Bring back TransactionalRewrite (needs test, copies twice)

src/hugr/rewrite.rs

@@ -31,7 +31,7 @@ pub trait Rewrite {
  /// being preferred.
  fn apply(self, h: &mut impl HugrMut) -> Result<(), Self::Error>;
 }
-/*
+
 /// Wraps any rewrite into a transaction (i.e. that has no effect upon failure)
 pub struct Transactional<R> {
  underlying: R,
@@ -43,21 +43,30 @@ impl<R: Rewrite> Rewrite for Transactional<R> {
  type Error = R::Error;
  const UNCHANGED_ON_FAILURE: bool = true;
 
- fn verify(&self, h: &Hugr) -> Result<(), Self::Error> {
+ fn verify(&self, h: &impl HugrView) -> Result<(), Self::Error> {
  self.underlying.verify(h)
  }
 
- fn apply(self, h: &mut Hugr) -> Result<(), Self::Error> {
+ fn apply(self, h: &mut impl HugrMut) -> Result<(), Self::Error> {
  if R::UNCHANGED_ON_FAILURE {
  return self.underlying.apply(h);
  }
- let backup = h.clone();
+ let mut backup = Hugr::new(h.root_type().clone());
+ backup.insert_from_view(backup.root(), h).unwrap();
  let r = self.underlying.apply(h);
  if r.is_err() {
- // drop the old h, it was undefined
- let _ = mem::replace(h, backup);
+ h.replace_op(h.root(), backup.root_type().clone());
+ loop {
+ let ch = h.children(h.root()).next();
+ match ch {
+ Some(child) => h.remove_node(child).unwrap(),
+ None => {
+ break;
+ }
+ }
+ }
+ h.insert_from_view(h.root(), &backup).unwrap();
  }
- r
+ Ok(())
  }
 }
-*/