fix lifetime bug with presolve result

[tmckayus]

During local development and testing I had reproducible crashes during presolve tests. This appears to be because the presolve result was declared in an inner scope, but the reduced problem is referenced by problem.original_problem_ptr in an outer block at line 1279.

I have not seen this in CI, but timing and memory layout on my local machine may have exposed a latent issue. This change resolved the crash for me.

Summary by CodeRabbit

• Refactor
  • Reorganized variable scope management in the solver to improve clarity; clarified lifetime and error-flow with comments while preserving existing behavior.
  • Enabled default move semantics for optimization problem objects to allow efficient resource transfers without copying.
  • Preserved external interfaces and runtime behavior.

[coderabbitai]

📝 Walkthrough

Walkthrough

Moved a presolver result variable in the PDLP solver from inner to outer scope to persist its value across control flow, and added defaulted move constructor and move assignment to the optimization_problem_t class to enable move semantics; no other behavioral changes introduced.

Changes

Cohort / File(s)	Summary
PDLP solver scope change cpp/src/pdlp/solve.cu	Presolver result variable relocated from inner/local scope to an outer optional to retain value across control flow; assignment updated and clarifying comment about lifetime/reduced-problem reference added.
Optimization problem move semantics cpp/include/cuopt/linear_programming/optimization_problem.hpp	Added defaulted move constructor and move assignment operator to optimization_problem_t to enable move semantics (optimization_problem_t(optimization_problem_t&&) = default, operator=(optimization_problem_t&&) = default).

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and specifically identifies the main change: fixing a lifetime bug with the presolve result, which is precisely what the changeset addresses.

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✨ Finishing Touches

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[coderabbitai]

🧹 Nitpick comments (1)

cpp/src/pdlp/solve.cu (1)

1206-1206: Add an explicit <optional> include to cpp/src/pdlp/solve.cu.

std::optional is used directly at line 1206; relying on transitive includes from third_party_presolve.hpp is brittle and can break with unrelated header changes.

🤖 Prompt for AI Agents

Verify each finding against the current code and only fix it if needed.

In `@cpp/src/pdlp/solve.cu` at line 1206, Add an explicit include for the
<optional> header in the file that defines the use of std::optional (so that
std::optional is available for the declaration of
std::optional<detail::third_party_presolve_result_t<i_t, f_t>> result;). Locate
the top-of-file includes in solve.cu and insert `#include` <optional> (before or
alongside other standard headers) so the code no longer relies on transitive
includes from third_party_presolve.hpp.

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Nitpick comments:
In `@cpp/src/pdlp/solve.cu`:
- Line 1206: Add an explicit include for the <optional> header in the file that
defines the use of std::optional (so that std::optional is available for the
declaration of std::optional<detail::third_party_presolve_result_t<i_t, f_t>>
result;). Locate the top-of-file includes in solve.cu and insert `#include`
<optional> (before or alongside other standard headers) so the code no longer
relies on transitive includes from third_party_presolve.hpp.

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ℹ️ Review info

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between 65afe84 and d21b147.

📒 Files selected for processing (2)

• cpp/include/cuopt/linear_programming/optimization_problem.hpp
• cpp/src/pdlp/solve.cu

[tmckayus]

actually it looks like this exact fix is need on #908

perhaps the memory model changes set up the right conditions

[tmckayus]

this change was included in #908