Some way to apply a rewrite to a region of a Hugr

[acl-cqc]

The simplest way might be:

• Make Rewrite::apply operate on any &mut impl HugrMut, rather than only on a &mut Hugr
• impl HugrMut for FlatRegionView. This would presumably have to do some sanity checking that the relevant/affected/passed-in Nodes were "in" the region. I'm not sure how straightforward these would be, this might be the killer...
• (Trivial) add HugrMut::apply_rewrite, as a Trait-default method I think (and moved from impl Hugr, in fact)

[aborgna-q]

HugrMut is pub(crate). apply_rewrite could go into a Rewritable : HugrMut or something similar, with some small changes to seal HugrMut.

impl HugrMut for FlatRegionView is not that straightforward since the views are immutable by default, but we can apply the rewrite on the underlying hugr and recreate the view (it's almost free).

[acl-cqc]

HugrMut is pub(crate)

Ah yes good point. So pub(crate) would do for now but perhaps we do need a new, pub mutator trait....

but we can apply the rewrite on the underlying hugr and recreate the view (it's almost free)

Yes. One loses any protection against the rewrite changing anything outside the region, and thus indeed, any protection against the rewrite removing the region altogether, but....

[aborgna-q]

We can still run the checks before unwrapping (I think we'd only require the removed set to be in the graph)?

[acl-cqc]

We can still run the checks before unwrapping (I think we'd only require the removed set to be in the graph)?

That works nicely for Replace, I think, yes. But there are other rewrite too, e.g. (Inline/Outline)(C/D)FG. We could write checks for each of those individually, and (hmmm) yes we probably will have to in order to determine in advance whether the rewrite will stay within a region or not.

We may also be able to make all Rewrites (not just Replace) do all their operations through a HugrView / MutRegion, so at least we can have some protection against accidents when the rewrite is being applied even if not in advance.

[acl-cqc]

Done in #522 and #519 via SiblingMut