Called
.take_while(..) on an iterator produced by HashSet::drain276 let mut area = HashSet::new();
277 loop {
278 if queue.is_empty() {
279 break area.drain().take_while(|pt: &MapPoint| pt.x > 0).collect();280 } else {
281 let last = queue.pop().unwrap_or_else(Default::default);
282 area.insert(last);
Description
Iterator methods such as .skip_while(_) and .take_while(_) are usually used
to manipulate a section of an ordered iterator. However, iteration methods upon
std::collections::HashMap and std::collections::HashSet iterate in an
arbitrary order, which can result in non-deterministic behaviour when paired
with .skip_while(_) or .take_while(_).
The following methods are typically used upon ordered iterators:
skip_whiletake_whilemap_whilenthpositionrpositionrevstep_bylast
The following methods produce arbitrarily ordered iterators:
HashMap::iterorHashSet::iterHashMap::into_iterorHashSet::into_iterHashMap::drainorHashSet::drainHashMap::drain_filterorHashSet::drin_filterHashMap::keysHashMap::valuesHashMap::into_keysHashMap::into_valuesHashMap::values_mutHashMap::iter_mutHashSet::differenceHashSet::intersectionHashSet::symmetric_differenceHashSet::union
Using methods from both categories in conjunction can result in
non-deterministic behaviour. Consider using a structure where the ordering
between elements is well-defined, such as std::collections::BTreeMap or
std::collections::BTreeSet.
Bad practice
let mut squares: HashMap<usize, usize> = [(1, 1), (2, 4), (3, 9)].into();
// this may or may not refer to (2, 4)
let second = squares.iter().nth(1).unwrap();
Recommended
let mut squares: BTreeMap<usize, usize> = [(1, 1), (2, 4), (3, 9)].into();
// always refers to (2, 4), the ordering is predictable
let second = squares.iter().nth(1).unwrap();