interpret: read_discriminant: only return VariantIdx

compiler/rustc_const_eval/src/const_eval/mod.rs

@@ -101,7 +101,7 @@ pub(crate) fn try_destructure_mir_constant_for_diagnostics<'tcx>(
             return None;
         }
         ty::Adt(def, _) => {
-            let variant = ecx.read_discriminant(&op).ok()?.1;
+            let variant = ecx.read_discriminant(&op).ok()?;
             let down = ecx.project_downcast(&op, variant).ok()?;
             (def.variants()[variant].fields.len(), Some(variant), down)
         }

compiler/rustc_const_eval/src/const_eval/valtrees.rs

@@ -130,7 +130,7 @@ pub(crate) fn const_to_valtree_inner<'tcx>(
                 bug!("uninhabited types should have errored and never gotten converted to valtree")
             }
 
-            let Ok((_, variant)) = ecx.read_discriminant(&place.into()) else {
+            let Ok(variant) = ecx.read_discriminant(&place.into()) else {
                 return Err(ValTreeCreationError::Other);
             };
             branches(ecx, place, def.variant(variant).fields.len(), def.is_enum().then_some(variant), num_nodes)

compiler/rustc_const_eval/src/interpret/discriminant.rs

@@ -1,6 +1,6 @@
 //! Functions for reading and writing discriminants of multi-variant layouts (enums and generators).
 
-use rustc_middle::ty::layout::{LayoutOf, PrimitiveExt};
+use rustc_middle::ty::layout::{LayoutOf, PrimitiveExt, TyAndLayout};
 use rustc_middle::{mir, ty};
 use rustc_target::abi::{self, TagEncoding};
 use rustc_target::abi::{VariantIdx, Variants};
@@ -93,7 +93,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
     pub fn read_discriminant(
         &self,
         op: &OpTy<'tcx, M::Provenance>,
-    ) -> InterpResult<'tcx, (Scalar<M::Provenance>, VariantIdx)> {
+    ) -> InterpResult<'tcx, VariantIdx> {
         trace!("read_discriminant_value {:#?}", op.layout);
         // Get type and layout of the discriminant.
         let discr_layout = self.layout_of(op.layout.ty.discriminant_ty(*self.tcx))?;
@@ -106,30 +106,22 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         // straight-forward (`TagEncoding::Direct`) or with a niche (`TagEncoding::Niche`).
         let (tag_scalar_layout, tag_encoding, tag_field) = match op.layout.variants {
             Variants::Single { index } => {
+                // Do some extra checks on enums.
+                if op.layout.ty.is_enum() {
                     // Hilariously, `Single` is used even for 0-variant enums.
                     // (See https://github.com/rust-lang/rust/issues/89765).
-                if matches!(op.layout.ty.kind(), ty::Adt(def, ..) if def.variants().is_empty()) {
+                    if matches!(op.layout.ty.kind(), ty::Adt(def, ..) if def.variants().is_empty())
+                    {
                         throw_ub!(UninhabitedEnumVariantRead(index))
                     }
-                let discr = match op.layout.ty.discriminant_for_variant(*self.tcx, index) {
-                    Some(discr) => {
-                        // This type actually has discriminants.
-                        assert_eq!(discr.ty, discr_layout.ty);
-                        Scalar::from_uint(discr.val, discr_layout.size)
-                    }
-                    None => {
-                        // On a type without actual discriminants, variant is 0.
-                        assert_eq!(index.as_u32(), 0);
-                        Scalar::from_uint(index.as_u32(), discr_layout.size)
-                    }
-                };
                     // For consisteny with `write_discriminant`, and to make sure that
                     // `project_downcast` cannot fail due to strange layouts, we declare immediate UB
                     // for uninhabited variants.
-                if op.layout.ty.is_enum() && op.layout.for_variant(self, index).abi.is_uninhabited() {
+                    if op.layout.for_variant(self, index).abi.is_uninhabited() {
                         throw_ub!(UninhabitedEnumVariantRead(index))
                     }
-                return Ok((discr, index));
+                }
+                return Ok(index);
             }
             Variants::Multiple { tag, ref tag_encoding, tag_field, .. } => {
                 (tag, tag_encoding, tag_field)
@@ -155,7 +147,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         trace!("tag value: {}", tag_val);
 
         // Figure out which discriminant and variant this corresponds to.
-        let (discr, index) = match *tag_encoding {
+        let index = match *tag_encoding {
             TagEncoding::Direct => {
                 let scalar = tag_val.to_scalar();
                 // Generate a specific error if `tag_val` is not an integer.
@@ -183,7 +175,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 }
                 .ok_or_else(|| err_ub!(InvalidTag(Scalar::from_uint(tag_bits, tag_layout.size))))?;
                 // Return the cast value, and the index.
-                (discr_val, index.0)
+                index.0
             }
             TagEncoding::Niche { untagged_variant, ref niche_variants, niche_start } => {
                 let tag_val = tag_val.to_scalar();
@@ -241,13 +233,33 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 // Compute the size of the scalar we need to return.
                 // No need to cast, because the variant index directly serves as discriminant and is
                 // encoded in the tag.
-                (Scalar::from_uint(variant.as_u32(), discr_layout.size), variant)
+                variant
             }
         };
         // For consisteny with `write_discriminant`, and to make sure that `project_downcast` cannot fail due to strange layouts, we declare immediate UB for uninhabited variants.
         if op.layout.for_variant(self, index).abi.is_uninhabited() {
             throw_ub!(UninhabitedEnumVariantRead(index))
         }
-        Ok((discr, index))
+        Ok(index)
+    }
+
+    pub fn discriminant_for_variant(
+        &self,
+        layout: TyAndLayout<'tcx>,
+        variant: VariantIdx,
+    ) -> InterpResult<'tcx, Scalar<M::Provenance>> {
+        let discr_layout = self.layout_of(layout.ty.discriminant_ty(*self.tcx))?;
+        Ok(match layout.ty.discriminant_for_variant(*self.tcx, variant) {
+            Some(discr) => {
+                // This type actually has discriminants.
+                assert_eq!(discr.ty, discr_layout.ty);
+                Scalar::from_uint(discr.val, discr_layout.size)
+            }
+            None => {
+                // On a type without actual discriminants, variant is 0.
+                assert_eq!(variant.as_u32(), 0);
+                Scalar::from_uint(variant.as_u32(), discr_layout.size)
+            }
+        })
     }
 }

compiler/rustc_const_eval/src/interpret/intrinsics.rs

@@ -226,8 +226,9 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             }
             sym::discriminant_value => {
                 let place = self.deref_operand(&args[0])?;
-                let discr_val = self.read_discriminant(&place.into())?.0;
+                let variant = self.read_discriminant(&place.into())?;
-                self.write_scalar(discr_val, dest)?;
+                let discr = self.discriminant_for_variant(place.layout, variant)?;
+                self.write_scalar(discr, dest)?;
             }
             sym::exact_div => {
                 let l = self.read_immediate(&args[0])?;

compiler/rustc_const_eval/src/interpret/step.rs

@@ -302,8 +302,9 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
 
             Discriminant(place) => {
                 let op = self.eval_place_to_op(place, None)?;
-                let discr_val = self.read_discriminant(&op)?.0;
+                let variant = self.read_discriminant(&op)?;
-                self.write_scalar(discr_val, &dest)?;
+                let discr = self.discriminant_for_variant(op.layout, variant)?;
+                self.write_scalar(discr, &dest)?;
             }
         }
 

compiler/rustc_const_eval/src/interpret/validity.rs

@@ -657,7 +657,7 @@ impl<'rt, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> ValueVisitor<'mir, 'tcx, M>
     ) -> InterpResult<'tcx, VariantIdx> {
         self.with_elem(PathElem::EnumTag, move |this| {
             Ok(try_validation!(
-                this.ecx.read_discriminant(op).map(|(_, idx)| idx),
+                this.ecx.read_discriminant(op),
                 this.path,
                 InvalidTag(val) => InvalidEnumTag {
                     value: format!("{val:x}"),

compiler/rustc_const_eval/src/interpret/visitor.rs

@@ -23,7 +23,7 @@ pub trait ValueVisitor<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>>: Sized {
     /// `read_discriminant` can be hooked for better error messages.
     #[inline(always)]
     fn read_discriminant(&mut self, v: &Self::V) -> InterpResult<'tcx, VariantIdx> {
-        Ok(self.ecx().read_discriminant(&v.to_op(self.ecx())?)?.1)
+        Ok(self.ecx().read_discriminant(&v.to_op(self.ecx())?)?)
     }
 
     /// This function provides the chance to reorder the order in which fields are visited for