Auto merge of #147687 - cjgillot:noshallow-init-box, r=nnethercote

Forbid ShallowInitBox after box deref elaboration.

MIR currently contains a `ShallowInitBox` rvalue. Its principal usage is to allow for in-place initialization of boxes. Having it is necessary for drop elaboration to be correct with that in-place initialization.

As part of analysis->runtime MIR lowering, we canonicalize deref of boxes to use the stored raw pointer. But we did not perform the same change to the construction of the box.

This PR replaces `ShallowInitBox` by the pointer manipulation it represents.

Alternatives:
- fully remove `ShallowInitBox` and implement `Box` in-place initialization differently;
- remove the `ElaborateBoxDeref` pass and keep dereferencing `Box` in runtime MIR.

Author: bors

src/base.rs

@@ -829,13 +829,6 @@ fn codegen_stmt<'tcx>(fx: &mut FunctionCx<'_, '_, 'tcx>, cur_block: Block, stmt:
                         fx.bcx.ins().nop();
                     }
                 }
-                Rvalue::ShallowInitBox(ref operand, content_ty) => {
-                    let content_ty = fx.monomorphize(content_ty);
-                    let box_layout = fx.layout_of(Ty::new_box(fx.tcx, content_ty));
-                    let operand = codegen_operand(fx, operand);
-                    let operand = operand.load_scalar(fx);
-                    lval.write_cvalue(fx, CValue::by_val(operand, box_layout));
-                }
                 Rvalue::NullaryOp(ref null_op, ty) => {
                     assert!(lval.layout().ty.is_sized(fx.tcx, fx.typing_env()));
                     let layout = fx.layout_of(fx.monomorphize(ty));
@@ -924,6 +917,7 @@ fn codegen_stmt<'tcx>(fx: &mut FunctionCx<'_, '_, 'tcx>, cur_block: Block, stmt:
                     lval.write_cvalue_transmute(fx, operand);
                 }
                 Rvalue::CopyForDeref(_) => bug!("`CopyForDeref` in codegen"),
+                Rvalue::ShallowInitBox(..) => bug!("`ShallowInitBox` in codegen"),
             }
         }
         StatementKind::StorageLive(_)