Tracking issue for missing Iterator::size_hint implementations #49205

size_hint() can sometimes allow for more efficient memory allocations, and other good performance things.

I intend to make a PR to implement the trivial / easy ones, at least. I've already made a PR for the first batch, #49201. The ones in that PR are omitted from this list.

This list will be updated as I sort through for false-positives.

Liballoc:

• Range<'a, K, V> (Add size_hints for btree iterators #49550) This is hard, but doable, and useful.
• RangeMut<'a, K, V> (Add size_hints for btree iterators #49550) This is hard, but doable, and useful.
• MergeIter<K, V, I> (Add size_hints for btree iterators #49550) This is hard, but doable, and useful.
• Range<'a, T> (Add size_hints for btree iterators #49550) This is hard, but doable, and useful.

Libcore:

• Pattern: $forward_iterator<'a, P> Impossible to provide upper or lower bounds.
• Pattern: $reverse_iterator<'a, P> Impossible to provide upper or lower bounds.

Libfmt_macros:

• Parser<'a> Both impossible and irrelevant.

Libproc_macro:

• TokenTreeIter Implementation is not trivial. It would be nice to do it eventually, though.

Librustc:

• NodesMatchingSuffix<'o, 'tcx>
• EnumerateAndAdjust<I> (Implement some trivial size_hints for various iterators #49201)
• TraitObligationStackList<'o, 'tcx>
• Ancestors Not trivial, and probably going to change.
• Elaborator<'cx, 'gcx, 'tcx> No way to define an upper bound.
• SupertraitDefIds<'cx, 'gcx, 'tcx> No way to define an upper bound.
• TypeWalker<'tcx> No way to define an upper bound.
• Iter<'dom, Node>
• Iter<'a, T>
• Iter<'a>
• Prefixes<'cx, 'gcx, 'tcx>
• ValueIter
• Autoderef<'a, 'gcx, 'tcx>

Librustdoc:

• CodeBlocks<'a, I>
• LinkReplacer<'a, 'b, I>
• HeadingLinks<'a, 'b, I>
• SummaryLine<'a, I>
• Footnotes<'a, I>
• Parser<T>

Libstd:

• fs
  • ReadDir (Implement size_hint on various sys iterators #49552)(implement size_hint on sys iterators #71308)
• io
  • Bytes<R> There's no way to do this.
  • Chars<R> There's no way to do this.
  • Split<B> There's no way to do this.
• net
  • Lines<B> Depreciated.
  • Incoming<'a> There's no way to provide anything better than (1, None)
• path
  • Iter<'a> (Implement size_hint on various sys iterators #49552)
  • Components<'a> Writing a size_hint for this is hard. But it would be useful; this is often collect()-ed.
  • Ancestors<'a> Writing a size_hint for this is hard, and not very useful.
• sync
  • Iter<'a, T> Inherently unbounded iterator.
  • TryIter<'a, T> Inherently unbounded iterator.
  • IntoIter<T> Inherently unbounded iterator.
  • Packets Impossible to write an upper bound.
• sys
  • cloudabi
    • ReadDir (Implement size_hint on various sys iterators #49552) (implement size_hint on sys iterators #71308)
    • LookupHost (Implement size_hint on various sys iterators #49552) (implement size_hint on sys iterators #71308)
    • SplitPaths<'a> (Implement size_hint on various sys iterators #49552)(implement size_hint on sys iterators #71308)
  • redox
    • ReadDir (Implement size_hint on various sys iterators #49552)
    • LookupHost (Implement size_hint on various sys iterators #49552)
  • unix
    • ReadDir No cheap way to do this.
    • LookupHost (Implement size_hint on various sys iterators #49552)
  • wasm
    • ReadDir (Implement size_hint on various sys iterators #49552) (implement size_hint on sys iterators #71308)
    • LookupHost (Implement size_hint on various sys iterators #49552) (implement size_hint on sys iterators #71308)
    • SplitPaths<'a> (Implement size_hint on various sys iterators #49552) (implement size_hint on sys iterators #71308)
    • Env (Implement size_hint on various sys iterators #49552) (implement size_hint on sys iterators #71308)
  • windows
    • ReadDir There's no cheap way to do this.
    • Env There's no safe way to do this.
    • SplitPaths<'a> (Implement size_hint on various sys iterators #49552) (implement size_hint on sys iterators #71308)
• sys_common
  • LookupHost There's no safe way to do this.
• unicode
  • ToLowercase (Implement size_hint for several unicode-based iterators. #50208)
  • ToUppercase (Implement size_hint for several unicode-based iterators. #50208)
  • CaseMappingIter (Implement size_hint for several unicode-based iterators. #50208)
  • Utf8LossyChunksIter<'a> (Implement size_hint for several unicode-based iterators. #50208)
  • SplitWhitespace<'a> (Implement size_hint for several unicode-based iterators. #50208)

Libsyntax:

• Frame
• Cursor

Cargo:
rust-lang/cargo#5211

I found them by running grep -r -n "impl.*\bIterator for \|fn size_hint" src > size_hint_todo.txt. Then, delete any pairs of an Iterator line followed by a size_hint line, and manually check the remaining Iterator lines for false positives.

It may be a good idea to make a lint for implementing size_hint. It would be opt-in, of course. It's really easy to forget, even if you're trying to optimize performance.