Adjust computation of place types to detect more invalid places

compiler/rustc_const_eval/src/transform/validate.rs

@@ -3,8 +3,8 @@
 use rustc_index::bit_set::BitSet;
 use rustc_infer::infer::TyCtxtInferExt;
 use rustc_middle::mir::interpret::Scalar;
-use rustc_middle::mir::traversal;
 use rustc_middle::mir::visit::{PlaceContext, Visitor};
+use rustc_middle::mir::{traversal, Place};
 use rustc_middle::mir::{
     AggregateKind, BasicBlock, Body, BorrowKind, Local, Location, MirPass, MirPhase, Operand,
     PlaceElem, PlaceRef, ProjectionElem, Rvalue, SourceScope, Statement, StatementKind, Terminator,
@@ -240,6 +240,14 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
         self.super_projection_elem(local, proj_base, elem, context, location);
     }
 
+    fn visit_place(&mut self, place: &Place<'tcx>, _: PlaceContext, location: Location) {
+        // Set off any `bug!`s in the type computation code
+        let ty = place.ty(&self.body.local_decls, self.tcx);
+        if ty.variant_index.is_some() {
+            self.fail(location, "Top level places may not have their variant index set!");
+        }
+    }
+
     fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) {
         match &statement.kind {
             StatementKind::Assign(box (dest, rvalue)) => {

compiler/rustc_middle/src/mir/tcx.rs

@@ -76,6 +76,9 @@ impl<'tcx> PlaceTy<'tcx> {
         V: ::std::fmt::Debug,
         T: ::std::fmt::Debug + Copy,
     {
+        if self.variant_index.is_some() && !matches!(elem, ProjectionElem::Field(..)) {
+            bug!("cannot use non field projection on downcasted place")
+        }
         let answer = match *elem {
             ProjectionElem::Deref => {
                 let ty = self