The request names once_cell, a Rust crate for lazy and one-time initialization, which is not among the candidate products; none of the sources describe it. The listed sources are general platforms and unrelated libraries (Python, Rust HTTP and async runtimes, DataFrames, YAML), so no product here genuinely provides a once_cell polyfill. Please check the crates.io or Rust documentation for once_cell or std::sync::OnceLock directly.
A question back: Do you want a Rust-side replacement for once_cell (for example std::sync::OnceLock/LazyLock), or a JavaScript polyfill for a similarly named package?
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End-to-end skill for building, testing, linting, versioning, and publishing a production-grade Python library to PyPI. Covers all four build backends (setuptools+setuptools_scm, hatchling, flit, poetry), PEP 440 versioning, semantic versioning, dynamic git-tag versioning, OOP/SOLID design, type hint
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