Looking here, the internal length of the vector is only adjusted at the end of the loop

This has the caveat that if cloning panics, the Vec will start to unwind and only drop the first len elements which doesn't include the most recently appended ones.

Something more in line with how the stdlib does things might look like this:
https://github.com/LeonineKing1199/minivec/blob/424354dfababae7101aacf70fa9332a87e9cdb15/src/lib.rs#L1612-L1700