Add variable-handling methods to QuantumCircuit
#10962
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Changelog: New Feature
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This adds the representation of `expr.Var` nodes that own their own storage locations, and consequently are not backed by existing Qiskit objects (`Clbit` or `ClassicalRegister`). This is the base of the ability for Qiskit to represent manual classical-value storage in `QuantumCircuit`, and the base for how manual storage will be implemented.
This does not yet add the implementation of `QuantumCircuit.store`, which will come later as part of expanding the full API of `QuantumCircuit` to be able to support these runtime variables. The `is_lvalue` helper is added generally to the `classical.expr` module because it's generally useful, while `types.cast_kind` is moved from being a private method in `expr` to a public-API function so `Store` can use it. These now come with associated unit tests.
This adds all the new `QuantumCircuit` methods discussed in the variable-declaration RFC[^1], and threads the support for them through the methods that are called in turn, such as `QuantumCircuit.append`. It does yet not add support to methods such as `copy` or `compose`, which will be done in a follow-up. The APIs discussed in the RFC necessitated making `Var` nodes hashable. This is done in this commit, as it is logically connected. These nodes now have enforced immutability, which is technically a minor breaking change, but in practice required for the properties of such expressions to be tracked correctly through circuits. A helper attribute `Var.standalone` is added to unify the handling of whether a variable is an old-style existing-memory wrapper, or a new "proper" variable with its own memory. [^1]: Qiskit/RFCs#50
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Summary
This adds all the new
QuantumCircuitmethods discussed in the variable-declaration RFC1, and threads the support for them through the methods that are called in turn, such asQuantumCircuit.append. It does yet not add support to methods such ascopyorcompose, which will be done in a follow-up.The APIs discussed in the RFC necessitated making
Varnodes hashable. This is done in this commit, as it is logically connected. These nodes now have enforced immutability, which is technically a minor breaking change, but in practice required for the properties of such expressions to be tracked correctly through circuits.A helper attribute
Var.standaloneis added to unify the handling of whether a variable is an old-style existing-memory wrapper, or a new "proper" variable with its own memory.Details and comments
Depends on #10946
Close #10924
Two changelog entries are added for the semi-orthogonal changes, but the main thrust will come as with most things at the end of the epic.
Footnotes
https://github.com/Qiskit/RFCs/pull/50 ↩