Are we sure that this is the only spot? Can you match on packed structs in match clauses? I suppose we're already guaranteed that the literals themselves work, just trying to think of wherever static structs can be used...

git grep -C 5 'C_struct' shows a few instances of C_struct(...) seemingly without a corresponding C_packed_struct, although I guess many are related to enums rather than structs.

(FWIW, it seems like it would be reasonable to just merge C_struct and C_packed_struct with a packed: bool parameter; it'd certainly make bugs like this less common if the author has to think about what to put there.)