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Add variable-handling methods to QuantumCircuit #10962

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Add variable-handling methods to QuantumCircuit #10962

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Commits on Nov 24, 2023

  1. Add representation of storage-owning Var nodes 

    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.
    @jakelishman
    jakelishman committed Nov 24, 2023
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  2. Add definition of Store instruction 

    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.
    @jakelishman
    jakelishman committed Nov 24, 2023
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  3. Add variable-handling methods to QuantumCircuit 

    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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    jakelishman committed Nov 24, 2023
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