[mypyc] Fix undefined behavior related to empty vecs#21094
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p-sawicki approved these changes Mar 23, 2026
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We used to use
&vec.buf->items, which causes undefined behavior for an empty vec that can be triggered on GCC when using -O3, sincebufis NULL for an empty vec. Useoffsetofinstead to calculate struct field offset, since it doesn't trigger UB if pointer is NULL.