Fix aligment downcast check in TryAsAligned

[alexlarsson]

We can't use core::mem::align_of on the Alignment types, they are always aligned 1. Instead use ::ALIGNMENT.

Also adds a unit test of this, which breaks before and now works.

[gemini-code-assist]

Summary of Changes

Hello @alexlarsson, 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 fixes an issue in the alignment downcast check within the TryAsAligned and TryAsAlignedMut implementations. It replaces the incorrect usage of core::mem::align_of with the correct FromA::ALIGNMENT to ensure proper alignment checks. Additionally, a unit test has been added to validate the fix and prevent regressions.

Highlights

• Alignment Check: Replaces core::mem::align_of with FromA::ALIGNMENT for alignment checks in TryAsAligned and TryAsAlignedMut implementations.
• Unit Test: Adds a new unit test to verify the correct behavior of TryAsAligned with aligned and misaligned data.

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[alexlarsson]

Unclear why the CI failed, but seems unrelated

[gemini-code-assist]

Code Review

This pull request correctly fixes a bug in the alignment downcast check where core::mem::align_of was used on ZSTs, which always returned 1. The fix correctly uses the ALIGNMENT associated constant instead. A new unit test is added to verify the fix and prevent regressions. My review includes a suggestion to improve the new test by using a safer, more explicit method for creating aligned data, which avoids using unsafe code.

[wmanley]

I will have broken this originally in 8492cad. Thanks for the fix.