From ad7179d2a409faaf45465862f44efe7f989cd71e Mon Sep 17 00:00:00 2001
From: Ralf Jung <post@ralfj.de>
Date: Sat, 23 May 2020 17:24:33 +0200
Subject: [PATCH] fix discriminant_ty for non-enums

---
 src/librustc_middle/mir/tcx.rs        |  4 +---
 src/librustc_middle/ty/mod.rs         |  5 +++++
 src/librustc_middle/ty/sty.rs         | 13 +++++++++----
 src/librustc_mir/interpret/operand.rs |  4 ++--
 4 files changed, 17 insertions(+), 9 deletions(-)

diff --git a/src/librustc_middle/mir/tcx.rs b/src/librustc_middle/mir/tcx.rs
index 174da7db1753d..3e172bd8d2a76 100644
--- a/src/librustc_middle/mir/tcx.rs
+++ b/src/librustc_middle/mir/tcx.rs
@@ -173,9 +173,7 @@ impl<'tcx> Rvalue<'tcx> {
                 tcx.intern_tup(&[ty, tcx.types.bool])
             }
             Rvalue::UnaryOp(UnOp::Not | UnOp::Neg, ref operand) => operand.ty(local_decls, tcx),
-            Rvalue::Discriminant(ref place) => {
-                place.ty(local_decls, tcx).ty.discriminant_type(tcx)
-            }
+            Rvalue::Discriminant(ref place) => place.ty(local_decls, tcx).ty.discriminant_ty(tcx),
             Rvalue::NullaryOp(NullOp::Box, t) => tcx.mk_box(t),
             Rvalue::NullaryOp(NullOp::SizeOf, _) => tcx.types.usize,
             Rvalue::Aggregate(ref ak, ref ops) => match **ak {
diff --git a/src/librustc_middle/ty/mod.rs b/src/librustc_middle/ty/mod.rs
index aad3c6889c3ce..4d050a7b48eac 100644
--- a/src/librustc_middle/ty/mod.rs
+++ b/src/librustc_middle/ty/mod.rs
@@ -2040,6 +2040,8 @@ impl ReprOptions {
         self.flags.contains(ReprFlags::HIDE_NICHE)
     }
 
+    /// Returns the discriminant type, given these `repr` options.
+    /// This must only be called on enums!
     pub fn discr_type(&self) -> attr::IntType {
         self.int.unwrap_or(attr::SignedInt(ast::IntTy::Isize))
     }
@@ -2272,6 +2274,7 @@ impl<'tcx> AdtDef {
 
     #[inline]
     pub fn eval_explicit_discr(&self, tcx: TyCtxt<'tcx>, expr_did: DefId) -> Option<Discr<'tcx>> {
+        assert!(self.is_enum());
         let param_env = tcx.param_env(expr_did);
         let repr_type = self.repr.discr_type();
         match tcx.const_eval_poly(expr_did) {
@@ -2308,6 +2311,7 @@ impl<'tcx> AdtDef {
         &'tcx self,
         tcx: TyCtxt<'tcx>,
     ) -> impl Iterator<Item = (VariantIdx, Discr<'tcx>)> + Captures<'tcx> {
+        assert!(self.is_enum());
         let repr_type = self.repr.discr_type();
         let initial = repr_type.initial_discriminant(tcx);
         let mut prev_discr = None::<Discr<'tcx>>;
@@ -2340,6 +2344,7 @@ impl<'tcx> AdtDef {
         tcx: TyCtxt<'tcx>,
         variant_index: VariantIdx,
     ) -> Discr<'tcx> {
+        assert!(self.is_enum());
         let (val, offset) = self.discriminant_def_for_variant(variant_index);
         let explicit_value = val
             .and_then(|expr_did| self.eval_explicit_discr(tcx, expr_did))
diff --git a/src/librustc_middle/ty/sty.rs b/src/librustc_middle/ty/sty.rs
index 6cee224219b63..aacca710dd037 100644
--- a/src/librustc_middle/ty/sty.rs
+++ b/src/librustc_middle/ty/sty.rs
@@ -2097,7 +2097,9 @@ impl<'tcx> TyS<'tcx> {
         variant_index: VariantIdx,
     ) -> Option<Discr<'tcx>> {
         match self.kind {
-            TyKind::Adt(adt, _) => Some(adt.discriminant_for_variant(tcx, variant_index)),
+            TyKind::Adt(adt, _) if adt.is_enum() => {
+                Some(adt.discriminant_for_variant(tcx, variant_index))
+            }
             TyKind::Generator(def_id, substs, _) => {
                 Some(substs.as_generator().discriminant_for_variant(def_id, tcx, variant_index))
             }
@@ -2106,11 +2108,14 @@ impl<'tcx> TyS<'tcx> {
     }
 
     /// Returns the type of the discriminant of this type.
-    pub fn discriminant_type(&self, tcx: TyCtxt<'tcx>) -> Ty<'tcx> {
+    pub fn discriminant_ty(&self, tcx: TyCtxt<'tcx>) -> Ty<'tcx> {
         match self.kind {
-            ty::Adt(adt_def, _) => adt_def.repr.discr_type().to_ty(tcx),
+            ty::Adt(adt, _) if adt.is_enum() => adt.repr.discr_type().to_ty(tcx),
             ty::Generator(_, substs, _) => substs.as_generator().discr_ty(tcx),
-            _ => bug!("{:?} does not have a discriminant", self),
+            _ => {
+                // This can only be `0`, for now, so `u8` will suffice.
+                tcx.types.u8
+            }
         }
     }
 
diff --git a/src/librustc_mir/interpret/operand.rs b/src/librustc_mir/interpret/operand.rs
index 139871310fbf3..f546f6236d777 100644
--- a/src/librustc_mir/interpret/operand.rs
+++ b/src/librustc_mir/interpret/operand.rs
@@ -581,10 +581,10 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         trace!("read_discriminant_value {:#?}", op.layout);
 
         // Get type and layout of the discriminant.
-        let discr_layout = self.layout_of(op.layout.ty.discriminant_type(*self.tcx))?;
+        let discr_layout = self.layout_of(op.layout.ty.discriminant_ty(*self.tcx))?;
         trace!("discriminant type: {:?}", discr_layout.ty);
 
-        // We use "discriminant" to refer to the value associated with a particualr enum variant.
+        // We use "discriminant" to refer to the value associated with a particular enum variant.
         // This is not to be confused with its "variant index", which is just determining its position in the
         // declared list of variants -- they can differ with explicitly assigned discriminants.
         // We use "tag" to refer to how the discriminant is encoded in memory, which can be either