[derive] Allow TryFromBytes on non-Immutable unions

[joshlf]

Makes progress on #5
Closes #1831
Closes #1832

---

• 　 Simplify try_transmute! (and friends) internals #2938
• 　 [util] use more items to make code more concise #2942
• 👉 [derive] Allow TryFromBytes on non-Immutable unions #2876

Latest Update: v86 — Compare vs v85

📚 Full Patch History

Links show the diff between the row version and the column version.

Version	v85	v84	v83	v82	v81	v80	v79	v78	v77	v76	v75	v74	v73	v72	v71	v70	v69	v68	v67	v66	v65	v64	v63	v62	v61	v60	v59	v58	v57	v56	v55	v54	v53	v52	v51	v50	v49	v48	v47	v46	v45	v44	v43	v42	v41	v40	v39	v38	v37	v36	v35	v34	v33	v32	v31	v30	v29	v28	v27	v26	v25	v24	v23	v22	v21	v20	v19	v18	v17	v16	v15	v14	v13	v12	v11	v10	v9	v8	v7	v6	v5	v4	v3	v2	v1	Base
v86	v85	v84	v83	v82	v81	v80	v79	v78	v77	v76	v75	v74	v73	v72	v71	v70	v69	v68	v67	v66	v65	v64	v63	v62	v61	v60	v59	v58	v57	v56	v55	v54	v53	v52	v51	v50	v49	v48	v47	v46	v45	v44	v43	v42	v41	v40	v39	v38	v37	v36	v35	v34	v33	v32	v31	v30	v29	v28	v27	v26	v25	v24	v23	v22	v21	v20	v19	v18	v17	v16	v15	v14	v13	v12	v11	v10	v9	v8	v7	v6	v5	v4	v3	v2	v1	Base
v85		v84																																																																																				Base
v84			v83																																																																																			Base
v83				v82																																																																																		Base
v82					v81																																																																																	Base
v81						v80																																																																																Base
v80							v79																																																																															Base
v79								v78																																																																														Base
v78									v77																																																																													Base
v77										v76																																																																												Base
v76											v75																																																																											Base
v75												v74																																																																										Base
v74													v73																																																																									Base
v73														v72																																																																								Base
v72															v71																																																																							Base
v71																v70																																																																						Base
v70																	v69																																																																					Base
v69																		v68																																																																				Base
v68																			v67																																																																			Base
v67																				v66																																																																		Base
v66																					v65																																																																	Base
v65																						v64																																																																Base
v64																							v63																																																															Base
v63																								v62																																																														Base
v62																									v61																																																													Base
v61																										v60																																																												Base
v60																											v59																																																											Base
v59																												v58																																																										Base
v58																													v57																																																									Base
v57																														v56																																																								Base
v56																															v55																																																							Base
v55																																v54																																																						Base
v54																																	v53																																																					Base
v53																																		v52																																																				Base
v52																																			v51																																																			Base
v51																																				v50																																																		Base
v50																																					v49																																																	Base
v49																																						v48																																																Base
v48																																							v47																																															Base
v47																																								v46																																														Base
v46																																									v45																																													Base
v45																																										v44																																												Base
v44																																											v43																																											Base
v43																																												v42																																										Base
v42																																													v41																																									Base
v41																																														v40																																								Base
v40																																															v39																																							Base
v39																																																v38																																						Base
v38																																																	v37																																					Base
v37																																																		v36																																				Base
v36																																																			v35																																			Base
v35																																																				v34																																		Base
v34																																																					v33																																	Base
v33																																																						v32																																Base
v32																																																							v31																															Base
v31																																																								v30																														Base
v30																																																									v29																													Base
v29																																																										v28																												Base
v28																																																											v27																											Base
v27																																																												v26																										Base
v26																																																													v25																									Base
v25																																																														v24																								Base
v24																																																															v23																							Base
v23																																																																v22																						Base
v22																																																																	v21																					Base
v21																																																																		v20																				Base
v20																																																																			v19																			Base
v19																																																																				v18																		Base
v18																																																																					v17																	Base
v17																																																																						v16																Base
v16																																																																							v15															Base
v15																																																																								v14														Base
v14																																																																									v13													Base
v13																																																																										v12												Base
v12																																																																											v11											Base
v11																																																																												v10										Base
v10																																																																													v9									Base
v9																																																																														v8								Base
v8																																																																															v7							Base
v7																																																																																v6						Base
v6																																																																																	v5					Base
v5																																																																																		v4				Base
v4																																																																																			v3			Base
v3																																																																																				v2		Base
v2																																																																																					v1	Base
v1																																																																																						Base

[gemini-code-assist]

Summary of Changes

Hello @joshlf, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the flexibility of the zerocopy crate by removing the Immutable trait requirement for deriving TryFromBytes, FromZeros, and FromBytes for unions. This change allows unions containing types like UnsafeCell, which are not inherently Immutable, to correctly implement these traits, broadening the utility of the derive macros without compromising memory safety.

Highlights

• Removed Immutable Trait Bound for Unions: The Immutable trait bound has been removed for TryFromBytes, FromZeros, and FromBytes derives on unions, allowing these derives to be used with unions containing non-Immutable types.
• Updated Safety Justification: The safety comment for TryFromBytes on unions has been updated to clarify that safety is ensured by ReadOnly<Self>: Immutable, which holds unconditionally, rather than Self: Immutable.
• New Test Cases for UnsafeCell Unions: New test cases have been added for unions containing UnsafeCell to validate the updated derive behavior for FromBytes, FromZeros, and TryFromBytes.

🧠 New Feature in Public Preview: You can now enable Memory to help Gemini Code Assist learn from your team's feedback. This makes future code reviews more consistent and personalized to your project's style. Click here to enable Memory in your admin console.

Using Gemini Code Assist

The full guide for Gemini Code Assist can be found on our documentation page, here are some quick tips.

Invoking Gemini

You can request assistance from Gemini at any point by creating a comment using either /gemini <command> or @gemini-code-assist <command>. Below is a summary of the supported commands on the current page.

Feature	Command	Description
Code Review	/gemini review	Performs a code review for the current pull request in its current state.
Pull Request Summary	/gemini summary	Provides a summary of the current pull request in its current state.
Comment	@gemini-code-assist	Responds in comments when explicitly tagged, both in pull request comments and review comments.
Help	/gemini help	Displays a list of available commands.

Customization

To customize Gemini Code Assist for GitHub experience, repository maintainers can create a configuration file and/or provide a custom code review style guide (such as PEP-8 for Python) by creating and adding files to a .gemini/ folder in the base of the repository. Detailed instructions can be found here.

Limitations & Feedback

Gemini Code Assist may make mistakes. Please leave feedback on any instances where its feedback is incorrect or counter productive. You can react with 👍 and 👎 on @gemini-code-assist comments. If you're interested in giving your feedback about your experience with Gemini Code Assist for Github and other Google products, sign up here.

You can also get AI-powered code generation, chat, as well as code reviews directly in the IDE at no cost with the Gemini Code Assist IDE Extension.

Footnotes

1. Review the Privacy Notices, Generative AI Prohibited Use Policy, Terms of Service, and learn how to configure Gemini Code Assist in GitHub here. Gemini can make mistakes, so double check it and use code with caution. ↩

[gemini-code-assist]

Code Review

This pull request correctly removes the Immutable bound requirement for fields of unions when deriving TryFromBytes, FromZeros, and FromBytes. This is a significant improvement that allows unions containing types with interior mutability (like UnsafeCell) to use these derives. The implementation ensures safety by using a ReadOnly wrapper during validity checks, and the corresponding safety comments are updated accordingly. The change is supported by new tests that verify the new functionality. The implementation is sound and the code quality is high.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.30%. Comparing base (a7d3fda) to head (f0942c1).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2876   +/-   ##
=======================================
  Coverage   92.30%   92.30%           
=======================================
  Files          19       19           
  Lines        5849     5849           
=======================================
  Hits         5399     5399           
  Misses        450      450           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[joshlf]

/gemini review

[gemini-code-assist]

Code Review

This pull request successfully addresses the FIXME(#5) comments by removing the Immutable bound from the derive_from_zeros_union, derive_from_bytes_union, and derive_try_from_bytes_union functions. The safety comments have been updated to reflect this change, and new test cases for UnsafeCellUnion have been added to validate that unions containing non-Immutable types can now correctly derive these traits. The changes are well-implemented and thoroughly tested.