Rollup merge of #122018 - RalfJung:box-custom-alloc, r=oli-obk

only set noalias on Box with the global allocator

As discovered in rust-lang/miri#3341, `noalias` and custom allocators don't go well together.

rustc can now check whether a Box uses the global allocator. This replaces the previous ad-hoc and rather unprincipled check for a zero-sized allocator.

This is the rustc part of fixing that; Miri will also need a patch.

Author: matthiaskrgr

compiler/rustc_abi/src/lib.rs

@@ -1612,8 +1612,9 @@ pub enum PointerKind {
     SharedRef { frozen: bool },
     /// Mutable reference. `unpin` indicates the absence of any pinned data.
     MutableRef { unpin: bool },
-    /// Box. `unpin` indicates the absence of any pinned data.
-    Box { unpin: bool },
+    /// Box. `unpin` indicates the absence of any pinned data. `global` indicates whether this box
+    /// uses the global allocator or a custom one.
+    Box { unpin: bool, global: bool },
 }
 
 /// Note that this information is advisory only, and backends are free to ignore it.
@@ -1622,6 +1623,8 @@ pub enum PointerKind {
 pub struct PointeeInfo {
     pub size: Size,
     pub align: Align,
+    /// If this is `None`, then this is a raw pointer, so size and alignment are not guaranteed to
+    /// be reliable.
     pub safe: Option<PointerKind>,
 }
 

compiler/rustc_codegen_cranelift/example/mini_core.rs

@@ -525,8 +525,11 @@ pub struct Unique<T: ?Sized> {
 impl<T: ?Sized, U: ?Sized> CoerceUnsized<Unique<U>> for Unique<T> where T: Unsize<U> {}
 impl<T: ?Sized, U: ?Sized> DispatchFromDyn<Unique<U>> for Unique<T> where T: Unsize<U> {}
 
+#[lang = "global_alloc_ty"]
+pub struct Global;
+
 #[lang = "owned_box"]
-pub struct Box<T: ?Sized, A = ()>(Unique<T>, A);
+pub struct Box<T: ?Sized, A = Global>(Unique<T>, A);
 
 impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Box<U>> for Box<T> {}
 
@@ -536,7 +539,7 @@ impl<T> Box<T> {
             let size = intrinsics::size_of::<T>();
             let ptr = libc::malloc(size);
             intrinsics::copy(&val as *const T as *const u8, ptr, size);
-            Box(Unique { pointer: NonNull(ptr as *const T), _marker: PhantomData }, ())
+            Box(Unique { pointer: NonNull(ptr as *const T), _marker: PhantomData }, Global)
         }
     }
 }

compiler/rustc_codegen_cranelift/src/unsize.rs

@@ -74,10 +74,6 @@ fn unsize_ptr<'tcx>(
         | (&ty::RawPtr(ty::TypeAndMut { ty: a, .. }), &ty::RawPtr(ty::TypeAndMut { ty: b, .. })) => {
             (src, unsized_info(fx, *a, *b, old_info))
         }
-        (&ty::Adt(def_a, _), &ty::Adt(def_b, _)) if def_a.is_box() && def_b.is_box() => {
-            let (a, b) = (src_layout.ty.boxed_ty(), dst_layout.ty.boxed_ty());
-            (src, unsized_info(fx, a, b, old_info))
-        }
         (&ty::Adt(def_a, _), &ty::Adt(def_b, _)) => {
             assert_eq!(def_a, def_b);
 

compiler/rustc_codegen_gcc/example/mini_core.rs

@@ -472,6 +472,7 @@ pub trait Allocator {
 
 impl Allocator for () {}
 
+#[lang = "global_alloc_ty"]
 pub struct Global;
 
 impl Allocator for Global {}

compiler/rustc_codegen_llvm/src/debuginfo/metadata.rs

@@ -454,9 +454,13 @@ pub fn type_di_node<'ll, 'tcx>(cx: &CodegenCx<'ll, 'tcx>, t: Ty<'tcx>) -> &'ll D
         ty::RawPtr(ty::TypeAndMut { ty: pointee_type, .. }) | ty::Ref(_, pointee_type, _) => {
             build_pointer_or_reference_di_node(cx, t, pointee_type, unique_type_id)
         }
-        // Box<T, A> may have a non-1-ZST allocator A. In that case, we
-        // cannot treat Box<T, A> as just an owned alias of `*mut T`.
-        ty::Adt(def, args) if def.is_box() && cx.layout_of(args.type_at(1)).is_1zst() => {
+        // Some `Box` are newtyped pointers, make debuginfo aware of that.
+        // Only works if the allocator argument is a 1-ZST and hence irrelevant for layout
+        // (or if there is no allocator argument).
+        ty::Adt(def, args)
+            if def.is_box()
+                && args.get(1).map_or(true, |arg| cx.layout_of(arg.expect_ty()).is_1zst()) =>
+        {
             build_pointer_or_reference_di_node(cx, t, t.boxed_ty(), unique_type_id)
         }
         ty::FnDef(..) | ty::FnPtr(_) => build_subroutine_type_di_node(cx, unique_type_id),

compiler/rustc_codegen_ssa/src/mir/operand.rs

@@ -204,6 +204,7 @@ impl<'a, 'tcx, V: CodegenObject> OperandRef<'tcx, V> {
 
     pub fn deref<Cx: LayoutTypeMethods<'tcx>>(self, cx: &Cx) -> PlaceRef<'tcx, V> {
         if self.layout.ty.is_box() {
+            // Derefer should have removed all Box derefs
             bug!("dereferencing {:?} in codegen", self.layout.ty);
         }
 

compiler/rustc_const_eval/src/interpret/place.rs

@@ -437,6 +437,7 @@ where
         trace!("deref to {} on {:?}", val.layout.ty, *val);
 
         if val.layout.ty.is_box() {
+            // Derefer should have removed all Box derefs
             bug!("dereferencing {}", val.layout.ty);
         }
 

compiler/rustc_const_eval/src/interpret/terminator.rs

@@ -359,14 +359,8 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             Ok(Some(match ty.kind() {
                 ty::Ref(_, ty, _) => *ty,
                 ty::RawPtr(mt) => mt.ty,
-                // We should only accept `Box` with the default allocator.
-                // It's hard to test for that though so we accept every 1-ZST allocator.
-                ty::Adt(def, args)
-                    if def.is_box()
-                        && self.layout_of(args[1].expect_ty()).is_ok_and(|l| l.is_1zst()) =>
-                {
-                    args[0].expect_ty()
-                }
+                // We only accept `Box` with the default allocator.
+                _ if ty.is_box_global(*self.tcx) => ty.boxed_ty(),
                 _ => return Ok(None),
             }))
         };

compiler/rustc_hir/src/lang_items.rs

@@ -267,6 +267,8 @@ language_item_table! {
     EhCatchTypeinfo,         sym::eh_catch_typeinfo,   eh_catch_typeinfo,          Target::Static,         GenericRequirement::None;
 
     OwnedBox,                sym::owned_box,           owned_box,                  Target::Struct,         GenericRequirement::Minimum(1);
+    GlobalAlloc,             sym::global_alloc_ty,     global_alloc_ty,            Target::Struct,         GenericRequirement::None;
+
     // Experimental language item for Miri
     PtrUnique,               sym::ptr_unique,          ptr_unique,                 Target::Struct,         GenericRequirement::Exact(1);
 

compiler/rustc_middle/src/ty/layout.rs

@@ -969,6 +969,8 @@ where
         }
     }
 
+    /// Compute the information for the pointer stored at the given offset inside this type.
+    /// This will recurse into fields of ADTs to find the inner pointer.
     fn ty_and_layout_pointee_info_at(
         this: TyAndLayout<'tcx>,
         cx: &C,
@@ -1068,15 +1070,17 @@ where
                     }
                 }
 
-                // FIXME(eddyb) This should be for `ptr::Unique<T>`, not `Box<T>`.
+                // Fixup info for the first field of a `Box`. Recursive traversal will have found
+                // the raw pointer, so size and align are set to the boxed type, but `pointee.safe`
+                // will still be `None`.
                 if let Some(ref mut pointee) = result {
-                    if let ty::Adt(def, _) = this.ty.kind() {
-                        if def.is_box() && offset.bytes() == 0 {
-                            let optimize = tcx.sess.opts.optimize != OptLevel::No;
-                            pointee.safe = Some(PointerKind::Box {
-                                unpin: optimize && this.ty.boxed_ty().is_unpin(tcx, cx.param_env()),
-                            });
-                        }
+                    if offset.bytes() == 0 && this.ty.is_box() {
+                        debug_assert!(pointee.safe.is_none());
+                        let optimize = tcx.sess.opts.optimize != OptLevel::No;
+                        pointee.safe = Some(PointerKind::Box {
+                            unpin: optimize && this.ty.boxed_ty().is_unpin(tcx, cx.param_env()),
+                            global: this.ty.is_box_global(tcx),
+                        });
                     }
                 }