Repeat iterator always returns the same element and behaves the same way
backwards and forwards. Take iterator can trivially implement backwards
iteration over Repeat inner iterator by simply doing forwards iteration.
DoubleEndedIterator is not currently implemented for Take<Repeat<T>>
because Repeat doesn’t implement ExactSizeIterator which is a required
bound on DEI implementation for Take.
Similarly, since Repeat is an infinite iterator which never stops, Take
can trivially know how many elements it’s going to return. This allows
implementing ExactSizeIterator on Take<Repeat<T>>.
While at it, observe that ExactSizeIterator can also be implemented for
Take<RepeatWhile<F>> so add that implementation too. Since in contrast
to Repeat, RepeatWhile doesn’t guarante to always return the same value,
DoubleEndedIterator isn’t implemented.
Those changes render core::iter::repeat_n somewhat redundant.
Issue: https://github.com/rust-lang/rust/issues/104434
Issue: https://github.com/rust-lang/rust/issues/104729
Most modules have such a blank line, but some don't. Inserting the blank
line makes it clearer that the `//!` comments are describing the entire
module, rather than the `use` declaration(s) that immediately follows.
The addition of `core::iter::zip` (#82917) set a precedent for adding
plain functions for iterator adaptors. Adding `chain` makes it a little
easier to `chain` two iterators.
```
for (x, y) in chain(xs, ys) {}
// vs.
for (x, y) in xs.into_iter().chain(ys) {}
```
Implement iterator specialization traits on more adapters
This adds
* `TrustedLen` to `Skip` and `StepBy`
* `TrustedRandomAccess` to `Skip`
* `InPlaceIterable` and `SourceIter` to `Copied` and `Cloned`
The first two might improve performance in the compiler itself since `skip` is used in several places. Constellations that would exercise the last point are probably rare since it would require an owning iterator that has references as Items somewhere in its iterator pipeline.
Improvements for `Skip`:
```
# old
test iter::bench_skip_trusted_random_access ... bench: 8,335 ns/iter (+/- 90)
# new
test iter::bench_skip_trusted_random_access ... bench: 2,753 ns/iter (+/- 27)
```
detects redundant imports that can be eliminated.
for #117772 :
In order to facilitate review and modification, split the checking code and
removing redundant imports code into two PR.
This is inherited from the old PR.
This method returns an iterator over mapped windows of the starting
iterator. Adding the more straight-forward `Iterator::windows` is not
easily possible right now as the items are stored in the iterator type,
meaning the `next` call would return references to `self`. This is not
allowed by the current `Iterator` trait design. This might change once
GATs have landed.
The idea has been brought up by @m-ou-se here:
https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/Iterator.3A.3A.7Bpairwise.2C.20windows.7D/near/224587771
Co-authored-by: Lukas Kalbertodt <lukas.kalbertodt@gmail.com>
For ranges < usize we determine the number of items
StepBy would yield and then store that in the range.end
instead of the actual end. This significantly
simplifies calculation of the loop induction variable
especially in cases where StepBy::step (an usize)
could overflow the Range's item type
A successful advance is now signalled by returning `0` and other values now represent the remaining number
of steps that couldn't be advanced as opposed to the amount of steps that have been advanced during a partial advance_by.
This simplifies adapters a bit, replacing some `match`/`if` with arithmetic. Whether this is beneficial overall depends
on whether `advance_by` is mostly used as a building-block for other iterator methods and adapters or whether
we also see uses by users where `Result` might be more useful.
Refactor iteration logic in the `Flatten` and `FlatMap` iterators
The `Flatten` and `FlatMap` iterators both delegate to `FlattenCompat`:
```rust
struct FlattenCompat<I, U> {
iter: Fuse<I>,
frontiter: Option<U>,
backiter: Option<U>,
}
```
Every individual iterator method that `FlattenCompat` implements needs to carefully manage this state, checking whether the `frontiter` and `backiter` are present, and storing the current iterator appropriately if iteration is aborted. This has led to methods such as `next`, `advance_by`, and `try_fold` all having similar code for managing the iterator's state.
I have extracted this common logic of iterating the inner iterators with the option to exit early into a `iter_try_fold` method:
```rust
impl<I, U> FlattenCompat<I, U>
where
I: Iterator<Item: IntoIterator<IntoIter = U>>,
{
fn iter_try_fold<Acc, Fold, R>(&mut self, acc: Acc, fold: Fold) -> R
where
Fold: FnMut(Acc, &mut U) -> R,
R: Try<Output = Acc>,
{ ... }
}
```
It passes each of the inner iterators to the given function as long as it keep succeeding. It takes care of managing `FlattenCompat`'s state, so that the actual `Iterator` methods don't need to. The resulting code that makes use of this abstraction is much more straightforward:
```rust
fn next(&mut self) -> Option<U::Item> {
#[inline]
fn next<U: Iterator>((): (), iter: &mut U) -> ControlFlow<U::Item> {
match iter.next() {
None => ControlFlow::CONTINUE,
Some(x) => ControlFlow::Break(x),
}
}
self.iter_try_fold((), next).break_value()
}
```
Note that despite being implemented in terms of `iter_try_fold`, `next` is still able to benefit from `U`'s `next` method. It therefore does not take the performance hit that implementing `next` directly in terms of `Self::try_fold` causes (in some benchmarks).
This PR also adds `iter_try_rfold` which captures the shared logic of `try_rfold` and `advance_back_by`, as well as `iter_fold` and `iter_rfold` for folding without early exits (used by `fold`, `rfold`, `count`, and `last`).
Benchmark results:
```
before after
bench_flat_map_sum 423,255 ns/iter 414,338 ns/iter
bench_flat_map_ref_sum 1,942,139 ns/iter 2,216,643 ns/iter
bench_flat_map_chain_sum 1,616,840 ns/iter 1,246,445 ns/iter
bench_flat_map_chain_ref_sum 4,348,110 ns/iter 3,574,775 ns/iter
bench_flat_map_chain_option_sum 780,037 ns/iter 780,679 ns/iter
bench_flat_map_chain_option_ref_sum 2,056,458 ns/iter 834,932 ns/iter
```
I added the last two benchmarks specifically to demonstrate an extreme case where `FlatMap::next` can benefit from custom internal iteration of the outer iterator, so take it with a grain of salt. We should probably do a perf run to see if the changes to `next` are worth it in practice.
The `advance_by(n)` docs state that in the error case `Err(k)` that k is always less than n.
It also states that `advance_by(0)` may return `Err(0)` to indicate an exhausted iterator.
These statements are inconsistent.
Since only one implementation (Skip) actually made use of that I changed it to return Ok(()) in that case too.
While adding some tests I also found a bug in `Take::advance_back_by`.
Due to #20400 the corresponding TrustedLen impls need a helper trait
instead of directly adding `Item = &[T;N]` bounds.
Since TrustedLen is a public trait this in turn means
the helper trait needs to be public. Since it's just a workaround
for a compiler deficit it's marked hidden, unstable and unsafe.
This only works if arrays are passed directly instead of array iterators
because we need to be sure that they have not been advanced before
Flatten does its size calculation.
This was unsound since a panic in a.next_back() would result in the
length not being updated which would then lead to the same element
being revisited in the side-effect preserving code.
