In a deep subdirectory, from the compiled result I find the file "entities.rs" with (among other things) this content:
impl LwPolyline {
pub fn get_is_closed(&self) -> bool {
self.flags & 1 != 0
}
pub fn set_is_closed(&mut self, val: bool) {
if val {
self.flags |= 1;
}
else {
self.flags &= !1;
}
}
pub fn get_is_pline_gen(&self) -> bool {
self.flags & 128 != 0
}
pub fn set_is_pline_gen(&mut self, val: bool) {
if val {
self.flags |= 128;
}
else {
self.flags &= !128;
}
}
}So the information, if a polyline is closed or not, is already available in the code ... somewhere.
But with NO knowledge of rust and the internals of dxf2elmt it's hard for me ... 
