Stop propagating to locals that were marks as unpropagatable.

We used to erase these values immediately after propagation, but some
things slipped through and it caused us to still initialize huge locals.

Author: oli-obk

src/librustc_mir/transform/const_prop.rs

@@ -157,14 +157,19 @@ struct ConstPropMachine<'mir, 'tcx> {
     written_only_inside_own_block_locals: FxHashSet<Local>,
     /// Locals that need to be cleared after every block terminates.
     only_propagate_inside_block_locals: BitSet<Local>,
+    can_const_prop: IndexVec<Local, ConstPropMode>,
 }
 
 impl<'mir, 'tcx> ConstPropMachine<'mir, 'tcx> {
-    fn new(only_propagate_inside_block_locals: BitSet<Local>) -> Self {
+    fn new(
+        only_propagate_inside_block_locals: BitSet<Local>,
+        can_const_prop: IndexVec<Local, ConstPropMode>,
+    ) -> Self {
         Self {
             stack: Vec::new(),
             written_only_inside_own_block_locals: Default::default(),
             only_propagate_inside_block_locals,
+            can_const_prop,
         }
     }
 }
@@ -243,6 +248,9 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for ConstPropMachine<'mir, 'tcx>
         local: Local,
     ) -> InterpResult<'tcx, Result<&'a mut LocalValue<Self::PointerTag>, MemPlace<Self::PointerTag>>>
     {
+        if ecx.machine.can_const_prop[local] == ConstPropMode::NoPropagation {
+            throw_machine_stop_str!("tried to write to a local that is marked as not propagatable")
+        }
         if frame == 0 && ecx.machine.only_propagate_inside_block_locals.contains(local) {
             ecx.machine.written_only_inside_own_block_locals.insert(local);
         }
@@ -287,7 +295,6 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for ConstPropMachine<'mir, 'tcx>
 struct ConstPropagator<'mir, 'tcx> {
     ecx: InterpCx<'mir, 'tcx, ConstPropMachine<'mir, 'tcx>>,
     tcx: TyCtxt<'tcx>,
-    can_const_prop: IndexVec<Local, ConstPropMode>,
     param_env: ParamEnv<'tcx>,
     // FIXME(eddyb) avoid cloning these two fields more than once,
     // by accessing them through `ecx` instead.
@@ -347,7 +354,7 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
             tcx,
             span,
             param_env,
-            ConstPropMachine::new(only_propagate_inside_block_locals),
+            ConstPropMachine::new(only_propagate_inside_block_locals, can_const_prop),
             (),
         );
 
@@ -373,7 +380,6 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
             ecx,
             tcx,
             param_env,
-            can_const_prop,
             // FIXME(eddyb) avoid cloning these two fields more than once,
             // by accessing them through `ecx` instead.
             source_scopes: body.source_scopes.clone(),
@@ -1031,7 +1037,7 @@ impl<'mir, 'tcx> MutVisitor<'tcx> for ConstPropagator<'mir, 'tcx> {
         let source_info = statement.source_info;
         self.source_info = Some(source_info);
         if let StatementKind::Assign(box (place, ref mut rval)) = statement.kind {
-            let can_const_prop = self.can_const_prop[place.local];
+            let can_const_prop = self.ecx.machine.can_const_prop[place.local];
             if let Some(()) = self.const_prop(rval, source_info, place) {
                 // This will return None if the above `const_prop` invocation only "wrote" a
                 // type whose creation requires no write. E.g. a generator whose initial state

src/test/mir-opt/const_prop/mutable_variable_aggregate_mut_ref.main.ConstProp.diff

@@ -23,15 +23,13 @@
                                            // + ty: i32
                                            // + val: Value(Scalar(0x0000002a))
                                            // mir::Constant
--                                          // + span: $DIR/mutable_variable_aggregate_mut_ref.rs:5:18: 5:20
-+                                          // + span: $DIR/mutable_variable_aggregate_mut_ref.rs:5:17: 5:25
+                                           // + span: $DIR/mutable_variable_aggregate_mut_ref.rs:5:18: 5:20
                                            // + literal: Const { ty: i32, val: Value(Scalar(0x0000002a)) }
                                            // ty::Const
                                            // + ty: i32
                                            // + val: Value(Scalar(0x0000002b))
                                            // mir::Constant
--                                          // + span: $DIR/mutable_variable_aggregate_mut_ref.rs:5:22: 5:24
-+                                          // + span: $DIR/mutable_variable_aggregate_mut_ref.rs:5:17: 5:25
+                                           // + span: $DIR/mutable_variable_aggregate_mut_ref.rs:5:22: 5:24
                                            // + literal: Const { ty: i32, val: Value(Scalar(0x0000002b)) }
           StorageLive(_2);                 // scope 1 at $DIR/mutable_variable_aggregate_mut_ref.rs:6:9: 6:10
           _2 = &mut _1;                    // scope 1 at $DIR/mutable_variable_aggregate_mut_ref.rs:6:13: 6:19