make StorageLive kill the current value of the local

Author: RalfJung

src/eval_context.rs

@@ -1720,15 +1720,12 @@ impl<'tcx> Frame<'tcx> {
         return Ok(());
     }
 
-    pub fn storage_live(&mut self, local: mir::Local) -> EvalResult<'tcx> {
+    pub fn storage_live(&mut self, local: mir::Local) -> EvalResult<'tcx, Option<Value>> {
         trace!("{:?} is now live", local);
-        if self.locals[local.index() - 1].is_some() {
-            // The variables comes live now, but was already accessed previously, when it was still dead
-            return Err(EvalError::DeadLocal);
-        } else {
-            self.locals[local.index() - 1] = Some(Value::ByVal(PrimVal::Undef));
-        }
-        return Ok(());
+
+        let old = self.locals[local.index() - 1];
+        self.locals[local.index() - 1] = Some(Value::ByVal(PrimVal::Undef)); // StorageLive *always* kills the value that's currently stored
+        return Ok(old);
     }
 
     /// Returns the old value of the local

src/step.rs

@@ -132,13 +132,12 @@ impl<'a, 'tcx> EvalContext<'a, 'tcx> {
                     Lvalue::Local{ frame, local, field: None } if self.stack.len() == frame+1 => (frame, local),
                     _ => return Err(EvalError::Unimplemented("Stroage annotations must refer to locals of the topmost stack frame.".to_owned())) // FIXME maybe this should get its own error type
                 };
-                match stmt.kind {
+                let old_val = match stmt.kind {
                     StorageLive(_) => self.stack[frame].storage_live(local)?,
-                    _ =>  {
-                        let old_val = self.stack[frame].storage_dead(local)?;
-                        self.deallocate_local(old_val)?;
-                    }
+                    StorageDead(_) =>  self.stack[frame].storage_dead(local)?,
+                    _ => bug!("We already checked that we are a storage stmt")
                 };
+                self.deallocate_local(old_val)?;
             }
 
             // Defined to do nothing. These are added by optimization passes, to avoid changing the