quite surprising and requires dedicated work-arounds e.g. in c2rust.
- Casting a `&mut T` to a `*const T` results in a read-only pointer, which regularly startles people.
- On the implementation side, this means we have to recognize all places where "safe" pointers turn into raw pointers, which turns out to be tricky for `Box`.

TB fully mitigates this by having a raw pointer inherit the tag from its parent pointer. I think we should do the same for SB. However, this requires a bit of hackery to prevent code like this from becoming UB:
```rust
fn foo(x: &mut [i32]) {
  let ptr = x.as_mut_ptr();
  let len = x.len();
  assert!(len > 0);
  ptr.write(0);
}
```
Under SB, `x.len()` causes a read of the entire slice which invalidates `ptr` since that is derived from a child of `x` (created as part of the implicit retag when calling `as_mut_ptr`).

So this experiment adds some special magic hackery to `len` to avoid `x.len()` from retagging anything. `len` is already special in invisible ways by having the function call be replaced by a MIR op; this extends that magic for the purposes of our alias tracking.

We also do one further change to the SB access logic: writing to a `Unique` does not invalidate SRW above this item. This, too, matches Tree Borrows. It does not affect local reasoning since owners of a `Unique` know whether any SRW have been pushed on top.

This is definitely not the final answer, but it is a minimal step that lets us make SB slightly cleaner, thus unlocking some patterns in the ecosystem that Miri has so far rejected, and some cleanup in the compiler. Longer-term plans include a new attribute one can put on functions to avoid retags -- putting that attribute on `len` *or* `as_mut_ptr` would fix the above example.

The main potential downside of this PR is that it will lead Miri to accept code like `let x = 5; (&raw const x).cast_mut().write(0);` by default. This is discussed in rust-lang/unsafe-code-guidelines#400. Depending on how likely we are to make this UB in the future, we may want Miri to keep people away from those patterns. However, rejecting such code in an operational way is non-trivial; the only proposal we have is [arguably a hack](rust-lang/unsafe-code-guidelines#400 (comment)). Also, note that destructors will receive an `&mut self` pointing to such an immutable local variable, so operationally, it seems a bit futile to pretend that they are immutable.

TODO:
- Check which further TB tests should be moved to "both borrows" now.
- Entirely remove `RetagKind::Raw`.
- Add mir-opt tests checking the MIR around `x.len()` looks the way we want it to.

try-job: `*gnu*aux`

Author: rust-bors[bot]

compiler/rustc_middle/src/mir/mod.rs

@@ -1108,7 +1108,7 @@ pub enum LocalInfo<'tcx> {
     /// A temporary created during evaluating `if` predicate, possibly for pattern matching for `let`s,
     /// and subject to Edition 2024 temporary lifetime rules
     IfThenRescopeTemp { if_then: HirId },
-    /// A temporary created during the pass `Derefer` to avoid it's retagging
+    /// A temporary created during the pass `Derefer` to avoid its retagging
     DerefTemp,
     /// A temporary created for borrow checking.
     FakeBorrow,

compiler/rustc_mir_transform/src/add_retag.rs

@@ -4,7 +4,6 @@
 //! of MIR building, and only after this pass we think of the program has having the
 //! normal MIR semantics.
 
-use rustc_hir::LangItem;
 use rustc_middle::mir::*;
 use rustc_middle::ty::{self, Ty, TyCtxt};
 
@@ -28,7 +27,6 @@ fn may_contain_reference<'tcx>(ty: Ty<'tcx>, depth: u32, tcx: TyCtxt<'tcx>) -> b
         // References and Boxes (`noalias` sources)
         ty::Ref(..) => true,
         ty::Adt(..) if ty.is_box() => true,
-        ty::Adt(adt, _) if tcx.is_lang_item(adt.did(), LangItem::PtrUnique) => true,
         // Compound types: recurse
         ty::Array(ty, _) | ty::Slice(ty) => {
             // This does not branch so we keep the depth the same.
@@ -153,6 +151,13 @@ impl<'tcx> crate::MirPass<'tcx> for AddRetag {
                                 }
                             }
                             Rvalue::Ref(..) => None,
+                            // Also skip coercions whose source is already a local. This is crucial
+                            // to avoid unnecessary retags in `my_array.len()` calls.
+                            Rvalue::Cast(
+                                CastKind::PointerCoercion(_, CoercionSource::Implicit),
+                                Operand::Copy(p) | Operand::Move(p),
+                                _,
+                            ) if p.as_local().is_some() => None,
                             _ => {
                                 if needs_retag(place) {
                                     Some(RetagKind::Default)

compiler/rustc_mir_transform/src/lib.rs

@@ -628,6 +628,7 @@ fn run_runtime_lowering_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
 fn run_runtime_cleanup_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
     let passes: &[&dyn MirPass<'tcx>] = &[
         &lower_intrinsics::LowerIntrinsics,
+        &lower_slice_len::LowerSliceLenCalls,
         &remove_place_mention::RemovePlaceMention,
         &simplify::SimplifyCfg::PreOptimizations,
     ];
@@ -671,9 +672,6 @@ pub(crate) fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'
             &check_null::CheckNull,
             // Before inlining: trim down MIR with passes to reduce inlining work.
 
-            // Has to be done before inlining, otherwise actual call will be almost always inlined.
-            // Also simple, so can just do first.
-            &lower_slice_len::LowerSliceLenCalls,
             // Perform instsimplify before inline to eliminate some trivial calls (like clone
             // shims).
             &instsimplify::InstSimplify::BeforeInline,

compiler/rustc_mir_transform/src/lower_slice_len.rs

@@ -2,14 +2,18 @@
 //! It should run before inlining!
 
 use rustc_hir::def_id::DefId;
+use rustc_index::IndexVec;
 use rustc_middle::mir::*;
-use rustc_middle::ty::TyCtxt;
+use rustc_middle::ty::adjustment::PointerCoercion;
+use rustc_middle::ty::{Ty, TyCtxt};
 
 pub(super) struct LowerSliceLenCalls;
 
 impl<'tcx> crate::MirPass<'tcx> for LowerSliceLenCalls {
     fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
-        sess.mir_opt_level() > 0
+        // This pass also removes some otherwise-problematic retags, so we
+        // always enable it when retags matter.
+        sess.mir_opt_level() > 0 || sess.opts.unstable_opts.mir_emit_retag
     }
 
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
@@ -23,7 +27,7 @@ impl<'tcx> crate::MirPass<'tcx> for LowerSliceLenCalls {
         let basic_blocks = body.basic_blocks.as_mut_preserves_cfg();
         for block in basic_blocks {
             // lower `<[_]>::len` calls
-            lower_slice_len_call(block, slice_len_fn_item_def_id);
+            lower_slice_len_call(tcx, &mut body.local_decls, block, slice_len_fn_item_def_id);
         }
     }
 
@@ -32,7 +36,12 @@ impl<'tcx> crate::MirPass<'tcx> for LowerSliceLenCalls {
     }
 }
 
-fn lower_slice_len_call<'tcx>(block: &mut BasicBlockData<'tcx>, slice_len_fn_item_def_id: DefId) {
+fn lower_slice_len_call<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    local_decls: &mut IndexVec<Local, LocalDecl<'tcx>>,
+    block: &mut BasicBlockData<'tcx>,
+    slice_len_fn_item_def_id: DefId,
+) {
     let terminator = block.terminator();
     if let TerminatorKind::Call {
         func,
@@ -50,11 +59,12 @@ fn lower_slice_len_call<'tcx>(block: &mut BasicBlockData<'tcx>, slice_len_fn_ite
         // perform modifications from something like:
         //     _5 = core::slice::<impl [u8]>::len(move _6) -> bb1
         // into:
-        //     _5 = PtrMetadata(move _6)
+        //     _5 = PtrMetadata(move _6);
         //     goto bb1
 
         // make new RValue for Len
-        let r_value = Rvalue::UnaryOp(UnOp::PtrMetadata, arg.node.clone());
+        let arg = arg.node.clone();
+        let r_value = Rvalue::UnaryOp(UnOp::PtrMetadata, arg.clone());
         let len_statement_kind = StatementKind::Assign(Box::new((*destination, r_value)));
         let add_statement =
             Statement { kind: len_statement_kind, source_info: terminator.source_info };
@@ -64,5 +74,75 @@ fn lower_slice_len_call<'tcx>(block: &mut BasicBlockData<'tcx>, slice_len_fn_ite
 
         block.statements.push(add_statement);
         block.terminator_mut().kind = new_terminator_kind;
+
+        if tcx.sess.opts.unstable_opts.mir_emit_retag {
+            // If we care about retags, we want there to be *no retag* for this `len` call.
+            // That means we have to clean up the MIR a little: the MIR will now often be:
+            //     _6 = &*_4;
+            //     _5 = PtrMetadata(move _6);
+            // Change that to:
+            //     _6 = &raw const *_4;
+            //     _5 = PtrMetadata(move _6);
+            let len = block.statements.len();
+            if len >= 2
+                && let &mut StatementKind::Assign(box (rebor_dest, ref mut rebor_r)) =
+                    &mut block.statements[len - 2].kind
+                && let Some(retag_local) = rebor_dest.as_local()
+                && !local_decls[retag_local].is_user_variable()
+                // The LHS of this previous assignment is the `arg` from above.
+                && matches!(arg, Operand::Copy(p) | Operand::Move(p) if p.as_local() == Some(retag_local))
+                // The RHS is a reference-taking operation.
+                && let Rvalue::Ref(_, BorrowKind::Shared, rebor_orig) = *rebor_r
+            {
+                let orig_inner_ty = rebor_orig.ty(local_decls, tcx).ty;
+                // Change the previous statement to use `&raw` instead of `&`.
+                *rebor_r = Rvalue::RawPtr(RawPtrKind::FakeForPtrMetadata, rebor_orig);
+                // Change the type of the local to match.
+                local_decls[retag_local].ty = Ty::new_ptr(tcx, orig_inner_ty, Mutability::Not);
+            }
+            // There's a second pattern we need to recognize for `array.len()` calls. There,
+            // the MIR is:
+            //     _4 = &*_3;
+            //     _6 = move _4 as &[T] (PointerCoercion(Unsize, Implicit));
+            //     StorageDead(_4);
+            //     _5 = PtrMetadata(move _6);
+            // Change that to:
+            //     _4 = &raw const *_3;
+            //     _6 = move _4 as *const [T] (PointerCoercion(Unsize, Implicit));
+            //     StorageDead(_4);
+            //     _5 = PtrMetadata(move _6);
+            if len >= 4
+                && let [reborrow_stmt, unsize_stmt, storage_dead_stmt] =
+                    block.statements.get_disjoint_mut([len - 4, len - 3, len - 2]).unwrap()
+                // Check that the 3 statements have the right shape.
+                && let &mut StatementKind::Assign(box (rebor_dest, ref mut rebor_r)) = &mut reborrow_stmt.kind
+                && let Some(retag_local) = rebor_dest.as_local()
+                && !local_decls[retag_local].is_user_variable()
+                && let &mut StatementKind::Assign(box (unsized_dest, ref mut unsize_r)) = &mut unsize_stmt.kind
+                && let Some(unsized_local) = unsized_dest.as_local()
+                && !local_decls[unsized_local].is_user_variable()
+                && storage_dead_stmt.kind == StatementKind::StorageDead(retag_local)
+                // And check that they have the right operands.
+                && matches!(arg, Operand::Copy(p) | Operand::Move(p) if p.as_local() == Some(unsized_local))
+                && let &mut Rvalue::Cast(
+                    CastKind::PointerCoercion(PointerCoercion::Unsize, CoercionSource::Implicit),
+                    ref cast_op,
+                    ref mut cast_ty,
+                ) = unsize_r
+                && let Operand::Copy(unsize_src) | Operand::Move(unsize_src) = cast_op
+                && unsize_src.as_local() == Some(retag_local)
+                && let Rvalue::Ref(_, BorrowKind::Shared, rebor_orig) = *rebor_r
+            {
+                let orig_inner_ty = rebor_orig.ty(local_decls, tcx).ty;
+                let unsize_inner_ty = cast_ty.builtin_deref(true).unwrap();
+                // Change the reborrow to use `&raw` instead of `&`.
+                *rebor_r = Rvalue::RawPtr(RawPtrKind::FakeForPtrMetadata, rebor_orig);
+                // Change the unsize coercion in the same vein.
+                *cast_ty = Ty::new_ptr(tcx, unsize_inner_ty, Mutability::Not);
+                // Change the type of the locals to match.
+                local_decls[retag_local].ty = Ty::new_ptr(tcx, orig_inner_ty, Mutability::Not);
+                local_decls[unsized_local].ty = *cast_ty;
+            }
+        }
     }
 }

library/alloctests/tests/sync.rs

@@ -332,15 +332,15 @@ fn weak_self_cyclic() {
 #[test]
 fn drop_arc() {
     let mut canary = AtomicUsize::new(0);
-    let x = Arc::new(Canary(&mut canary as *mut AtomicUsize));
+    let x = Arc::new(Canary(&raw mut canary));
     drop(x);
     assert!(canary.load(Acquire) == 1);
 }
 
 #[test]
 fn drop_arc_weak() {
     let mut canary = AtomicUsize::new(0);
-    let arc = Arc::new(Canary(&mut canary as *mut AtomicUsize));
+    let arc = Arc::new(Canary(&raw mut canary));
     let arc_weak = Arc::downgrade(&arc);
     assert!(canary.load(Acquire) == 0);
     drop(arc);