Redefine CustomOperation::to_matrix

[davidfcohen]

This PR redefines CustomOperation::matrix as fallible CustomOperation::to_matrix. Different custom operations can fail for a different set of reasons. So, I used type-erased Box<dyn std::error::Error>. I would have liked to return Self::Err, however, because of the way CustomOperation is used dynamically, I boxed the error instead. We use the form Result<Option<T>, E> to cover the common "unimplemented" case.

closes #16862

AI/LLM disclosure

• No part of this submission is LLM generated.
• Some written text was generated by:
• Some submitted code was generated by:

[qiskit-bot]

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

• @Qiskit/terra-core

[davidfcohen]

I'm unsure whether this function should Result as well. Let me know!

[coveralls]

Coverage Report for CI Build 33206100477

Coverage decreased (-0.008%) to 87.736%

Details

• Coverage decreased (-0.008%) from the base build.
• Patch coverage: 14 uncovered changes across 2 files (21 of 35 lines covered, 60.0%).
• 21 coverage regressions across 4 files.

Uncovered Changes

File	Changed	Covered	%
crates/circuit/src/packed_instruction.rs	9	0	0.0%
crates/circuit/src/operations.rs	26	21	80.77%

Coverage Regressions

21 previously-covered lines in 4 files lost coverage.

File	Lines Losing Coverage	Coverage
crates/qasm2/src/parse.rs	12	97.16%
crates/qasm2/src/lex.rs	6	91.77%
crates/circuit/src/packed_instruction.rs	2	86.02%
crates/circuit/src/operations.rs	1	83.68%

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Coverage Stats

Relevant Lines:	131095
Covered Lines:	115018
Line Coverage:	87.74%
Coverage Strength:	981056.38 hits per line

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💛 - Coveralls