Auto merge of #95576 - DrMeepster:box_erasure, r=oli-obk

Remove dereferencing of Box from codegen

Through #94043, #94414, #94873, and #95328, I've been fixing issues caused by Box being treated like a pointer when it is not a pointer. However, these PRs just introduced special cases for Box. This PR removes those special cases and instead transforms a deref of Box into a deref of the pointer it contains.

Hopefully, this is the end of the Box<T, A> ICEs.

compiler/rustc_codegen_ssa/src/mir/operand.rs

@@ -118,22 +118,20 @@ 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() {
+            bug!("dereferencing {:?} in codegen", self.layout.ty);
+        }
+
         let projected_ty = self
             .layout
             .ty
             .builtin_deref(true)
             .unwrap_or_else(|| bug!("deref of non-pointer {:?}", self))
             .ty;
+
         let (llptr, llextra) = match self.val {
             OperandValue::Immediate(llptr) => (llptr, None),
-            OperandValue::Pair(llptr, llextra) => {
-                // if the box's allocator isn't a ZST, then "llextra" is actually the allocator
-                if self.layout.ty.is_box() && !self.layout.field(cx, 1).is_zst() {
-                    (llptr, None)
-                } else {
-                    (llptr, Some(llextra))
-                }
-            }
+            OperandValue::Pair(llptr, llextra) => (llptr, Some(llextra)),
             OperandValue::Ref(..) => bug!("Deref of by-Ref operand {:?}", self),
         };
         let layout = cx.layout_of(projected_ty);

compiler/rustc_codegen_ssa/src/mir/place.rs

@@ -446,35 +446,15 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                         mir::PlaceRef { projection: &place_ref.projection[..elem.0], ..place_ref },
                     );
 
-                    // a box with a non-zst allocator should not be directly dereferenced
-                    if cg_base.layout.ty.is_box() && !cg_base.layout.field(cx, 1).is_zst() {
-                        // Extract `Box<T>` -> `Unique<T>` -> `NonNull<T>` -> `*const T`
-                        let ptr =
-                            cg_base.extract_field(bx, 0).extract_field(bx, 0).extract_field(bx, 0);
-
-                        ptr.deref(bx.cx())
-                    } else {
-                        cg_base.deref(bx.cx())
-                    }
+                    cg_base.deref(bx.cx())
                 } else {
                     bug!("using operand local {:?} as place", place_ref);
                 }
             }
         };
         for elem in place_ref.projection[base..].iter() {
             cg_base = match *elem {
-                mir::ProjectionElem::Deref => {
-                    // a box with a non-zst allocator should not be directly dereferenced
-                    if cg_base.layout.ty.is_box() && !cg_base.layout.field(cx, 1).is_zst() {
-                        // Project `Box<T>` -> `Unique<T>` -> `NonNull<T>` -> `*const T`
-                        let ptr =
-                            cg_base.project_field(bx, 0).project_field(bx, 0).project_field(bx, 0);
-
-                        bx.load_operand(ptr).deref(bx.cx())
-                    } else {
-                        bx.load_operand(cg_base).deref(bx.cx())
-                    }
-                }
+                mir::ProjectionElem::Deref => bx.load_operand(cg_base).deref(bx.cx()),
                 mir::ProjectionElem::Field(ref field, _) => {
                     cg_base.project_field(bx, field.index())
                 }

compiler/rustc_const_eval/src/interpret/place.rs

@@ -313,6 +313,11 @@ where
     ) -> InterpResult<'tcx, MPlaceTy<'tcx, M::PointerTag>> {
         let val = self.read_immediate(src)?;
         trace!("deref to {} on {:?}", val.layout.ty, *val);
+
+        if val.layout.ty.is_box() {
+            bug!("dereferencing {:?}", val.layout.ty);
+        }
+
         let mplace = self.ref_to_mplace(&val)?;
         self.check_mplace_access(mplace, CheckInAllocMsg::DerefTest)?;
         Ok(mplace)

compiler/rustc_const_eval/src/transform/validate.rs

@@ -240,65 +240,79 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
         context: PlaceContext,
         location: Location,
     ) {
-        if let ProjectionElem::Index(index) = elem {
-            let index_ty = self.body.local_decls[index].ty;
-            if index_ty != self.tcx.types.usize {
-                self.fail(location, format!("bad index ({:?} != usize)", index_ty))
+        match elem {
+            ProjectionElem::Index(index) => {
+                let index_ty = self.body.local_decls[index].ty;
+                if index_ty != self.tcx.types.usize {
+                    self.fail(location, format!("bad index ({:?} != usize)", index_ty))
+                }
             }
-        }
-        if let ProjectionElem::Field(f, ty) = elem {
-            let parent = Place { local, projection: self.tcx.intern_place_elems(proj_base) };
-            let parent_ty = parent.ty(&self.body.local_decls, self.tcx);
-            let fail_out_of_bounds = |this: &Self, location| {
-                this.fail(location, format!("Out of bounds field {:?} for {:?}", f, parent_ty));
-            };
-            let check_equal = |this: &Self, location, f_ty| {
-                if !this.mir_assign_valid_types(ty, f_ty) {
-                    this.fail(
+            ProjectionElem::Deref if self.mir_phase >= MirPhase::GeneratorsLowered => {
+                let base_ty = Place::ty_from(local, proj_base, &self.body.local_decls, self.tcx).ty;
+
+                if base_ty.is_box() {
+                    self.fail(
+                        location,
+                        format!("{:?} dereferenced after ElaborateBoxDerefs", base_ty),
+                    )
+                }
+            }
+            ProjectionElem::Field(f, ty) => {
+                let parent = Place { local, projection: self.tcx.intern_place_elems(proj_base) };
+                let parent_ty = parent.ty(&self.body.local_decls, self.tcx);
+                let fail_out_of_bounds = |this: &Self, location| {
+                    this.fail(location, format!("Out of bounds field {:?} for {:?}", f, parent_ty));
+                };
+                let check_equal = |this: &Self, location, f_ty| {
+                    if !this.mir_assign_valid_types(ty, f_ty) {
+                        this.fail(
                         location,
                         format!(
                             "Field projection `{:?}.{:?}` specified type `{:?}`, but actual type is {:?}",
                             parent, f, ty, f_ty
                         )
                     )
-                }
-            };
-            match parent_ty.ty.kind() {
-                ty::Tuple(fields) => {
-                    let Some(f_ty) = fields.get(f.as_usize()) else {
-                        fail_out_of_bounds(self, location);
-                        return;
-                    };
-                    check_equal(self, location, *f_ty);
-                }
-                ty::Adt(adt_def, substs) => {
-                    let var = parent_ty.variant_index.unwrap_or(VariantIdx::from_u32(0));
-                    let Some(field) = adt_def.variant(var).fields.get(f.as_usize()) else {
-                        fail_out_of_bounds(self, location);
-                        return;
-                    };
-                    check_equal(self, location, field.ty(self.tcx, substs));
-                }
-                ty::Closure(_, substs) => {
-                    let substs = substs.as_closure();
-                    let Some(f_ty) = substs.upvar_tys().nth(f.as_usize()) else {
-                        fail_out_of_bounds(self, location);
-                        return;
-                    };
-                    check_equal(self, location, f_ty);
-                }
-                ty::Generator(_, substs, _) => {
-                    let substs = substs.as_generator();
-                    let Some(f_ty) = substs.upvar_tys().nth(f.as_usize()) else {
-                        fail_out_of_bounds(self, location);
-                        return;
-                    };
-                    check_equal(self, location, f_ty);
-                }
-                _ => {
-                    self.fail(location, format!("{:?} does not have fields", parent_ty.ty));
+                    }
+                };
+
+                match parent_ty.ty.kind() {
+                    ty::Tuple(fields) => {
+                        let Some(f_ty) = fields.get(f.as_usize()) else {
+                            fail_out_of_bounds(self, location);
+                            return;
+                        };
+                        check_equal(self, location, *f_ty);
+                    }
+                    ty::Adt(adt_def, substs) => {
+                        let var = parent_ty.variant_index.unwrap_or(VariantIdx::from_u32(0));
+                        let Some(field) = adt_def.variant(var).fields.get(f.as_usize()) else {
+                            fail_out_of_bounds(self, location);
+                            return;
+                        };
+                        check_equal(self, location, field.ty(self.tcx, substs));
+                    }
+                    ty::Closure(_, substs) => {
+                        let substs = substs.as_closure();
+                        let Some(f_ty) = substs.upvar_tys().nth(f.as_usize()) else {
+                            fail_out_of_bounds(self, location);
+                            return;
+                        };
+                        check_equal(self, location, f_ty);
+                    }
+                    ty::Generator(_, substs, _) => {
+                        let substs = substs.as_generator();
+                        let Some(f_ty) = substs.upvar_tys().nth(f.as_usize()) else {
+                            fail_out_of_bounds(self, location);
+                            return;
+                        };
+                        check_equal(self, location, f_ty);
+                    }
+                    _ => {
+                        self.fail(location, format!("{:?} does not have fields", parent_ty.ty));
+                    }
                 }
             }
+            _ => {}
         }
         self.super_projection_elem(local, proj_base, elem, context, location);
     }

compiler/rustc_index/src/bit_set.rs

@@ -340,6 +340,12 @@ impl<T: Idx> BitRelations<BitSet<T>> for BitSet<T> {
     }
 }
 
+impl<T: Idx> From<GrowableBitSet<T>> for BitSet<T> {
+    fn from(bit_set: GrowableBitSet<T>) -> Self {
+        bit_set.bit_set
+    }
+}
+
 /// A fixed-size bitset type with a partially dense, partially sparse
 /// representation. The bitset is broken into chunks, and chunks that are all
 /// zeros or all ones are represented and handled very efficiently.
@@ -1542,6 +1548,12 @@ impl<T: Idx> GrowableBitSet<T> {
     }
 }
 
+impl<T: Idx> From<BitSet<T>> for GrowableBitSet<T> {
+    fn from(bit_set: BitSet<T>) -> Self {
+        Self { bit_set }
+    }
+}
+
 /// A fixed-size 2D bit matrix type with a dense representation.
 ///
 /// `R` and `C` are index types used to identify rows and columns respectively;

compiler/rustc_middle/src/mir/mod.rs

@@ -195,6 +195,7 @@ pub enum MirPhase {
     /// Beginning with this phase, the following variants are disallowed:
     /// * [`TerminatorKind::Yield`](terminator::TerminatorKind::Yield)
     /// * [`TerminatorKind::GeneratorDrop`](terminator::TerminatorKind::GeneratorDrop)
+    /// * [`ProjectionElem::Deref`] of `Box`
     GeneratorsLowered = 6,
     Optimized = 7,
 }

compiler/rustc_mir_dataflow/src/elaborate_drops.rs

@@ -410,7 +410,18 @@ where
     fn open_drop_for_box(&mut self, adt: ty::AdtDef<'tcx>, substs: SubstsRef<'tcx>) -> BasicBlock {
         debug!("open_drop_for_box({:?}, {:?}, {:?})", self, adt, substs);
 
-        let interior = self.tcx().mk_place_deref(self.place);
+        // drop glue is sent straight to codegen
+        // box cannot be directly dereferenced
+        let unique_ty = adt.non_enum_variant().fields[0].ty(self.tcx(), substs);
+        let nonnull_ty =
+            unique_ty.ty_adt_def().unwrap().non_enum_variant().fields[0].ty(self.tcx(), substs);
+        let ptr_ty = self.tcx().mk_imm_ptr(substs[0].expect_ty());
+
+        let unique_place = self.tcx().mk_place_field(self.place, Field::new(0), unique_ty);
+        let nonnull_place = self.tcx().mk_place_field(unique_place, Field::new(0), nonnull_ty);
+        let ptr_place = self.tcx().mk_place_field(nonnull_place, Field::new(0), ptr_ty);
+        let interior = self.tcx().mk_place_deref(ptr_place);
+
         let interior_path = self.elaborator.deref_subpath(self.path);
 
         let succ = self.box_free_block(adt, substs, self.succ, self.unwind);