facet-rs / facet / 19884224451

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Fix UB in PeekEnum::discriminant() for RustNPO ZSTs

PeekEnum::discriminant() previously fell through to a default case
that read a u32 from memory for EnumRepr::RustNPO types. For zero-sized
types like Option<Opaque<Infallible>>, this caused undefined behavior
by reading from a 0-byte allocation.

This fix explicitly handles RustNPO in the match, returning 0 (the
declared discriminant for NPO variants) instead of reading memory.

Fixes: https://github.com/facet-rs/facet/issues/998#issuecomment-3605191431

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