use ALIGN for Push::alignment in struct types

[bkietz]

This PR adds an override of the default Push::alignment(), which is incorrectly 1 for structs because they are byte arrays and this doesn't accurately reflect the type's alignment requirements.

fixes #8150

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[bkietz]

@evgenyx00 @tustvold

[alamb]

I wonder if we should add a test to this PR to make sure there are no regressions

[bkietz]

I was not sure where to add such a test, but I'd be happy to. I opened apache/arrow-rs#6426 since that seemed like a good way to check for regressions (I didn't see any). @aardappel @CasperN any recommendations?

[aardappel]

@bkietz yes we generally should have tests :) Not too familiar with the Rust tests though, have a look.

[bkietz]

I've regenerated and added a test.

• struct_vec3_is_written_with_correct_alignment_in_table did not catch this bug before; adding a struct to a Table also goes through the Push trait. I could extend it (for example using a loop over multiple preceding strings to ensure at least one iteration will require padding for alignment)
• The pull request template suggests that building should also regenerate. However, AFAICT cmake will only regenerate for C++. Perhaps cmake should instead invoke generate_code.py?
• Could a reviewer please approve the CI workflows so we can see if I've missed anything?

[alamb]

Thank you @bkietz

I am not familiar with this codebase, but I did verify that the test you added fails without the code in this PR

To test I did:

cd flatbuffers/tests/rust_usage_test
cargo test --test integration_test

The test fails like this without the PR

---- flatbuffers_tests::push_impls::push_u8_generated_struct_alignment stdout ----
thread 'flatbuffers_tests::push_impls::push_u8_generated_struct_alignment' panicked at tests/integration_test.rs:2360:9:
assertion `left == right` failed
  left: [10, 0, 20, 0, 0, 1]
 right: [10, 0, 20, 0, 1]

Seem like an improvement to me (the 1u8 is now aligned at a byte boundary rather than a u16 boundary)

[bkietz]

@dbaileychess could you review and/or approve the CI workflows?

[bkietz]

@aardappel whom can we ask to review this?

[aardappel]

I'd say either @CasperN or @dbaileychess

[bkietz]

Thanks!