Implement ProgramNode

[ihincks]

This PR closes #16029 by defining a trait called ProgramNode. It also implements a simple core node (Store) just to show the idea of what a simple node definition will be, albeit a slightly weird one in that it requires an empty input tree.

PR Stack

• Parent: Add DType, Tensor & friends to providers crate #15993
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AI/LLM disclosure

• I didn't use LLM tooling, or only used it privately.
• I used the following tool to help write this PR description:
• I used the following tool to generate or modify code: Claude 4.6

[qiskit-bot]

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

• @Qiskit/terra-core

[coveralls]

Coverage Report for CI Build 25382081104

Coverage decreased (-0.1%) to 87.501%

Details

• Coverage decreased (-0.1%) from the base build.
• Patch coverage: 182 uncovered changes across 3 files (259 of 441 lines covered, 58.73%).
• 6 coverage regressions across 2 files.

Uncovered Changes

File	Changed	Covered	%
crates/providers/src/tensor.rs	341	170	49.85%
crates/providers/src/store.rs	86	76	88.37%
crates/providers/src/data_tree.rs	11	10	90.91%

Coverage Regressions

6 previously-covered lines in 2 files lost coverage.

File	Lines Losing Coverage	Coverage
crates/qasm2/src/lex.rs	5	91.52%
crates/circuit/src/parameter/symbol_expr.rs	1	74.21%

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Coverage Stats

Relevant Lines:	122267
Covered Lines:	106985
Line Coverage:	87.5%
Coverage Strength:	960960.62 hits per line

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💛 - Coveralls

[mtreinish]

Could you move the static definition outside of the method definition and give it a comment about having a single empty data tree definition to avoid reallocation on each store. The other thing is why do we need a OnceLock here? Couldn't we use a OnceCell, unless you're thinking we're going to be calling this from a threaded context. The other thought is lazy_static is an option here too.

[ihincks]

Could you move the static definition outside of the method definition

I'm happy to, but I'd like to know why. Isn't it a bit more readable to put it inside the method when only one method needs it?

The other thing is why do we need a OnceLock here? Couldn't we use a OnceCell

It's because static variables are always Sync and OnceCell is explicitly not Sync. Looking into lazy_static, it looks like it's been superseded by LazyOnce and LazyCell.

I made a change in 1c2eb52, but am happy to iterate it. It's worth getting the pattern right because many other nodes will want to copy it.

[mtreinish]

I normally move it to the top of the module just to make the scoping clear. A static is global so it can be a bit confusing to see it inline. It's not a big deal either way.

[qiskit-bot]

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

• @Qiskit/terra-core