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Replace once_cell with std::sync::OnceLock/core::cell::OnceCell#7077

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youknowone merged 2 commits intoRustPython:mainfrom
youknowone:once-cell
Feb 11, 2026
Merged

Replace once_cell with std::sync::OnceLock/core::cell::OnceCell#7077
youknowone merged 2 commits intoRustPython:mainfrom
youknowone:once-cell

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@youknowone youknowone commented Feb 11, 2026 •
edited by coderabbitai bot
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  • Replace once_cell::sync::{Lazy, OnceCell} with std::sync::{LazyLock, OnceLock}
  • Replace once_cell::unsync::{Lazy, OnceCell} with core::cell::{LazyCell, OnceCell}
  • Inline get_or_try_init at call sites (unstable in std as of 1.93)
  • Replace OnceCell::with_value() with OnceCell::from() in codecs.rs
  • Remove once_cell direct dependency from common and vm crates

Summary by CodeRabbit

  • Chores
    • Removed an external dependency and refactored internal lazy initialization patterns throughout the codebase.

@youknowone
Replace once_cell with std::sync::OnceLock/core::cell::OnceCell
a5ca61b 
- Replace `once_cell::sync::{Lazy, OnceCell}` with
  `std::sync::{LazyLock, OnceLock}`
- Replace `once_cell::unsync::{Lazy, OnceCell}` with
  `core::cell::{LazyCell, OnceCell}`
- Inline `get_or_try_init` at call sites (unstable in std as of 1.93)
- Replace `OnceCell::with_value()` with `OnceCell::from()` in codecs.rs
- Remove `once_cell` direct dependency from common and vm crates
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coderabbitai bot commented Feb 11, 2026 •
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📝 Walkthrough

Walkthrough

The PR removes the once_cell crate dependency from the project by migrating to standard library equivalents: std::sync::LazyLock/OnceLock for threading contexts and core::cell::LazyCell/OnceCell for non-threaded contexts. Implementation details across multiple VM and common modules are updated to use explicit lazy initialization patterns.

Changes

Cohort / File(s) Summary
Dependency Removals
Cargo.toml, crates/common/Cargo.toml, crates/vm/Cargo.toml		 Removed once_cell dependencies from workspace and crate manifests.
Lock Module Reexports
crates/common/src/lock.rs		 Updated public re-exports to alias stdlib types: LazyLock as Lazy and OnceLock as OnceCell (threaded), or LazyCell as Lazy and OnceCell (non-threaded), replacing once_cell crate references.
Static Cell Implementation
crates/common/src/static_cell.rs		 Replaced get_or_try_init delegation with explicit initialization: check for existing value via get(), compute if absent, store via set(), and return reference.
VM Lazy Initialization
crates/vm/src/builtins/function.rs, crates/vm/src/builtins/memory.rs, crates/vm/src/codecs.rs, crates/vm/src/stdlib/os.rs		 Replaced get_or_try_init closures with explicit check-compute-cache patterns using get() and set() calls. Updated initialization of exception cells to use From-based construction instead of with_value.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Possibly related PRs

Poem

🐰 The once\_cell is gone, hooray, hooray!
We've swapped the crate for stdlib's way
LazyLock and OnceLock now take the stage
Explicit get and set fill every page
Built-in types, no external cage! ✨

🚥 Pre-merge checks | ✅ 2 | ❌ 1
❌ Failed checks (1 warning)
Check name Status Explanation Resolution
Docstring Coverage ⚠️ Warning Docstring coverage is 45.45% 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 accurately summarizes the main change: replacing the once_cell crate with standard library equivalents (std::sync::OnceLock and core::cell::OnceCell).

✏️ Tip: You can configure your own custom pre-merge checks in the settings.

✨ Finishing touches
  • 📝 Generate docstrings
🧪 Generate unit tests (beta)
  • Create PR with unit tests
  • Post copyable unit tests in a comment

No actionable comments were generated in the recent review. 🎉

🧹 Recent nitpick comments
crates/common/src/static_cell.rs (1)

30-40: Threading variant: f() may be invoked concurrently by multiple threads.

Unlike OnceLock::get_or_try_init (which blocks competing callers), this manual get/set pattern allows multiple threads to evaluate f() simultaneously, discarding all but the winner's result. This is functionally safe but differs from the previous once_cell behavior which serialized initialization. If f() is expensive or has side effects, callers should be aware.

Given this is an accepted trade-off for avoiding unstable APIs, just flagging for awareness.

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github-actions bot commented Feb 11, 2026

Code has been automatically formatted

The code in this PR has been formatted using:

  • cargo fmt --all
    Please pull the latest changes before pushing again:
git pull origin once-cell

@youknowone youknowone marked this pull request as ready for review February 11, 2026 01:58
@youknowone youknowone merged commit c06cf56 into RustPython:main Feb 11, 2026
13 checks passed
@youknowone youknowone deleted the once-cell branch February 11, 2026 02:08
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