Account for alignment directives in `derive(IntoBytes)` by jswrenn · Pull Request #3064 · google/zerocopy

jswrenn · 2026-02-23T22:11:46Z

This fixes #3063 by threading alignment directives into the padding
check macros, and fixes #3067 by unconditionally emitting the padding
check even in the absence of fields (thus ensuring that there exists a
padding check into which alignment directives can be threaded).

Fixes #3063, #3067

👉 Account for alignment directives in derive(IntoBytes) #3064

Latest Update: v7 — Compare vs v6

📚 Full Patch History

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

Version	v6	v5	v4	v3	v2	v1	Base
v7	vs v6	vs v5	vs v4	vs v3	vs v2	vs v1	vs Base
v6		vs v5	vs v4	vs v3	vs v2	vs v1	vs Base
v5			vs v4	vs v3	vs v2	vs v1	vs Base
v4				vs v3	vs v2	vs v1	vs Base
v3					vs v2	vs v1	vs Base
v2						vs v1	vs Base
v1							vs Base

⬇️ Download this PR

Branch

git fetch origin refs/heads/Gf18655827d1c30e4421c83579b962bf7aa453969 && git checkout -b pr-Gf18655827d1c30e4421c83579b962bf7aa453969 FETCH_HEAD

Checkout

git fetch origin refs/heads/Gf18655827d1c30e4421c83579b962bf7aa453969 && git checkout FETCH_HEAD

Cherry Pick

git fetch origin refs/heads/Gf18655827d1c30e4421c83579b962bf7aa453969 && git cherry-pick FETCH_HEAD

Pull

git pull origin refs/heads/Gf18655827d1c30e4421c83579b962bf7aa453969

Stacked PRs enabled by GHerrit.

gemini-code-assist · 2026-02-23T22:12:09Z

Summary of Changes

Hello @jswrenn, 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 enhances the derive(IntoBytes) macro to correctly interpret and apply #[repr(align)] and #[repr(packed)] attributes when calculating padding for dynamically sized types (DSTs). It introduces new parameters to the underlying padding calculation macros and updates the derive macro to extract these alignment directives from the type's attributes, ensuring more accurate memory layout computations and addressing a known issue.

Highlights

Macro Signature Update: Updated the signatures of internal padding calculation macros (struct_padding!, union_padding!, enum_padding!, repr_c_struct_has_padding!) to accept new align and packed arguments.
Attribute Parsing: Modified the derive(IntoBytes) macro to parse #[repr(align)] and #[repr(packed)] attributes from the derived type.
Alignment Directive Propagation: Passed the newly parsed align and packed directives from the type's attributes to the internal padding calculation macros for more accurate layout analysis.
Test Updates: Adjusted existing test calls for padding macros to conform to their new argument signatures, passing None for align and packed where not explicitly specified.
UI Test Output Adjustment: Updated the expected error output for union.msrv.stderr UI tests to reflect changes in macro parsing behavior and error reporting.

🧠 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.

Changelog

src/macros.rs
- Updated calls to struct_padding! and union_padding! within cryptocorrosion_derive_traits! to include None, None for new align and packed arguments.
src/util/macro_util.rs
- Modified struct_padding!, repr_c_struct_has_padding!, union_padding!, and enum_padding! macros to accept $align and $packed expressions.
- Updated repr_c_struct_has_padding! to pass the new align and packed arguments to DstLayout::for_repr_c_struct.
- Adjusted test calls for all padding macros to pass None, None for the new align and packed parameters.
zerocopy-derive/src/util.rs
- Imported NonZeroU32 and quote_spanned for attribute parsing.
- Implemented logic to parse #[repr(align)] and #[repr(packed)] attributes from the derived type's attributes.
- Passed the extracted repr_align and repr_packed values to the validator_macro! call.
zerocopy-derive/tests/ui/union.msrv.stderr
- Modified the expected output for UI tests to reflect a new macro parsing error and the removal of an old PaddingFree trait bound error.

Activity

The pull request is currently marked as Work In Progress ([wip]).
The author has noted that regression tests are still pending (TODO: Regression tests.).
This change is intended to fix issue derive(IntoBytes) accepts DSTs with trailing padding #3063.

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.

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 aims to account for alignment directives on Dynamically Sized Types (DSTs) when deriving IntoBytes. The changes involve updating internal macros to accept alignment and packing parameters and modifying the procedural macro logic to extract and pass these values. While the overall direction is correct, there are several issues to address: the use of unwrap() in the procedural macro is risky and redundant, the current implementation causes a regression in MSRV tests due to how paths are passed to macros, and the use of TODO comments violates the repository's policy of using FIXME for non-blocking issues to avoid CI failures.

zerocopy-derive/src/util.rs

src/util/macro_util.rs

zerocopy-derive/src/util.rs

codecov-commenter · 2026-02-23T23:20:38Z

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.87%. Comparing base (b7fc71d) to head (ea77523).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3064   +/-   ##
=======================================
  Coverage   91.87%   91.87%           
=======================================
  Files          20       20           
  Lines        6057     6057           
=======================================
  Hits         5565     5565           
  Misses        492      492

☔ 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.

jswrenn · 2026-02-25T18:04:05Z

@joshlf This test:

zerocopy/zerocopy-derive/src/output_tests/mod.rs

Lines 248 to 278 in b7fc71d

    
           #[test] 
        
           fn test_into_bytes_enum() { 
        
               macro_rules! test_repr { 
        
                   ($(#[$attr:meta])*) => { 
        
                       $(test! { 
        
                           IntoBytes { 
        
                               #[$attr] 
        
                               enum Foo { 
        
                                   Bar, 
        
                               } 
        
                           } expands to "expected/into_bytes_enum.expected.rs" 
        
                       })* 
        
                   }; 
        
               } 
        
               test_repr! { 
        
                   #[repr(C)] 
        
                   #[repr(u8)] 
        
                   #[repr(u16)] 
        
                   #[repr(u32)] 
        
                   #[repr(u64)] 
        
                   #[repr(u128)] 
        
                   #[repr(usize)] 
        
                   #[repr(i8)] 
        
                   #[repr(i16)] 
        
                   #[repr(i32)] 
        
                   #[repr(i64)] 
        
                   #[repr(i128)] 
        
                   #[repr(isize)] 
        
               } 
        
           }

...no longer passes, because the sub-tests no longer expand to the same thing (the different repr is now reflected in the expansion). Do you have a preference between either:

splatting to different output files
slimming down the test to just exercise one repr

joshlf · 2026-02-25T18:56:03Z

@joshlf This test:

zerocopy/zerocopy-derive/src/output_tests/mod.rs

Lines 248 to 278 in b7fc71d

#[test]

fn test_into_bytes_enum() {

macro_rules! test_repr {

($(#[$attr:meta])*) => {

$(test! {

IntoBytes {

#[$attr]

enum Foo {

Bar,

}

} expands to "expected/into_bytes_enum.expected.rs"

})*

};

}

test_repr! {

#[repr(C)]

#[repr(u8)]

#[repr(u16)]

#[repr(u32)]

#[repr(u64)]

#[repr(u128)]

#[repr(usize)]

#[repr(i8)]

#[repr(i16)]

#[repr(i32)]

#[repr(i64)]

#[repr(i128)]

#[repr(isize)]

}

}

...no longer passes, because the sub-tests no longer expand to the same thing (the different repr is now reflected in the expansion). Do you have a preference between either:

splatting to different output files

slimming down the test to just exercise one repr

Let's go with (1) – should be easy to get an agent to grind through it for you.

This fixes #3063 by threading alignment directives into the padding check macros, and fixes #3067 by unconditionally emitting the padding check even in the absence of fields (thus ensuring that there exists a padding check into which alignment directives can be threaded). Fixes #3063, #3067 gherrit-pr-id: Gf18655827d1c30e4421c83579b962bf7aa453969

joshlf

Please address these before merging, but otherwise LGTM!

joshlf · 2026-02-26T18:05:49Z

src/util/macro_util.rs

 #[macro_export]
 macro_rules! struct_padding {
-    ($t:ty, [$($ts:ty),*]) => {
+    ($t:ty, $align:expr, $packed:expr, [$($ts:ty),*]) => {{


Suggested change

($t:ty, $align:expr, $packed:expr, [$($ts:ty),*]) => {{

($t:ty, $_align:expr, $_packed:expr, [$($ts:ty),*]) => {{

joshlf · 2026-02-26T18:06:12Z

src/util/macro_util.rs

 #[macro_export]
 macro_rules! union_padding {
-    ($t:ty, [$($ts:ty),*]) => {{
+    ($t:ty, $align:expr, $packed:expr, [$($ts:ty),*]) => {{


Suggested change

($t:ty, $align:expr, $packed:expr, [$($ts:ty),*]) => {{

($t:ty, $_align:expr, $_packed:expr, [$($ts:ty),*]) => {{

joshlf · 2026-02-26T18:06:39Z

src/util/macro_util.rs

 #[macro_export]
 macro_rules! enum_padding {
-    ($t:ty, $disc:ty, $([$($ts:ty),*]),*) => {{
+    ($t:ty, $align:expr, $packed:expr, $disc:ty, $([$($ts:ty),*]),*) => {{


Suggested change

($t:ty, $align:expr, $packed:expr, $disc:ty, $([$($ts:ty),*]),*) => {{

($t:ty, $_align:expr, $packed:expr, $disc:ty, $([$($ts:ty),*]),*) => {{

joshlf · 2026-02-26T18:35:14Z

Mark #3072 (which releases 0.8.40) as merge-when-ready once this PR is in the merge queue?

gemini-code-assist bot reviewed Feb 23, 2026

View reviewed changes

zerocopy-derive/src/util.rs Outdated Show resolved Hide resolved

src/util/macro_util.rs Outdated Show resolved Hide resolved

zerocopy-derive/src/util.rs Show resolved Hide resolved

zerocopy-derive/src/util.rs Outdated Show resolved Hide resolved

jswrenn force-pushed the Gf18655827d1c30e4421c83579b962bf7aa453969 branch 2 times, most recently from 93d17cc to 1b397a6 Compare February 24, 2026 16:59

jswrenn mentioned this pull request Feb 25, 2026

derive(IntoBytes) accepts enums with padding #3067

Open

jswrenn force-pushed the Gf18655827d1c30e4421c83579b962bf7aa453969 branch from 1b397a6 to 0e20369 Compare February 25, 2026 17:58

jswrenn changed the title ~~[wip] Account for alignment directives on DSTs in derive(IntoBytes)~~ Account for alignment directives in derive(IntoBytes) Feb 26, 2026

jswrenn force-pushed the Gf18655827d1c30e4421c83579b962bf7aa453969 branch 2 times, most recently from 964c653 to fd1fc83 Compare February 26, 2026 16:12

jswrenn force-pushed the Gf18655827d1c30e4421c83579b962bf7aa453969 branch from fd1fc83 to ea77523 Compare February 26, 2026 17:56

jswrenn requested a review from joshlf February 26, 2026 18:01

joshlf approved these changes Feb 26, 2026

View reviewed changes

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Account for alignment directives in `derive(IntoBytes)`#3064

Account for alignment directives in `derive(IntoBytes)`#3064
jswrenn wants to merge 1 commit intomainfrom
Gf18655827d1c30e4421c83579b962bf7aa453969

jswrenn commented Feb 23, 2026 •

edited

Loading

Uh oh!

gemini-code-assist bot commented Feb 23, 2026

Uh oh!

gemini-code-assist bot left a comment

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

codecov-commenter commented Feb 23, 2026 •

edited

Loading

Uh oh!

jswrenn commented Feb 25, 2026

Uh oh!

joshlf commented Feb 25, 2026

Uh oh!

joshlf left a comment

Uh oh!

joshlf Feb 26, 2026

Uh oh!

joshlf Feb 26, 2026

Uh oh!

joshlf Feb 26, 2026

Uh oh!

joshlf commented Feb 26, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

3 participants

	($t:ty, $align:expr, $packed:expr, [$($ts:ty),*]) => {{
	($t:ty, $_align:expr, $_packed:expr, [$($ts:ty),*]) => {{

	($t:ty, $align:expr, $packed:expr, $disc:ty, $([$($ts:ty),]),) => {{
	($t:ty, $_align:expr, $packed:expr, $disc:ty, $([$($ts:ty),]),) => {{

Conversation

jswrenn commented Feb 23, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

gemini-code-assist bot commented Feb 23, 2026

Summary of Changes

Highlights

Footnotes

Uh oh!

gemini-code-assist bot left a comment

Choose a reason for hiding this comment

Code Review

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

codecov-commenter commented Feb 23, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Codecov Report

Uh oh!

jswrenn commented Feb 25, 2026

Uh oh!

joshlf commented Feb 25, 2026

Uh oh!

joshlf left a comment

Choose a reason for hiding this comment

Uh oh!

joshlf Feb 26, 2026

Choose a reason for hiding this comment

Uh oh!

joshlf Feb 26, 2026

Choose a reason for hiding this comment

Uh oh!

joshlf Feb 26, 2026

Choose a reason for hiding this comment

Uh oh!

joshlf commented Feb 26, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

3 participants

jswrenn commented Feb 23, 2026 •

edited

Loading

codecov-commenter commented Feb 23, 2026 •

edited

Loading