Missing size_hint overrides in some Iterators implementations

[SimonSapin]

Some implementations of Iterator don’t override the size_hint method (and keep the conservative (0, None) default), but should. This includes at least:

impl<A: Int> Iterator for StepBy<A, ::ops::Range<A>>
impl<A: Int> Iterator for ::ops::RangeFrom<A>
impl<A: Int> Iterator for RangeStepInclusive<A>

CC @alexcrichton

[alexcrichton]

cc @aturon, this may some day tie into std::num stabilization and how Int is going away.

[SimonSapin]

Yes, these implementations (and other) will need to change if Int is going away, but isn’t that orthogonal to size_hint?

[alexcrichton]

The bounds may change somewhat if we expect to be able to define size_hint, but it is largely orthogonal, yes.

[Mark-Simulacrum]

I've made a list of the things which are currently missing size_hint overrides. Some of these may be fine with the default.

List found with rg 'impl.*\bIterator for ', though some items have been eliminated.

• impl<I: Iterator<Item = u8>> Iterator for DecodeUtf8<I>
• impl<T, Iter: Iterator<Item=Option<T>>> Iterator for Adapter<Iter>
• impl<I, T, E> Iterator for ResultShunt<I, E>
• impl<'a, T> Iterator for CycleIter<'a, T>
• impl<'a, P: Pattern<'a>> Iterator for $forward_iterator<'a, P>
• impl<'a, P: Pattern<'a>> Iterator for $reverse_iterator<'a, P>
• impl<'a, K, V> Iterator for Range<'a, K, V>
• impl<'a, K, V> Iterator for RangeMut<'a, K, V>
• impl<K: Ord, V, I: Iterator<Item = (K, V)>> Iterator for MergeIter<K, V, I>
• impl<'a, T> Iterator for Range<'a, T>
• impl<I> Iterator for EnumerateAndAdjust<I> where I: Iterator
• impl<'a> Iterator for Iter<'a>
• impl<'a, 'tcx> Iterator for Preorder<'a, 'tcx> {
• impl<'a, 'tcx> Iterator for Postorder<'a, 'tcx>
• impl<'a, 'tcx> Iterator for ReversePostorder<'a, 'tcx>
• impl<'tcx,I:Iterator<Item=ty::Predicate<'tcx>>> Iterator for FilterToTraits<I>
• impl<'a> Iterator for BitVectorIter<'a>
• impl<'a, 'hir> Iterator for NodesMatchingUII<'a, 'hir>
• impl<'g, N: Debug> Iterator for EnumeratedNodes<'g, N>
• impl<'g, E: Debug> Iterator for EnumeratedEdges<'g, E>
• impl<'g, N: Debug, E: Debug> Iterator for AdjacentTargets<'g, N, E>
• impl<'g, N: Debug, E: Debug> Iterator for AdjacentSources<'g, N, E>
• impl<'a> Iterator for AllTraits<'a>
• impl<'a> Iterator for Substitutions<'a>
• impl<'a> Iterator for Substitutions<'a>
• impl<'a> Iterator for ListAttributesIter<'a>

[Phlosioneer]

CycleIter is used just for testing, so it's irrelevant.
There is no sensible implementation of size_hint for $forward_iterator and $reverse_iterator, as far as I can tell.
The rest are addressed in my PR.

I've compiled a more complete list of iterators without size_hints, and will make a new tracking issue for them.

[Phlosioneer]

New tracking issue for expanded list: #49205

[SimonSapin]

Closing for #49205.