Work around for rust-lang#64506

Author: wesleywiser

src/librustc/mir/interpret/error.rs

@@ -397,6 +397,10 @@ pub enum UnsupportedOpInfo<'tcx> {
     /// while evaluating `const` items.
     ReadOfStaticInConst,
 
+    /// FIXME(#64506) Error used to work around accessing projections of
+    /// uninhabited types.
+    UninhabitedValue,
+
     // -- Everything below is not categorized yet --
     FunctionAbiMismatch(Abi, Abi),
     FunctionArgMismatch(Ty<'tcx>, Ty<'tcx>),
@@ -564,6 +568,8 @@ impl fmt::Debug for UnsupportedOpInfo<'tcx> {
                 write!(f, "{}", msg),
             ReadOfStaticInConst =>
                 write!(f, "tried to read from a static during const evaluation"),
+            UninhabitedValue =>
+                write!(f, "tried to use an uninhabited value"),
         }
     }
 }

src/librustc_mir/interpret/place.rs

@@ -9,7 +9,7 @@ use rustc::mir;
 use rustc::mir::interpret::truncate;
 use rustc::ty::{self, Ty};
 use rustc::ty::layout::{
-    self, Size, Align, LayoutOf, TyLayout, HasDataLayout, VariantIdx, PrimitiveExt
+    self, Size, Abi, Align, LayoutOf, TyLayout, HasDataLayout, VariantIdx, PrimitiveExt
 };
 use rustc::ty::TypeFoldable;
 
@@ -385,6 +385,10 @@ where
                 stride * field
             }
             layout::FieldPlacement::Union(count) => {
+                // FIXME(#64506) `UninhabitedValue` can be removed when this issue is resolved
+                if base.layout.abi == Abi::Uninhabited {
+                    throw_unsup!(UninhabitedValue);
+                }
                 assert!(field < count as u64,
                         "Tried to access field {} of union with {} fields", field, count);
                 // Offset is always 0