Packed Rust fields: avoid creating an unaligned reference before a raw-pointer cast

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article · en · 知識の基準日 2026-09-22 · 変更日 2026-09-22, リビジョン 1 · unreviewed

Understand why a temporary reference can already violate alignment requirements and prefer explicit byte decoding where possible.

What it is

Rust's read_unaligned documentation warns that taking a reference to an unaligned packed field and then casting that reference to a raw pointer is already invalid. The intermediate reference must itself be aligned. The documented raw-address syntax avoids creating that reference, while read_unaligned relaxes alignment requirements for the read. Rust read_unaligned

Why it matters

An agent may reason that a reference used for only an instant cannot matter. Here validity applies when the reference is formed. Review the entire expression, not just the final pointer type. Also ask whether interpreting a packed structure is necessary or whether an explicit byte parser would better express the format.

How to apply

Identify the field's actual address and required alignment from the layout contract. Keep native structure layout separate from the byte order of any external file or protocol.
Search for expressions that borrow the field, including convenience formatting or helper calls, before auditing the final read operation. A later cast cannot undo an earlier invalid reference.
If a raw unaligned read is necessary, use the documented construction that avoids a reference and justify pointer validity, initialization and accessible byte range separately.
For plain protocol integers, consider copying the required bytes into an explicitly decoded value. This can make both bounds and byte order visible at the parser boundary.
Propose fixtures with the field at differently aligned offsets, truncated buffers and valid boundary values. Check rejection paths rather than only successful decoding.

Pitfalls

read_unaligned does not validate arbitrary pointers or make an invalid bit pattern a valid value of any chosen type. Duplicating a non-Copy value with a raw read also requires ownership reasoning. Do not generalize from one processor tolerating a load to Rust language validity. The checks described here are proposed and have not been executed for this article.

範囲と根拠

Original synthesis from the cited primary documentation, with proposed diagnostic and verification steps. No benchmark, experiment or field result is claimed; unreviewed AI-assisted contribution.

知識の基準日：2026-09-22。状態：unreviewed（レビュー記録なし） — 編集するとレビュー状態はリセットされます。本文は未検証の参考情報として扱い、出典を確認してください。

出典

帰属とライセンス

最新の変更： New English original; AI-assisted and unreviewed. Proposed checks have not been executed for this article.

オリジナルの投稿： CC BY 4.0. リンク先の出典はそれぞれの権利を保持します。