Circuit need to keep track of their Extensions

[aborgna-q]

We have moved to using Packages to interchange hugr definitions between the hugrverse projects. The main reasons for this is i) to embed the required extensions to validate each hugr, and ii) to bundle multiple hugrs that should be linked on the target.

In tket2 we mostly care about i). We need to be able to read hugrs+extensions from elsewhere and keep that info around.

A separate problem is also storing the entrypoint node pointer in the wire format.

Old idea

Swap the current Hugr + Node pointer used as support for a Circuit, and use a Package + {module_id: usize, entrypoint: Node} instead.

• Do we need something like the NodePointer traits from hugr-py?
  https://github.com/CQCL/hugr/blob/cb613d56dbe60d98fe1df681f85d51e60f16a634/hugr-py/src/hugr/package.py#L129
• How do we serialize the node pointer, now that the circuit may be deeply nested inside a hugr?

Current solution

After some discussion, we decided that extensions will be embedded in the HUGR themselves, and the entrypoint node stored in the root's metadata. See CQCL/hugr#1613.

Since that change won't be implemented until hugr-model is done, we need a patch solution in the meantime:

• Keep using a Hugr + Node for Circuit.
• Store circuits as Hugrs with a root-level metadata entry indicating the entry function.
• Add an extensions: Option<Vec<Extensions>> field to Circuit.
• Hardcode a default extension set and use it when reading extension-less hugrs.
  • Support loading either Packages or Hugrs, using Package::from_json, and add the default extension set as needed.
    https://github.com/CQCL/hugr/blob/8d9da5f6371469055c75378748a06d7290098aaf/hugr-core/src/package.rs#L143-L147
  • Support writing Hugr files. Add a warning if the extension set is not the default?