Fix Target.instruction_supported when target.num_qubits == None

[ElePT]

Summary

This PR fixes a small bug found in Target.instruction_supported, where target.num_qubits == None would cause Target.instruction_supported to always evaluate to False. This was handled in the original Python implementation by artificially overwriting qargs, but this would require a mutable variable in Rust, so I just added an additional condition to account for this case.

Details and comments

Bug discovered through #12850.

[qiskit-bot]

One or more of the following people are relevant to this code:

• @Qiskit/terra-core

[raynelfss]

LGTM but before I can approve it, I have one question. Would it be easier to just discard the qargs argument at the beginning the way it was done in Python? You could avoid the mutable reference by just re-defining the variable, for example:

// Handle case where num_qubits is None by always checking globally supported operations
// I don't think it has to be mut, but in case it's needed.
let mut qargs: Option<Qargs> = if self.num_qubits.is_none() {
    None
} else {
    qargs
};

I do see why this would be less clean but it would avoid having to check the same condition multiple times.

[ElePT]

LGTM but before I can approve it, I have one question. Would it be easier to just discard the qargs argument at the beginning the way it was done in Python? You could avoid the mutable reference by just re-defining the variable.

Sure, I was in between the two approaches, and I don't really have a strong preference, so I can totally change it.

[coveralls]

Pull Request Test Coverage Report for Build 12750239425

Details

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• 12 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.5%) to 88.442%

---

Files with Coverage Reduction	New Missed Lines	%
crates/accelerate/src/unitary_synthesis.rs	1	93.18%
crates/qasm2/src/lex.rs	5	92.73%
crates/qasm2/src/parse.rs	6	97.62%

Totals
Change from base Build 12744574228:	-0.5%
Covered Lines:	53802
Relevant Lines:	60833

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💛 - Coveralls

[raynelfss]

LGTM, I wanted to add that this was being done in py_instruction_supported already, but when I split off these functions to have a rust-native parallel, the condition got lost for the rust-native one.