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Constructing a reference to an invalid value is always UB, according to this documentation.
In addition, even just the
set_len
call can cause UB if we panic immediately after it (and thus try to drop the invalid value).The new implementation is taken from
Vec::push
. Hat tip to @guswynn , @doy-materialize, and @petrosagg for talking me through why it was indeed UB, and the latter for also suggesting theVec::push
implementation.