rfold is the reverse version of fold.
Fold allows iterators to implement a different (non-resumable) internal
iteration when it is more efficient than the external iteration
implemented through the next method. (Common examples are VecDeque and
.chain()).
Introduce rfold() so that the same customization is available for
reverse iteration. This is achieved by both adding the method, and by
having the Rev<I> adaptor connect Rev::rfold -> I::fold, Rev::fold -> I::rfold.
Adaptors are things that take iterators and adapt them into other
iterators. With this definition, fold is just a usual method, because it
doesn't normally make an iterator.
Add an overflow check in the Iter::next() impl for Range<_> to help with vectorization.
This helps with vectorization in some cases, such as (0..u16::MAX).collect::<Vec<u16>>(),
as LLVM is able to change the loop condition to use equality instead of less than and should help with #43124. (See also my [last comment](https://github.com/rust-lang/rust/issues/43124#issuecomment-319098625) there.) This PR makes collect on ranges of u16, i16, i8, and u8 **significantly** faster (at least on x86-64 and i686), and pretty close, though not quite equivalent to a [manual unsafe implementation](https://is.gd/nkoecB). 32 ( and 64-bit values on x86-64) bit values were already vectorized without this change, and they still are. This PR doesn't seem to help with 64-bit values on i686, as they still don't vectorize well compared to doing a manual loop.
I'm a bit unsure if this was the best way of implementing this, I tried to do it with as little changes as possible and avoided changing the step trait and the behavior in RangeFrom (I'll leave that for others like #43127 to discuss wider changes to the trait). I tried simply changing the comparison to `self.start != self.end` though that made the compiler segfault when compiling stage0, so I went with this method instead for now.
As for `next_back()`, reverse ranges seem to optimise properly already.
This helps with vectorization in some cases, such as (0..u16::MAX).collect::<Vec<u16>>(),
as LLVM is able to change the loop condition to use equality instead of less than
Add `Iterator::for_each`
This works like a `for` loop in functional style, applying a closure to
every item in the `Iterator`. It doesn't allow `break`/`continue` like
a `for` loop, nor any other control flow outside the closure, but it may
be a more legible style for tying up the end of a long iterator chain.
This was tried before in #14911, but nobody made the case for using it
with longer iterators. There was also `Iterator::advance` at that time
which was more capable than `for_each`, but that no longer exists.
The `itertools` crate has `Itertools::foreach` with the same behavior,
but thankfully the names won't collide. The `rayon` crate also has a
`ParallelIterator::for_each` where simple `for` loops aren't possible.
> I really wish we had `for_each` on seq iterators. Having to use a
> dummy operation is annoying. - [@nikomatsakis][1]
[1]: https://github.com/nikomatsakis/rayon/pull/367#issuecomment-308455185
Replaced by adding extra imports, adding hidden code (`# ...`), modifying
examples to be runnable (sorry Homura), specifying non-Rust code, and
converting to should_panic, no_run, or compile_fail.
Remaining "```ignore"s received an explanation why they are being ignored.
Change the for-loop desugar so the `break` does not affect type inference. Fixes#42618
Rewrite the `for` loop desugaring to avoid contaminating the inference results. Under the older desugaring, `for x in vec![] { .. }` would erroneously type-check, even though the type of `vec![]` is unconstrained. (written by @nikomatsakis)
The benefit of using internal iteration is shown in new benchmarks:
test iter::bench_for_each_chain_fold ... bench: 635,110 ns/iter (+/- 5,135)
test iter::bench_for_each_chain_loop ... bench: 2,249,983 ns/iter (+/- 42,001)
test iter::bench_for_each_chain_ref_fold ... bench: 2,248,061 ns/iter (+/- 51,940)
This works like a `for` loop in functional style, applying a closure to
every item in the `Iterator`. It doesn't allow `break`/`continue` like
a `for` loop, nor any other control flow outside the closure, but it may
be a more legible style for tying up the end of a long iterator chain.
This was tried before in #14911, but nobody made the case for using it
with longer iterators. There was also `Iterator::advance` at that time
which was more capable than `for_each`, but that no longer exists.
The `itertools` crate has `Itertools::foreach` with the same behavior,
but thankfully the names won't collide. The `rayon` crate also has a
`ParallelIterator::for_each` where simple `for` loops aren't possible.
> I really wish we had `for_each` on seq iterators. Having to use a
> dummy operation is annoying. - [@nikomatsakis][1]
[1]: https://github.com/nikomatsakis/rayon/pull/367#issuecomment-308455185
Change for-loop desugar to not borrow the iterator during the loop
This is enables the use of suspend points inside for-loops in movable generators. This is illegal in the current desugaring as `iter` is borrowed across the body.
Override size_hint and propagate ExactSizeIterator for iter::StepBy
Generally useful, but also a prerequisite for moving a bunch of unit tests off `Range*::step_by`.
A small non-breaking subset of https://github.com/rust-lang/rust/pull/42110 (which I closed).
Includes two small documentation changes @ivandardi requested on that PR.
r? @alexcrichton
Make RangeInclusive just a two-field struct
Not being an enum improves ergonomics and consistency, especially since NonEmpty variant wasn't prevented from being empty. It can still be iterable without an extra "done" bit by making the range have !(start <= end), which is even possible without changing the Step trait.
Implements merged https://github.com/rust-lang/rfcs/pull/1980; tracking issue https://github.com/rust-lang/rust/issues/28237.
This is definitely a breaking change to anything consuming `RangeInclusive` directly (not as an Iterator) or constructing it without using the sugar. Is there some change that would make sense before this so compilation failures could be compatibly fixed ahead of time?
r? @aturon (as FCP proposer on the RFC)
