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Document unsafety in core::{panicking, alloc::layout, hint, iter::adapters::zip}
Helps with #66219.
r? @Mark-Simulacrum do you want to continue reading safety comments? :D
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iter::adapters::zip}`
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Don't fuse Chain in its second iterator
Only the "first" iterator is actually set `None` when exhausted,
depending on whether you iterate forward or backward. This restores
behavior similar to the former `ChainState`, where it would transition
from `Both` to `Front`/`Back` and only continue from that side.
However, if you mix directions, then this may still set both sides to
`None`, totally fusing the iterator.
Fixes #71375
r? @scottmcm
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Only the "first" iterator is actually set `None` when exhausted,
depending on whether you iterate forward or backward. This restores
behavior similar to the former `ChainState`, where it would transition
from `Both` to `Front`/`Back` and only continue from that side.
However, if you mix directions, then this may still set both sides to
`None`, totally fusing the iterator.
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Dogfood or_patterns in the standard library
We can start using `or_patterns` in the standard library as a step toward stabilization.
cc #54883 @Centril
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Hides default fns inside Fuse impl to avoid exposing it to any crate
Fixes #70796
@cuviper I've added some default, private traits to do the job for us. If required, I can expose them to a specific visibility if you want to call these functions for #70332
r? @cuviper
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Removed unnecessarry empty impls.
Moved code to organise it better
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FuseExactSizeIteratorImpl to avoid exposing default functions outside of the current crate.
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The iterators are now "fused" with `Option` so we don't need separate
state to track which part is already exhausted, and we may also get
niche layout for `None`. We don't use the real `Fuse` adapter because
its specialization for `FusedIterator` unconditionally descends into the
iterator, and that could be expensive to keep revisiting stuff like
nested chains. It also hurts compiler performance to add more iterator
layers to `Chain`.
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Match options directly in the Fuse implementation
Rather than using `as_ref()`, `as_mut()`, and `?`, we can use `match` directly to save a lot of generated code. This was mentioned as a possibility in https://github.com/rust-lang/rust/pull/70366#issuecomment-603462546, and I found that it had a very large impact on #70332 using `Fuse` within `Chain`. Let's evaluate this change on its own first.
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- Added `Iterator::fold_first`, which is like `fold`, but uses the first element in the iterator as the initial accumulator
- Includes doc and doctest
- Rebase commit; see #65222 for details
Co-Authored-By: Tim Vermeulen <tvermeulen@me.com>
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The former `done` flag was roughly similar to an `Option` tag, but left
the possibity of misuse. By using a real `Option`, we can set `None`
when the iterator is exhausted, removing any way to call it again. We
also allow niche layout this way, so the `Fuse` may be smaller.
The `FusedIterator` specialization does want to ignore the possibility
of exhaustion though, so it uses `unsafe { intrinsics::unreachable() }`
to optimize that branch away. The entire `Fuse` implementation is now
isolated in its own module to contain that unsafety.
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r=LukasKalbertodt
Remove `finished` flag from `MapWhile`
This PR removes `finished` flag from `MapWhile` as been proposed in https://github.com/rust-lang/rust/pull/66577#discussion_r370958025.
This also resolves open questions of the tracking issue (#68537):
- `MapWhile` can't implement both
+ `DoubleEndedIterator` (discussed in https://github.com/rust-lang/rust/pull/66577#discussion_r370947990 and following comments)
+ `FusedIterator` (this pr removes `finished` flag, so `MapWhile` isn't fused anymore)
- Debug output (this pr removes `finished` flag, so there is no question in including it in debug output)
r? @Mark-Simulacrum
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Fix bugs in Peekable and Flatten when using non-fused iterators
I fixed a couple of bugs with regard to the `Peekable` and `Flatten`/`FlatMap` iterators when the underlying iterator isn't fused. For testing, I also added a `NonFused` iterator wrapper that panics when `next` or `next_back` is called on an iterator that has returned `None` before, which will hopefully make it easier to spot these mistakes in the future.
### Peekable
`Peekable::next_back` was implemented as
```rust
self.iter.next_back().or_else(|| self.peeked.take().and_then(|x| x))
```
which is incorrect because when the `peeked` field is `Some(None)`, then `None` has already been returned from the inner iterator and what it returns from `next_back` can no longer be relied upon. `test_peekable_non_fused` tests this.
### Flatten
When a `FlattenCompat` instance only has a `backiter` remaining (i.e. `self.frontiter` is `None` and `self.iter` is empty), then `next` will call `self.iter.next()` every time, so the `iter` field needs to be fused. I fixed it by giving it the type `Fuse<I>` instead of `I`, I think this is the only way to fix it. `test_flatten_non_fused_outer` tests this.
Furthermore, previously `FlattenCompat::next` did not set `self.frontiter` to `None` after it returned `None`, which is incorrect when the inner iterator type isn't fused. I just delegated it to `try_fold` because that already handles it correctly. `test_flatten_non_fused_inner` tests this.
r? @scottmcm
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Implement nth, last, and count for iter::Copied
Implement nth, last and count for iter::Copied.
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Remove spotlight
I had a few comments saying that this feature was at best misunderstood or not even used so I decided to organize a poll about on [twitter](https://twitter.com/imperioworld_/status/1232769353503956994). After 87 votes, the result is very clear: it's not useful. Considering the amount of code we have just to run it, I think it's definitely worth it to remove it.
r? @kinnison
cc @ollie27
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use question mark operator in a few places.
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Improve documentation on iterators length
Attempts to resolve #66491. @the8472 does this help?
r? @steveklabnik
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Fix doc of `Iterator::map_while` so it would
be clearer that it isn't fused.
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Simplify `Skip::nth` and `Skip::last` implementations
The main improvement is to make `last` no longer recursive.
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Remove some unsound specializations
This removes the unsound and exploitable specializations in the standard library
* The `PartialEq` and `Hash` implementations for `RangeInclusive` are changed to avoid specialization.
* The `PartialOrd` specialization for slices now specializes on a limited set of concrete types.
* Added some tests for the soundness problems.
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Iterators typically don't implement `Copy` and this shouldn't be an exception.
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