Remove ShallowInitBox in runtime MIR.

Author: cjgillot

compiler/rustc_codegen_ssa/src/mir/rvalue.rs

@@ -724,22 +724,16 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                     }
                 }
             }
-            mir::Rvalue::ShallowInitBox(ref operand, content_ty) => {
-                let operand = self.codegen_operand(bx, operand);
-                let val = operand.immediate();
-
-                let content_ty = self.monomorphize(content_ty);
-                let box_layout = bx.cx().layout_of(Ty::new_box(bx.tcx(), content_ty));
-
-                OperandRef { val: OperandValue::Immediate(val), layout: box_layout }
-            }
             mir::Rvalue::WrapUnsafeBinder(ref operand, binder_ty) => {
                 let operand = self.codegen_operand(bx, operand);
                 let binder_ty = self.monomorphize(binder_ty);
                 let layout = bx.cx().layout_of(binder_ty);
                 OperandRef { val: operand.val, layout }
             }
             mir::Rvalue::CopyForDeref(_) => bug!("`CopyForDeref` in codegen"),
+            mir::Rvalue::ShallowInitBox(..) => {
+                bug!("ShallowInitBox must not appear in runtime MIR")
+            }
         }
     }
 

compiler/rustc_const_eval/src/interpret/step.rs

@@ -255,12 +255,6 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
                 self.write_immediate(*val, &dest)?;
             }
 
-            ShallowInitBox(ref operand, _) => {
-                let src = self.eval_operand(operand, None)?;
-                let v = self.read_immediate(&src)?;
-                self.write_immediate(*v, &dest)?;
-            }
-
             Cast(cast_kind, ref operand, cast_ty) => {
                 let src = self.eval_operand(operand, None)?;
                 let cast_ty =
@@ -281,6 +275,8 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
                 let op = self.eval_operand(op, None)?;
                 self.copy_op_allow_transmute(&op, &dest)?;
             }
+
+            ShallowInitBox(..) => bug!("ShallowInitBox must not appear in runtime MIR"),
         }
 
         trace!("{:?}", self.dump_place(&dest));

compiler/rustc_middle/src/mir/syntax.rs

@@ -129,8 +129,8 @@ pub enum RuntimePhase {
     /// disallowed:
     /// * [`TerminatorKind::Yield`]
     /// * [`TerminatorKind::CoroutineDrop`]
-    /// * [`Rvalue::Aggregate`] for any `AggregateKind` except `Array`
     /// * [`Rvalue::CopyForDeref`]
+    /// * [`Rvalue::ShallowInitBox`]
     /// * [`PlaceElem::OpaqueCast`]
     /// * [`LocalInfo::DerefTemp`](super::LocalInfo::DerefTemp)
     ///

compiler/rustc_mir_transform/src/dataflow_const_prop.rs

@@ -489,11 +489,11 @@ impl<'a, 'tcx> ConstAnalysis<'a, 'tcx> {
             | Rvalue::Cast(..)
             | Rvalue::BinaryOp(..)
             | Rvalue::Aggregate(..)
-            | Rvalue::ShallowInitBox(..)
             | Rvalue::WrapUnsafeBinder(..) => {
                 // No modification is possible through these r-values.
                 return ValueOrPlace::TOP;
             }
+            Rvalue::ShallowInitBox(..) => bug!("ShallowInitBox must not appear in runtime MIR"),
         };
         ValueOrPlace::Value(val)
     }

compiler/rustc_mir_transform/src/elaborate_box_derefs.rs

@@ -86,6 +86,20 @@ impl<'a, 'tcx> MutVisitor<'tcx> for ElaborateBoxDerefVisitor<'a, 'tcx> {
 
         self.super_place(place, context, location);
     }
+
+    fn visit_rvalue(&mut self, rvalue: &mut Rvalue<'tcx>, location: Location) {
+        if let Rvalue::ShallowInitBox(operand, ty) = rvalue {
+            let ty = Ty::new_box(self.tcx, *ty);
+            let operand = std::mem::replace(
+                operand,
+                // Just a meaningless placeholder.
+                Operand::Copy(RETURN_PLACE.into()),
+            );
+            *rvalue = Rvalue::Cast(CastKind::Transmute, operand, ty)
+        }
+
+        self.super_rvalue(rvalue, location);
+    }
 }
 
 pub(super) struct ElaborateBoxDerefs;

compiler/rustc_mir_transform/src/gvn.rs

@@ -1073,8 +1073,9 @@ impl<'body, 'a, 'tcx> VnState<'body, 'a, 'tcx> {
             }
 
             // Unsupported values.
-            Rvalue::ThreadLocalRef(..) | Rvalue::ShallowInitBox(..) => return None,
+            Rvalue::ThreadLocalRef(..) => return None,
             Rvalue::CopyForDeref(_) => bug!("`CopyForDeref` in runtime MIR"),
+            Rvalue::ShallowInitBox(..) => bug!("ShallowInitBox must not appear in runtime MIR"),
         };
         let ty = rvalue.ty(self.local_decls, self.tcx);
         Some(self.insert(ty, value))

compiler/rustc_mir_transform/src/validate.rs

@@ -1173,6 +1173,13 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                 }
             }
             Rvalue::ShallowInitBox(operand, _) => {
+                if self.body.phase >= MirPhase::Runtime(RuntimePhase::Initial) {
+                    self.fail(
+                        location,
+                        "`ShallowInitBox` should have been removed after box elaboration phase",
+                    );
+                }
+
                 let a = operand.ty(&self.body.local_decls, self.tcx);
                 check_kinds!(a, "Cannot shallow init type {:?}", ty::RawPtr(..));
             }

tests/mir-opt/box_expr.rs

@@ -7,7 +7,7 @@
 fn move_from_inner() {
     // CHECK-LABEL: fn move_from_inner(
     // CHECK:   debug x => [[x:_.*]];
-    // CHECK:   [[box:_.*]] = ShallowInitBox(
+    // CHECK:   [[box:_.*]] = move {{_.*}} as std::boxed::Box<S> (Transmute);
     // CHECK:   [[ptr:_.*]] = copy (([[box]].0: std::ptr::Unique<S>).0: std::ptr::NonNull<S>) as *const S (Transmute);
     // CHECK:   (*[[ptr]]) = S::new() -> [return: [[ret:bb.*]], unwind: [[unwind:bb.*]]];
     // CHECK: [[ret]]: {
@@ -34,7 +34,7 @@ fn move_from_inner() {
 // EMIT_MIR box_expr.main.ElaborateDrops.diff
 fn main() {
     // CHECK-LABEL: fn main(
-    // CHECK:   [[box:_.*]] = ShallowInitBox(
+    // CHECK:   [[box:_.*]] = move {{_.*}} as std::boxed::Box<S> (Transmute);
     // CHECK:   [[ptr:_.*]] = copy (([[box]].0: std::ptr::Unique<S>).0: std::ptr::NonNull<S>) as *const S (Transmute);
     // CHECK:   (*[[ptr]]) = S::new() -> [return: [[ret:bb.*]], unwind: [[unwind:bb.*]]];
     // CHECK: [[ret]]: {

tests/mir-opt/const_prop/boxes.main.GVN.panic-abort.diff

@@ -31,7 +31,7 @@
 
       bb1: {
           StorageLive(_7);
-          _7 = ShallowInitBox(move _6, i32);
+          _7 = move _6 as std::boxed::Box<i32> (Transmute);
           _8 = copy ((_7.0: std::ptr::Unique<i32>).0: std::ptr::NonNull<i32>) as *const i32 (Transmute);
           (*_8) = const 42_i32;
           _3 = move _7;

tests/mir-opt/const_prop/boxes.main.GVN.panic-unwind.diff

@@ -31,7 +31,7 @@
 
       bb1: {
           StorageLive(_7);
-          _7 = ShallowInitBox(move _6, i32);
+          _7 = move _6 as std::boxed::Box<i32> (Transmute);
           _8 = copy ((_7.0: std::ptr::Unique<i32>).0: std::ptr::NonNull<i32>) as *const i32 (Transmute);
           (*_8) = const 42_i32;
           _3 = move _7;