Review comments.

Author: cjgillot

compiler/rustc_mir_transform/src/elaborate_box_derefs.rs

@@ -1,6 +1,9 @@
 //! This pass transforms derefs of Box into a deref of the pointer inside Box.
 //!
 //! Box is not actually a pointer so it is incorrect to dereference it directly.
+//!
+//! `ShallowInitBox` being a device for drop elaboration to understand deferred assignment to box
+//! contents, we do not need this any more on runtime MIR.
 
 use rustc_abi::{FieldIdx, VariantIdx};
 use rustc_index::{IndexVec, indexvec};

compiler/rustc_mir_transform/src/validate.rs

@@ -1177,11 +1177,12 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                 }
             }
             Rvalue::ShallowInitBox(operand, _) => {
-                let a = operand.ty(&self.body.local_decls, self.tcx);
-                check_kinds!(a, "Cannot shallow init type {:?}", ty::RawPtr(..));
                 if self.body.phase >= MirPhase::Runtime(RuntimePhase::Initial) {
                     self.fail(location, format!("ShallowInitBox after ElaborateBoxDerefs"))
                 }
+
+                let a = operand.ty(&self.body.local_decls, self.tcx);
+                check_kinds!(a, "Cannot shallow init type {:?}", ty::RawPtr(..));
             }
             Rvalue::Cast(kind, operand, target_type) => {
                 let op_ty = operand.ty(self.body, self.tcx);