Handle inactive enum variants in MaybeUninitializedPlaces

src/librustc_mir/dataflow/drop_flag_effects.rs

@@ -1,6 +1,7 @@
 use crate::util::elaborate_drops::DropFlagState;
 use rustc_middle::mir::{self, Body, Location};
 use rustc_middle::ty::{self, TyCtxt};
+use rustc_target::abi::VariantIdx;
 
 use super::indexes::MovePathIndex;
 use super::move_paths::{InitKind, LookupResult, MoveData};
@@ -228,3 +229,42 @@ pub(crate) fn for_location_inits<'tcx, F>(
         }
     }
 }
+
+/// Calls `handle_inactive_variant` for each descendant move path of `enum_place` that contains a
+/// `Downcast` to a variant besides the `active_variant`.
+///
+/// NOTE: If there are no move paths corresponding to an inactive variant,
+/// `handle_inactive_variant` will not be called for that variant.
+pub(crate) fn on_all_inactive_variants<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    body: &mir::Body<'tcx>,
+    move_data: &MoveData<'tcx>,
+    enum_place: mir::Place<'tcx>,
+    active_variant: VariantIdx,
+    mut handle_inactive_variant: impl FnMut(MovePathIndex),
+) {
+    let enum_mpi = match move_data.rev_lookup.find(enum_place.as_ref()) {
+        LookupResult::Exact(mpi) => mpi,
+        LookupResult::Parent(_) => return,
+    };
+
+    let enum_path = &move_data.move_paths[enum_mpi];
+    for (variant_mpi, variant_path) in enum_path.children(&move_data.move_paths) {
+        // Because of the way we build the `MoveData` tree, each child should have exactly one more
+        // projection than `enum_place`. This additional projection must be a downcast since the
+        // base is an enum.
+        let (downcast, base_proj) = variant_path.place.projection.split_last().unwrap();
+        assert_eq!(enum_place.projection.len(), base_proj.len());
+
+        let variant_idx = match *downcast {
+            mir::ProjectionElem::Downcast(_, idx) => idx,
+            _ => unreachable!(),
+        };
+
+        if variant_idx != active_variant {
+            on_all_children_bits(tcx, body, move_data, variant_mpi, |mpi| {
+                handle_inactive_variant(mpi)
+            });
+        }
+    }
+}

src/librustc_mir/dataflow/impls/mod.rs

@@ -12,7 +12,7 @@ use super::MoveDataParamEnv;
 
 use crate::util::elaborate_drops::DropFlagState;
 
-use super::move_paths::{HasMoveData, InitIndex, InitKind, LookupResult, MoveData, MovePathIndex};
+use super::move_paths::{HasMoveData, InitIndex, InitKind, MoveData, MovePathIndex};
 use super::{AnalysisDomain, BottomValue, GenKill, GenKillAnalysis};
 
 use super::drop_flag_effects_for_function_entry;
@@ -124,11 +124,23 @@ pub struct MaybeUninitializedPlaces<'a, 'tcx> {
     tcx: TyCtxt<'tcx>,
     body: &'a Body<'tcx>,
     mdpe: &'a MoveDataParamEnv<'tcx>,
+
+    mark_inactive_variants_as_uninit: bool,
 }
 
 impl<'a, 'tcx> MaybeUninitializedPlaces<'a, 'tcx> {
     pub fn new(tcx: TyCtxt<'tcx>, body: &'a Body<'tcx>, mdpe: &'a MoveDataParamEnv<'tcx>) -> Self {
-        MaybeUninitializedPlaces { tcx, body, mdpe }
+        MaybeUninitializedPlaces { tcx, body, mdpe, mark_inactive_variants_as_uninit: false }
+    }
+
+    /// Causes inactive enum variants to be marked as "maybe uninitialized" after a switch on an
+    /// enum discriminant.
+    ///
+    /// This is correct in a vacuum but is not the default because it causes problems in the borrow
+    /// checker, where this information gets propagated along `FakeEdge`s.
+    pub fn mark_inactive_variants_as_uninit(mut self) -> Self {
+        self.mark_inactive_variants_as_uninit = true;
+        self
     }
 }
 
@@ -350,27 +362,16 @@ impl<'tcx> GenKillAnalysis<'tcx> for MaybeInitializedPlaces<'_, 'tcx> {
         _adt: &ty::AdtDef,
         variant: VariantIdx,
     ) {
-        let enum_mpi = match self.move_data().rev_lookup.find(enum_place.as_ref()) {
-            LookupResult::Exact(mpi) => mpi,
-            LookupResult::Parent(_) => return,
-        };
-
-        // Kill all move paths that correspond to variants other than this one
-        let move_paths = &self.move_data().move_paths;
-        let enum_path = &move_paths[enum_mpi];
-        for (mpi, variant_path) in enum_path.children(move_paths) {
-            trans.kill(mpi);
-            match variant_path.place.projection.last().unwrap() {
-                mir::ProjectionElem::Downcast(_, idx) if *idx == variant => continue,
-                _ => drop_flag_effects::on_all_children_bits(
+        // Kill all move paths that correspond to variants we know to be inactive along this
+        // particular outgoing edge of a `SwitchInt`.
+        drop_flag_effects::on_all_inactive_variants(
             self.tcx,
             self.body,
             self.move_data(),
-                    mpi,
+            enum_place,
+            variant,
             |mpi| trans.kill(mpi),
-                ),
-            }
-        }
+        );
     }
 }
 
@@ -443,6 +444,30 @@ impl<'tcx> GenKillAnalysis<'tcx> for MaybeUninitializedPlaces<'_, 'tcx> {
             },
         );
     }
+
+    fn discriminant_switch_effect(
+        &self,
+        trans: &mut impl GenKill<Self::Idx>,
+        _block: mir::BasicBlock,
+        enum_place: mir::Place<'tcx>,
+        _adt: &ty::AdtDef,
+        variant: VariantIdx,
+    ) {
+        if !self.mark_inactive_variants_as_uninit {
+            return;
+        }
+
+        // Mark all move paths that correspond to variants other than this one as maybe
+        // uninitialized (in reality, they are *definitely* uninitialized).
+        drop_flag_effects::on_all_inactive_variants(
+            self.tcx,
+            self.body,
+            self.move_data(),
+            enum_place,
+            variant,
+            |mpi| trans.gen(mpi),
+        );
+    }
 }
 
 impl<'a, 'tcx> AnalysisDomain<'tcx> for DefinitelyInitializedPlaces<'a, 'tcx> {

src/librustc_mir/transform/elaborate_drops.rs

@@ -48,6 +48,7 @@ impl<'tcx> MirPass<'tcx> for ElaborateDrops {
                 .into_results_cursor(body);
 
             let uninits = MaybeUninitializedPlaces::new(tcx, body, &env)
+                .mark_inactive_variants_as_uninit()
                 .into_engine(tcx, body, def_id)
                 .dead_unwinds(&dead_unwinds)
                 .iterate_to_fixpoint()