Fix nits

src/librustc_mir/transform/const_prop.rs

@@ -59,7 +59,7 @@ impl MirPass for ConstProp {
 
         trace!("ConstProp starting for {:?}", source.def_id());
 
-        // steal some data we need from `body`
+        // Steal some data we need from `body`.
         let source_scope_local_data = std::mem::replace(
             &mut body.source_scope_local_data,
             ClearCrossCrate::Clear
@@ -188,31 +188,49 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
         }
     }
 
-    fn release_stolen_data(self) ->
-        (ClearCrossCrate<IndexVec<SourceScope, SourceScopeLocalData>>,
-         IndexVec<Promoted, Body<'tcx>>)
-    {
+    fn release_stolen_data(
+        self,
+    ) -> (
+        ClearCrossCrate<IndexVec<SourceScope, SourceScopeLocalData>>,
+        IndexVec<Promoted, Body<'tcx>>,
+    ) {
         (self.source_scope_local_data, self.promoted)
     }
 
     fn get_const(&self, local: Local) -> Option<Const<'tcx>> {
         let l = &self.ecx.frame().locals[local];
-        if l.value == LocalValue::Uninitialized || l.value == LocalValue::Dead {
+
+        // If the local is `Unitialized` or `Dead` then we haven't propagated a value into it.
+        //
+        // `InterpretCx::access_local()` mostly takes care of this for us however, for ZSTs,
+        // it will synthesize a value for us. In doing so, that will cause the
+        // `get_const(l).is_empty()` assert right before we call `set_const()` in `visit_statement`
+        // to fail.
+        if let LocalValue::Uninitialized | LocalValue::Dead = l.value {
             return None;
         }
 
         self.ecx.access_local(self.ecx.frame(), local, None).ok()
     }
 
-    fn set_const(&mut self, local: Local, c: Option<Const<'tcx>>) {
-        self.ecx.frame_mut().locals[local] =
-            match c {
-                Some(op_ty) => LocalState {
-                    value: LocalValue::Live(*op_ty),
-                    layout: Cell::new(Some(op_ty.layout)),
-                },
-                None => LocalState { value: LocalValue::Uninitialized, layout: Cell::new(None) },
-            };
+    fn set_const(&mut self, local: Local, c: Const<'tcx>) {
+        let frame = self.ecx.frame_mut();
+
+        if let Some(layout) = frame.locals[local].layout.get() {
+            debug_assert_eq!(c.layout, layout);
+        }
+
+        frame.locals[local] = LocalState {
+            value: LocalValue::Live(*c),
+            layout: Cell::new(Some(c.layout)),
+        };
+    }
+
+    fn remove_const(&mut self, local: Local) {
+        self.ecx.frame_mut().locals[local] = LocalState {
+            value: LocalValue::Uninitialized,
+            layout: Cell::new(None),
+        };
     }
 
     fn use_ecx<F, T>(
@@ -768,7 +786,7 @@ impl<'mir, 'tcx> MutVisitor<'tcx> for ConstPropagator<'mir, 'tcx> {
                         if self.can_const_prop[local] {
                             trace!("storing {:?} to {:?}", value, local);
                             assert!(self.get_const(local).is_none());
-                            self.set_const(local, Some(value));
+                            self.set_const(local, value);
 
                             if self.should_const_prop() {
                                 self.replace_with_const(
@@ -808,7 +826,7 @@ impl<'mir, 'tcx> MutVisitor<'tcx> for ConstPropagator<'mir, 'tcx> {
                                     place = &proj.base;
                                 }
                                 if let Place::Base(PlaceBase::Local(local)) = *place {
-                                    self.set_const(local, None);
+                                    self.remove_const(local);
                                 }
                             },
                             Operand::Constant(_) => {}