Use std::move_only_function for parallel evel work items

[edolstra]

Motivation

This avoids having to put RootValues in std::shared_ptrs.

Context

Summary by CodeRabbit

• Performance
  • Improved efficiency during parallel evaluation and deep value processing.
  • Reduced overhead when handling concurrent tasks and recursively processing complex values.
  • Improved responsiveness for workloads involving extensive parallel computation or JSON conversion.
• Reliability
  • Enhanced handling of parallel work for safer execution.
  • Improved support for non-copyable tasks, reducing potential issues during concurrent processing.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: bd4e71ea-6efa-47eb-bbb6-41a45ed2cddc

📥 Commits

Reviewing files that changed from the base of the PR and between 78339f6 and 539d5f9.

📒 Files selected for processing (3)

• src/libexpr/include/nix/expr/parallel-eval.hh
• src/libutil/include/nix/util/meson.build
• src/libutil/include/nix/util/move-only-function.hh

🚧 Files skipped from review as they are similar to previous changes (2)

• src/libutil/include/nix/util/meson.build
• src/libexpr/include/nix/expr/parallel-eval.hh

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📝 Walkthrough

Walkthrough

The parallel evaluation work-item type now accepts move-only callables through a portable MoveOnlyFunction wrapper. Single-item spawning forwards an explicit work-item vector. Evaluation and JSON deep-forcing callbacks capture RootValue directly.

Changes

Parallel evaluation ownership

Layer / File(s)	Summary
Move-only function implementation src/libutil/include/nix/util/move-only-function.hh, src/libutil/include/nix/util/meson.build	Adds the MoveOnlyFunction wrapper with a standard-library implementation path and a move-only type-erased fallback. Registers the header in the public header list.
Move-only work-item API src/libexpr/include/nix/expr/parallel-eval.hh	Executor::work_t now uses MoveOnlyFunction<void()>. The single-item FutureVector::spawn overload constructs and forwards an Executor::WorkItems vector.
Direct RootValue callback captures src/libexpr/parallel-eval.cc, src/libexpr/value-to-json.cc	Callbacks capture RootValue directly and use a single dereference when forcing values.

Estimated code review effort: 2 (Simple) | ~15 minutes

Possibly related PRs

• DeterminateSystems/nix-src#125: Introduces the parallel evaluator APIs refined by the move-only work-item changes.
• DetermateSystems/nix-src#546: Contains related RootValue handling in parallel evaluation code.

Suggested reviewers: cole-h

🚥 Pre-merge checks | ✅ 4 | ❌ 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 (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly describes the main change to use move-only functions for parallel evaluator work items, despite a minor typo in "evel."
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches 💡 1

📝 Generate docstrings 💡

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch move-work-items

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[coderabbitai]

Actionable comments posted: 1

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Verify each finding against current code. Fix only still-valid issues, skip the
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Inline comments:
In `@src/libutil/include/nix/util/move-only-function.hh`:
- Around line 42-43: Update the fallback `Impl` constructor in
`move-only-function.hh` to use forwarding-reference deduction while constraining
construction against the decayed callable type, so copyable lvalue callables are
accepted. Preserve move construction for rvalues and initialize the stored `f`
member through the forwarded argument.

🪄 Autofix

Fix all unresolved CodeRabbit comments on this PR:

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: c7ce8a66-099d-47f6-a501-00be6c01225b

📥 Commits

Reviewing files that changed from the base of the PR and between d9ef669 and 78339f6.

📒 Files selected for processing (3)

• src/libexpr/include/nix/expr/parallel-eval.hh
• src/libutil/include/nix/util/meson.build
• src/libutil/include/nix/util/move-only-function.hh

🚧 Files skipped from review as they are similar to previous changes (1)

• src/libexpr/include/nix/expr/parallel-eval.hh