Add a Rust representation of Expr

[pdhoolia]

The Expr class for representing classical expressions is part of the core data model for control flow which is defined in Python. With https://github.com/Qiskit/qiskit/issues/13271 moving controflow expressions to rust we'll also need to move the Expr class to rust otherwise control flow ops will rely on calling back to Python when working with control flow using expressions.

[pdhoolia]

To address the issue of moving the Expr class from Python to Rust, we will need to make several changes across the suggested files. Below is a detailed plan for each file, including the necessary changes and reasoning:

1. expr.py

Reason: This file likely contains the implementation of the Expr class. We need to transition this functionality to Rust.

Suggested Changes:

• Identify the Expr class and its methods in expr.py.
• Create a Rust equivalent of the Expr class. This will involve writing a Rust module that mimics the behavior and structure of the Expr class in Python.
• Ensure that the Rust code is compiled into a shared library that can be called from Python using FFI (Foreign Function Interface).
• Remove or comment out the Expr class in expr.py and replace it with a Python wrapper that calls the Rust implementation.

2. control_flow_ops.py

Reason: This file may use the Expr class, so it will need updates to interact with the Rust implementation.

Suggested Changes:

• Update any imports or references to Expr to use the new Rust-based implementation.
• Modify functions or methods that create or manipulate Expr objects to use the FFI interface to the Rust code.
• Ensure that any logic that specifically relies on Python-based Expr features is compatible with the Rust implementation.

3. data_model.py

Reason: This file might define related structures that interact with Expr.

Suggested Changes:

• Review the data structures in this file to check for any dependencies on Expr.
• If there are dependencies, modify these structures to be compatible with the Rust-based Expr.
• Consider whether any data serialization/deserialization needs to be adjusted to accommodate the Rust implementation.

4. utils.py

Reason: Utilities for control flow might interact with Expr.

Suggested Changes:

• Identify any utility functions that interact with the Expr class.
• Update these utility functions to work with the Rust-based Expr implementation.
• Ensure that any utility logic is compatible with the constraints and features of the Rust implementation.

5. tests/test_expr.py

Reason: Tests for Expr will need updates to ensure compatibility with Rust changes.

Suggested Changes:

• Update test cases to work with the Rust-based Expr implementation.
• Use Python's FFI capabilities to call the Rust code from the test cases.
• Ensure that tests cover both the correctness and performance of the Rust implementation.
• Add any new test cases necessary to validate the functionality of the Rust Expr class.

New Rust Code

Reason: We need a Rust implementation of the Expr class.

Suggested New Code:

• Create a new Rust crate that implements the Expr class. This crate should define the same methods and behavior as the Python class.
• Use Rust's FFI capabilities to expose the necessary functions to Python.
• Compile this crate into a shared library that can be loaded by Python.

Integration

• Set up a build system to compile the Rust code and link it with the Python code.
• Ensure that the Python code can dynamically load the Rust library and call its functions.

By following these steps, we can effectively transition the Expr class from Python to Rust while maintaining compatibility and performance within the Qiskit control flow module.