Handle discriminants in dataflow-const-prop.

compiler/rustc_mir_dataflow/src/lib.rs

@@ -1,6 +1,7 @@
 #![feature(associated_type_defaults)]
 #![feature(box_patterns)]
 #![feature(exact_size_is_empty)]
+#![feature(let_chains)]
 #![feature(min_specialization)]
 #![feature(once_cell)]
 #![feature(stmt_expr_attributes)]

compiler/rustc_mir_dataflow/src/value_analysis.rs

@@ -65,10 +65,8 @@ pub trait ValueAnalysis<'tcx> {
             StatementKind::Assign(box (place, rvalue)) => {
                 self.handle_assign(*place, rvalue, state);
             }
-            StatementKind::SetDiscriminant { .. } => {
-                // Could treat this as writing a constant to a pseudo-place.
-                // But discriminants are currently not tracked, so we do nothing.
-                // Related: https://github.com/rust-lang/unsafe-code-guidelines/issues/84
+            StatementKind::SetDiscriminant { box ref place, .. } => {
+                state.flood_discr(place.as_ref(), self.map());
             }
             StatementKind::Intrinsic(box intrinsic) => {
                 self.handle_intrinsic(intrinsic, state);
@@ -447,26 +445,29 @@ impl<V: Clone + HasTop + HasBottom> State<V> {
     }
 
     pub fn flood_with(&mut self, place: PlaceRef<'_>, map: &Map, value: V) {
-        if let Some(root) = map.find(place) {
-            self.flood_idx_with(root, map, value);
-        }
-    }
-
-    pub fn flood(&mut self, place: PlaceRef<'_>, map: &Map) {
-        self.flood_with(place, map, V::top())
-    }
-
-    pub fn flood_idx_with(&mut self, place: PlaceIndex, map: &Map, value: V) {
         let StateData::Reachable(values) = &mut self.0 else { return };
-        map.preorder_invoke(place, &mut |place| {
+        map.for_each_aliasing_place(place, None, &mut |place| {
             if let Some(vi) = map.places[place].value_index {
                 values[vi] = value.clone();
             }
         });
     }
 
-    pub fn flood_idx(&mut self, place: PlaceIndex, map: &Map) {
-        self.flood_idx_with(place, map, V::top())
+    pub fn flood(&mut self, place: PlaceRef<'_>, map: &Map) {
+        self.flood_with(place, map, V::top())
+    }
+
+    pub fn flood_discr_with(&mut self, place: PlaceRef<'_>, map: &Map, value: V) {
+        let StateData::Reachable(values) = &mut self.0 else { return };
+        map.for_each_aliasing_place(place, Some(TrackElem::Discriminant), &mut |place| {
+            if let Some(vi) = map.places[place].value_index {
+                values[vi] = value.clone();
+            }
+        });
+    }
+
+    pub fn flood_discr(&mut self, place: PlaceRef<'_>, map: &Map) {
+        self.flood_discr_with(place, map, V::top())
     }
 
     /// Copies `source` to `target`, including all tracked places beneath.
@@ -474,7 +475,9 @@ impl<V: Clone + HasTop + HasBottom> State<V> {
     /// If `target` contains a place that is not contained in `source`, it will be overwritten with
     /// Top. Also, because this will copy all entries one after another, it may only be used for
     /// places that are non-overlapping or identical.
-    pub fn assign_place_idx(&mut self, target: PlaceIndex, source: PlaceIndex, map: &Map) {
+    ///
+    /// The target place must have been flooded before calling this method.
+    fn assign_place_idx(&mut self, target: PlaceIndex, source: PlaceIndex, map: &Map) {
         let StateData::Reachable(values) = &mut self.0 else { return };
 
         // If both places are tracked, we copy the value to the target. If the target is tracked,
@@ -492,26 +495,28 @@ impl<V: Clone + HasTop + HasBottom> State<V> {
             let projection = map.places[target_child].proj_elem.unwrap();
             if let Some(source_child) = map.projections.get(&(source, projection)) {
                 self.assign_place_idx(target_child, *source_child, map);
-            } else {
-                self.flood_idx(target_child, map);
             }
         }
     }
 
     pub fn assign(&mut self, target: PlaceRef<'_>, result: ValueOrPlace<V>, map: &Map) {
+        self.flood(target, map);
         if let Some(target) = map.find(target) {
             self.assign_idx(target, result, map);
-        } else {
-            // We don't track this place nor any projections, assignment can be ignored.
         }
     }
 
+    pub fn assign_discr(&mut self, target: PlaceRef<'_>, result: ValueOrPlace<V>, map: &Map) {
+        self.flood_discr(target, map);
+        if let Some(target) = map.find_discr(target) {
+            self.assign_idx(target, result, map);
+        }
+    }
+
+    /// The target place must have been flooded before calling this method.
     pub fn assign_idx(&mut self, target: PlaceIndex, result: ValueOrPlace<V>, map: &Map) {
         match result {
             ValueOrPlace::Value(value) => {
-                // First flood the target place in case we also track any projections (although
-                // this scenario is currently not well-supported by the API).
-                self.flood_idx(target, map);
                 let StateData::Reachable(values) = &mut self.0 else { return };
                 if let Some(value_index) = map.places[target].value_index {
                     values[value_index] = value;
@@ -526,6 +531,14 @@ impl<V: Clone + HasTop + HasBottom> State<V> {
         map.find(place).map(|place| self.get_idx(place, map)).unwrap_or(V::top())
     }
 
+    /// Retrieve the value stored for a place, or ⊤ if it is not tracked.
+    pub fn get_discr(&self, place: PlaceRef<'_>, map: &Map) -> V {
+        match map.find_discr(place) {
+            Some(place) => self.get_idx(place, map),
+            None => V::top(),
+        }
+    }
+
     /// Retrieve the value stored for a place index, or ⊤ if it is not tracked.
     pub fn get_idx(&self, place: PlaceIndex, map: &Map) -> V {
         match &self.0 {
@@ -582,7 +595,6 @@ impl Map {
     /// This is currently the only way to create a [`Map`]. The way in which the tracked places are
     /// chosen is an implementation detail and may not be relied upon (other than that their type
     /// passes the filter).
-    #[instrument(skip_all, level = "debug")]
     pub fn from_filter<'tcx>(
         tcx: TyCtxt<'tcx>,
         body: &Body<'tcx>,
@@ -614,7 +626,7 @@ impl Map {
 
     /// Potentially register the (local, projection) place and its fields, recursively.
     ///
-    /// Invariant: The projection must only contain fields.
+    /// Invariant: The projection must only contain trackable elements.
     fn register_with_filter_rec<'tcx>(
         &mut self,
         tcx: TyCtxt<'tcx>,
@@ -623,21 +635,46 @@ impl Map {
         ty: Ty<'tcx>,
         filter: &mut impl FnMut(Ty<'tcx>) -> bool,
     ) {
-        if filter(ty) {
-            // We know that the projection only contains trackable elements.
-            let place = self.make_place(local, projection).unwrap();
+        // We know that the projection only contains trackable elements.
+        let place = self.make_place(local, projection).unwrap();
 
-            // Allocate a value slot if it doesn't have one.
-            if self.places[place].value_index.is_none() {
-                self.places[place].value_index = Some(self.value_count.into());
-                self.value_count += 1;
+        // Allocate a value slot if it doesn't have one, and the user requested one.
+        if self.places[place].value_index.is_none() && filter(ty) {
+            self.places[place].value_index = Some(self.value_count.into());
+            self.value_count += 1;
+        }
+
+        if ty.is_enum() {
+            let discr_ty = ty.discriminant_ty(tcx);
+            if filter(discr_ty) {
+                let discr = *self
+                    .projections
+                    .entry((place, TrackElem::Discriminant))
+                    .or_insert_with(|| {
+                        // Prepend new child to the linked list.
+                        let next = self.places.push(PlaceInfo::new(Some(TrackElem::Discriminant)));
+                        self.places[next].next_sibling = self.places[place].first_child;
+                        self.places[place].first_child = Some(next);
+                        next
+                    });
+
+                // Allocate a value slot if it doesn't have one.
+                if self.places[discr].value_index.is_none() {
+                    self.places[discr].value_index = Some(self.value_count.into());
+                    self.value_count += 1;
+                }
             }
         }
 
         // Recurse with all fields of this place.
         iter_fields(ty, tcx, |variant, field, ty| {
-            if variant.is_some() {
-                // Downcasts are currently not supported.
+            if let Some(variant) = variant {
+                projection.push(PlaceElem::Downcast(None, variant));
+                let _ = self.make_place(local, projection);
+                projection.push(PlaceElem::Field(field, ty));
+                self.register_with_filter_rec(tcx, local, projection, ty, filter);
+                projection.pop();
+                projection.pop();
                 return;
             }
             projection.push(PlaceElem::Field(field, ty));
@@ -694,13 +731,77 @@ impl Map {
         Some(index)
     }
 
+    /// Locates the given place, if it exists in the tree.
+    pub fn find_discr(&self, place: PlaceRef<'_>) -> Option<PlaceIndex> {
+        let index = self.find(place)?;
+        self.apply(index, TrackElem::Discriminant)
+    }
+
     /// Iterate over all direct children.
     pub fn children(&self, parent: PlaceIndex) -> impl Iterator<Item = PlaceIndex> + '_ {
         Children::new(self, parent)
     }
 
+    /// Invoke a function on the given place and all places that may alias it.
+    ///
+    /// In particular, when the given place has a variant downcast, we invoke the function on all
+    /// the other variants.
+    ///
+    /// `tail_elem` allows to support discriminants that are not a place in MIR, but that we track
+    /// as such.
+    fn for_each_aliasing_place(
+        &self,
+        place: PlaceRef<'_>,
+        tail_elem: Option<TrackElem>,
+        f: &mut impl FnMut(PlaceIndex),
+    ) {
+        let Some(&Some(mut index)) = self.locals.get(place.local) else {
+            // The local is not tracked at all, nothing to invalidate.
+            return;
+        };
+        let elems = place
+            .projection
+            .iter()
+            .map(|&elem| elem.try_into())
+            .chain(tail_elem.map(Ok).into_iter());
+        for elem in elems {
+            let Ok(elem) = elem else { return };
+            let sub = self.apply(index, elem);
+            if let TrackElem::Variant(..) | TrackElem::Discriminant = elem {
+                // Writing to an enum variant field invalidates the other variants and the discriminant.
+                self.for_each_variant_sibling(index, sub, f);
+            }
+            if let Some(sub) = sub {
+                index = sub
+            } else {
+                return;
+            }
+        }
+        self.preorder_invoke(index, f);
+    }
+
+    /// Invoke the given function on all the descendants of the given place, except one branch.
+    pub fn for_each_variant_sibling(
+        &self,
+        parent: PlaceIndex,
+        preserved_child: Option<PlaceIndex>,
+        f: &mut impl FnMut(PlaceIndex),
+    ) {
+        for sibling in self.children(parent) {
+            let elem = self.places[sibling].proj_elem;
+            // Only invalidate variants and discriminant. Fields (for generators) are not
+            // invalidated by assignment to a variant.
+            if let Some(TrackElem::Variant(..) | TrackElem::Discriminant) = elem
+                // Only invalidate the other variants, the current one is fine.
+                && Some(sibling) != preserved_child
+            {
+                self.preorder_invoke(sibling, f);
+            }
+        }
+    }
+
     /// Invoke a function on the given place and all descendants.
-    pub fn preorder_invoke(&self, root: PlaceIndex, f: &mut impl FnMut(PlaceIndex)) {
+    fn preorder_invoke(&self, root: PlaceIndex, f: &mut impl FnMut(PlaceIndex)) {
         f(root);
         for child in self.children(root) {
             self.preorder_invoke(child, f);
@@ -759,6 +860,7 @@ impl<'a> Iterator for Children<'a> {
 }
 
 /// Used as the result of an operand or r-value.
+#[derive(Debug)]
 pub enum ValueOrPlace<V> {
     Value(V),
     Place(PlaceIndex),
@@ -776,6 +878,8 @@ impl<V: HasTop> ValueOrPlace<V> {
 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
 pub enum TrackElem {
     Field(Field),
+    Variant(VariantIdx),
+    Discriminant,
 }
 
 impl<V, T> TryFrom<ProjectionElem<V, T>> for TrackElem {
@@ -784,6 +888,7 @@ impl<V, T> TryFrom<ProjectionElem<V, T>> for TrackElem {
     fn try_from(value: ProjectionElem<V, T>) -> Result<Self, Self::Error> {
         match value {
             ProjectionElem::Field(field, _) => Ok(TrackElem::Field(field)),
+            ProjectionElem::Downcast(_, idx) => Ok(TrackElem::Variant(idx)),
             _ => Err(()),
         }
     }
@@ -900,6 +1005,12 @@ fn debug_with_context_rec<V: Debug + Eq>(
     for child in map.children(place) {
         let info_elem = map.places[child].proj_elem.unwrap();
         let child_place_str = match info_elem {
+            TrackElem::Discriminant => {
+                format!("discriminant({})", place_str)
+            }
+            TrackElem::Variant(idx) => {
+                format!("({} as {:?})", place_str, idx)
+            }
             TrackElem::Field(field) => {
                 if place_str.starts_with('*') {
                     format!("({}).{}", place_str, field.index())

compiler/rustc_mir_transform/src/dataflow_const_prop.rs

@@ -13,6 +13,7 @@ use rustc_mir_dataflow::value_analysis::{Map, State, TrackElem, ValueAnalysis, V
 use rustc_mir_dataflow::{lattice::FlatSet, Analysis, ResultsVisitor, SwitchIntEdgeEffects};
 use rustc_span::DUMMY_SP;
 use rustc_target::abi::Align;
+use rustc_target::abi::VariantIdx;
 
 use crate::MirPass;
 
@@ -30,6 +31,7 @@ impl<'tcx> MirPass<'tcx> for DataflowConstProp {
 
     #[instrument(skip_all level = "debug")]
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+        debug!(def_id = ?body.source.def_id());
         if tcx.sess.mir_opt_level() < 4 && body.basic_blocks.len() > BLOCK_LIMIT {
             debug!("aborted dataflow const prop due too many basic blocks");
             return;
@@ -63,14 +65,31 @@ impl<'tcx> MirPass<'tcx> for DataflowConstProp {
     }
 }
 
-struct ConstAnalysis<'tcx> {
+struct ConstAnalysis<'a, 'tcx> {
     map: Map,
     tcx: TyCtxt<'tcx>,
+    local_decls: &'a LocalDecls<'tcx>,
     ecx: InterpCx<'tcx, 'tcx, DummyMachine>,
     param_env: ty::ParamEnv<'tcx>,
 }
 
-impl<'tcx> ValueAnalysis<'tcx> for ConstAnalysis<'tcx> {
+impl<'tcx> ConstAnalysis<'_, 'tcx> {
+    fn eval_disciminant(
+        &self,
+        enum_ty: Ty<'tcx>,
+        variant_index: VariantIdx,
+    ) -> Option<ScalarTy<'tcx>> {
+        if !enum_ty.is_enum() {
+            return None;
+        }
+        let discr = enum_ty.discriminant_for_variant(self.tcx, variant_index)?;
+        let discr_layout = self.tcx.layout_of(self.param_env.and(discr.ty)).ok()?;
+        let discr_value = Scalar::try_from_uint(discr.val, discr_layout.size)?;
+        Some(ScalarTy(discr_value, discr.ty))
+    }
+}
+
+impl<'tcx> ValueAnalysis<'tcx> for ConstAnalysis<'_, 'tcx> {
     type Value = FlatSet<ScalarTy<'tcx>>;
 
     const NAME: &'static str = "ConstAnalysis";
@@ -79,6 +98,25 @@ impl<'tcx> ValueAnalysis<'tcx> for ConstAnalysis<'tcx> {
         &self.map
     }
 
+    fn handle_statement(&self, statement: &Statement<'tcx>, state: &mut State<Self::Value>) {
+        match statement.kind {
+            StatementKind::SetDiscriminant { box ref place, variant_index } => {
+                state.flood_discr(place.as_ref(), &self.map);
+                if self.map.find_discr(place.as_ref()).is_some() {
+                    let enum_ty = place.ty(self.local_decls, self.tcx).ty;
+                    if let Some(discr) = self.eval_disciminant(enum_ty, variant_index) {
+                        state.assign_discr(
+                            place.as_ref(),
+                            ValueOrPlace::Value(FlatSet::Elem(discr)),
+                            &self.map,
+                        );
+                    }
+                }
+            }
+            _ => self.super_statement(statement, state),
+        }
+    }
+
     fn handle_assign(
         &self,
         target: Place<'tcx>,
@@ -87,17 +125,22 @@ impl<'tcx> ValueAnalysis<'tcx> for ConstAnalysis<'tcx> {
     ) {
         match rvalue {
             Rvalue::Aggregate(kind, operands) => {
-                let target = self.map().find(target.as_ref());
-                if let Some(target) = target {
-                    state.flood_idx_with(target, self.map(), FlatSet::Bottom);
-                    let field_based = match **kind {
-                        AggregateKind::Tuple | AggregateKind::Closure(..) => true,
-                        AggregateKind::Adt(def_id, ..) => {
-                            matches!(self.tcx.def_kind(def_id), DefKind::Struct)
+                state.flood_with(target.as_ref(), self.map(), FlatSet::Bottom);
+                if let Some(target_idx) = self.map().find(target.as_ref()) {
+                    let (variant_target, variant_index) = match **kind {
+                        AggregateKind::Tuple | AggregateKind::Closure(..) => {
+                            (Some(target_idx), None)
                         }
-                        _ => false,
+                        AggregateKind::Adt(def_id, variant_index, ..) => {
+                            match self.tcx.def_kind(def_id) {
+                                DefKind::Struct => (Some(target_idx), None),
+                                DefKind::Enum => (Some(target_idx), Some(variant_index)),
+                                _ => (None, None),
+                            }
+                        }
+                        _ => (None, None),
                     };
-                    if field_based {
+                    if let Some(target) = variant_target {
                         for (field_index, operand) in operands.iter().enumerate() {
                             if let Some(field) = self
                                 .map()
@@ -108,15 +151,20 @@ impl<'tcx> ValueAnalysis<'tcx> for ConstAnalysis<'tcx> {
                             }
                         }
                     }
+                    if let Some(variant_index) = variant_index
+                        && let Some(discr_idx) = self.map().apply(target_idx, TrackElem::Discriminant)
+                    {
+                        let enum_ty = target.ty(self.local_decls, self.tcx).ty;
+                        if let Some(discr_val) = self.eval_disciminant(enum_ty, variant_index) {
+                            state.assign_idx(discr_idx, ValueOrPlace::Value(FlatSet::Elem(discr_val)), &self.map);
+                        }
+                    }
                 }
             }
             Rvalue::CheckedBinaryOp(op, box (left, right)) => {
+                state.flood(target.as_ref(), self.map());
+
                 let target = self.map().find(target.as_ref());
-                if let Some(target) = target {
-                    // We should not track any projections other than
-                    // what is overwritten below, but just in case...
-                    state.flood_idx(target, self.map());
-                }
 
                 let value_target = target
                     .and_then(|target| self.map().apply(target, TrackElem::Field(0_u32.into())));
@@ -195,6 +243,9 @@ impl<'tcx> ValueAnalysis<'tcx> for ConstAnalysis<'tcx> {
                 FlatSet::Bottom => ValueOrPlace::Value(FlatSet::Bottom),
                 FlatSet::Top => ValueOrPlace::Value(FlatSet::Top),
             },
+            Rvalue::Discriminant(place) => {
+                ValueOrPlace::Value(state.get_discr(place.as_ref(), self.map()))
+            }
             _ => self.super_rvalue(rvalue, state),
         }
     }
@@ -268,12 +319,13 @@ impl<'tcx> std::fmt::Debug for ScalarTy<'tcx> {
     }
 }
 
-impl<'tcx> ConstAnalysis<'tcx> {
-    pub fn new(tcx: TyCtxt<'tcx>, body: &Body<'tcx>, map: Map) -> Self {
+impl<'a, 'tcx> ConstAnalysis<'a, 'tcx> {
+    pub fn new(tcx: TyCtxt<'tcx>, body: &'a Body<'tcx>, map: Map) -> Self {
         let param_env = tcx.param_env(body.source.def_id());
         Self {
             map,
             tcx,
+            local_decls: &body.local_decls,
             ecx: InterpCx::new(tcx, DUMMY_SP, param_env, DummyMachine),
             param_env: param_env,
         }
@@ -466,6 +518,21 @@ impl<'tcx, 'map, 'a> Visitor<'tcx> for OperandCollector<'tcx, 'map, 'a> {
             _ => (),
         }
     }
+
+    fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {
+        match rvalue {
+            Rvalue::Discriminant(place) => {
+                match self.state.get_discr(place.as_ref(), self.visitor.map) {
+                    FlatSet::Top => (),
+                    FlatSet::Elem(value) => {
+                        self.visitor.before_effect.insert((location, *place), value);
+                    }
+                    FlatSet::Bottom => (),
+                }
+            }
+            _ => self.super_rvalue(rvalue, location),
+        }
+    }
 }
 
 struct DummyMachine;

tests/mir-opt/dataflow-const-prop/enum.mutate_discriminant.DataflowConstProp.diff

@@ -0,0 +1,26 @@
+- // MIR for `mutate_discriminant` before DataflowConstProp
++ // MIR for `mutate_discriminant` after DataflowConstProp
+  
+  fn mutate_discriminant() -> u8 {
+      let mut _0: u8;                      // return place in scope 0 at $DIR/enum.rs:+0:29: +0:31
+      let mut _1: std::option::Option<NonZeroUsize>; // in scope 0 at $SRC_DIR/core/src/intrinsics/mir.rs:LL:COL
+      let mut _2: isize;                   // in scope 0 at $SRC_DIR/core/src/intrinsics/mir.rs:LL:COL
+  
+      bb0: {
+          discriminant(_1) = 1;            // scope 0 at $DIR/enum.rs:+4:13: +4:34
+          (((_1 as variant#1).0: NonZeroUsize).0: usize) = const 0_usize; // scope 0 at $DIR/enum.rs:+6:13: +6:64
+          _2 = discriminant(_1);           // scope 0 at $SRC_DIR/core/src/intrinsics/mir.rs:LL:COL
+          switchInt(_2) -> [0: bb1, otherwise: bb2]; // scope 0 at $DIR/enum.rs:+9:13: +12:14
+      }
+  
+      bb1: {
+          _0 = const 1_u8;                 // scope 0 at $DIR/enum.rs:+15:13: +15:20
+          return;                          // scope 0 at $DIR/enum.rs:+16:13: +16:21
+      }
+  
+      bb2: {
+          _0 = const 2_u8;                 // scope 0 at $DIR/enum.rs:+19:13: +19:20
+          unreachable;                     // scope 0 at $DIR/enum.rs:+20:13: +20:26
+      }
+  }
+  

tests/mir-opt/dataflow-const-prop/enum.rs

@@ -1,13 +1,52 @@
 // unit-test: DataflowConstProp
 
-// Not trackable, because variants could be aliased.
+#![feature(custom_mir, core_intrinsics, rustc_attrs)]
+
+use std::intrinsics::mir::*;
+
 enum E {
     V1(i32),
     V2(i32)
 }
 
-// EMIT_MIR enum.main.DataflowConstProp.diff
-fn main() {
+// EMIT_MIR enum.simple.DataflowConstProp.diff
+fn simple() {
     let e = E::V1(0);
     let x = match e { E::V1(x) => x, E::V2(x) => x };
 }
+
+#[rustc_layout_scalar_valid_range_start(1)]
+#[rustc_nonnull_optimization_guaranteed]
+struct NonZeroUsize(usize);
+
+// EMIT_MIR enum.mutate_discriminant.DataflowConstProp.diff
+#[custom_mir(dialect = "runtime", phase = "post-cleanup")]
+fn mutate_discriminant() -> u8 {
+    mir!(
+        let x: Option<NonZeroUsize>;
+        {
+            SetDiscriminant(x, 1);
+            // This assignment overwrites the niche in which the discriminant is stored.
+            place!(Field(Field(Variant(x, 1), 0), 0)) = 0_usize;
+            // So we cannot know the value of this discriminant.
+            let a = Discriminant(x);
+            match a {
+                0 => bb1,
+                _ => bad,
+            }
+        }
+        bb1 = {
+            RET = 1;
+            Return()
+        }
+        bad = {
+            RET = 2;
+            Unreachable()
+        }
+    )
+}
+
+fn main() {
+    simple();
+    mutate_discriminant();
+}

tests/mir-opt/dataflow-const-prop/enum.simple.DataflowConstProp.diff

@@ -1,8 +1,8 @@
-- // MIR for `main` before DataflowConstProp
-+ // MIR for `main` after DataflowConstProp
+- // MIR for `simple` before DataflowConstProp
++ // MIR for `simple` after DataflowConstProp
   
-  fn main() -> () {
-      let mut _0: ();                      // return place in scope 0 at $DIR/enum.rs:+0:11: +0:11
+  fn simple() -> () {
+      let mut _0: ();                      // return place in scope 0 at $DIR/enum.rs:+0:13: +0:13
       let _1: E;                           // in scope 0 at $DIR/enum.rs:+1:9: +1:10
       let mut _3: isize;                   // in scope 0 at $DIR/enum.rs:+2:23: +2:31
       scope 1 {
@@ -25,8 +25,10 @@
           StorageLive(_1);                 // scope 0 at $DIR/enum.rs:+1:9: +1:10
           _1 = E::V1(const 0_i32);         // scope 0 at $DIR/enum.rs:+1:13: +1:21
           StorageLive(_2);                 // scope 1 at $DIR/enum.rs:+2:9: +2:10
-          _3 = discriminant(_1);           // scope 1 at $DIR/enum.rs:+2:19: +2:20
-          switchInt(move _3) -> [0: bb3, 1: bb1, otherwise: bb2]; // scope 1 at $DIR/enum.rs:+2:13: +2:20
+-         _3 = discriminant(_1);           // scope 1 at $DIR/enum.rs:+2:19: +2:20
+-         switchInt(move _3) -> [0: bb3, 1: bb1, otherwise: bb2]; // scope 1 at $DIR/enum.rs:+2:13: +2:20
++         _3 = const 0_isize;              // scope 1 at $DIR/enum.rs:+2:19: +2:20
++         switchInt(const 0_isize) -> [0: bb3, 1: bb1, otherwise: bb2]; // scope 1 at $DIR/enum.rs:+2:13: +2:20
       }
   
       bb1: {
@@ -50,7 +52,7 @@
       }
   
       bb4: {
-          _0 = const ();                   // scope 0 at $DIR/enum.rs:+0:11: +3:2
+          _0 = const ();                   // scope 0 at $DIR/enum.rs:+0:13: +3:2
           StorageDead(_2);                 // scope 1 at $DIR/enum.rs:+3:1: +3:2
           StorageDead(_1);                 // scope 0 at $DIR/enum.rs:+3:1: +3:2
           return;                          // scope 0 at $DIR/enum.rs:+3:2: +3:2